Thurrock Council - Air Quality Assessment s02

Detailed Assessment of SO2

for the Thurrock Council

July 2005 ERG, King’s College, London 4th Floor, Franklin Wilkins Building, Stamford Street, London SE1 9NN Tel: 020 7848 4011

Thurrock Council – Detailed Assessment of SO2

ERG, King’s College London 2



Executive Summary This report provides supplementary information regarding sulphur dioxide (SO2) from the Coryton Refinery to that of the Council’s Detailed Assessment for 2004. The Council were due to produce a Progress report this year but based on recent findings in relation to the area close to the refinery, it has instead opted to undertake Detailed Assessment for SO2, without delay, as indicated in the government’s PRG 03 guidance. The above findings indicate that there is a risk that the air quality objectives will be exceeded where there is risk of public exposure. The report utilises additional information beyond that available for the previous report, including both monitoring results and information relating to refinery operations. The monitoring indicates that the 15-minute mean, 1-hour mean and 24 hour mean objectives were exceeded at both BP’s site at Oozedam Farm and Coryton in 2004. Further analysis is presented of the episodes at the monitoring sites in terms of time day and year. Analysis using wind data confirms that the refinery is the source of pollution. The objectives were not exceeded at the more distant monitoring sites run by the Council at Corringham (Thurrock 3) and a site in Castle Point on Canvey Island. This report uses reports from BP to models emissions from the refinery. The results indicate that the 15 minute mean objective will be exceeded and show good agreement with modelling undertaken by BP. Predictions made for 2004 however do not provide good agreement with the 2004 monitoring results. Possible for reasons for this include uncertainty with the emissions data (based on a lack of time varying data), the complexity of the refinery and its emission sources and the general difficulty in predicting peak concentrations. In this instance a suitable adjustment factor was found not to be appropriate. As a result of the findings in this report the Council is recommended to undertake the following actions, in respect of the findings for the statutory sulphur dioxide objectives:

To investigate and confirm the potential for public exposure in areas predicted to exceed the SO2 15 minute objective in Figure 14 and Figure 15. Based on a confirmation that public exposure does exist, to designate an AQMA, based on these findings. Undertake further consultation on the findings arising from this report with the statutory and other consultees as required. To continue to use the outcome of the consultation process to determine the need for remedial measures, which the Environment Agency will enforce, regarding exceedences from the sources at Coryton Refinery.



To seek additional information regarding time varying emissions from sources at the refinery.

Continue its SO2 monitoring programme as part of its LAQM actions



Table of Contents

1 INTRODUCTION TO DETAILED ASSESSMENT OF SO2 .............................. 9

1.1 OVERVIEW TO THE REPORT................................................................................. 9 1.2 BACKGROUND .................................................................................................... 9

2 SO2 AIR POLLUTION MEASUREMENTS IN THE THURROCK AREA.... 11

2.1 MONITORING UPDATE ...................................................................................... 11 2.2 SUMMARY OF SULPHUR DIOXIDE MEASUREMENTS ............................................ 12 2.3 FURTHER EPISODE ANALYSIS ............................................................................ 16

2.3.1 Time of day .............................................................................................. 16 2.3.2 Monthly distribution ................................................................................ 17 2.3.3 Analysis of episodes and wind factors ..................................................... 18 2.3.4 Influences at Thurrock 3 and Castle Point............................................... 21

3 PREDICTIONS OF SULPHUR DIOXIDE (SO2)............................................................ 24 3.1 Outline of modelling developments.............................................................. 24

3.2 DISPERSION MODEL .......................................................................................... 24 3.3 FIGURES SHOWING MODEL PREDICTIONS........................................................... 24 3.5 MODELLED CONCENTRATIONS USING BP EMISSIONS DATA ............................... 25 3.6 PREDICTED 99.9TH PERCENTILE CONCENTRATIONS ............................................ 25 3.7 PREDICTED 99.73TH PERCENTILE CONCENTRATIONS .......................................... 27 3.8 PREDICTED 99.18TH PERCENTILE CONCENTRATIONS .......................................... 29 3.9 COMPARISON WITH BP PREDICTIONS ................................................................ 31 3.10 COMPARISON OF MODELLING AT THE MONITORING SITES .................................. 31 3.11 COMPARISON OF MONITORED AND MODELLED RESULTS.................................... 32 3.12 SENSITIVITY TESTS ........................................................................................... 34 3.13 PREDICTIONS OF MAIN SOURCES IN REFINERY ................................................... 35 3.14 CHANGING EMISSIONS PARAMETERS................................................................. 37 3.15 CHANGED SCENARIOS....................................................................................... 38 3.15 COMMENTARY ON MODEL VERIFICATION.......................................................... 45

CONCLUSION.............................................................................................................. 46

RECOMMENDATIONS .............................................................................................. 47

REFERENCES: ............................................................................................................ 48

APPENDIX A ................................................................................................................ 49

METEOROLOGY INFORMATION USED FOR THE MODELLING .................. 49

APPENDIX B ................................................................................................................ 50

MODELLING APPROACH ........................................................................................ 50

Area of interest........................................................................................................ 50 Dispersion model and parameters used................................................................... 50 Limitations in predicting short term objectives ....................................................... 51 Treatment of background SO2 ................................................................................. 51

APPENDIX C ................................................................................................................ 51



EMISSIONS FROM CORYTON REFINERY........................................................... 51

INTRODUCTION ............................................................................................................ 51 ASSUMPTIONS USED IN THE REPORT.............................................................................. 52

APPENDIX D ................................................................................................................ 53

QA/QC DETAILS FOR BP MONITORING SITES.................................................. 53



List of Figures Figure 1 Monitoring sites near Coryton refinery ............................................................. 11 Figure 2 SO2 monitoring at Coryton and Oozedam Farm sites (15 minute objective) ..... 13 Figure 3 SO2 monitoring at Coryton and Oozedam Farm sites (1 hour mean objective) . 13 Figure 4 SO2 monitoring at Coryton and Oozedam Farm sites (24 hour mean objective)14 Figure 5 Distribution of episodes throughout the day at the Oozedam Farm site ............ 16 Figure 6 Distribution of episodes throughout the day at the Coryton site........................ 17 Figure 7 Monthly distributions of episodes at the Oozedam Farm and Coryton sites...... 18 Figure 8 Location of monitoring sites and refinery sources............................................. 19 Figure 9 Wind directions and speed for episodes monitored at Oozedam Farm in 2004 . 20 Figure 10 Wind directions and speed for episodes monitored at Coryton site in 2004 .... 20 Figure 11 Main sources of emissions in the refinery (see Table 13)................................ 21 Figure 12 Monitoring results and wind direction for Thurrock 3 (in 2004) ..................... 22 Figure 13 Monitoring results and wind direction for Castle Point (in 2004) ................... 23 Figure 14 Predicted 99.9th percentile SO2 15-minute mean concentration (μg m-3) for the

southeast of Thurrock (using 2002 meteorology from Manston)............................. 26 Figure 15 Predicted 99.9th percentile SO2 15-minute mean concentration (μg m-3) for the

southeast of Thurrock (using 2004 meteorology from Heathrow) ........................... 26 Figure 16 Comparison of above 15-minute predictions................................................... 27 Figure 17 Predicted 99.73th percentile SO2 1 hour mean concentration (μg m-3) for the

southeast of Thurrock (using 2002 meteorology from Manston)............................. 28 Figure 18 Predicted 99.73th percentile SO2 1 hour mean concentration (μg m-3) for the

southeast of Thurrock (using 2004 meteorology from Heathrow) ........................... 28 Figure 19 Comparison of 1hour predictions .................................................................... 29 Figure 20 Predicted 99.18th percentile SO2 24 hour mean concentration (μg m-3) for the

southeast of Thurrock (using 2002 meteorology from Manston)............................. 30 Figure 21 Predicted 99.18th percentile SO2 24 hour mean concentration (µg m-3) for the

southeast of Thurrock (using 2004 meteorology from Heathrow) ........................... 30 Figure 22 Comparison of predicted 24-hour mean concentrations .................................. 31 Figure 23 Predicted 99.9th percentile SO2 15-minute mean concentration (µg m-3) for the

southeast of Thurrock (using 2004 Heathrow met data) using maximum release rates................................................................................................................................ 40

Figure 24 Predicted 99.9th percentile SO2 15-minute mean concentration (µg m-3) for the southeast of Thurrock (using 2004 Heathrow met data) using increased release rates................................................................................................................................ 40

Figure 25 Predicted 99.73th percentile SO2 1 hour mean concentration (µg m-3) for the southeast of Thurrock (using 2004 Heathrow met data) using maximum release rates................................................................................................................................ 42

Figure 26 Predicted 99.73th percentile SO2 1 hour mean concentration (µg m-3) for the southeast of Thurrock (using 2004 Heathrow met data) using increased release rates................................................................................................................................ 42





Figure 29 Wind rose for Manston and Heathrow 2002.................................................... 49 Figure 30 Coryton and Oozedam Farm 2004 monitoring (hourly mean concentrations).55 List of Tables Table 1 U.K Air quality objectives relevant to LAQM for SO2....................................... 12 Table 2 SO2 monitoring results for Thurrock 3 and Castle Point (1999 to 2004 inclusive)

................................................................................................................................ 15 Table 3 Equivalent 99.9th percentile concentrations of 15 minute mean SO2 (μg m-3) .... 15 Table 4 Equivalent 2004 percentile concentrations of 15 minute mean SO2 for Oozedam

Farm and Coryton sites (μg m-3).............................................................................. 16 Table 5 Comparison of predictions at monitoring sites (µg m-3) ..................................... 32 Table 6 2004 Measurements from the monitoring sites (µg m-3)..................................... 33 Table 7 Predicted concentrations for all refinery stacks (µg m-3) .................................... 35 Table 8 Predicted concentrations for refinery stacks 2, 4, 5, 17 and 18 (µg m-3)............. 35 Table 9 Predicted percentile concentrations for individual stacks (µg m-3) ..................... 36 Table 10 Predicted percentile concentrations for stacks 17 and 18 (µg m-3).................... 37 Table 11 Predicted percentile concentrations of changed parameters (µg m-3)................ 37 Table 12 Predicted percentile concentrations of changed parameters (µg m-3)................ 45 Table 13 Analysis of wind sectors based on the monitoring sites.................................... 50 Table 14 Details of Coryton sources and stack parameters.............................................. 52 Table 15 Refinery emission rates .................................................................................... 53



1 Introduction to Detailed Assessment of SO2 1.1 Overview to the report

This report provides further information to supplement the Detailed Assessment of air quality produced by the Thurrock Council in 2004. Additional emissions information relating to the Coryton refinery, beyond that available for the previous report is now utilised. This report also updates the continuous monitoring results in the area and its interpretation. The report is intended to further aid the Council’s Local Air Quality Management (LAQM) progress in respect of this pollutant. The Council’s original Detailed Assessment focused on monitoring results and recognised that action was in hand to limit emissions to ensure that the government’s air quality objectives were met. This report is intended to assess the extent that these objectives are exceeded for the purposes of local air quality management.

1.2 Background

Since the production of the Detailed Assessment, further information relating to SO2 emissions from the refinery has become available to the Council. This has mainly been derived from the Environment Agency and BP, who operate the refinery. The BP Management Plan reports (dated January 2003 ands March 2004) have also been used. In these reports, the operator assessed the likely air quality impacts of emissions from the refinery as part of the IPPC regulation of the refinery. It used specific emissions data acquired from the specific processes in operation at the refinery. Key points to note from these reports are:

1. The BP Management Plan reports (BP report) provide detailed emissions information obtained from a 20-month period when the plant was operating normally. Periods of plant shutdown and significant periods of down time were excluded from the assessment (see Appendix 5 of the BP report). The BP report provides the average release rate for the stacks at the refinery. All stacks operate on continuous basis and time varying factors were not supplied, however maximum release rates were provided for information.

2. The BP report used factoring to convert predicted hourly 99.9th percentile

concentrations to 15-minute 99.9th percentile concentrations.

3. The BP report modelled the refinery using ADMS3.1, with meteorological data from Manston in Kent for 1999 (considered as worst case).

Formatted: Bullets andNumbering



4. The BP report predicted the 15-minute mean air quality objective for SO2 to be exceeded in an area surrounding the refinery. The area identified as exceeding this objective extends almost 1km beyond the refinery boundary. The equivalent 99.9th percentile predicted at the maximum point of impact was 557 µg m-3. The modelling indicated that the one hour and twenty-four hour objectives were not exceeded (page 9 of BP report).

Further details regarding the emissions of SO2 are given in the appendices.



2 SO2 Air Pollution Measurements in the Thurrock area 2.1 Monitoring Update

This section of the report provides a further update on recent air pollution monitoring undertaken close to the Coryton refinery, beyond that provided in the Detailed Assessment. The fixed monitoring sites all the use continuously operated UV-fluorescence analysers. Monitoring data are available for 4 sites in the vicinity of the refinery, including: 1) The Thurrock Council’s site at Corringham (TK3) 2) The Castle Point D.C site on Canvey Island (CP1) 3) The two BP operated sites at Coryton refinery and the nearby Oozedam Farm The sites are all shown in Figure 1. The TK3 and CP1 sites are both part of the London Air Quality Network and therefore the QA/QC standards for the sites meet those required by the government TG03 guidance. The QA/QC details provided for the BP sites at Oozedam Farm and Coryton are provided in Appendix 2 of the BP report and are reproduced in Appendix D of this report. This shows that the quality of the data is to be comparable to AUN sites. In all instances it is considered that the results are representative, although it should be noted that the data for 2004 from the Council’s and the Castle Point sites are currently not fully ratified. Confirmation details of any final ratification of the BP sites are not known.

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Figure 1 Monitoring sites near Coryton refinery




2.2 Summary of sulphur dioxide measurements

The figures below expand and update the information provided in the Council’s earlier report. It should be noted that the data are summarised for calendar years in line with normal reporting practice. The data are also reported for the government’s three SO2 objectives (see Table 1 below).

Table 1 U.K Air quality objectives relevant to LAQM for SO2

Concentration Measured as Date to be achieved by

266μg m-3 (100 ppb) not be exceeded more than 35 times a

year

15 minute mean 31-Dec-05

350μg m-3 (131 ppb) not be exceeded more than 24 times a

year

1 hour mean 31-Dec-04

125μg m-3 (47 ppb) not be exceeded more than 3 times a year

24 hour mean 31-Dec-04

Monitoring results for the BP sites at Coryton and Oozedam Farm were only available for 2003 and 2004. The data capture for the sites in 2004 was 77.7% for Coryton site and 95.1% for Oozedam Farm site (based on 15 minute measurements). Details of the data capture for 2003 are not known. The TG03 guidance indicates that 90% data capture is ideally required for short-term objective comparison, except in cases where the objective is exceeded with a lower data capture rate. Figure 2 shows that the 15-minute objective was easily exceeded at the two monitoring sites, in both 2003 and 2004. In 2003 the Oozedam Farm site monitored more periods that exceeded the objective than the Coryton site. In 2004 both sites monitored similar numbers of periods, with slightly more at Coryton than Oozedam Farm. The increase in the number of periods at Coryton almost doubled between 2003 and 2004. The increase at Oozedam Farm for the same period was only 11%. Figure 3 shows that the one-hour objective was easily exceeded at the Oozedam Farm site in both 2003 and 2004. The Coryton site only exceeded the objective in 2004, having approached the objective in 2003 with 20 periods that exceeded the objective. There was a 70% increase in periods that exceeded between 2003 and 2004 for this site. For both years the Oozedam Farm site monitored more periods. Figure 4 shows that the 24-hour objective was exceeded at the Oozedam Farm and Coryton sites in 2004 only. The objective was not exceeded in 2003 at either site, all both recorded 2 periods when the 24 hour standard was exceeded.



Figure 2 SO2 monitoring at Coryton and Oozedam Farm sites (15 minute objective)

Figure 3 SO2 monitoring at Coryton and Oozedam Farm sites (1 hour mean objective)

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Figure 4 SO2 monitoring at Coryton and Oozedam Farm sites (24 hour mean objective)

The Thurrock 3 site started operating in 2003, with 2004 being the first full year of monitoring. The Castle Point site started operating in 1996 and the monitoring reported in this report are since 1999. The data capture for all years from 1999 to 2004 exceeded 90%. The results for the Thurrock 3 and Castle Point sites are provided in Table 2. The 15 minute, one hour and 24 hour objectives were not exceeded at either site. There were also no periods that exceeded the 15-minute objective standard of 266 µg m-3 at Thurrock 3 and at the Castle Point site (since 2002). In 2002, 2000 and 1999 there were 2, 2 and 5 periods each year respectively. The one hour standard was exceeded on one occasion only at the Castle Point site during the six-year period reported (in 1999). The equivalent percentile concentrations for the sites are given in Table 3 and Table 4. The 99.9th percentile for Castle Point site has reduced from greater than 110 µg m-3 in 1999 and 2000 to approximately 80 µg m-3 in 2003 and 2004. The 99.9th percentile for Thurrock 3 in 2004 was also less than 100 µg m-3. The equivalent percentile concentrations for the one and 24-hour objectives are not given, although all are less than the 99.9th percentile values.

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The 99.9th percentile concentrations for Coryton and Oozedam Farm for 2004 were 673 and 1046 µg m-3 respectively, easily exceeding the objective standard of 266 µg m-3. The equivalent percentile for the one hour objective for both sites exceeded 500 µg m-3 and therefore also exceeded the one hour objective standard of 350 µg m-3. For the 24-hour objective the equivalent percentile concentrations were 148 µg m-3 for Coryton and 128 µg m-3 for Oozedam Farm. These concentrations exceeded the 24 hour objective standard of 125 µg m-3. (Note – summary data only were provided for 2003, hence the equivalent percentiles cannot be reported here). The differences between the Oozedam Farm and Coryton results are further discussed in section 3.xx.

Table 2 SO2 monitoring results for Thurrock 3 and Castle Point (1999 to 2004 inclusive)

Year

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means > 266 μg m-3

Maximum 15 minute mean

μg m-3

No of 1 hr means >

350 μg m-3 Data capture

% Castle Point

2004* 0 167 0 94 2003 0 147 0 97 2002 2 343 0 96 2001 0 223 0 98 2000 2 337 0 94 1999 5 915 1 92

Thurrock 3 2004* 0 187 0 97 2003 0 147 0 34

(Note - * indicates that data not fully ratified)

Table 3 Equivalent 99.9th percentile concentrations of 15 minute mean SO2 (μg m-3)

Year Castle Point Thurrock 3 2004 83 88 2003 77 81 2002 99 no 2001 92 no 2000 121 no 1999 116 no

(no indicates not in operation)



Table 4 Equivalent 2004 percentile concentrations of 15 minute mean SO2 for Oozedam Farm and Coryton sites (μg m-3)

Site 99.9th

percentile 99.73th

percentile 99.18th

percentile Coryton refinery 673.0 500.6 148.0 Oozedam Farm 1046.7 579.8 128.6

2.3 Further episode analysis 2.3.1 Time of day

Further analysis has been undertaken of the episodes of high SO2 pollution that arose at the two BP monitoring sites. This is intended to show whether or not episodes arise throughout the 24-hour day and night period as might be expected from continuous production processes. In all instances episodes greater than the 15-minute mean standard have been plotted against the time of day for the different pollution years.

Figure 5 Distribution of episodes throughout the day at the Oozedam Farm site

Figure 5 indicates that at least 1 episode arose during all times of the day and night, with the only exceptions being six periods, late at night when there were no episodes, including a period between 01:00 and 01:30 hours. The peak period that

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episodes arose was around 07:00 hours. On average there were about two episodes that arose for every 15-minute period throughout the 24-hour period at this site. This finding confirms that the refinery processes were operating more or less continuously as might be expected. Episodes at the Coryton site also arose throughout the 24 hours. There were 3 periods when there were no episodes, including a period between 01:00 and 01:30 hours. This finding further confirms that refinery processes were operating continuously.

Figure 6 Distribution of episodes throughout the day at the Coryton site

2.3.2 Monthly distribution

The distribution of high pollution episodes can often vary dependent on seasonal meteorological factors. To understand this for the Oozedam Farm and Coryton sites, the 15-minute episodes > 266 µg m-3 have been grouped into monthly totals. Figure 7 shows the distribution of episodes through the year for both sites for 2004. Episodes arose at the Oozedam Farm site throughout each month of the year apart from February. The number of episodes was reduced for the winter period with only 10 episodes during the three months of January, February and December. The peak month for episodes was October, with 50 episodes. There were more than 35 episodes during four other months (May, June, August and November). The pattern of episodes for this site does not strongly suggest any seasonal effect, other

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than perhaps stronger winds during the winter months causing greater dispersion and lower concentrations for these months. The distribution for the Coryton site was however markedly different from the Oozedam Farm, with 85% of episodes in two months of September and October. In September, episodes arose on 13 days and in October episodes arose on 9 days. There were also four months with no periods exceeding the standard at the site. Without further information about emissions and analysis at this stage it is difficult to postulate why the episodes were so concentrated within these two months.

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Figure 7 Monthly distributions of episodes at the Oozedam Farm and Coryton sites

2.3.3 Analysis of episodes and wind factors

The Council’s previous Detailed Assessment used wind data from the Bexley 2 LAQN site in southeast London as being representative of the Thurrock area. These wind data have further been used to develop an understanding of the wind conditions at the two BP monitoring sites. Figure 8 shows the location of the monitoring sites in relation to the refinery sources. Bexley 2 data are reported for 15-minute mean intervals.



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Figure 8 Location of monitoring sites and refinery sources

The Oozedam Farm site is located approximately 1km northwest of the main cluster of sources and Coryton is located approximately 500m to the northeast of the main cluster (i.e. stacks 1,17, 18, 19, 7, 8 and 14 based on Figure 11 and Appendix B). To investigate conditions when high pollution episodes arise, both wind direction and wind speed have been plotted together for the 15-minute episodes > 266 µg m-3 that arose at each site. Wind data were not available for all episodes. For the Oozedam Farm episodes there was 67.2% availability and for Coryton there was 91.3%. Figure 9 shows that the majority of episodes (77%) for the Oozedam Farm site arose between 90° to 135° for all wind speeds. If winds greater than 1 m s-1 were only to be considered then this proportion increases to 90%. The mean wind direction for all episodes was 122° indicating that the source is southeast of the site and therefore corresponding with the refinery. Episodes arose with a mean wind speed of 1.4 m s-1 and a maximum speed of 4 m s-1. There was a group of episodes located around 350° to 10°; these arose mostly during periods with wind speeds less than 1 m s-1. These low wind speeds can be considered to indicate calm conditions and under such conditions plumes can meander greatly and also be re-circulated. This figure also indicates that the flare (sited separate from the main part of the refinery and to the north of the site) is not an important source of high episodes.



Figure 9 Wind directions and speed for episodes monitored at Oozedam Farm in 2004

Figure 10 Wind directions and speed for episodes monitored at Coryton site in 2004

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Episodes for the Coryton monitoring site (Figure 10) were not as normally distributed as at the Oozedam Farm site. Instead as winds increased from 0 to 2 m s-1 the wind direction changes from the south (180°) to southwest (240° to 250°). However if wind speeds greater than 1 m s-1 only were considered (representing 92% of the episodes) the mean wind direction was 239° (i.e. from the southwest). Figure 11 shows that this relates to a group of stacks only (1,17,18, 19, 7, 8 and 14) and not to all stacks in the refinery. This suggests that that the emission sources were the specific point sources of emissions. The processes that the stacks relate to are shown in Table 14 and Table 15 and discussed further in the BP report. The mean winds speed for the episodes was 3.2 m s-1 and a maximum speed of 8 m s-1. In both instances these were higher than the wind speeds associated with the Oozedam Farm episodes. The higher wind speeds suggest that the plumes were bent down for these episodes.

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Figure 11 Main sources of emissions in the refinery (see Table 14) (Notes – the location of the point sources was taken from the BP report and the grid reference is to the

nearest 10m, hence some sources appear as overlapping. There is not a source numbered 3) 2.3.4 Influences at Thurrock 3 and Castle Point

The Thurrock 3 and Castle Point sites are located approximately 5 km directly west and 6 km east of the refinery, respectively. As already seen there were no instances of periods of high pollution where the 15-minute standard of 266 µg m-3 was exceeded in the past two years. To determine whether the refinery influenced concentrations, the 15-minute mean results for the sites have been plotted against wind direction for 2004. Only data where the wind speed was greater than 1 m s-1 have been included.



Figure 12 shows the 2004 results for the Thurrock 3 site. This indicates that elevated concentrations predominantly were below 20 µg m-3. Higher concentrations above this level arose when the wind was from two main prevailing directions, when the wind was from the 90° to 120° sector and 210° to 240° sector. For both sectors the concentrations peaked at approximately 120 µg m-3. This suggests that influencing sources were east of the site and southwest of the site. The refinery is located to east of the site and this analysis suggests that it does influence concentrations at the site, although not so as to cause 15 minute mean concentrations to exceed the 266 µg m-3 standard.

Figure 12 Monitoring results and wind direction for Thurrock 3 (in 2004)

Thurrock 3 (wind>1m/s) 2004

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Figure 13 Monitoring results and wind direction for Castle Point (in 2004)

The results for the Castle Point site are shown in Figure 13. As with the Thurrock 3 site the majority of results were below 20 µg m-3, with two peaks, although one is more pronounced than the other. The most pronounced peak relates to the wind sector 240° to 270° (peaking at approximately 100 µg m-3), with the less distinct peak relating 90° to 105° (peaking at just over 40 µg m-3). Other peak concentrations arose when the wind was from 135° and 150° although these appear more isolated occurrences. The refinery is located to the west of this monitoring site and this analysis suggests that it influences concentrations, although again not to the extent that the 15 minute mean standard of 266 µg m-3 was exceeded.

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3 Predictions of Sulphur Dioxide (SO2) 3.1 Outline of modelling developments

This report supplements the Council’s previous Detailed Assessment for SO2 and utilises the emissions data now available from BP, as the refinery operator. The overall aim of the air pollution modelling is to produce a sound method whereby concentrations of SO2 can be determined and in conjunction with the continuous monitoring results be used to produce equivalent percentile values that can be compared with the AQS objectives. An explanation of the methods used in the report, including the assumptions and emissions information used is given in the appendices.

3.2 Dispersion model

The dispersion model used for this report was the same as that used by the refinery operator and previously used by the Council for the Detailed Assessment; i.e. ADMS 3.1. This is a well-established dispersion model that meets the requirements of TG03 for point sources. It is one of a new generation of models that are considered most appropriate for practical dispersion modelling; hence it is widely used by both regulatory bodies and the private sector. ADMS 3.1 incorporates a description of the boundary layer using boundary layer depth and the Monin-Obukhov length. The model has also been well validated for use with industrial point sources.

3.3 Figures showing model predictions

The modelled predictions of the three SO2 objectives are shown in the form of pollution maps for the Thurrock area1. It should be noted that the objectives relate to the number of occasions, which a standard based on a mean concentration for a stated time period is exceeded over a year. These statistics can only be fully determined using measurements. The government’s guidance however permit approximate equivalent percentiles to be used to assess pollution levels where modelling is necessary. For the 15 minute objective the equivalent percentile is the 99.9th percentile. This relates to the 36th highest 15-minute mean concentration arising over a year. Equivalent percentiles used for the one hour and 24 hour objectives respectively are the 99.73th and 99.18th percentiles.

1 The pollution plots are reproduced from Ordnance Survey material with the permission of Ordnance Survey on behalf of the Controller of Her Majesty’s Stationery Office© Crown copyright. Unauthorised reproduction infringes Crown copyright and may lead to prosecution or civil proceedings Thurrock Council Licence No. LA079766.




3.5 Modelled concentrations using BP emissions data

Two sets of modelling predictions were undertaken using the average release rate of stacks at Coryton (from the BP repot). Details of emissions used are given in Appendix C. The first sets of predictions were produced using 2002 meteorological data from Manston. This site was used for comparison purposes with the modelling undertaken by BP in its report. The second set used 2004 meteorological data from Heathrow. The 2004 data were used to enable comparisons with both the results for the 2002 data from Manston and to enable a comparison with 2004 monitored results. The plots for the three objectives are given in the sets of figures from Figure 14 to Figure 22. Additional figures are included to provide comparisons of the plots showing the differences for the two meteorological years. (Note – the plots include the BP monitoring sites, marked with grey crosses).

3.6 Predicted 99.9th percentile concentrations

The prediction of the area exceeding the 15-minute mean of 266 μg m-3 based on the 99.9th percentile is shown in Figure 14. This assumes that the meteorology for 2002 at Manston is repeated. The equivalent predictions for 2004 based on Heathrow met data are shown in Figure 15. The BP monitoring sites are also shown on these figures as dark grey crosses.

Both sets of predictions confirm that the 15-minute mean air quality objective will be exceeded in areas close to and around the refinery.

From Figure 14 the area predicted to exceed this objective forms an approximate oval shape with the longest axis in an approximate southeast to northwest direction. The area extends approximately 3 km from the centre of the refinery. There is a small area predicted not to exceed the objective in the refinery centred on the main sources modelled From Figure 15 the area predicted to exceed is a more rectangular shape with an area of lower concentration extending from near the main sources of emission in the refinery towards the north (this looks like an “indent” on the plot). The comparison of areas greater than the objective (using red and blue lines) shows that the extent of both sets of predictions is roughly similar, apart from the 2004 “indent”. The 200 µg m-3 and 350 µg m-3 contours are also shown and these are similarly located for both sets.




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Figure 14 Predicted 99.9th percentile SO2 15-minute mean concentration (μg m-3) for the southeast of Thurrock (using 2002 meteorology from Manston)

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Figure 15 Predicted 99.9th percentile SO2 15-minute mean concentration (μg m-3) for the

southeast of Thurrock (using 2004 meteorology from Heathrow)



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Figure 16 Comparison of above 15-minute predictions (Red line is 266 µg m-3 isopleth for Manston 2002 and blue line is Heathrow 2004 equivalent)


The prediction of the 1-hour mean of 350 µg m-3 based on the 99.73th percentile is shown in Figure 17. This assumes that the meteorology for 2002 at Manston is repeated. The equivalent prediction based on Heathrow 2004 met data is shown in Figure 18. Figure 17 and Figure 18 both show that the objective is not exceeded, although in both instances there are small areas that approach the 350 µg m-3 standard. The predicted concentration in Figure 18 is slightly larger and peaks at a higher concentration. In both figures the areas showing the highest concentrations arise within the refinery. A comparison of the two sets of predictions is shown in Figure 19 and this indicates broad similarity although the predictions differ at the highest concentrations nearest the stacks in the refinery.

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Figure 17 Predicted 99.73th percentile SO2 1 hour mean concentration (μg m-3) for the

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Figure 19 Comparison of 1hour predictions (Orange represents 2002 and pink 2004 predictions)


The prediction of the 24-hour mean of 125 µg m-3 based on the 99.18th percentile is shown in Figure 20. This assumes that the meteorology for 2002 at Manston is repeated. The equivalent prediction based on Heathrow 2004 met data is shown in Figure 21. The predictions both show that the objective is not exceeded, although in both instances there are small areas that approach the 125 µg m-3 standard. The predicted concentration in Figure 21 is slightly larger and peaks at a higher concentration than that in Figure 20. In both figures the areas showing the highest concentrations arise within the refinery. The comparison of predictions in Figure 22 shows only the isopleths at the 50 and 75 µg m-3 levels. These indicate some agreement in overall dimension but the distribution varies markedly around the refinery.

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Figure 21 Predicted 99.18th percentile SO2 24 hour mean concentration (µg m-3) for the

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Figure 22 Comparison of predicted 24-hour mean concentrations

(Orange represents 2002 and pink 2004 predictions) 3.9 Comparison with BP predictions

The BP report provided predictions for the 15-minute objective only using met data from Manston for 1999 for a number of potential air quality management scenarios. A comparison of the base case prediction (Case 1 of the BP report) with above the Manston 2002 prediction (Figure 14) indicates reasonable agreement, with the BP prediction, which also exceeded the objective beyond the refinery and beyond the Oozedam Farm monitoring site. A comparison with the prediction made using Heathrow 2004 data shows that there is less good agreement mainly due to the “indent” i.e. the area of lower concentrations in the refinery close the main emission sources as described in section 3.7.

3.10 Comparison of modelling at the monitoring sites

A comparison of the modelled predictions at the four monitoring sites (i.e. Thurrock 3, Castle Point, Oozedam Farm and Coryton refinery) is given in the Table 5 for the three SO2 objectives. The comparison is based on the predictions made using both 2002 Manston and 2004 Heathrow data. An initial comparison of the modelling between the two local authority sites, i.e. Thurrock 3 and Castle Point, for the three objectives indicates that there is reasonable agreement. This is as indicated earlier in sections 3.7 to 3.9. A corresponding comparison for the two BP sites however does not show such good agreement with the predictions based on 2002 Manston met data having higher

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concentrations for all objectives than those for the 2004 Heathrow met data. For the Coryton site the prediction for the 15-minute objective is almost 40% higher. The plots of the objectives above indicate that there is an area of lower concentrations closer to the main emission sources for those predictions using 2004 Heathrow met data. This is most clearly seen in Figure 15. From this it can be seen that both monitoring sites are located close to this area of lower concentrations, hence both sites have lower concentrations. The lower predictions arise as a result of the difference in the meteorological conditions between the two sets of data used. Despite this the plots as already indicated do show some similarity for the rest of the area modelled.

Table 5 Comparison of predictions at monitoring sites (µg m-3)

Site 99.9th%ile 99.73th%ile 99.18th%ile Manston 2002 Oozedam Farm 348.3 239.2 85.2 Coryton 356.6 199.1 65.3 Thurrock 3 105.1 62.5 39.0 Castle Point 154.0 74.0 30.8 Heathrow 2004 Oozedam Farm 272.3 170.2 64.4 Coryton 256.5 166.1 64.0 Thurrock 3 116.3 63.3 38.9 Castle Point 154.6 95.2 37.0

3.11 Comparison of monitored and modelled results

The purpose of this comparison is to give confidence in the modelling of the refinery. The 2004 monitoring results based on the equivalent percentiles are given in Table 6. A comparison of the 2004 modelled and monitored data at the BP sites indicates that the modelling has substantially under predicted concentrations, indicating that the modelling has not performed well. For the 15-minute objective the Oozedam Farm and Coryton site monitored results are approximately four times the Heathrow 2004 predictions. These differences reduce for the 1-hour objective and reduce even further for the 24-hour objective at both sites. The equivalent comparison for the two local authority sites indicates that the modelling over predicts the monitored results, with the Castle Point prediction based on 2004 met data being almost twice that of the monitored prediction for the 15-minute objective. This is the reverse of the findings for the BP sites. The predictions for the other objectives also vary and do not indicate a good comparison with the monitored results.



Table 6 2004 Measurements from the monitoring sites (µg m-3)

Site 99.9th%ile 99.73th%ile 99.18th%ileOozedam Farm 1046.7 579.8 128.6 Coryton 673.0 500.6 148.0 Thurrock 3 88.3 70.3 20.8 Castle Point 83.1 60.7 20.8

There are many possible reasons for these differences and these include the following either in isolation or combination (note – these are not listed in any order): 1) Uncertainty relating to the emissions The emissions are modelled at an average release rate rather than at time varying rates for all sources. Information for time varying emissions was not provided. The emissions information provided however was mainly based on the direct monitoring of emissions, and where this was not possible it was based on calculations agreed with the Environment Agency. The processes involved are also continuous processes (apart from stack 16, which is not a major source – see Table 15). BP also provided maximum release rates, although there was no indication as to the frequency when these releases arose. The use of average emission rates only is likely to under predict concentrations, as there is no consideration of the peak emissions. 2) Difficulty in predicting peak concentration periods The SO2 objectives requiring assessment relate to short-term peak periods of high concentrations. For the 15-minute objective an assessment of the 36 highest concentrations based on 15 minute averages are required out of a period of a year (having more than 35,000 such periods). Many dispersion models perform less well when the highest concentrations arise, as in many instances the peak concentrations also arise during calm conditions with wind speeds of less than 1 m s-1. As a consequence of the difficulty in modelling these periods, concentrations are likely to be under predicted. The modelling uses hourly sequential meteorological data, this means that meteorological factors are averaged over an hour. A factor has been applied to improve accuracy in predicting the 15-minute objective; this factor has been applied to the 99.9th percentile of hourly predictions and was based on a factor used by the Environment Agency (i.e. 1.36). Previous examinations of this factor by the ERG, based on comparisons from monitored data have indicated that it is too high. However reducing this factor will lead to a reduction in predicted concentrations. 3) Effects not modelled – coastal effects, complexity of site



The oil refinery is extremely complex in design and layout. As a consequence the BP modelling did not utilise the buildings module as it was considered that it would not reflect downwash accurately. Buildings can have significant impact on the dispersion of sources; this can result in increased ground level concentrations. The main effect is to entrain pollutants into the cavity region in the leeward side of the building, bringing them rapidly down to ground level. This increases concentrations near to buildings but decreases them further away. The modelled dispersion will only be affected if the buildings are more than one third the height of the modelled stacks. Building information was not available for the modelling in this report and hence it was not included, despite this both monitoring sites are more than 5 stack lengths from the tallest stack. As with the BP modelling, the modelling in the report also did not incorporate potential coastal effects. Typically it might expect that the winds are greater near the coast, leading to greater dispersion of pollutants. 4) Dispersion and monitoring results The monitoring results for the site indicate that higher concentrations were measured at Oozedam Farm than Coryton, despite the Oozedam Farm site being further from the main sources (stacks 17 and 18) in the refinery as shown in Figure 8. The Oozedam Farm site is located approximately 1 km northwest and Coryton to 500 m to the northeast. The proximity of the stacks is such that concentrations monitored at the Coryton site might be expected to exceed those monitored at Oozedam Farm rather than the opposite as seen. This would reflect typical dispersion of the pollutant with distance. Reasons as to why this is not case might be associated with the complexity of the site (discussed above) and the precise location of the monitoring site within it. Other explanations are that the data capture for the Coryton site is only 77% for the year compared to more than 90% at Oozedam Farm. Figure 30 shows the hourly data capture for the sites. The periods where there were missing data include a prolonged period during the summer, (from 20th July to 1st September) when episodes might have been expected to arise. Episodes did arise during this period at the Oozedam Farm site. The predictions for 2004 based on Heathrow met data (Figure 15) indicate that concentrations are higher at Oozedam Farm than Coryton, although this was not repeated with 2002 Manson met data. This underlines the importance and relevance of the meteorological data in determining concentrations.

3.12 Sensitivity tests

To better understand how the model reacts a series of sensitivity tests were undertaken, the purpose of which is to try to establish how the modelling predictions might be improved. The tests included establishing the major sources in the refinery and the extent of their importance and also changing modelling parameters (surface roughness, emissions, and temperature).



3.13 Predictions of main sources in refinery The emissions and release parameters are given in the Appendix B. The modelling was undertaken in long-term mode and no account was taken of building height (as with the original modelling). The predictions of all sources using both Manston 2002 and Heathrow 2004 met data are given in Table 7. The predictions of the main five sources at the refinery are given in Table 8. A comparison of the results indicates the predictions for the five main sources are only slightly less than for all refinery stacks, confirming that these are the main sources.

Table 7 Predicted concentrations for all refinery stacks (µg m-3)

99.9th%ile 99.73th%ile 99.18th%ileManston 2002 Oozedam 348.3 239.2 85.2 Coryton 356.6 199.1 65.3 Thurrock 3 105.1 62.5 39.0 Castle Point 154.0 74.0 30.8 Heathrow 2004 Oozedam 272.3 170.2 64.4 Coryton 256.5 166.1 64.0 Thurrock 3 116.3 63.3 38.9 Castle Point 154.6 95.2 37.0

Table 8 Predicted concentrations for refinery stacks 2, 4, 5, 17 and 18 (µg m-3)

99.9th%ile 99.73th%ile 99.18th%ileManston 2002 Oozedam 346.7 238.3 84.2 Coryton 354.6 197.6 64.8 Thurrock 3 104.6 61.7 38.6 Castle Point 152.1 73.3 30.6 Heathrow 2004 Oozedam 270.9 169.5 64.1 Coryton 255.5 165.0 63.6 Thurrock 3 114.9 62.5 38.4 Castle Point 152.6 94.5 36.6

To further understand the importance of the sources, the five main sources were modelled individually using Heathrow 2004 met data only. The predicted



concentrations at the monitoring sites for the five main stacks from which the bulk of emissions arise are given in Table 9.

Table 9 Predicted percentile concentrations for individual stacks (µg m-3)

99.9th%ile 99.73th%ile 99.18th%ile Stack 17 Oozedam 80.7 54.1 24.6 Coryton 99.1 56.0 23.1 Thurrock 3 50.2 32.6 22.2 Castle Point 57.4 41.5 21.9 Stack 18 Oozedam 141.1 90.1 37.7 Coryton 173.9 101.4 34.5 Thurrock 3 44.0 33.4 23.3 Castle Point 63.6 39.4 22.9 Stack 4 Oozedam 66.7 49.5 25.5 Coryton 143.7 101.9 45.3 Thurrock 3 28.8 25.1 18.9 Castle Point 34.5 27.6 19.9 Stack 2 Oozedam 36.9 28.0 18.5 Coryton 30.2 23.4 17.6 Thurrock 3 23.8 19.9 17.7 Castle Point 27.3 23.2 17.8 Stack 5 Oozedam 55.7 37.4 20.7 Coryton 46.8 29.0 18.7 Thurrock 3 28.6 22.3 18.7 Castle Point 36.2 27.0 18.8

The highest concentrations arose from stacks 17 and 18 for all monitoring sites; these stacks produced the greatest emissions of the refinery stacks. The lowest concentrations were seen with the stack 2 emissions. The emissions from stacks 2, 4 and 5 were all less than half of stacks 17 and 18, with an average release rate of between 20 and 34 g s-1. Stacks 4 and 5 had similar predicted concentrations apart the stack 4 prediction for the Coryton site. The dispersion of pollutants is affected greatly by stack height. The height of stack 4 is approximately half that of the other stacks and as a result the predictions suggested that this stack had a significant influence on the Coryton site, which was sited upwind of the source. This extent of the influence approached that for the stack 18 emissions. The importance of stacks 17 and 18 together was also modelled using Manton 2002 met data only and the results are given below in Table 10. These results can be



compared to Table 7. From this comparison it can be seen that the predicted concentrations were approximately two thirds of the concentrations for all stacks. The ratio of modelled predictions of stacks 17 and 18 against all stacks are also given in the Table 10. The ratio fro all sites was approximately two thirds to three quarters of the total. The ratio for the Coryton site however was higher (over 80%) than that for the other sites, this may as a result of the closer proximity of this site to the sources.

Table 10 Predicted percentile concentrations for stacks 17 and 18 (µg m-3)

99.9th%ile 99.73th%ile 99.18th%ile Manston 2002 Oozedam 258.0 168.0 56.8 Coryton 299.6 169.1 58.4 Thurrock 3 71.7 45.3 30.3 Castle Point 101.3 50.7 24.9 Ratio Oozedam 74.1% 70.2% 66.7% Coryton 84.0% 84.9% 89.6% Thurrock 3 68.2% 72.5% 77.7% Castle Point 65.8% 68.5% 80.8%

3.14 Changing emissions parameters

Sensitivity tests to investigate the importance of changing parameters were also undertaken. The purpose of these tests to try to understand how the model works with changed parameters. The changes were not intended to reflect any proposed or planned changes at the refinery. The tests investigated the effect of increasing emissions to maximum release rate for stacks 17 and 18, reducing the temperature of releases (to 100° and 150°) and reducing stack height (of stacks 17 and 18). These tests were undertaken for the emissions from the main emission sources only using Manston 2002 met data. The tests were modelled for the monitoring site locations and the results are given in Table 11.

Table 11 Predicted percentile concentrations of changed parameters (µg m-3)

Manston 2002 99.9th%ile 99.73th%ile 99.18th%ile Max release rate Oozedam 542.1 363.8 118.0 Coryton 557.4 326.8 101.6 Thurrock 3 148.4 85.6 50.3 Castle Point 222.1 102.3 37.6 Reduced temp



Oozedam 388.4 258.7 89.9 Coryton 380.8 222.9 70.4 Thurrock 3 111.5 64.2 39.6 Castle Point 156.9 77.2 30.9 Reduced stack height Oozedam 403.3 290.6 114.2 Coryton 519.3 355.1 166.9 Thurrock 3 97.9 68.1 35.3 Castle Point 112.9 76.3 29.0 Base Oozedam 348.3 239.2 85.2 Coryton 356.6 199.1 65.3 Thurrock 3 105.1 62.5 39.0 Castle Point 154.0 74.0 30.8

The results show how concentrations increased above the base rate in all instances. Increasing the emission rate to the maximum release values caused the greatest increase in concentrations. Reducing the stack height and release temperature each had the effect of reducing dispersion, leading to increased concentrations at the locations.

3.15 Changed scenarios

The earlier modelling was based on the average emission release rates to reflect the modelling undertaken in the BP report. To start to understand the impact of increased emissions two further scenarios were run. These were based on increasing emissions to the maximum release rates for stacks 17 and 18 only. These stacks as already indicated are the two main sources at the refinery (see Table 15). In addition the maximum release rates for both were also the highest of the sources. The maximum release rates were a factor of 1.56 and 2.02 above the average rates for stacks 17 and 18 respectively. The modelling undertaken assumes that i) The average release rate is the maximum release rate for each stack ii) The average release rate is increased to the average of the maximum and

average release rates. The results for the 15-minute objective are shown in



Figure 23 and Figure 24. The white crosses indicate the two BP monitoring sites.



Figure 23 Predicted 99.9th percentile SO2 15-minute mean concentration (µg m-3) for the southeast of Thurrock (using 2004 Heathrow met data) using maximum release rates

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Figure 24 Predicted 99.9th percentile SO2 15-minute mean concentration (µg m-3) for the southeast of Thurrock (using 2004 Heathrow met data) using increased release rates

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These plots both show large increases in the areas predicted to exceed the objective. It is notable that the area increases to both southwards into the Thames Estuary and eastwards to the western edge of the built up area of Canvey Island. Both plots also show a small area close to the main source of emissions where the objective is not exceeded. The predictions for the 1-hour objective using these increased emission rates are given in



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Figure 25 Predicted 99.73th percentile SO2 1 hour mean concentration (µg m-3) for the southeast of Thurrock (using 2004 Heathrow met data) using maximum release rates

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Figure 26 Predicted 99.73th percentile SO2 1 hour mean concentration (µg m-3) for the southeast of Thurrock (using 2004 Heathrow met data) using increased release rates

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These plots indicate areas that exceed the 1-hour objective with the areas predicted to exceed located to the east of the main emission sources.

The predictions for the 24-hour objective using these increased emission rates are given in



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These plots indicate areas that exceed the 24-hour objective with the areas predicted to exceed east of the main emission sources. A comparison of the predictions with monitored data at the monitoring sites (Table 12) indicates that modelling at the two BP sites substantially under predicts the monitored concentrations. Conversely the predictions for the two local authority sites over predict the concentrations greatly.



Table 12 Predicted percentile concentrations of changed parameters (µg m-3)

99.3th%ile 99.73th%ile 99.18th%ile Maximum Oozedam 417.6 262.4 91.7 Coryton 431.5 248.9 76.1 Thurrock 3 166.3 87.5 50.3 Castle Point 222.8 133.7 47.0 Increased Oozedam 353.9 210.7 73.1 Coryton 349 207.3 66.9 Thurrock 3 132 74.2 43 Castle Point 194.4 113.4 41.5 Monitored Oozedam 1046.7 579.8 128.6 Coryton 673.0 500.6 148.0 Thurrock 3 88.3 70.3 20.8 Castle Point 83.1 60.7 20.8

3.15 Commentary on model verification

DEFRA’s technical guidance (TG03) indicates that local authorities need to have confidence in their results, where they will be used to determine whether or not objectives are exceeded and subsequently to declare an AQMA. The guidance however recognises that for point sources the verification of modelled against monitored results may be difficult to carry out. The use of model verification is recommended. In a typical straightforward instance, predicted and modelled results are compared. From this comparison an adjustment factor is derived to apply to the modelled results. In those instances where the modelled under predict monitored results the factor applied would increase modelled results. Where it is not appropriate to use an adjustment factor the guidance suggests that a suitably validated model may be sufficient. This however depends on accurately quantified source emissions. In this instance it has not been possible to use appropriate adjustment factors, however an understanding of the changes in factoring concentrations upwards can be gained from the modelled predictions made using the increased emissions above.



Conclusion

This report provides additional support to the Council’s earlier Detailed Assessment of sulphur dioxide close to the Coryton refinery. It has updated and further interpreted the recent monitoring results undertaken in the vicinity of the refinery and modelled emissions from the refinery. The monitoring indicates that the 15-minute mean, 1-hour mean and 24 hour mean objectives were easily exceeded at BP’s monitoring sites at Oozedam Farm and Coryton in 2004. The 15-minute mean was also exceeded at both sites in 2003, as was the 1-hour mean objective at the Oozedam Farm site. Further analysis of the episodes at these monitoring sites indicates that episodes arise both throughout the 24-hour day and night period and in each month of the year. An analysis of episodes using wind data also confirms that the refinery is the source of emissions. The SO2 objectives were not exceeded at the Thurrock 3 and Castle Point sites. Episodes of higher concentrations were analysed using wind data and this shows that emissions from the refinery do influence measurements. To understand emissions from the refinery the BP Management Plan reports have been used. These reports provided information on the average emission release rates as well as other release parameters for all the main sources at the refinery. Details of time varying emissions were not provided. The modelling undertaken varies dependent on met data used for different years, although it does approximate to the modelling undertaken in the BP reports. The predictions indicate that the 15-minute mean is exceeded beyond the refinery boundary but the other objectives. A separate comparison of predicted data with monitored results however indicates provides poor agreement. Reasons for this include uncertainty relating to the emissions information available, the complexity of the site and the difficulty in predicting objectives that relate to peak concentrations. Additional modelling showed the impact from the main sources within the refinery. The comparison of modelled and monitored results confirmed that the use of a suitable adjustment factor was problematic and therefore one has not been derived. To help overcome the lack of time varying emission information additional scenarios were modelled using increased emissions based on the maximum emission rates in the BP reports for stacks 17 and 18. These predictions indicate that all objectives are exceeded. The 15-minute objective has increased areas that exceed to the east of the refinery approaching the built up area of Canvey Island.



Recommendations

The Council is recommended to undertake the following actions, in respect of the findings for the sulphur dioxide statutory objectives:

1. To investigate and confirm the potential for public exposure in areas predicted to exceed the SO2 15 minute objective in Figure 14 and Figure 15. Based on a confirmation that public exposure does exist, to designate an AQMA, based on these findings.

2. Undertake further consultation on the findings arising from this report

with the statutory and other consultees as required. 3. To continue to use the outcome of the consultation process to determine

the need for remedial measures, which the Environment Agency will enforce, regarding exceedences from the sources at Coryton Refinery.

4. To seek additional information regarding time varying emissions from

sources at the refinery.

5. Continue its SO2 monitoring programme as part of its LAQM actions.



References: Air Quality (England) Regulations 2000. Air Quality (Amendment) Regulations 2002. DEFRA, 2003. Local Air Quality Management Technical Guidance (LAQM.TG (03). DEFRA, London. BP Coryton Refinery Management Plan January 2003 and March 2004. Response to Improvement Condition 8.13 of Variation BM4171. BP Oil UK Ltd. Thurrock Council, 2004. Detailed Assessment of Air Quality, September 2004. Thurrock Council



Appendix A Meteorology information used for the modelling The BP Management Plan used meteorological data from Manston and modelled five years from 1995 to 1999. The report considered this site as representative for the area near the refinery. Previous modelling studies for the Council have used Heathrow data and modelling, hence data from both sites were used in this report. The modelling used hourly sequential meteorological data and these were obtained from the Met Office in ADMS compatible format for 2002 and 2004. For 2002 comparison purposes, the number of hours that wind data were lacking at Manston was 183 (representing just over 2% of the years measurements) and Heathrow 27 (i.e. 0.3%). The wind roses are shown in Figure 29.

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Figure 29 Wind rose for Manston and Heathrow 2002

Both wind roses indicate predominant winds from the southwest. Table 13 provides a comparison of the winds from identified sectors for 2002, based on the number of hours for each wind sector. The sectors relate to those derived from the analysis of episodes earlier in the report. The frequency of winds from the southwest are approximately 13-14% of the total measurements for both sites based on the narrow sector in line with the Coryton site for each year and 7-8% for the Oozedam Farm site. This indicates reasonable agreement between the Manston and Heathrow sites.



Table 13 Analysis of wind sectors based on the monitoring sites

Wind sector (°) Manston (hours)

Heathrow (hours)

100-130 709 8.1% 577 6.6% 230-255 1226 14.0% 1104 12.6%

Total (hours) 8577 8733 Appendix B Modelling Approach The overall objective of the air pollution dispersion modelling in this report is to produce a sound method whereby concentrations of SO2 can be determined and developed in conjunction with the continuous monitoring results to produce percentile values that can be compared to the AQS objectives. In summary the main aims are to:

1) Use a dispersion model to make predictions of SO2 over the time scales relevant to the AQS objectives

2) Undertake modelling of the releases of SO2 over the area of interest and compare the model performance against air pollution measurements.

3) To use locally available meteorological data over recent years to take into account year to year variation.

Area of interest The area of interest was limited to the southeast corner of the Council’s area and the immediate surrounding area surrounding the Coryton refinery. This was based on the findings of Council’s earlier Detailed Assessment. Dispersion model and parameters used The surface roughness (s.r) length of 0.25m used was in the study (in line with the BP Management Plan). A higher roughness length would produce higher concentrations. The development of emissions information used for modelling the area is dealt with separately in Appendix C. The model was set up to produce percentile values in long term mode, since these are output required. The effects of buildings were not considered due to their relatively low height compared to the stacks. Terrain and coastal effects were also not included within the model set up.



Limitations in predicting short term objectives It is very important to note that the AQS objective considered in this report presents difficulties for dispersion models. The peak percentiles for SO2 pose a particular challenge since all models are limited in their ability to predict over such short time scales (especially for the 15 minute objective) and this should be borne in mind when considering the results. It should also be noted that the ADMS formatted files give wind direction by 10° sectors. The data are also hourly sequential, which means that they are averaged for each hour of the year. The conditions that can lead to the highest pollutant concentrations i.e. very low wind speeds are also the very conditions that dispersion modelling start to break down. When wind speeds are less than 1m/s, conditions are considered calm and thus cannot be robustly modelled. It is therefore extremely beneficial that high quality measurements are being made. This is most relevant for the BF1 site as it is closest to the brickworks and the analysis of monitoring data highlights that many episodes arose during low wind speeds. For the KH site however the analysis of episodes earlier showed that over 70% of episodes arose when the wind speed is greater than 2.5m/s. Treatment of background SO2 The BP Management Plan used background concentrations of SO2 that were derived from other monitoring sites in the area, including both Thurrock 1 and Castle Point. The final estimated background concentration was based on the Rochester monitoring site in Kent and for 1999 it was assumed as 16 µg m-3 for all objectives modelled. National expectations from TG03 are that background concentrations have reduced; it recommends that 2004/2005 concentrations be considered 75% of 2001 concentrations. Analysis of the Thurrock 3 site for 2004 also indicates a mean 15-minute concentration of 5.5 µg m-3 and median of 4.3 µg m-3. For comparison purposes the background concentration from the BP Management Plan has been assumed. Appendix C Emissions from Coryton Refinery Introduction The understanding of emissions arising from the refinery has been based on the BP Management Plan, which used real plant information as much as possible. The average release rate in the report used data for the 20-month period from January 2001 to October 2002, excluding plant shutdown and significant turndown periods. In the case of stack A2 FCC Regen empirical factors developed for Coryton were used. The BP report indicates that these factors were agreed with the Environment Agency.




Assumptions used in the report

1) All SO2 emissions are calculated on the basis of grammes emitted per second (g/s), as required by dispersion models.

2) The emissions are discharged from the 19 different chimneystacks at the refinery.

Specific details of the stacks are given in Table 14.

Table 14 Details of Coryton sources and stack parameters

Model stack

Release point ID Description Height Int Dia

Efflux temp

Easting Northing

1 FCC (1A) FCC stack first flue 117.04 2.9 202 574250 182480 2 Fuels (A4) Fuels complex stack 91.4 4.11 254 574590 182430 4 Boiler 3 (A8) Boiler 3 stack 45.72 2.44 182 574720 182340 5 Boiler 4/5 (A9) Boilers 4 & 5 stack 106.7 2.44 160 574720 182310 6 Boiler 6 (A10) Boilers 6 stack catalytic 106.7 2.19 150 574720 182290 7 CHD (A5) Hydrodesulphurisation stack 36.6 1.14 300 574470 182410 8 2 PTR (A6) No 2 pre-heater stack 23.2 0.91 320 574480 182530 9 Asphalt (A11) Asphalt stack 30.48 1.07 527 574870 182170 10 Furfural B2 (A12) Furfural B2 stack 51.82 1.68 427 575000 182180 11 Furfural B1 (A13) Furfural B1 stack 45.72 0.91 427 575020 182190 12 WHU (A14) Wax hydro finishing stack 25.91 0.34 648 575260 182250 13 LHU (A15) Lube hydro finishing stack 30.48 0.61 621 575250 182280 14 CCR (A3) Continuous catalytic reformer 80.01 4.05 160 574570 182600 15 GTG (A16) Gas turbine generator/ heat

recovery steam boiler 45.72 3.05 150 574650 182360

16 Dgen (A17) Diesel generator stack 15.73 1.05 355 575750 182220 17 FCC Regen (A2) FCC stack second flue 117.04 2.6 173 574250 182480 18 SRU Sulphur recovery units 91.5 1.22 270 574400 182410 19 GSPL Gasoline splitter stack 44.7 0.87 360 574400 182410 20 Flare Refinery south flare stack 100 1.2 500 574630 183380

(Note – there is no stack 3)

3) The emission rates for all stacks were obtained from the BP Management Plan report and these are provided in Table 15.

4) The modelled estimates for the 15-minute objective were derived from the

modelled 99.9th percentile of one-hour outputs. To obtain these a factor of 1.36 was used, this is consistent with the BP Management Plan.



Table 15 Refinery emission rates

Model stack

Release point ID SO2 average release rate (g s-1)

SO2 maximum release rate (g s-1)

Volumetric flow rate (m3 s-1)

1 FCC (1A) 0.42 3.68 34.38 2 Fuels (A4) 23.35 136.64 73.46 4 Boiler 3 (A8) 20.94 34.7 12.86 5 Boiler 4/5 (A9) 33.76 53.67 36.68 6 Boiler 6 (A10) 10.37 25.01 22.73 7 CHD (A5) 0.1 1.01 2.62 8 2 PTR (A6) 0.02 0.46 0.54 9 Asphalt (A11) 0.01 0.15 0.91 10 Furfural B2 (A12) 0.01 0.02 0.91 11 Furfural B1 (A13) 0.04 0.07 2.75 12 WHU (A14) 0 0.01 0.06 13 LHU (A15) 0 0.03 0.41 14 CCR (A3) 0.22 4.86 33.51 15 GTG (A16) 0.14 12.09 61.51 16 Dgen (A17) 0.12 1.99 8.57 17 FCC Regen (A2) 90.4 141.4 68.2 18 SRU 69.86 141.2 14.14 19 GSPL 0.03 0.04 0.75 20 Flare 3.62 40 0.65

Appendix D QA/QC details for BP monitoring sites Extracted from BP Coryton Management Plan report – January 2003 Section 6 – Data Quality “The availability of monitoring equipment is targeted to be greater than 90% for each monitored species. Data quality will be comparable to those expected from DEFRA affiliated sites. To achieve these data coverage and data quality objectives the following procedures will be used:

• Analysers purchased from Monitor Europe use UV fluorescence for sulphur oxides and chemiluminescence for oxides of nitrogen.

• SO2 and NOx instruments will visually checked for calibration errors every 2 weeks and adjusted manually as required. Calibration using a zero air source and a cylinder of 450ppb SO2 and 450ppb NO. The cylinders will be obtained from a



reputable supplier with the gas concentrations certified to accuracy, traceable to national standards, by NPL or similar accreditation laboratory.

• A daily, overnight calibration cycle for SO2 and NOx will be completed automatically under the control of the data logger.

• Inlet filters will be changed every two weeks. • Each output channel from the analysers will be scanned every 10 seconds or less

and these readings will used to calculate the minute averages for each channel which will be stored within the site data logger. The minute averages will be used to calculate mean values for each measured species over 15 minutes and or 1 hour as appropriate.

• Site log books and 1 minute raw data, calibration data, factors and final calibrated data will be made available to enable a high probability of >90% final data coverage for all species on an annual basis.

• Both sites will be checked and the analyses recalibrated as necessary, this calibration will be completed half yearly by the analyser provider.”



Figure 30 Coryton and Oozedam Farm 2004 monitoring (hourly mean concentrations)

-20.0

80.0

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480.0

580.0

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CorytonOozedam

Documents

Thurrock Council - Air Quality Assessment s02