Ind ian Journal of Rad io & Space Phys ics Vol. 34, February 2005, pp. 42-49

Chemistry of sulphur and nitrogen species and other major cations/anions in fog water

Anita Lakhani , G S Satsangi*, R S Parmar & Satya Prakash Department of Chemi stry, Facul ty of Science, Daya lbagh Ed ucati onal Inst itute, Dayalbagh, Agra 282 005

* Department of Chemistry, St. John 's College, Agra 282 002

Received 10 March 2004; rel'isea 8 Jun e 2004; accepted 21 September 2004

Fog water, aerosols and gas phase samples were co llected during the months of December 2000 and January 200 I at Daya lbagh, Agra. Major cations (Ca~+. Mg~+. Na+ and K+) and ani ons (F, cr. N03. and so}·) were analysed by ICP-AES and ion chromatography in the fog and aeroso l samples, while gaseous species like so~. HN0 3, NH 3 (as H/ ) and 0 3 were measured by colori metric method. The spec ies NH4 +. N03. and so}· were the dominant ions in fog water, whi ch was in fer red as the inn uence of agric ul tura l and fuel combustion ac tiv ities on fog composition. In all the samples, Ca2+/Mg~+ rati o was almost constant indicating their ori gin from so il. while the Ca2+/K+ rati os was highly va ri able indicati ng release of K+ fro m combustion sources. High concentration of so}· in fog water sa mples are attribu ted to its fo rmation by the aqueous phase ox ida tion of S02. Calcul ated values show that ox idation of S02 is fac ilitated by 0 3 concent ra tion rather than Mn2+ catalyzed reactions. High concentrati ons of N03. and NH/ in fog water samples may be due to dissolution of N03- and NH4 +aeroso ls.

Keywords: Aeroso ls, Cations, Anions. Fog water, Chromatography

PACS Nos: 92.60.Jq; 92.60. Mt

IPC Code: GO IWI /02

1 Introduction Concentrations of major ions in non-precipitating

clouds and fogs 1.2 have been reported to be signi ficantly higher than those commonl y observed in acidi c precipitation. Moreover, in Los Angeles, fog water was reported to have ac iditi es hundred times higher than those observed previously in rainwater!.". Mechanis ms responsible for the chemi cal compos ition of fog water and the origin of chemi cal components have been recently inves tigated3

.5

. Lower diluti ons and higher scavengi ng effi ciencies cl ue to reduced mass transfer limitations of gas absorption and longer res idence times explain , in part, the higher concentrati ons fo und in fog with droplet size ranging between I and 100 f.J.m in contrast to rain with droplet size between 0.1 and 3.0 f.J.m . Hi gh concentration of nitrate and sulphate and very low p H values have been fo und in fog water studies in Californi a3

·5

.

Important mechanisms controlli ng fog water compositi on involve the interaction of fog droplets with aerosol and gases. However. the resulting concen trat ions of fog water components are strongly dependent upon the growth and evaporati on of the fog droplets, as reflected by the liquid water content during the fog event.

Aerosols prov ide nuclei for the growth of the fog droplets. They may also be parti all y di ssolved and contribute to the chemical compos ition of fog water. Upon formation of fog, soluble gaseous air components are taken up into the aqueous phase; the extent of thi s uptake depends on the physical solubility of the gases which is governed by Henry's law and on their reac tivity in the aqueous phase. The resulting concentrations in the aqueous phase also depend on the volume of water per volume of air referred to as liquid water con tent (L WC). The components HCI , HN03, H20 2, CH20 are readily soluble in water, while the solubility of S02 and NH3

is strongly p H dependent. Ammonia is released directly to the atmosphere from a vari ety of sources but H2S04 and HN03 are mostly produced by atmospheric ox idati on of reduced sulphur and nit rogen compounds. Sulphuric acid is presen t as an aerosol under usual atmospheric conditions, but HN0 3 and NH3 have substantial vapour pressures over ammonium nitrate aerosol6. The atmospheric residence times of HN03 and NH3 arc strongly dependent on thei r partit ion ing between the gas phase and the aerosol.

LAKHANI e1 a/.: SULPHUR & NITROGEN SPECIES IN FOG WATER 43

In add ition to the major inorgan ic ions, fog and cloud water contain important organi c species derived from pollutant and natural sources. Carboxyls such as formaldehyde, glyoxal and methylglyoxal and organ ic acids such as formic and acetic acids are produced by photochemical oxidation of hydrocarbons and are emitted directly. These hi gb ly soluble organ ic species have been observed at hi gh concentrations in fog and cloud water2·5·

7·8

. Studies on the chemical co mpositi on of fog water are very sparse in lndia9

·10

. Studies carried out in United States and Europe1·5-

11 and Japan 12 have reported highly ac idic fog and have attributed hi gh concentration of SO/- and N03- as a major cause of acidity. In Indi a, however, high levels of Ca2

+ and Mg2+ have been observed in particulates

in the atmosphere. Moreover, these alkaline particulates have influenced the composition of

. d . . lk ,. 13 14 ra111water, ren enng It an a a 1ne nature · . In the present paper, an attempt has been made to

interpret fog water compos 1t1on collected at Dayalbagh, Agra. Agra is the site of several foundries, which release S02 and NO,. In additi on, agricultural act ivi ties and animal wastes from a nearby live stock provide important sources of NH 3 . Stagnation episodes, associated with persistent fog, occur frequently during the wi nter months. These stagnation epi sodes are caused by temperature inversions which result in a build-up of pollutant concentrati on clue to cessation of both the ventilation mechanisms of the city, viz. hori zontal wind and vertical convection. An attempt has also been made to relate the field observations during fog events to the thermodyn amic equilibria of fog water with gaseous HN03, NH3 and to the transformation of S02 into H2S04 in the aqueous phase.

2 Methodology

2.1 Description of sampling site

All samples were collected on the roof of Science Faculty building situated in the suburban area of

Dayalbagh, located about 5 km to the north of Agra city. It is primarily a vegetated area with no industrial activity. There is barren land to the E and W of the sampling site; res idential and commercial areas are situated at sou thern end, whi le the north of the sampling site is fully culti vated.

2.2 Fog collection and measurement A fog water collector was designed and built.

Figure I represents a schematic drawing of the fog water-sampling instrument. The design of the sampling unit is similar to that described by Collett et a/. 11 and consists of a I m long wind tu nnel (length = 100 em, wid th = 25 em and height = 30 em, corresponding to a volume of 750 dm3

) of stainless steel in which an air stream containing the fog droplets is created by a fan located in the rear part of the tunnel. The droplets impact on a series of Teflon strings of 2 mm thickness which are arranged in the form of screens strung on two frames inclined at an angle of 90° with respect to the direction of flow. These strings are spaced 4 mm from each other. The collected droplets coalesce and flow down the strands by gravi ty and aerodynamic drag into the co llection funnel and sampling bottle. The 52% collection efficiency was found to correspond to a droplet di ameter of 4 f.Lm. A mini aturized anemometer measured the speed of the air behind the sampling string frames and these data were used for total flow calculation. The mean speed of the air through the tunnel was 1.3 m s-1 corresponding to an airflow of 7.2 m3 min -I . The collector was sealed at both ends by two covers, which was manually opened when sampling was to be performed.

Fog in Agra occurs mostly in the months of December and January, with fog frequency nearly 25% of the time during winter. Fog water samples were collected in the years 2000 and 200 I. All fog events were sampled, but sufficient volume for chemical analysis was obtained only in 25 sampl es.

-8 Flow measurements -D

Ill \

-Air Flow --

Rear

LlJ Front

Cove Cover

Sample Collection

Fig. !-Schematic layout of fabricated fog collector

44 I DIAN J RADIO & SPACE PHYS, FEBRUARY 2005

The fog collector was installed on a 1 m high iron stand, 8 m above the ground. Before sampling, the collector was washed with detergent so lution and then rinsed thoroughly with de- ionized water. Sampling was commenced with fog formation and fog collector was sw itched off when fog di ss ipated . During fog collection, the velocity of air through the sampler and the time of collection were monitored. The liquid water content of air was calculated as the vo lume of collected water divided by the volume of ai r. Volume of air is eq ual to the product of collection time, the veloci ty of air, surface area of the sampler face a11d collection efficiency. After sampling, the collected fog water was transferred into a clean pre-weighed polyethylene bottle , so that the volume of sampl es could be calculated from its mass. The samp le vo lume varied between 60 and 300 mi.

In addition to this, aerosols and gas phase measurements of S02, 0 3, HN03 and H3 were al so cmTied out. Aerosol samples were collected on desiccated (to eliminate humidity effects) and pre­weighed Whatman 41 filter papers by a four stage Cascade Particle Separator (Kimoto CPS-I 05, Japan). Detailed description of sampling and ana lysis of aerosols and gas phase measurements of S02, 0 3,

1- 16 HN03 and H3 are given e lsewhere '· .

Meteorological data, i.e. temperature, wind direction, wind speed, relative humidity and solar intensity were recorded by an automatic data logger (Dynalab model no WDLI002) placed on the roof of the Science Faculty building at Dayalbagh during the entire course of this s tudy.

2.3 Analysis

The pH and conductivity were determined 1n an aliquot of fog water sampl e immediate ly after collection. The concentration of H/ was also determined immed iately , us ing indophenol blue method by colorimetric technique 17

. The samples were then centrifuged (model Remi R 324) at I 0,000 revolutions per minute (r.p.m.) and filtered using 0.45

~lm pore size Nylon membrane filters . The filtrate was transferred into two c lean polye thy lene bottles. One part (bo ttl e) was refrigerated after add ition of CHCI .1 to reduce microbial degradation 1' . Thi s part was used for the analysis of anions (F-. Cl-, o.~- , SO/ -). The o ther part was acidified to pH 2 with H 0 1 and stored in an acid leached bottl e. This was used for analysis o f the cations (Na+, K+, c}+, Mg~+).

The major inorganic ~mions (F, Cr, N03-, SO}-) were analyzed by ion chromatography (Dionex OX 500). Separation was accomplished using a separator column (AS4A-SC) with self-regenerating suppressor (SRS), which ensured the lowest possible background and detection limit. The column was protected upstream by a guard column (AG4A). A sample of I 0 J.l.l was injected. The e luen t was a mixture of sodium carbonate ( 1.8 mM) and sodium bicarbonate ( 1.7 mM) passed at a flow rate o f I ml min-1. Concentration of bicarbonate ion wa ca lcul ated from the pH va lues using the re lati on [HC03-]= (K.K11.P/[W] correcting the value of K11 at 278 K (Ref 19). Here K11 is the Henry's Jaw constant for C02

solubility , K the first dissociation constant for H2C03

and P the partial pressure of C02 taken here to be 0.00029 atm.

The maj or cations (Na+, K+, Ca2+, Mg2+) were

analyzed using TCP-AES (Jobin Yuon Panorama 46P Inductively Coupled Plasma-Atomic Emission Spectroscopy). The analytical uncertainties , viz. precision, accuracy and detection limits of measured species as analyzed by ion chromatography, ICP-AES :111d spectrophotometry are presented in Table I.

The H 0 3 concentratio:. was determined by chromatography as nitrate ion, while ammonia was determined as NH/ by indophenol blue method . Species S02 and 0 3 were estimated spectrophotomctrically by the West and Gaeke and the neutral buffered Kl method, respectively.

Table !- Analytical protocol

Variables Detection Prec ision Accuracy Technique used

F

CI-

NO-'-

504~­

Na+

K+

Ca2+

Mg~+

11-1/

502

limit o/o o/o neq ml- 1

8

5

I

8.3

7

2

3

37

I.J

0.78

0.05

13

2

3

8

6

0.2

0.05

0.6

0.7

3

2.5

.f.O

98 Ion ch romatography

99 Ion chromatography

92 Ion chromatography

96 I n chromatography

99 ICP-AES

98 ICP-AES

98 ICP-AES

98 ICP-AES

84 UV-Vi s Spec: lrophotomctcr

82 UV-Vi s Spec:1mphotome1ry

92 Ion chromatography

89 UV-Vis Spec I rophotomel ry

LAKHANI eta/.: SULPHUR & NITROGEN SPECIES IN FOG WATER 45

3 Results and discussion 3.1 Fog water composition

Volume-weighted mean values of fog water concentration are presented in Fig. 2. The pH of fog water samples ranged between 6.9 and 7.6 with volume-weighted mean of 7.2, indicating its alkaline nature. In India, pH of fog water samples collected at Delhi has also shown an alkaline nature with values ranging between 6.2 and 6.9 (Refs 9, I 0). The HC03-

concentrations were theoreticall y calculated. Close ionic balance was observed for most of the samples with an average cation/anion rati o of 1.1. The species

H/, N03- and SO}- were the most important components present and indicate the influence of agriculture ( 1H/) and fuel combustion (N03- and SO/) activities on the composition of fog water. Occasionally, Ca2+, Mg2+ and Cl" also contributed significantly to the total equivalent loading of fog droplets. The ratio of concentration of one component to another did not vary much from sample to sample, which implies that stable atmospheric conditions prevailed and the air masses were generally homogenous . As evident from Fig. 3, highest ionic strength (12,270 11eq r' ) was observed in fog events associated with low LWC (0.02 g m-3

). In contrast, lowest ionic strength (1850 11eq r ') was associated with high LWC (0.13 g m-3

).

Among the soil derived species, the concentration of Ca2+, Mg2+ and Na+ vary by a factor of 2 and 3, while that of K+ varies by a factor of 4. Both N03-

and SO}- vary by a factor of 3, while NH/ and cr vary by factors of 6. Further, Ca2+/Mg2+ ratio in

2000~--------------------------------~

"L 0" <)

::1.

;z Q !-<I: 0:: !-;z

1 UJ u ;z 0 u

~ ~ ~ >

0

Cl N03 504 Ca Mg K Na NH4 HC03

COMPONENTS

Fig . 2-Volumc we igh ted (V.W. ) mea n ionic concen trati on in fog w:J ter at Dayalbagh, Agra

individual samples remained nearly constant with an average ratio of 2.3, suggesting the constant source strength and, therefore, indicates their origin to be from soil (Fig. 4). The Ca2+/K+ ratio is highly variable and varies between 90 and 37 (Fig. 4). Low Ca2+/K+ ratio indicates the release of K+ from wood burning activities, which is believed to be an important sou rce of fine mode potass ium20

. The 0 3-/SO/ - ratio in fog samples was nearly constant (0.45-0.89 with a mean ratio of 0 .72) indicating that the source strengths did not vary greatly. The rati os also indicate a dominance of SO/ - over N03-. It has been observed that in Indi a, some of the so}- and N03- are contributed from soil. The mass size di stribution of SO} - and 0 3-containing aerosols is bimodal. During the winters, 40% of so/ - and oJ- is observed in the sub-micron

0 14

0 13

0 12

0 11

0 10

i' 009

;, 0.08

u 0 07

~ 0.06 ....l

0 05

004

0.03

0.02

2000 4000 6000 8000 10000 12000 14000

IONIC STRENGTH , !' eq r'

Fig. }--Relatio nship betwee n ionic strength (J.L eq 1" 1) and LWC

(g nf3)

Vl 0 i=

xo

nO

~ 40

co

SAM PLES

Fig. 4--Sample-to-sample var iati on of different io ns (*d:na points represent rati os x 10 · ')

46 'DIAN J RADIO & SPACE PI-IYS, FEBRUARY 2005

mode, which is mostly from pollution sources and 60% of so}- and oJ- is in the coarse mode, which is derived from soi l11 . Based on this assumption, soil and industrial SO}- and N03- vvcrc calculated from I So ~-/M ~+ l o-/M ~+ . . . 111 R. I 1e 4- g- anc J g- ratiOS Ill SOl . aliOS

or ( 1H/ + Ca2+)/(No.~-+ so} -) llOil·'<lil are greater than I, indicating that acidity generated by SO}- and N03-

is fully neutralized by NH/ and C}+.

In order to understand the chemistry of fog water, the complex interaction between gas phase, aerosols and fog water has to be studi ed. Gases such as H 0 3(g) and HCI (g) arc highly so luble in water, whereas solubility of S02 and NH 3 is strongly pH dependent. Depending on pH, different amounts of so2 may be taken up into the aqueous phase and become available for subsequent oxidation. Aerosols and gases are also interacted by various reactions, like the formation of NH4N0.1 aerosols from the gases HN03 and NH 3. Hi gh dissolved concentrations in fog water can result from both the uptake of different so luble components and the release of these aerosols or gaseous components upon fog dissipation. Both the role of aerosols and gaseous species in establishing the composition of the fog water and the influence of the reactions in aqueous phase during fog on the concentration of aerosols components are considered.

3.2 Uptake of S02

The average gas phase concentration of S02 during the daytime was 14 flg m-3

, while the ni ghttime concentration was lower with an average of 9.4 flg m-J. The solubility of S02 in the liquid phase strongly depends on the pH , as it is governed by the acid-base equilibria giving HSOJ- and S03

2- according to the

following equilibria21

( l)

(2)

(3)

This shows that under acidic conditions the solubility of so2 would be low and is strongly increased at higher pH. Since in our conditions, the pH of fog samples is in the alkaline range, the uptake of gaseous S02 would be favoured. The total dissolved sulphur [S(IV)], which is given as [S(IV)) = [S02.H20J + [HSOJ-] + [S03

2-], was calculated. This can be related to the partial pressure of so2 over the solution by

(4)

where, khs is the Henry's law coefficient of S02, ( 1 and k,2 are the equi I i bri um constants for the I'' and 2"d dissociation equilibria. The equ ilibri um constants were adjusted for prevalent mean temperature, 5 °C, duri ng winters (T = 278,) with Van' t Hoff relationship

Idlnk = H;<Js felT R T~

... (5)

The H0 values at 298 K were obtained from literature. The total S (IV ) species was calculated to be 9.3 X 10-3 M.

The conversion of dissolved S(IV) to sulphate is an important route for forming su lphate. Th is oxidation may be carried out by several oxidizing species including dissolved 0 2, 0 3 and H20 2 and the reaction may be uncatalyzecl or catalyzed by dissolved metal ions like Fe and Mn. The components 0 3 and H20 2

are effective ox idants clue to their higher Henry 's law coefficients (9.4 x 10-3 M atm·1 and 7.1 x 104 M atm-1

at 298 K, respectively), combined with their unusual reactivity. However, oxidation by H20 2 predominates on ly at lower pH (p H < 5) and concentrations of H20 2

are generally not suffici en t to oxidize so2 clue to its rapid depletion under fog situation21 .

Oxidation of aqueous S02 by 0 3 is strongly pH clepenclent22 and is most efficient at high pH. We, therefore, speculate that in the present conditions, oxidation would proceed, to some extent, by OJ. Although OJ is photochemically produced, and theoretically its concentrations should drop to zero in the night, but we observed an average concentration of 25.5 flg m-3 during the nighttime. This could probably result from its transference from the inversion layers to the boundary layer. We also calculated the rate of oxidation of S02 by 0 3. The reaction of 0 3 with S(IV) proceeds through independent pathways that involve a nucleophilic attack on 0 3 by S02.H20, HOS02 and SO/ -. The order of reactivity of these species is SOJ2-> HSOJ-> S02.H20. The rate of oxidation can be expressed as

-d[S(JV)] = (koet.o+k1a1+k2a2) [S(IV)] [OJ] dt

(Hoffman2J, 1985) . . . (6)

LAKHA I eta/.: SULPHUR & NITROGEN SPECIES IN FOG WATER 47

where, CXo. a 1 and a3 are the mole fractions of the three S(IV) species (CXo = xS02 . a 1 = x.[HS03-], a2 = x SO/ -). The rate expression can, therefore, be expressed as

-d[~~IV)] = (kox[ S02] + k, x.[HS03-]

+k:_ X [SO)?.-] [03]) ... (7)

The values of k0 = 2.4 x 104 M-1 s-1, k 1 = 3.7 x I 05

M-1 s-1 and k2 = 1.5 x 109 M-1 s-1 (Ref. 23) and concentrations of [S02.H20], [HS03-] and [SO/-] are g1ven as

. .. (8)

. .. (9)

... ( 10)

An oxidation rate of 0.92 x 10-2 M s-1 was thus obtained with an average nighttime concentration of 0. I I x 10-9 M 0 3 and pSOz of 3.6 x 10-9 atm.

Another oxidation path that could probably be facilitated in the present conditions is oxidation by 0 2

in the presence of Fe and Mn. The elements Fe and Mn are the abundant transition metals present in the atmosphere. Important sources of Fe and Mn containing particles include soil dust, tly ash from oil and coal fired power plants, particulate emissions from industrial operations and exhaust from internal combustion engines. At this site, Fe and Mn concentrations have been found to be 23.7 ng m-3 and 20.7 ng m-3

, respectively, in aerosols and 0.58 flg r' and 0.98 flg r' in fog water. In the presence of 0 2, Fe in Fe(III) state has been shown to be a catalyst and the rate of the catalytic reaction depends on the concentration of S(IV) and Fe(III) in solution, pH, ionic strength and temperature. The reaction can also be sensitive to the presence of anions like so/ - and cations li ke Mn2

+ in solution. The so lubility of Fe decreases significantly above pH 4.5 and Fe probably exists as condensed Fe(OH)3 or Fe20 3. Thus, we infer that the catalytic reaction by Fe would be insignificant in the present conditi ons, as Fe would be precipitated in the alkaline samples (pH> 7).

The catalytic effectiveness of Mn(ll) is also dependent on its speciation in the droplet. The principal Mn species are Mn(H20)6

2+ and MnS04(aq).

Both of these species are equally effective as catalyst for S(TV) auto-oxidation . The rate of oxidation depends on the concentration of both S(IV) and Mn(ll). At concentration of S(IV) > 10-4 M and Mn(II) > 10-5 M, the rate expression is given as

(l l )

where

k _

2 109 M_, _, ~ _ [Mn20H3

+ ][W] d 1 - x s - , an

' [Mn 2+]

log~=- 9.8 at 298 K. . .. ( 12)

An oxidation rate of 0.62 x 10-2 M s-1 was obtained in the presence of Mn2

+ at a concentration of 0.98 flg r'. A comparison of the rate constant by 0 3 and Mn2

+

shows that oxidation by 0 3 is faster than Mn2+.

3.3 Uptake of HN03

The question of an aqueous phase oxidation producing N03 during fog event may also be considered. The absorption and aqueous phase disproportionate N02 to form nitrate and nitric acids can be major contributors to nitrate formation in fog droplets if allowed to reach equilibrium. However, because of the second order kinetics they are too slow to be important on the time scale of concern in fog21 .

Production of nitrate in the aqueous phase proceeds through gas phase formation of HN03 followed by dissolution and dissociation in the droplet, as it is scavenged efficiently (100%) by the droplets. Another possible source could be the dissolution of 0 3-

aerosols and oxidation of N02- from dissolution of HN02(gl· The solubility of H 0 2 is strongly pH dependent and concentration of HN02 are expected to be high at night due to the absence of photolytic destruction and its fo rmation favoured by high humidity. In few samples N02- concentration were observed to be 3.3 fleq r'of liquid phase or 0.2 neq m-3 of air. This accounts for less than I% of total fog water N03-.

Scavenging of HN03 gas thus could probably be the major source of N03- in fog water. We measured an average concentration of 1.3 f.J.. g m-3 during

48 INDI A J RADIO & SPACE PHYS, FEBRUARY 2005

daytime and relatively lower concen tration of 0.5 I p.g m-3 in the night. Al though HN03 is photochemicall y produced , difference in daytime and nighttime values indicate it s scavenging by fog. Jacob et a/.5 have reported that over 99% of H N03 is scavenged by fog water hav ing p l-1 values between 2 and 8 and LWC of the order of 0.0 1- 1 gm-:; and predict that no measurab le HN03(g) should ex ist in eq uilibrium with fog under these cond itions. Assum ing th at all atmos pheri c HN03 is disso lved in fog water, 1.3 p.g m-3 of HN03 (20 neq m-3

) would dissolve to give 10 3- concentration of 330 p. eq r 1 at a LWC of 0.06 g

m-3. The observed vo lume-weighted mean NO,­

concentration in fog sample is 720 ~p.eq r 1• This shov~s

th at disso lution of nitric acid roughly accounts for 47% of the observed fog water No.,-. Thus, it may be inferred that a major fraction of N03- is deri ved from dissolution of 0 3- of aeroso l origin.

3.-1 Uptake of NH .1

The concentrat ion of Nl-14 + in fog water may be derived from dissolution of NH 3cgl or ammonium containing aerosol, the concentration depending also on the L WC. Below pH 5, the droplets are essentially a total sink for NH3 but above pH 5, the parti ti oning of gas between the two phases is hi ghly pH dependent. The fraction of total NH3 scavenged by fog was calculated by

. .. ( 13)

under the present conditions of pH , temperature and L WC; where equilibrium constants k1 and k2 are the k298 values for dissolution of H3(gl as Nl-13<gl

~NH3(oql (k 1 = 7.4 X 10 M atm- 1) and NH3(oq) + 1-1+

~ Nl-14 (k2 = 1.7 x I 09 M- 1). The values of k1 and k2

were corrected to T = 278 K by using Van't Hoff relation and !1H298 values were taken from literature. Here L is the LWC in cubic meter (m3) of water per cubic meter (m3) of air, R the gas constant and T the absolute temperature. Value of [H+] was calculated from the mean pH value. The fraction scavenged is found to be 20% which agrees well with the observation of Jacob eta!. 5.

The concentrations of NH/ (aq) that wou ld result from di ssolution of NH3 gas was calculated as

... (14)

where, k 11 is the Henry's constant , k 1 the dissociation constant and N H3r the tota l 1-1 3 concentration per m3

of air. At this si te, an average dayt ime concentration of H3 was found to be 465 neq m-3

. The NH/ concen tration ca lcu lated from the above was about 25 p.eq r 1

, which accounts for only I % of tota l observed NH/ in fog water. This shows th at under alka line fog cond itions H1 remains mainly in the gas phase, and that major fracti on of H/ in fog water can, therefore, be attributed to dissolution of ammonium aerosols.

4 Conclusions Fog water provides a propitious env ironment for

the scavenging of particulate and gaseous forms of sulphur and nitrogen species . High concentrations in fog water were observed at Dayalbagh, Agra . The major contributors to the ionic load ing of the fog water were H/, SO}- and 0 3-. This can be attributed to ag ricultural activiti es and combustion processes. Oxidation of S02 proceeded mainly in the fog droplets by the reaction of S(IV) wi th 0 3 and 0 2 (aq) [catalyzed by Mn (II)]. The calculated ox idation rate of so2 by 0 3 was 0.92 X 10-2 M s -I, which is greater than the calcu lated oxidati on rate by Mn2

+

(0.62 x 10-2). The N03- concentration in fog water can be associated to the dissolution of HN03 and N03- aerosols, while NI-l/ concentration could be attributed to di ssolution of NH/ aerosols rather than dissolution of gas phase Nl-13.

Acknowledgements The authors are thankful to the Director, Dayalbagh

Educational Institute, fo r providing infrastructure facilities . This study was supported by the Department of Science and Technology (DST), ew Delhi, under project No. ESS/63/B 11- 12 1/94.

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LAKHANI eta/.: SULPHUR & NITROGEN SPECIES IN FOG WATER 49

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