Atmospheric Environment 37 (2003) 4837–4846

ARTICLE IN PRESS

AE International – Asia

*Correspondin

562-281226.

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1352-2310/$ - see

doi:10.1016/j.atm

Measurement of NO2, HNO3, NH3 and SO2 and relatedparticulate matter at a rural site in Rampur, India

Abha Gupta, Ranjit Kumar, K. Maharaj Kumari, S.S. Srivastava*

Faculty of Science, Department of Chemistry, Dayalbagh Educational Institute, Dayalbagh, Agra 282 005, India

Received 6 March 2003; received in revised form 26 June 2003; accepted 3 July 2003

Abstract

This paper presents the measurements of gaseous SO2, NO2, HNO3 and NH3 and particulate NH4+, NO3

�and SO42�

at Rampur, a rural site of semi-arid region of India and annual mean concentrations are 3.772.2, 7.373.7, 0.770.6and 6.774.2 and 1.070.4, 1.171.3 and 2.671.6 mgm�3, respectively. Seasonal variation with higher concentration in

winter is observed for gaseous SO2, NO2, and NH3 and particulate NH4+. The concentration of HNO3 and particulate

NO3�and SO4

2� are higher in summer. Summer to winter ratio of HNO3 and particulate NO3� are 3.8 and 1.8,

respectively, showing seasonal variation is more pronounced in case of HNO3 than particulate NO3�. The ratio of

gaseous NH3 to particulate NH4+ (NH3/NH4

+) is more than 1 probably due to basic nature of aerosol. Examination of

equilibrium between gaseous NH3 and HNO3 and particulate NH4NO3 shows observed equilibrium constant is lower

than theoretical equilibrium constant in summer and vice versa in winter.

r 2003 Elsevier Ltd. All rights reserved.

Keywords: Nitric acid; Ammonia; Sulphur dioxide; Aerosol; Nitrate; Sulphate; India

1. Introduction

Atmospheric releases of acidic pollutants, which

include sulphur and nitrogen compounds in both gas

and aerosol species, can cause adverse health effects and

have the potential to cause other environmental damage

(e.g. acid rain) (Lee et al., 1999). Once released into the

atmosphere by either man-made (anthropogenic) or

natural sources, these compounds can undergo several

different processes such as transformation due to

atmospheric reactions (e.g. gas to particle conversion),

transport associated with wind, and finally wet and dry

deposition. All of these processes can perturb the

environment with a host of beneficial and detrimental

effects, such as increased crop yields from nitrogen

loading or decreased visibility from increased aerosol

production (Aneja et al., 2001). In addition to inorganic

g author. Tel.: +91-562-281545; fax: +91-

ss: sssdei@yahoo.com (S.S. Srivastava).

front matter r 2003 Elsevier Ltd. All rights reserve

osenv.2003.07.008

species organic acids (formic and acetic acids) are

important trace compounds of the atmosphere and

may contribute significantly (upto 64%) to the free

acidity in remote regions of the world (Keene and

Galloway, 1984).

The deposition of acidic species from the atmosphere

is currently considered a threat to vegetation and

aquatic life in ecologically sensitive areas. The chemical

species involved consist primarily of SO2 and NOx

(NO+NO2) and their reaction products, SOx

(SO2+SO42�) and NOy (NOx+HNO3+NO3

�+PAN+

organic nitrates+HONO+N2O5).

Several oxidizing agents that are formed in the gas

phase reactions within the troposphere oxidize SO2 and

NO2 to acids. The important reactive species include

molecules such as O3, H2O2, methyl hydroperoxide and

peroxyacetic acid and reactive free radicals such as OH,

HO2 and NO3. SO2 may react immediately with hydroxyl

radical in the atmosphere to produce SO3, which in turn

reacts quickly with water vapour to produce sulphuric

acid or depending on the meteorological conditions and

d.

ARTICLE IN PRESSA. Gupta et al. / Atmospheric Environment 37 (2003) 4837–48464838

local availability of oxidizing substances, the SO2 may

be transported hundreds of kilometers before it reacts

(Erduran and Tuncel, 2001).

Nitrogen oxide reaction products, which are formed

in the atmosphere, include both gaseous and particulate

nitrates. The oxides of nitrogen, NOx (NO+NO2)

mainly emitted in the atmosphere as NO, which is

subsequently transformed into NO2 and other nitrogen-

ous species are very important constituents of air

pollution. NOx, volatile organic compounds (VOCs)

and OH are the precursors for formation of O3 and

other photooxidants.

Nitric acid and ammonia are important trace species

in the atmosphere. Both occur naturally but man’s

agricultural practices and industrial activities substan-

tially increase their concentration (Brost et al., 1988).

Nitric acid is the most important transformation

product of NOx. The measurement of gaseous HNO3in ambient air is of great importance because of its

significance in the acidification of the atmosphere and its

control of the levels of photooxidants. A number of

studies have been reported on HNO3 levels, formation

and destruction from US, Europe and UK and

Mediterranean region (Cadle et al., 1982; Cadle, 1985;

Allen and Harrison, 1989; Lee et al., 1999; Danalatos

and Glavas, 1999; Erduran and Tuncel, 2001). During

the daytime the most important source of nitric acid is

the reaction of NO2 with the hydroxyl radical. During

the night NO3 free radical is the source of tropospheric

HNO3. NO3 is relatively unimportant, during the day

light hours as it is destroyed readily through its rapid

photolysis in sunlight and by its rapid reaction with NO

(Stockwell et al., 1997).

NO3 either combines with NO2 to form N2O5, which

reacts with water to form HNO3, or it may form the acid

by H-atom abstraction from aldehyde and other reactive

compounds. When nitric acid is available in the atmo-

sphere, it has a tendency to react with basic species such

as ammonia gas or with crustal minerals and sodium

chloride particles. In the regions where soil is calcareous

and predominantly present in SPM, neutralization with

carbonates and bicarbonates of Ca2+ and Mg2+ is more

likely.

The reaction of HNO3 with NH3 to form NH4NO3 is

reversible and is believed to be the primary source of

particulate nitrate aerosol in urban air (Stockwell et al.,

2000)

HNO3ðgÞ þNH3ðgÞ"NH4NO3ðsÞ

The equilibrium constant for this reaction is both

relative humidity and temperature dependent. Forma-

tion of particulate ammonium nitrate is favoured under

conditions of high relative humidity and low tempera-

ture.

NH3 is the only common alkaline gas in the atmo-

sphere, which plays major part as neutralizing agent of

the atmospheric acids. This neutralization occurs pre-

dominantly in aerosols, a complicated system that

includes ammonia, sulphuric acid, nitric acid and water.

The affinity of sulphuric acid for ammonia is so much

larger than that of nitric acid for ammonia that available

ammonia is first taken up by sulphuric acid. Any excess

available ammonia may then react with available nitrate

(Seinfeld, 1986; Brost et al., 1988).

The contribution of NH3 to atmospheric nitrogen

deposition is governed in part by the gas to particle

conversion rate of NH3 to NH4+. Because of the short

lifetime of NH3 (1–5 days or less) in the atmosphere

(Warneck, 1988), low source height and relatively high

dry deposition velocity (Asman and van Jaarsveld, 1992;

Aneja et al., 2001), a substantial fraction (20–40%) will

likely deposit near its source. However, ammonium

(NH4+) aerosols, with atmospheric lifetimes of the order

of t ¼ 1215 days (Aneja and Murray, 1998; Aneja et al.,2000) will tend to deposit at larger distances downwind

of sources.

Rampur is a semi-arid rural site 75 km SE of Agra,

which has no major towns and cities in 35 km radius.

Earlier studies on aerosol composition in this region

show that aerosol is basic in nature and mainly

composed of soil dust (Kulshrestha et al., 1995; Satsangi

et al., 2002) and the size-segregated studies report

bimodal distribution with higher fraction in coarse

mode (52%). SO42� and NO3

� occur mainly in super-

micron particle, MMD of SO42� and NO3

� is 3.9 and 3.6,

respectively (Kulshrestha et al., 1998; Parmar et al.,

2001a; Kumar et al., 2003). Although there are several

reports of SO2, NO2 and NH3 levels from urban, rural

and marine sites of India (Khemani et al., 1987; CPCB,

1999; Das et al., 1997; Panwar et al., 1997; Parmar et al.,

1999; Kumar et al., 2002; Varshney and Singh, 2002)

there is only one report of HNO3 from suburban site of

India (Parmar et al., 2001b) and no study on simulta-

neous measurements of SO2, NO2, NH3 and HNO3 and

particulate NH4+, NO3

� and SO42�. Understanding the

chemistry of nitrogen compounds is a prerequisite to

understanding much of the remainder of the chemistry

of nonurban troposphere. Hence in this work we present

a simultaneous study of gaseous NH3, particulate NH4+,

gaseous SO2, particulate SO42�, gaseous NO2, HNO3 and

particulate NO3�.

2. Materials and methods

2.1. Sampling site

Rampur is located south east of Agra city (27�100N,

78�050E) at a distance of about 75 km. The population

density is about 100 people km�2. There are no major

cities or towns within a radius of 35 km. The sampling

site is located on the top of a sand dune about 25m in

ARTICLE IN PRESS

Fig. 1. Map showing sampling site.

A. Gupta et al. / Atmospheric Environment 37 (2003) 4837–4846 4839

height, which is sparsely covered with bushes and Acacia

trees. The immediate vicinity of sampling site has only

Acacia trees. River Yamuna is on the northern periphery

of the sampling site at Rampur. The NE, N, NW

directions of the sampling site are totally barren up to

15 km distance and agricultural fields are present at a

distance of 2 km in the S, SE and SW directions (Fig. 1).

2.2. Sample collection

A total number of 48 samples were collected 24 each

in summer and winter and on average about 6 samples

were collected per month for both gases and particulates

simultaneously.

2.2.1. Trace gases

Sampling of gaseous SO2, NO2, NH3 and HNO3 were

performed using SKC Anasorb sorbent sampling tubes

and are being used by OSHA and NIOSH researchers as

well as by federal state and local government in USA.

For collection of samples and analysis, NIOSH Manual

of Analytical Methods (NMAM, 1994) fourth Edition,

have been followed. To collect desired amount of each

gas in the specified sorbent tubes at the selected site,

sampling was performed for 36 h at the rate of

0.5 lmin�1.

NO2: NO2 was sampled using SKC sample tube (Cat.

No. 226-40-02) which is packed with tri ethanolamine

coated molecular sieve (13� , 30–40 mesh). After

sampling, sorbent was transferred to a volumetric flask

and volume made upto 50ml with absorbing solution

and shaken vigorously for 30 s. 0.2% H2O2, sulphani-

lamide and NEDA was added to 10ml of this solution

and mixed thoroughly. Color was developed when

allowed to stand for 10min which was measured at

540 nm using UV-VIS Spectrophotometer. Precision,

accuracy, LOD of the method are 2.6%, 714.6% and

1 mg NO2� per sample, respectively.

SO2: SO2 was sampled on a coated filter (SKC Cat

No. 225-9005). It consists of a short plastic tube

containing front cassette and back cassette. Front

cassette contains cellulose ester membrane filter of

0.8 mm pore size, which prevents particulates and allowscomplete passage of SO2, supported by back up pad.

Back cassette contains cellulose filter impregnated with

Na2CO3 and support pad. After sampling impregnated

cellulose filter was transferred to a clean vial and 10ml

of eluant was added and shaken vigorously for 30min.

1ml of 30% H2O2 was added and shaken well to ensure

complete oxidation of SO3� into SO4

2�. Sample was taken

into syringe fitted with an in-line filter and analysed as

SO42� using Ion Chromatograph. Precision, accuracy

and LOD of the method are 1.2%, 6.3% and

0.08 mgml�1, respectively.HNO3: HNO3 was sampled using SKC tube (Cat. No.

226-10-03) containing 400mg front section and 200mg

backup section of washed silica gel in glass tube of 7mm

OD and 11 cm length with sealed ends with plastic caps.

Front filter plug and section is retained with a glass fibre

filter plug and urethane plug separates and retains

backup section. After sampling glass fibre filter plug and

front section sorbent were transferred to a 15ml

ARTICLE IN PRESS

Table 1

Seasonal and annual average concentrations of the species

measured (mgm�3)

Species Summer

ðN ¼ 24ÞWinter

ðN ¼ 24ÞAnnual

ðN ¼ 48Þ

SO2 2.571.5 5.272.0 3.772.2NO2 4.772.9 9.674.0 7.373.7HNO3 1.370.8 0.370.3 0.770.7NH3 2.570.9 8.173.6 6.774.2NO3

� 1.971.8 1.170.9 1.171.3SO4

2� 3.272.3 2.371.2 2.671.6NH4

+ 0.970.4 1.170.4 1.070.4

Table 2

A comparison of average concentration of gaseous SO2, NO2,

NH3, and HNO3 of the present study and nearby cities and

locations in India

Site SO2 NO2 NH3 HNO3 Reference

Rampur 3.7 7.3 6.7 0.7 Present study

Agra 4.1 8.62 6.57 2.37 Kumar et al. (2003)

Delhi 17.7 37.4 –– –– CPCB (1999)

Hyderabad 11.6 46.2 –– –– CPCB (1999)

Ahmedabad 19.3 35.3 –– –– CPCB (1999)

Goa 11.2 20.5 –– –– CPCB (1999)

Shimla 6.1 9.9 –– –– CPCB (1999)

Faridabad 37.8 14.6 –– –– CPCB (1999)

Cochin 7.4 10.7 –– –– CPCB (1999)

Nagpur 10.7 14.0 –– –– CPCB (1999)

Bombay 21.7 30.7 –– –– CPCB (1999)

A. Gupta et al. / Atmospheric Environment 37 (2003) 4837–48464840

graduated centrifuge tube. 8ml of eluant was added and

heated on a boiling water bath for 10min. After cooling

volume was made upto 10ml with eluant and shaken

vigorously. Sample was taken in 10ml plastic syringe

fitted with an in-line filter and anlysed using Ion

Chromatograph as NO3�. Precision, accuracy and

LOD are 1.2%, 4.2% and 0.08mgml�1, respectively.NH3: NH3 was sampled by aspirating air through

50ml of 2.5mM H2SO4 in an impinger fitted with a

PTFE prefilter. After sampling collected NH3 samples

were transferred into polypropylene bottles, preserved in

refrigerator and analysed colourimetrically at 625 nm by

catalysed indophenol-blue method using UV-Visible

Spectrophotometer.

2.2.2. Aerosol

As no electricity is available at the sampling site,

SKC, PCXR8 battery operated universal pump with

programmable start/stop timer was used to collect

aerosols at the flow rate of 4.5 lmin�1 using GS cyclone

with cassette adapter 37mm (SKC Cat. No. 225-100).

GS cyclone has 50% cut points of 4.0 mm at a flow rateof 2.75 lmin�1 and estimated 50% cut point of 0.95mmat 4.5 lmin�1. Sampling was performed for 36 h to get

samples that could be analysed for SO42�, NO3

� and

NH4+.

The filter was extracted in 50ml deionized water on

ultrasonic bath for half an hour. Chloroform was added

to the supernatant for preservation and analysed for

NH4+, NO3

� and SO42�.

2.3. Chemical analysis

Dionex DX-500 Ion Chromatograph system equipped

with guard column (AS 11A), separator column (AS11

ASC), self-regenerating suppressor (SRS) and conduc-

tivity detector (CD-20) was used for the analysis of NO3�

and SO42� using 10mM NaOH as eluent at flow rate of

1.0mlmin�1. Gaseous NO2, and particulate NH4+ were

analysed by UV-VIS Spectrophotometer (Shimadzu

Model-1601) using NIOSH method, and indophenol

blue method (Harrison and Perry, 1986), respectively.

2.4. Uncertainty

The instrument Dionex DX-500 Ion Chromatograph

was calibrated daily with fresh working standard

solution of 2 ppm, prepared daily from 1000-ppm stock

standard solutions of NO3�, and SO4

2�. Although the

standard peak heights never changed by more than a few

percent throughout the day and the variation in peak area

was found to be less than 5%, instrument was

recalibrated after every five samples. Field blank for

aerosol samples were collected by sampling for 1min. The

concentration of ions was found to be below detection

limits. Field blank for vapour phase SO2, NO2, NH3 and

HNO3 were collected by sampling for 1min. Field blank

values were found to be below detection limit.

3. Results and discussion

Table 1 presents seasonal and annual mean values

(arithmetic mean and standard deviation) of SO2, NO2,

HNO3 and NH3 and particulate SO42�, NO3

� and NH4+.

Table 1 shows gaseous HNO3 concentration to be

relatively lower and gaseous NO2 to be relatively higher

than other species. Average concentration of SO2, NO2,

HNO3 and NH3 at the present site was compared with

other nearby cities and locations in India (Table 2). The

mean value of SO2, NO2 at the present site is lower than

that of Agra and other locations in India like, Delhi,

Hyderabad, Ahmedabad, Goa, Shimla, Faridabad,

Cochin, Nagpur and Bombay. HNO3 value is also

lower than Agra while concentration of NH3 in the

present study is higher than Agra.

Seasonally, mean value of SO2, NO2 and NH3 are

higher in winter (5.272.0, 9.674.0, 8.173.6mgm�3,

ARTICLE IN PRESS

Table 3

Average levels of pollutants in different region of the world and in present site (mgm�3)

Sampling site SO2 SO42� NH3 NH4

+ HNO3 NO3�

North Eastern Austria (rural) (Puxbaum et al., 1993) –– 8.1872.29 1.4970.87 4.5571.29 1.7870.53 4.6072.26Lithuania, Preila (rural) (Sopauskiene and

Budvytyte, 1994)

15.679.0 11.079.48 –– 1.7972.78 –– 1.5772.41

North America (rural) ((Sopauskiene and

Budvytyte, 1994)

–– 6.2476.05 0.5870.48 2.0172.09 2.6572.77 0.3170.37

NE Mediterranean, Turkey (coastal) (Erduran and

Tuncel, 2001)

2.0371.1 2.3572.01 0.9971.08 2.1971.17 0.4270.28 2.2871.78

Rampur (Present study) 3.772.2 2.671.6 6.774.2 1.070.4 0.770.7 1.171.3

A. Gupta et al. / Atmospheric Environment 37 (2003) 4837–4846 4841

respectively) and HNO3, particulate NO3� and SO4

2� are

higher in summer (1.370.8, 1.971.8, 3.272.3 mgm�3,

respectively), while particulate NH4+ does not show

much variation with respect to season. Analysis of

variance (ANOVA) is a statistical test to find differences

among the mean of components by examining the

amount of variation within each of these samples,

relative to variation between the samples (Kothari,

1997). On applying ANOVA to the data set seasonal

variation is not significant.

Higher concentration for SO2 and NO2 in winter may

be because of the high fuel usage in winter. Similar

observation has been made by others (SURE, 1981;

Shaw, 1983; Kelly et al., 1989; Scheff and Valiozis, 1990;

Barbiaux et al., 1992). The trend of seasonal variations

exhibited by particulate SO42� opposite to that of SO2.

Higher SO42� concentration in summer may be due to

higher temperature and solar insolation, which enhances

photochemical activity. Many workers have earlier

reported higher SO42� concentration in summer (Husar

and Patterson, 1980; Tanner and Leaderer, 1982;

Hussain et al., 1982; ASRC, 1985; Koutrakis et al.,

1988; Kelly et al., 1989).

Higher concentration of HNO3 in summer can be

explained by increased production of HNO3 from

gaseous precursors during photochemical activity and

a shift of equilibrium from particulate phase to gaseous

NH3 and HNO3 (HNO3+NH3"NH4NO3) (Stelson

and Seinfeld, 1982; Seinfeld, 1986). Summer average

concentration of NO3� is approximately 1.8 times greater

than winter average concentration. Similar variation

between summer and winter NO3� concentrations has

been reported at NE Mediterranean coastal site by

Erduran and Tuncel (2001). NO3� ion is mostly formed

through gas to particle conversion; therefore, increased

nitrate concentration during summer season can be

attributed to the oxidation of precursor gases to nitrate.

Seasonal fluctuation of NH3 is more pronounced than

the particulate NH4+. Ammonia is mainly emitted by

soil, livestock and vegetation. In this region summers are

dry and barren and winter is a period of maximum

vegetation growth, hence NH3 emissions are higher in

winter. However, as aerosol is basic in nature at this site

most of sulphuric and nitric acid reacts with these

aerosol particles and hence transformation of gaseous

ammonia into particulate ammonium is low.

Table 3 shows the comparison of the data of the

present site with other sampling sites in the world.

Enough data is not available for the gaseous and

particulate species from rural areas, with similar

parameters as the present study. We have therefore

included a NE Mediterranean coastal site in Table 3 as

the study reports similar parameters. On comparing with

other sites SO2, HNO3 and particulate SO42� at present

site are lower than the rural sites but higher than coastal

site. NO3� concentration at present site is lower than

other rural and coastal sites (except the rural site of

North America). The concentration of NH4+ at present

site is the lowest than that of all other sites while gaseous

NH3 is the highest.

3.1. HNO3 and particulate NO3�

Simultaneous measurements of HNO3 and particulate

NO3� show same order in levels and ratio of HNO3 to

particulate NO3� is less than unity at the present site

(0.38). Most of the ground-based measurements indicate

an excess of particulate NO3� so that HNO3/NO3

� ratio

is less than unity. In the free troposphere the relation is

reversed and the ratio is more than unity. The ratio is

also more than unity at certain urban (Claremont, CA—

1.35), rural (Abbeville, LA—2.1), semi rural (NC—2.3)

and suburban sites (Warren, MI—2.69) (Forrest et al.,

1982; Grosjean, 1983; Shaw et al., 1982; Cadle et al.,

1982). Huebert and Lazrus (1980) made measurements

of HNO3 and particulate nitrate over the North

American continent and over large parts of the Pacific

Ocean in the free troposphere and observed in both

northern and southern hemisphere 70% of the total

nitrate, on average is present in gas phase. At coastal

sites and sites (urban and nonurban) where aerosol is in

abundance in atmosphere and contains NaCl and soil

dust (carbonate and bicarbonate of Ca2+ and Mg2+),

HNO3/NO3� ratio is less than unity and percent

ARTICLE IN PRESS

Table 4

Annual and seasonal arithmetic mean (ppbv) values of total N

and ratios of different species

Ratio Winter Summer Annual

Total N 5.6 4.0 4.6

HNO3/NO3� 0.3 0.7 0.4

HNO3/total NO3� 0.2 0.3 0.3

NO3�/total NO3

� 0.8 0.7 0.7

Total NO3�/Total N 0.1 0.3 0.2

NH3/NH4+ 7.9 3.0 7.0

SO42�/SO2 0.2 0.6 0.4

Total NO3�=(HNO3+particulate NO3

�).

Total N=(NO2+HNO3+particulate NO3�).

A. Gupta et al. / Atmospheric Environment 37 (2003) 4837–48464842

contribution of HNO3 to total nitrate varies between

12% and 40% (Huebert and Lazrus, 1980). At present

site also ratio is less than unity and percent contribution

of HNO3 to total nitrate is 23% in winter and about

31% in summer (Table 4). Low ratio could be explained

on the basis of enhanced nighttime heterogeneous

formation of particulate NO3� on these calcareous

aerosols and irreversible neutralization reaction with

carbonates and bicarbonates of Ca2+ and Mg2+ and

with NaCl. The possibility of NO2 getting adsorbed on

these particles and its further oxidation to particulate

NO3� could not be ignored. Summer to winter ratio of

HNO3 and particulate NO3� are 3.8 and 1.79, respec-

tively, showing seasonal variation is more pronounced in

case of HNO3 than particulate NO3�. Aerosol is basic in

nature and its concentration is highest in summer. It is

expected that NO2 may get adsorbed on these aerosol

particles and get oxidized to particulate NO3� in addition

to N2O5 heterogeneous oxidation and condensation of

HNO3 on these particles while in winter condensation of

HNO3 may be more than in summer due to high RH

and low temperature. This may be the reason for low

summer to winter ratio for particulate NO3� than HNO3.

Significant correlation between particulate NO3� and

NO2 ðr ¼ 0:93Þ suggests formation of particulate NO3�

from NO2. Good correlation ðr ¼ 0:70Þ between parti-culate NO3

� and total NO3� indicates the dominance of

particulate NO3� in total NO3

�.

Total NO3� (HNO3+particulate NO3

�) is 1.78mgm�3

(Table 4). Reported values of total NO3� vary from

0.33mgm�3 over Pacific Ocean (Huebert, 1980) to

8.76mgm�3 at a polluted site in Germany (Meixner

et al., 1985) with rural sites averaging between 1 and

2mgm�3 (Shaw et al., 1982; Cadle et al., 1982). The

value of the present site is similar to values reported for

rural sites.

Total N (NO2+HNO3+particulate NO3�) is highest

in winter with total NO3� being only 10% of total N in

winter while total NO3� is about 31% of total N in

summer. Highest total N in winter may be because of

increased biomass burning, frequent ground level inver-

sions and maximum calm condition while the highest

level of total NO3� in summer may be because of high

photochemical activity.

3.2. NH3 and particulate NH4+

Annual mean values of gaseous NH3 and particulate

NH4+ are 6.774.2 and 1.070.4 mgm�3, respectively.

Both values are higher than reported for temperate sites

and marine sites and comparable to values reported for

suburban site (Parmar et al., 2001b) in this region. The

observed ratio of gaseous NH3 to particulate NH4+

(NH3/NH4+) is o1 at temperate countries (Georgii and

Muller, 1974; Georgii and Lenhard, 1978; Ferm, 1979)

and more than 1 at marine sites (Gras, 1983). At this site

ratio is more than 1. Similar ratio was found at suburban

site (Dayalbagh) in this region (Singh et al., 2001). The

reason for smaller ratio has been attributed to fast

transformation of gaseous NH3 into particulate NH4+

because of neutralization reaction of ammonia with

H2SO4 and HNO3 formed by atmospheric oxidation of

SO2 and NO2 (Aneja et al., 2001). The higher ratio at

marine sites and the sites where aerosol is basic in nature

due to calcareous nature of soil and road dust, (as in this

region) most of the sulphuric acid and nitric acid

preferentially react with these aerosol particles or SO42�

and NO3� are formed after adsorption of SO2 and NO2

on these calcareous particles. Hence transformation of

gaseous ammonia into particulate ammonium is less.

Similarly at marine sites HNO3 preferentially reacts with

NaCl. This is further validated by size-segregated study

for this region which reveals that SO42� and NO3

� occur

mainly in supermicron particle rather than submicron

(Kulshrestha et al., 1998; Kumar et al., 2003).

3.3. NH3 and HNO3 equillibria

HNO3 can react with gaseous ammonia to produce

particulate ammonium nitrate. It is an equilibrium

reaction

HNO3ðgÞ þNH3ðgÞ"NH4NO3ðsÞ

In order for NH4NO3 to be formed the concentration

product of HNO3 and NH3 expressed in (ppb2) must

exceed the equilibrium constant. The relationship

between NH4NO3 and its gaseous precursors NH3 and

HNO3 has been subject of several investigations. The

applicability of the equilibrium conditions based on

thermodynamic calculations has been checked in a series

of field measurements. Results of some studies have

shown that the NH4NO3 equilibrium constant is

consistent with atmospheric observations but large

deviations have also been observed mainly at relative

humidity below 60% and above 90% (Mamane and

Mehler, 1987; Yamamota et al., 1988; Allen and

ARTICLE IN PRESSA. Gupta et al. / Atmospheric Environment 37 (2003) 4837–4846 4843

Harrison, 1989; Pio et al., 1992; Harrison and Msibi,

1994; Mehlmann and Warneck, 1995; Danalatos et al.,

1995; Danalatos and Glavas, 1999; Stockwell et al.,

2000). If RH is less than RH of deliquescence at any

temperature, the relation

lnK ¼ 84:6�24220

T� 6:1 ln

T

298ð1Þ

is used to calculate equilibrium constant K in unit of

(ppb)2 (Pitts and Pitts, 1986). RH of deliquescence, is

calculated by

ln ðRHDÞ ¼732:7

Tþ 1:703; ð2Þ

where, RHD is deliquescence RH and T temperature in

Kelvin (Hammer and Wu, 1972).

For RH less than the RH of deliquescence where

ammonium nitrate is mainly in solid form disagreement

is observed between theoretical K and observed K with

less amounts of HNO3 and NH3 than predicted by

theory in summer and vice versa in winter. Similar

observation is made at present site (Table 5) i.e., the

observed K value is lower than predicted in summer and

higher than predicted in winter in most cases, although

on 3 December 2001 the Kobs is lower than Ktheo. This

has been observed by many workers (Cadle et al., 1982;

Daumer et al., 1987; Mamane and Mehler, 1987; Allen

and Harrison, 1989; Allergrini et al., 1992; Pio et al.,

1992; Harrison and Msibi, 1994; Danalatos et al., 1995;

Mehlmann and Warneck, 1995; Danalatos and Glavas,

1999). The following reasons have been suggested for

low values of observed K than theoretical K :

(i)

Table

Theo

form

Date

17-09

06-10

14-10

28-10

09-11

15-11

03-12

26-01

26-02

05-03

15-03

26-03

30-04

12-04

16-04

22-04

HNO3 deposition process is much more rapid than

the attainment of the thermodynamic equilibrium.

5

retical and observed equilibrium constant K (ppb2) for

ation of solid NH4NO3

Temperature RH obs RH cal Kobs Ktheo

-01 30.90 57.70 59.44 3.32 152.93

-01 29.80 60.01 59.96 3.02 111.05

-01 26.40 56.46 61.61 2.04 42.09

-01 27.80 56.11 60.92 8.36 62.80

-01 20.80 52.54 64.50 2.48 8.00

-01 13.20 60.91 68.80 1.32 0.76

-01 17.96 60.94 66.08 1.6 3.38

-02 18.83 61.59 65.59 5.6 4.39

-02 18.83 61.59 65.59 5.6 4.39

-02 21.17 51.92 64.32 0.46 8.93

-02 24.68 46.74 62.63 1.03 24.53

-02 26.16 42.62 61.73 0.42 39.25

-02 29.29 34.19 60.20 1.22 96.54

-02 31.92 31.02 58.97 2.02 152.00

-02 33.31 34.00 58.34 3.02 298.80

-02 32.10 31.77 58.68 2.46 242.25

(ii)

Preferential reaction of HNO3 with NaCl and soil

dust and bound with aerosol particles as relatively

nonvolatile salts. Thus no free nitrate is available

for thermodynamical equillibria with HNO3 and

NH3 (Hildemann et al., 1984; Danalatos et al.,

1995; Mehlmann and Warneck, 1995).

In this region, earlier studies (Kulshrestha et al., 1998;

Parmar et al., 2001a; Satsangi et al., 2002) on composi-

tion of aerosol and size-segregated aerosols show basic

nature of aerosols, dominated by soil components. Sum

of major cations is more than sum of major anions,

dominance coarse particles over fine particles and MMD

of NO3� in supermicron size (3.6) account for absence of

free nitrate in this region (Kulshrestha et al., 1998).

For higher values of observed K than theoretical K ;the reported reasons are

(i)

The possibility that kinetic constraints and trans-

port limitations prevent equilibrium being estab-

lished has been suggested in theoretical works

(Harrison and MacKenzie, 1990; Wexler and

Seinfeld, 1992).

(ii)

The formation of organic coatings at the surface of

aerosol droplets would occur, thus delaying and or

preventing the attainment of equilibrium (Daumer

et al., 1987).

3.4. SO2 and particulate SO42�

Oxidation of SO2 to SO42� is a complex process. Its

oxidation in both gaseous and aqueous phases takes

place with nearly equal rates although all processes are

not active at the same time. Higher SO42� concentration

(3.272.3mgm�3) has been observed in summer at this

site. Earlier studies also report higher SO42� in summer

(Waldman et al., 1991; Lee, 1995; Spengler et al., 1996;

Day et al., 1997). Higher SO42� concentration during

summer may be due to higher temperature and solar

insolation, which enhances photochemical activity,

especially the rise in OH levels during this period.

Wilson et al. (1981) and Meagher and Luria (1983) in

their study on oxidation rate of SO2 to SO42� in plumes

from electric power plants had demonstrated diurnal

and seasonal cycles with maximum value at midday and

minimum at night and also values maximum in summer

and minimum in winter and have concluded importance

of photochemical conversion. The concentration of SO2is the highest in winter in contrast to SO4

2�, which

exhibited the highest concentration in summer. As a

result, SO42� comprised a smaller fraction of SOx

(SO2+SO42�) in winter than in summer as shown by

the ratios of SO42�/SO2 (0.21 in winter, 0.59 in summer)

(Table 4). Correlation coefficient of SO42� with SO2

ranged from �0.14 in winter to 0.75 in summer, theformer significant at only 28% and latter at 97% level.

Similar results were reported earlier by Kelly et al.

ARTICLE IN PRESSA. Gupta et al. / Atmospheric Environment 37 (2003) 4837–48464844

(1989) with SO42� and SO2 correlations ranging from 0.2

in fall to 0.75 in summer, the former significant at 50%

level and latter at 99.9% levels.

4. Conclusion

The present study reports measurement of gaseous

SO2, NO2, HNO3 and NH3 and particulate NH4+,

NO3�and SO4

2� at Rampur, a rural site of semi-arid

region. Seasonally mean values of SO2 and NO2 are

higher in winter. The high concentration in winter may

be due to high fuel usage for heating purposes. The

concentration of HNO3, NO3� and SO4

2� are higher in

summer. Higher concentration of HNO3 is due to

increased production of HNO3 from gaseous precursors

during photochemical activity and a shift of equilibrium

from particulate phase to gaseous NH3 and HNO3(HNO3+NH3"NH4NO3). Higher SO4

2� concentration

in summer may be due to higher temperature and solar

insolation, which enhances the photochemical activity.

The measurements of HNO3 and particulate NO3� show

same order in levels and ratio of HNO3 to particulate

NO3� is less than unity. Summer to winter ratio of HNO3

and particulate NO3� are 3.8 and 1.8, respectively,

showing seasonal variation is more pronounced in case

of HNO3 than particulate NO3�. Total N (NO2+H-

NO3+particulate NO3�) is highest in winter with total

NO3� being only 10% of total N in winter while total

NO3� is about 31% of total N in summer. Highest total

N in winter may be because of increased biomass burning,

while the highest level of total NO3� in summer may be

because of high photochemical activity. Gaseous NH3 to

particulate NH4+ (NH3/NH4

+) is more than 1 probably

due to basic nature of aerosol. Examination of equili-

brium between gaseous NH3 and HNO3 and particulate

NH4NO3 shows observed equilibrium constant (Kobs) is

lower than theoretical equilibrium constant (Ktheo) in

summer and vice versa in winter. This observation is

similar to that reported earlier by other researchers.

Acknowledgements

We wish to thank Prof. Satya Prakash, Head, Depart-

ment of Chemistry of this Institute for providing necessary

facilities. Sampling assistance by Mr. Dayal Saran is

greatly appreciated. Financial assistance from DST (ESS/

63/166/98) is gratefully acknowledged. One of the authors

(RK) acknowledges CSIR, New Delhi for SRF.

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