Seasonal Evolution of Levels of Gaseous Pollutants in an Urban Area (Ciudad Real) in Central-Southern Spain: A Doas Study

SEASONAL EVOLUTION OF LEVELS OF GASEOUS POLLUTANTS INAN URBAN AREA (CIUDAD REAL) IN CENTRAL-SOUTHERN SPAIN:

A DOAS STUDY

ALFONSO SAIZ-LOPEZ1,2, ALBERTO NOTARIO1, ERNESTO MARTINEZ1

and JOSE ALBALADEJO1,∗1Departamento de Quımica Fısica, Facultad de Ciencias Quımicas, Universidad de Castilla-La

Mancha, Avenida Camilo Jose Cela 10, 13071 Ciudad Real, Spain; 2Now at School ofEnvironmental Sciences, University of East Anglia, Norwich, NR4 7TJ, United Kingdom

(∗author for correspondence, e-mail: [email protected], Tel: (+34) 926 29 53 27)

(Received 27 June 2005; accepted 4 November 2005)

Abstract. Long term continuous monitoring measurements of urban atmospheric concentrations of

O3, NO2, NO, and SO2 were performed for the first time in Ciudad Real, a city in central-southern

Spain. The measurements were carried out using the differential optical absorption spectroscopy

(DOAS) technique, with a commercial system (OPSIS, Lund-Sweden), covering the summer and

winter seasons (from 21st July 2000 to 23rd March 2001). Mean levels of O3, NO2 and SO2 monitored

during this period were: 27 μg m−3, 50 μg m−3 and 7 μg m−3 respectively. The highest hourly averaged

value of O3 (160 μg m−3) was measured during the summer period, while NO2 was enhanced in

wintertime (highest values 90 μg m−3). In the coldest period, when central heating installations were

operating, SO2 showed maximum levels of 20 μg m−3. The daily, weekly and seasonal analysis of

the data shows that photochemical air pollution dominates in this urban atmosphere and is strongly

influenced by levels of motor traffic and domestic heating system emissions. These measurements were

compared with other studies in Spain and Europe. Also, the long path averaged DOAS measurements

were compared with in situ observations made in Ciudad Real, from 23rd August 2000 to 25th

September 2000, using a mobile air pollution control station. All gas concentrations reported in

this paper are below the WHO guidelines and the different thresholds introduced by the European

Environmental Legislation.

Keywords: air monitoring, DOAS, ozone, seasonal variation, traffic emissions

1. Introduction

In urban areas, emissions of air pollutants by anthropogenic processes such astraffic, industry, power plants and domestic heating systems are the main sources ofpollution (Fenger, 1999). The massive growth in road traffic and in the use of fossilfuels during the last decades has changed the composition of urban air, increasingthe frequency of pollution episodes and the number of cities experiencing them. Theprimary traffic-related pollutants are carbon monoxide (CO), nitrogen monoxide(NO), dust and soot as well as various types of non-methane hydrocarbons (NMHC),principally aromatic compounds.

The high levels of solar irradiation observed in the Mediterranean countriesfavour, in general, the enhanced photochemical production of secondary oxidising

Water, Air, and Soil Pollution (2006) 171: 153–167

DOI: 10.1007/s11270-005-9029-2 C© Springer 2006

154 A. SAIZ-LOPEZ ET AL.

pollutants, including O3, nitrogen dioxide (NO2), and peroxyacetylnitrate (PAN).This has been further linked to certain respiratory diseases, and consequently hasbecome an issue of social concern in a number of cities in Southern Spain (e.g.Saez et al., 2002). O3 is an important oxidizing agent in the troposphere as hasbeen demonstrated in different laboratory studies (e.g. Atkinson, 1994; Martinez etal., 2000; Albaladejo et al., 2002; Jimenez et al., 2005). This species has also beenshown to be harmful to human health under episodes of elevated concentrationsoccurring within the surface layer in urban environments (e.g. Lippmann, 1991;Brunekreef et al., 1995).

Hence, monitoring of air pollution is necessary, not only to comply with theenvironmental directives, but also to provide public information and to measurethe effectiveness of emission control polices. It is also important to have a numberof measurements from different cities to help understand the role of the differ-ent physicochemical processes in the troposphere. In addition to meteorologicaland irradiance conditions, traffic patterns, volatile organic compounds (VOC)/NOx

emissions and industrial activities vary significantly between different locations.In this work, continuous monitoring of urban air pollutants has been the subject

of study for the first time in the city of Ciudad Real, where no permanent air qualitymonitoring stations exist. The city has some 63,000 inhabitants and is located inthe heart of La Mancha region in central-southern Spain (38◦59′N, 3◦55′W, atapproximately 628 m above sea level) in a fairly flat area, 200 km south of Madrid.With a low presence of industry and no power plants, traffic is likely to be the mostimportant source of air pollution in this city. Measurements were performed from21st July 2000 to 23rd March 2001 covering the summer and winter seasons.

Meteorologically, it has very hot and sunny weather during summer, dry andcold winters, with very low wind speed throughout the year in addition to frequenttemperature inversions. These conditions will play a crucial role in the evolutionand transformation of the polluting agents.

The aim of this paper is to identify, measure and study the daily, weekly and sea-sonal variation of the concentrations of important tropospheric constituents such asO3, NO, NO2 and SO2. The observations were made by using the DOAS technique.During the last three decades, the DOAS principle and instrumentation have provento be a powerful tool to simultaneously monitor relevant atmospheric trace gases(Platt, 1994; Plane and Saiz-Lopez, 2005). The use of light paths which range fromhundred of meters to several kilometres can avoid the problems of local influencesand surface effects, as the measured concentrations are averaged over the light pathand are barely influenced by small-scale perturbations. The technique is based onthe fact that all trace gases absorb electromagnetic radiation in some part of thespectrum. If the radiation of the appropriate frequency is transmitted through theatmosphere, the absorption features of each molecule in that spectral region allowthe identification and quantification of the gas concentration. To date, only a fewDOAS measurements of urban air pollution over long time periods have been re-ported (Lofgren, 1992; Barrenfors, 1996; Brocco et al., 1997; Kourtidis et al., 2000,

SEASONAL EVOLUTION OF LEVELS OF GASEOUS POLLUTANTS IN AN URBAN AREA 155

2002; Gobiet et al., 2000; Kim and Kim, 2001) and, to the best of our knowledge,this is the first one in Southern Spain. The long path averaged DOAS observationswere also compared with in situ measurements carried out from 23rd August 2000to 25th September 2000 using an air pollution control mobile station.

2. Experimental

A commercial DOAS system (OPSIS, model AR 500) was employed to simulta-neously monitor the gas concentration of O3, NO2, NO, and SO2, integrated alonga light path of 391 m. The DOAS instrument was placed at a fixed location insidethe campus of the University of Castilla-La Mancha in Ciudad Real, situated in theEastern part of the city and downwind of the main traffic of the city (see Figure 1).

The system used in this study consists of an emitter (EM 150) and receiver (RE150) in combination with the AR 500 analyzer. The emitter was located on theroof of the School of Agricultural Studies (denoted as B in Figure 1(c)) about 16 mabove the ground, and the receiver system was installed 12 m above street level,on a terrace on the School of Computing Science (denoted as A in Figure1(c)).The average altitude of the optical path was 14 m above ground level. The 391 mlight beam crosses over a small parking area and is nearby two interior city roadscontaining the heaviest traffic in the city. Also, a monitoring station of the NationalMeteorological Institute is located in the middle of the light path (see Figure 1(c))and provided surface meteorological data for the interpretation of the measurementsin this study.

The RE unit, interfaced with the AR system, was designed to conduct PC-basedspectral analysis. The light for the spectral analysis is generated from a 150 W Xenonlamp, incorporated in the transmitting telescope of the RE unit, and transmittedthrough the open atmosphere. The radiation is collected with the RE unit andfocused onto the entrance of a 10 m long fibre optic coupled to the AR analyzer.Analysis of the pollutants was made by a 0.5 m UV Czerny-Turner spectrometerwith a moveable grating in combination with a slotted scanning disk (rotatingat 300 rpm) placed in front of the detector – a photomultiplier tube (PMT). Therecording time required for a single spectrum is of the order of 10 ms, but toremove atmospheric turbulence effects on the spectral information, several spectrawere co-added, giving a total observation time of 5 min.

In the evaluation procedure (Platt, 1994; Plane and Saiz-Lopez, 2005), the wave-length region of the strongest absorption was used for each gas: 200–250 nm (NO),250–300 nm (O3), 360–440 nm (NO2) and 260–340 nm (SO2). In order to reducestray light and increase the performance of the spectrometer, an UV filter was in-stalled on the fibre pole of the receiving telescope when the system was monitoringNO. Nevertheless, it must be noted that measurements of NO over our optical path(391 m) could not be regularly carried out because of the strong O2 Rayleigh at-tenuation of light in the spectral region used to monitor NO. The calibration of the


Figure 1. (a) Map of the geographical location of Ciudad Real in Spain. (b) Location of the DOAS

monitoring system at the campus of the University of Castilla-La Mancha and the situation of the

RAQN mobile station. (c) An extension of the marked zone in part b) of this figure, showing the light

path and the location of the DOAS equipments: A – School of Computing Science, B – School of

Agricultural Studies, (�) Meteorological station.


Figure 2. Example of the evolution of NO, NO2 and O3 concentrations for a typical summer morning

(12th August 2000) in Ciudad Real measured by DOAS. Time scale is given in CET (Central European

Time), being UTC + 2 in summer.

DOAS system was made using the cell reference system provided by the manufac-turer (an OPSIS CB 100 calibration bench with cells of various lengths, a calibrationlamp CA 150 and an OPSIS OC 500 ozone calibrator unit). For the pollutants mon-itored, the manufacturer specifies a detection limit of the order of 1 μg m−3 andan accuracy of 2% for NO2, SO2 and O3 and 3%–15% for NO. Measurement datathat do not satisfy a selected light level (above 20% of the light intensity at highvisibility) were discarded prior to data analysis because of high photon noise. Thispaper reports only measurements with 1-σ uncertainties less than 10%, 12% and25%, for O3 and NO2, SO2, and NO, respectively.

3. Results and Discussion

Figure 2 shows the evolution of NO, NO2 and O3 concentrations observed for atypical summer morning (12th August 2000). The diurnal cycles of NO, NO2 andO3 registered in this study show that the urban atmosphere of this city of SouthernEurope is strongly influenced by motor traffic and photochemistry. Similar diurnalbehaviour of these pollutants have been observed in other urban areas in Spain (e.g.Pujadas et al., 2000; Toll and Baldasano, 2000; Palacios et al., 2002) and Europe(e.g. Gobiet et al., 2000; Vandaele et al., 2002; Kourtidis et al., 2002). The primaryNOx pollutants (mainly NO, as more than 95% of the NOx are emitted as NO)react in the presence of VOCs and sunlight to form a host of secondary pollutants,among them O3 and other oxidants such as PAN (Finlayson-Pitts and Pitts, 2000).The overall reaction is:

VOCs + NOx + hν → O3 + PAN + HNO3, etc (1)


Simultaneously to the VOCs oxidation in the troposphere, NO is oxidized to NO2

by the peroxy radicals formed and O3 is produced in the dominant photochemicalprocess, the photolysis of NO2:

NO2 + hν (λ < 398 nm) → NO + O (2)

O + O2(+M) → O3 (3)

Certain features of the time-concentration profiles observed in Figure 2 are typi-cal of cities where the air pollution episodes are dominated by traffic emissions (e.g.Pujadas et al., 2000; Kourtidis et al., 2002). These features include the following:(1) in the early morning, the concentration of NO rises and reaches a maximum at atime that approximately coincides with the maximum of NO emissions, in this cityat the rush hour of traffic (between 8:30 and 9:30); (2) subsequently, NO2 rises toa maximum; and (3) oxidant levels (e.g., O3), which are relatively low in the earlymorning, increase significantly about midday when the NO concentration drops tolower values. O3 then reaches a maximum after NO2. A rise in the morning O3 con-centrations due to fumigation from the reservoir layer above can be also observed.Downwind from urban centres, the profiles are shifted and O3 may peak in theafternoon or even after dark, depending on emissions and transport of an urban airparcel containing O3 and other secondary pollutants formed during daylight hours(Finlayson-Pitts and Pitts, 2000). The low NO concentration in the early afternoonpresented a handicap to continuous monitoring of NO continuously with the pathlength used in this study since it was normally below the detection limit of theinstrument.

In Figure 3 we present a summary of the daily averaged mixing ratios of the threetrace gases NO2, O3 and SO2, monitored continuously during the whole period ofmeasurements. As it can be seen, the annual cycle for these trace gases show thelargest values of O3 during summer while NO2 and SO2 exhibit maximum valuesin winter. On average, the production of O3 is reduced in winter by up to 60%compared to the summer months. Maximum monthly averages were: 35 μg m−3

(O3) in August, 65 μg m−3 (NO2) and 10 μg m−3 (SO2), both in December. Meanannual levels for O3, NO2 and SO2 were 27 μg m−3, 50 μg m−3 and 7 μg m−3

respectively. The values reported here are generally lower than those observedin previous studies in large urban areas in Spain, such as Madrid (Pujadas et al.,2000; Palacios et al., 2002) and Barcelona (Toll and Baldasano, 2000), where trafficemissions are considerably higher.

The hourly averaged diurnal variations of O3 and NO2 together with solar ra-diation intensities are shown in Figures 4(a) and 4(b) for a winter month (January2001) and a summer month (August 2000), respectively. The main features thatcan be observed in these figures are: (1) negative correlation between NO2 and O3,both in winter and in summertime; (2) an additional peak for NO2 is observed aftersunset which has been linked to the occurrence of a second rush hour in the traffic


Figure 3. Daily mean concentration data for O3, NO2 and SO2 obtained from the DOAS dataset for

the entire period of measurements (August 2000–March 2001).

circulation; (3) a more pronounced maximum ozone peak is observed in summer,while NO2 shows its highest values in winter. The highest diurnal O3 values are al-ways observed just after midday, with some delay in winter. In summer, the ozonepeak shows a plateau, which may be indicative of advection from a well-mixedlayer upwind.

In winter, the photolytic channel (2) will not be as efficient as in summer sincethe solar zenith angles are higher and there are less hours of photochemical activity.Consequently, the subsequent oxidation of the fresh emissions of NO from motortraffic and central heating systems will increase the concentration of NO2. Also,


Figure 4. Mean diurnal variation of the hourly averaged solar radiation intensity and concentration

of O3 and NO2 during: (a) January 2001, (b) August 2000, (c) Measured SO2 concentration data for

the same months. Time scale is given in CET (Central European Time), being UTC + 1 (Coordinated

Universal Time) in winter and UTC + 2 in summer.


the reduction of the photochemical production of O3 will favour the removal ofthis molecule via titration with NO to NO2 + O2, which in turn will contribute tothe enhancement of the NO2 values. In addition, the production of the hydroxylradical (OH) will be limited by the reduced photolytic and photochemical activity.This will lead to reduction in the efficiency of the conversion of NO2 to nitricacid (HNO3) via the termolecular reaction with OH radicals and also to enhancethe NO2 concentration. Another contribution to the winter peaks could also be thegeneral use of central heating during this cold season. Winter peaks of NO2 canalso be related to the lower development of the mixing layer due to colder surfacetemperatures. It is interesting to observe that, while levels of the morning NO2

peaks are very similar for both seasons, there is a difference for the evening peak,with higher levels in winter probably due to the lower thickness of the mixinglayer.

A diurnal profile of SO2 for summer and winter time is illustrated in Figure4(c). In winter, SO2 mean values were 2 times higher than in summer (20 μg m−3

compared with 10 μg m−3). The higher SO2 concentrations in winter were mainlyrelated to house heating and only at a small extent with road traffic. The maindifference between them arises in the later part of the day (i.e. 16 h to 22 h). In thecase of the January data, the concentration of SO2 increases up to three times in theearly evening hours, when central heating systems (widely employed in this cityduring the cold winter months) are in greater use. The diurnal variation of SO2 isalso characterised by a secondary small peak observed in the morning, at the rushhour of traffic. In the warm season, only this small peak at the rush hour is observed.This small peak is broader in August indicating a change in the traffic pattern inthis vacational month and/or another source of SO2. The SO2 levels in Ciudad Realwere found to be lower than those encountered in similar studies on other Europeancities (e.g. Gobiet et al., 2000; Vandaele et al., 2002; Kourtidis et al., 2002).

Meteorological conditions can affect the pollutant transport and distribution.Maximum temperatures in summer are more than 20 ◦C higher than in the winterseason. Mean wind speed is below 2 m s−1 in both seasons. Moreover, predominantwind direction (from WSW) comes from the city centre toward the location of theDOAS system, i.e. the system is placed downwind. In winter, some contribution ofNE winds is also observed. The wind dependence of the NO2 concentrations for thesummer and winter seasons is shown in Figure 5. As can be seen, the wind speeddoes not significantly influence the NO2 observations in winter. This correlateswith the steadier conditions frequently observed during this time of year whichcontribute to a low atmospheric dispersion and to the significant NO2 higher valuesdiscussed above. By contrast, in summer, higher influence of the wind speed in theNO2 concentrations is observed with a decreasing trend on NO2 as the wind speedincreases. The higher thermal convection (associated with larger eddy diffusioncoefficients) and the faster winds in summer will contribute to a more efficientmixing leading to a higher dependence of the NO2 values with wind speed asobserved by the DOAS.


Figure 5. Dependence of the measured NO2 concentrations on wind speed in winter (January 2001)

and in summer (August 2000).

With the aim of distinguishing weekly variations in the measurements, the diur-nal profiles of O3, NO2 and SO2, as averaged for the entire period of observations,are shown in Figure 6. Note that the diurnal cycle of NO2 has a double-sinusoidalpeak shape with levels of NO2 comparatively higher on weekdays than at week-ends. The two maxima correlate very precisely with the rush hours from Mondayto Friday. However, the NO2 profiles are significantly altered during the weekendindicating a change in the traffic pattern. Also, quantitatively, the decrease in theNO2 concentrations during weekends could be a consequence of the reduction inmotor traffic on those two days of the week. The diurnal cycles of O3 show a dis-tinct maximum in the afternoon (caused, as explained above, by photochemicalO3 formation), and a comparatively low secondary maximum in the early morn-ing which seems to be the result of downward transport of O3 from higher levelcontaining more O3. Other urban air pollution studies (e.g. Mayer, 1999) have alsoobserved this secondary O3 maximum, which is more distinct the more the airquality station is influenced by motor traffic, almost vanishing on Sundays due tothe low traffic emissions. Reduced emissions of SO2 are also observed at week-ends (note that the first SO2 peak almost vanishes), when the influence of trafficemissions is less pronounced or at least not concentrated at certain times of theday.

Finally, a comparison study was carried out between the long path averagedDOAS measurements and those made in situ by a mobile station of the RegionalAir Quality Network (RAQN). The results of a summer campaign (23rd August to25th September 2000) for O3, NO2 and SO2 are shown in Figure 7. The RAQNstation was situated 2 m above ground level on the opposite side of the town, upwindof the predominant wind direction, therefore, outside the flow of O3 precursorsfrom traffic emissions within the town. As with the DOAS instrument, the RAQNstation was located very close to a main ring road, and was therefore subject tosimilar traffic induced pollution patterns. Note that the concentration profiles for


Figure 6. Averaged weekly and diurnal cycles of O3, NO2 and SO2 for the entire period of measure-

ments (August 2000–March 2001).

the different species are very similar for both the in situ and the long-path averagedmeasurements. However, as it is reflected in Figure 7, the RAQN O3 concentrationswere systematically higher than the DOAS measurements and the opposite wasfound for NO2. This disparity may arise from the difference in location and airmasses being sampled. Since the DOAS instrument was downwind and about 100 mfrom the main road, the pollutants would have time to react (i.e. the emitted NOwould initiate the titration of O3).

The different locations of the instruments would also account for the higher NO2

values measured by the DOAS instrument as the plume containing the pollutants


Figure 7. Mean diurnal variation comparison of the hourly averaged concentration of O3, NO2 and

SO2 obtained from the DOAS instrument and from the in situ monitors (RAQN) during the period

23rd August 2000 to 25th September 2000.


would have time to evolve and oxidation of NO to NO2 would take place. Bycontrast, close to the source, the in situ system would observe lower values ofNO2, considering that most of the NOx (NO + NO2) emissions are in the form ofNO.

Finally, the SO2 concentrations measured at the DOAS site were marginallylower than those by the in situ monitors. The comparison with the DOAS mea-surements shows that no significant differences in concentration occur. This can beexplained by taking into account the relatively long lifetime of the molecule and themore efficient vertical mixing in the summer period. Note however, that the DOASmeasurements were slightly lower than the in situ because of the latter being closerto the emissions point.

In summary, considering the inherent differences between a point and a lineintegrating technique, for the pollutants monitored in this study, the in situ andthe DOAS instrument seemed to be sampling comparable qualitative behaviour inthe diurnal profiles. The distance to the emission source determines the concen-tration of the species since the plume has more time to evolve allowing reactionsbetween different contaminants. This is an indication of non-linear chemical pro-cesses occurring during urban air pollution events which will depend upon differentmeteorological and geographical parameters. This non-linearity in the urban air pol-lution chemistry needs to be taken into consideration when designing networks forair quality control and assessment in cities such as Ciudad Real.

4. Conclusions

Year round DOAS measurements of NO, NO2, O3 and SO2 have been performedfor the first time in a Southern Spain location. A comprehensive data set oftraffic induced pollutants and their daily, weekly and seasonal variations are re-ported. This exercise presents an example of the evolution and spatial distribu-tions of some motor vehicle contaminants within an urban air scenario largelydominated by photochemistry. The measurements showed that these pollutants ex-hibit a marked seasonal cycle which is typical of pollution behaviour in urbanareas. The highest hourly averaged values of O3 (160 μg m−3) were detectedduring the summer season. By contrast, NO2 was enhanced in wintertime withvalues up to 90 μg m−3. SO2 also showed the highest levels, 20 μg m−3, duringthe coldest period, while the average value during the summer season was only5 μg m3. A comparison between long path averaged DOAS and in situ measure-ments shows a fairly good agreement between both monitoring methods consid-ering the differences in the techniques and also in the altitude and location of theinstruments.

The values reported in this work are below the WHO guidelines and the dif-ferent thresholds introduced by the directives of the European EnvironmentalLegislation.


Acknowledgments

The authors gratefully thank the European Commission and the Spanish D.G.E.S.for supporting this work (Project 1FD97-1563).

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Documents

Seasonal Evolution of Levels of Gaseous Pollutants in an Urban Area (Ciudad Real) in Central-Southern Spain: A Doas Study