Measurements of formic and acetic acid levels in the vapour phase at Dayalbagh, Agra, India

Pergamon PII:

Atmospheric Environment Vol. 30, No. 20, pp. 3545-3550, 1996 Copyright ~) 1996 Elsevier Science Ltd Printed in Great Britain. All fights reserved 1352-2310/96 $15.00 + 0.00

S1352-2310(96)00042-8

MEASUREMENTS OF FORMIC A N D ACETIC ACID LEVELS IN THE VAPOUR PHASE AT DAYALBAGH, AGRA, INDIA NANDINI

K U M A R , U. C. K U L S H R E S T H A , P. K H A R E , A. S A X E N A , K. M. K U M A R I a n d S. S. S R I V A S T A V A * Department of Chemistry, Dayalbagh Educational Institute, Dayalbagh, Agra 282 005, India (First received 20 April 1995 and in final form 12 December 1995)

Abstract--Acetic and formic acid concentrations were determined in the vapour phase at ground level at Dayalbagh, Agra, during the winter and summer of 1992-1993. Ion chromatography was used to measure the species. Yearly means were 1.7 + 1.6 ppbv for acetic acid and 1.3 + 1.4 ppbv for formic acid (N = 137). Average winter mixing ratios (N = 60) for acetic acid and formic acid were 1.8 ___1.6 and 1.4 + 1.4 ppbv, respectively, and the corresponding summer levels (N = 77) were 1.3 + 1.2 and 1.1 + 1.2 ppbv but seasonal variation was not statisticallysignificant. These concentrations are within the range reported at other sites worldwide. The F/A (formate-to-acetate) ratio showed a diurnal variation with higher averages in the day (1.2) than at night (0.45). With respect to wind direction, the W sector appears to be the cleanest and is associated with the lowest mean concentrations of acetic and formic acids; local sources as well as the ones on a larger scale appear to contribute to the levels recorded in this study. Copyright © 1996 Elsevier Science Ltd Key word index." Formic acid, acetic acid, Agra.

INTRODUCTION The acidity of precipitation over continental areas in tropical regions and in countries of the Southern Hemisphere is dominated by formic and acetic acids (Sanhueza, 1992). There are fewer measurements on these acids in the vapour phase as compared to those in precipitation. Formic and acetic acids are constituents of the atmosphere in polluted (Grosjean, 1989, 1990, 1992; Schultz Tokos et al., 1992) and remote areas of the world (Keene and Galloway, 1986; Talbot et al., 1988; Andreae et al., 1988; Dawson and Farmer, 1988; Puxbaum et al., 1988; Keene et al., 1989; Schultz Tokos, 1989; Hartmann et al., 1991; Sanhueza et al., 1992) and are therefore postulated to originate from natural as well as anthropogenic sources. Direct or secondary emissions from vegetation could be important contributors to both the species while photochemical production might result from the oxidation of H C H O in cloud water (Chameides and Davis, 1983), the oxidation of isoprene emitted by plants (Talbot et al., 1988; Andreae et al., 1988; Jacob and Wofsy, 1988; Keene and Galloway, 1986), the reaction of ozone with olefins (Calvert and Stockwell, 1983), or, as recently proposed by Madronich et al. (1990), via the reactions of peroxy acetyl radicals, CH3-CO(OO.), with hydroperoxy (HOO-), methyl peroxy (CHaOO-),

* Corresponding author.

or other primary and secondary radicals. The model proposed for the latter mechanism accounts for some ppbv of acetic acid. Chemical pathways proposed earlier for the formation of H C O O H could not explain the values of CH3COOH frequently measured in rain from remote areas. Typical concentrations of H C O O H and CH3C O O H in the unpolluted continental troposphere range between about 0.1-4 ppbv (e.g. Andreae et al., 1988; Puxbaum et al., 1988; Dawson et al., 1980) while studies by Grosjean (1989) in Los Angeles and Schultz Tokos et al. (1992) in Yokohama report values in the range of 1-15 ppbv for both species. Data on vapour phase measurements are available from several areas of the world and include equatorial Africa (Helas, 1992a), South America (Talbot et al., 1990), the U.S.A. (Grosjean, 1989; Dawson et al., 1980; Dawson and Farmer, 1988; Talbot et al., 1988; Norton, 1992; Lawrence and Koutrakis, 1994), Japan (Schultz Tokos et al., 1992; Satsumabayashi et al., 1989), and some parts of Europe (Puxbaum et al., 1988; Winiwarter et al., 1988). Several of these reports have also identified diurnal cycles for H C O O H and C H s C O O H which follow daily variations in temperature and solar radiation. This observation strengthens the argument for vegetative and photochemical sources and also indicates that both species have short residence times, of the order of a few days. Seasonal differences have also been noted in the vapour phase and the winter concentrations are lower than the summer levels (Talbot et al., 1988; Keene and

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N. KUMAR et al.

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Galloway, 1986). These differences are t h o u g h t to arise from variations in g r o w t h p a t t e r n s between seasons.

Agra is a semi-rural site a n d studies o n the chemistry of precipitation, dry deposition a n d aerosols are part of o u r research activities. W e have previously reported m e a s u r e m e n t s of formate a n d acetate in r a i n w a t e r ( K u m a r et al., 1993) at D a y a l b a g h a n d extended the study to m e a s u r e m e n t s in the v a p o u r phase. This is mainly because n o reports are available o n formic a n d acetic acid levels from a n y part of India. D a y a l b a g h , where the institute is located, is a n open area a n d is exposed to a n u n o b s t r u c t e d flow of winds. It m a y be said to represent the c o n d i t i o n s at a semirural c o n t i n e n t a l site in India,

METHODS

Sampling location Sampling was carried out on the roof of the building of the Faculty of Science of our institute situated in the suburban area of Dayalbagh, which lies to the north of Agra. Agra lies in the semi-arid zone bordering the Thar desert of Rajasthan and its vegetation is characterised by thorny scrub. The soil in this region is loose, sandy and calcareous. Wood and other biomass fuels are widely used in the entire region for domestic purposes. Summer days are extremely hot and dry (mean daytime temperature, 39°C; nighttime temperature, 24°C, RH ranging between 25 and 30%, calm period ~ 35%) with winds from the west and southwesterly sectors. Winter (November-February) days are cool (mean temperature about 22°C) and nights are cold (mean 10°C). The RH in the winter averages to about 75%. Atmospheric conditions are stable in the winter (calm period ~75%) and the predominant wind directions are east, northeast and southwest. At Dayalbagh, winters are associated with the maximum vegetative growth and summers with the least, unlike the trend in temperate countries. The hot and dry conditions in the summer do not allow for active vegetative growth but in the winter high RH values and low temperatures foster green cover.

Sample collection HCOOH and CHaCOOH were collected from the atmosphere using the aqueous nebuliser (Cofer et al., 1985), or "mist chamber". In this method, the air to be sampled is drawn through one of two nebulisers enclosed in a round glass chamber which contains the extracting solution (in this case, water). As the air passes through the second nebulising nozzle, water from the chamber is aspirated into the air stream and fine mist generated. These droplets provide a large surface area for the adsorption of water-soluble gases from the sampled air. A hydrophobic membrane placed at the top of the chamber causes water droplets that have moved upwards to coalesce and flow back into the reservoir. A detailed discussion of the efficiency of this method for organic acid collection may be found in Talbot et al. (1988). The volume of water placed in the chamber varied between 10 and 30 ml depending upon the season. Particulate matter was collected on a PTFE filter (Sartorius, 1.2 #m pore size, 47 mm dia) placed upstream of the chamber. The apparatus was housed in a wooden enclosure to exclude light and placed on the concrete roof of our faculty building at a total height of approximately 8 m. The wind direction, relative humidity and air temperature were recorded for each sample. Collection was carried out four times in a day; early in the morning (around sunrise), after sunset (together referred to

as "nighttime" measurements; collection times were between 1900 and 0600 h in the winter and 2100 and 0500 h in the summer) and twice between 0900 and 1800 LT ("daytime" measurements). Samples corresponding to daytime numbered 28 in the winter, 39 in the summer, while those collected around sunrise and sunset were 32 in the winter and 38 in the summer. All samples were treated with spectroscopic grade chloroform for preservation and then frozen. Analysis was completed within a week after collection. The sampling duration was 30 min in the winter (19 December 1992-18 January 1993) and 120 min in the summer (19 May-22 June 1993). In the latter season, sampling for less than 120 min led to values that were below the detection limits of our ion chromatograph. Field blank measurements were carried out by pulling air through the mist chamber for a period of 60 s. This blank was subsequently handled in exactly the same way as other samples and analysed. Levels of formate and acetate in the blank were always below the detection limits of our instrument.

Analytical method The measurements of CH3COOH and HCOOH levels were made by bulking the solution in the chamber to a fixed volume. Samples were injected into the chromatograph without dilution. Ion chromatography (Dionex 2000i/SP) was used for the analysis of all samples. The eluent was 0.125 or 1.25 mM Na2B40~' 10H20 and the AS4A column was used for separation. The precision of the method under these conditions was 0.41% for acetate and 0.34% for formate. The lower limits of analytical detection corresponded to atmospheric concentrations of 0.14 ppbv for formic acid, 0.43 ppbv for acetic acid in the winter (average flow rate of 14 d min-l, sampling interval of 31 min). In the summer these limits were 0.04 ppbv for formic acid and 0.18 ppbv for acetic acid (average sampling interval of 117 min and a flow rate of 16 d min-1). Standards were prepared from sodium salts of all the acids (Aldrich, > 99%) and chloroform added for preservation.

RESULTS AND DISCUSSION

T h e m e a n levels of acetic a n d formic acids in the entire d a t a set (N = 137) were 1 . 7 ( _ l . 6 ) p p b v a n d 1 . 3 ( + l . 4 ) p p b v , respectively. In the winter m o n t h s these values (N = 60) averaged to 1.8(_+ 1.6) a n d 1.4(_+ 1.4)ppbv while the s u m m e r m e a n s (N = 77) were 1.3(+1.2) a n d 1 . 1 ( + l . 2 ) p p b v , respectively (Table 1). Reduced m a j o r axis (RMA) (Miller a n d K a h n , 1962) regression p a r a m e t e r s were calculated separately for the d a t a of b o t h seasons. In the R M A m e t h o d for representing a linear relation between points, the slope is represented by the ratio Sy/Sx where Sy is the s t a n d a r d deviation of the formic acid d a t a a n d Sx, t h a t of the acetic acid data. This m e t h o d is of particular use where there are two i n d e p e n d e n t variables b o t h of which are subject to m e a s u r e m e n t error (Keene a n d Galloway, 1986). T h e slopes (winter, 0.87; s u m m e r 1.0) were n o t f o u n d to be significantly different (95% confidence level) o n the application of the Z-test (Miller a n d K a h n , 1962) a n d the entire d a t a set was therefore treated as belonging to one population. The R M A regression analysis of the entire d a t a set (N = 137, Fig. 1) gave a slope of 0.87 + 0.07 a n d intercept of 0.17. Values of the slope less t h a n a b o u t 1.0 suggest -

Formic and acetic acid levels in the vapour phase

3547

Table 1. Mixing ratios (in ppbv) of acetic and formic acids observed at Dayalbagh, Agra Acetic a c i d

Formic acid

N

Duration of measurement

1.7 (1.6) 1.8 (1.6) 1.3 (1.2) 1.8 (1.4) 1.4 (1.3) 1.2 (0.9) 2.2 (2.4) 2.0 (1.9) 1.3 (1.1)

1.3 (1.4) 1.4 (1.4) 1.1 (1.2) 0.9 (1.0) 1.7 (1.8) 1.5 (1.4) 1.0 (1.3) 0.9 (1.1) 1.6 (1.6)

137 60 77 34 (a) 35 (b) 32 (c) 36 (d) 70 67

December 1992-June 1993 19 December 1992-18 January 1993, (winter) 19 May-22 June 1993 (summer) Early morning (around sunrise) Between 1000 and 1600 LT Between 1000 and 1600 LT After sunset Nighttime (a + d) Daytime (b + c)

Values in parentheses are standard deviations.

10 o

,b

.~ 5 E

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•

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•

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.l.

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m-

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•

•

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~.'?" 0

"','%.'.

3

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.

,

,

6 9 Acetic acid mixing ratio (ppbv)

12

Fig. 1. RMA regression analysis of the entire data set (N = 137); slope: 0.87 + 0.07; intercept: -0.17.

a dominance of sources for acetic acid over those for formic acid. The mean levels of formic and acetic acids as observed at this site are within the range reported for these species worldwide; they are intermediate in nature, somewhat higher than the levels reported at remote continental and marine locations (0.2-1 ppbv) (e.g. Keene and Galloway, 1986; Norton, 1992; Arlander et al., 1990; Schultz Tokos, 1989; Helas et al., 1992a) but lower than values at polluted sites and in urban areas (2-5 ppbv) (e.g. Schultz Tokos et al., 1992; Grosjean, 1989; Kawamura and Kaplan, 1985). They are lower than values measured during the dry season in central Amazonia (both acids about 2 ppbv, Talbot et al., 1990) and Central Africa (formic acid 3.7 ppbv, acetic acid 2.7 ppbv, Helas et al., 1992a) but significantly larger than those measured during the rainy season in central Amazonia (formic acid 0.4 ppbv, acetic acid 0.3 ppbv, Talbot et al., 1990). A weak correlation coefficient of 0.33 (significant at the 99% confidence level) was obtained between formic and acetic acid. This is different from observations in most other studies where both acids are found to correlate well; values of at least 0.6 are reported

(Talbot et al., 1988; Norton, 1992 and references therein). However, a recent study also reported a relatively weak correlation of 0.44 at Uniontown, U.S.A. (Lawrence and Koutrakis, 1994). Daily fluctuations (i.e. as measured in the early morning, before noon, late afternoon and after sunset) of formic and acetic acids were not statistically significant and had high variabilities. In view of this large variation, the data were clubbed into day and night periods (Table 1) and are discussed thus; no further speculation is made about sources of the two acids at this site. Instead, some notable features of our data which are like as well as unlike those observed by other workers are highlighted below. The mean daytime values for HCOOHg (1.6 ppbv) in our data set (N -- 67) are lower than those reported for contaminated air (4.4 ppbv) by Schultz Tokos et al. (1992) but similar to values obtained at Virginia (about 1.5 ppbv) by Talbot et al. (1988). Daytime acetic acid levels (1.3 ppbv, N = 67) were similar to those obtained at Yokohama (1.4ppbv, Schultz Tokos et al., 1992). F / A ratios plotted as a function of time (season-wise) show a diurnal variation with a greater occurrence of values > 1 during the day

3548

N. KUMAR et al.

( F / A ratio of average concentrations, 1.2) and < 1 at night ( F / A ratio of average concentrations, 0.45). Daytime formic acid means always exceeded those of acetic acid while acetic acid was the dominant acid in the night and early morning hours (Table 1). A similar observation has been recorded by Martin et al. (1991). Mean levels of formic acid in the daytime (1.6 ppbv) were higher than those recorded early in the morning or after sunset (0.9 ppbv, mean). This trend is similar to that reported by other workers (Table 2) (Hartmann et al., 1991; Talbot et al., 1988) and is thought to occur because of increased emissions from vegetation as well as photochemical activity. Acetic acid, however, did not exhibit the same behaviour; instead, daytime mean values (1.3 ppbv) were a little lower than nighttime levels (2 ppbv). This exceptional pattern may be a consequence of the influence of a high winter nighttime mean of 3.6 ppbv (N = 16) on the overall nighttime average (N = 67). Increased biomass burning in the winter coupled with stable meteorological conditions are thought to be possible contributions to the 3.6 ppbv value.

Dew has been proposed (Hartmann et al., 1991; Talbot et al., 1988; Helas et al., 1992b; Pierson et al., 1998; Pierson and Brachaczek, 1990) as an important sink for the vapours of these two gaseous species and might have resulted in the fact that winter mixing ratios of both acids in the early morning were lower than the previous night's values. While due formation is a common feature at Dayalbagh in the winter we did not verify this possibility any further. A closer analysis of morning levels of both acids over the previous night's concentrations showed that the levels of both species at dawn in the summer were higher than the previous night's values in contrast with the winter observations. The average decrease of morning levels over the previous night's concentrations was 4 in the winter for formic acid (1.6/0.4); 2.5 for acetic acid, (3.4/1.3) and in the summer, 0.4 for acetic acid (1.0/2.2) as well as formic acid (0.4/1.0). The values in parentheses are averages of the night and morning concentrations, respectively. An analysis of the data with respect to wind direction showed that our sampling site may be influenced

Table 2. Daytime and nighttime concentrations (in ppb) of formic and acetic acid in the vapour phase at different locations worldwide Daytime

Nighttime

F

A

F

A

Place

Reference

1.8 4.2 1.5 10 4.4 0.45 1.6

1.4 3.6 0.5 4 1.4 0.37 1.3

0.9 3.4 0.5 8.4 3.8 0.06 0.9

1.1 3.3 0.4 3.7 1.1 0.09 2.0

Virginia, U.S.A. Los Angeles Venezuelan savannah (dry period) Pennsylvania, U.S.A Yokohama, Japan Hawaii (3400 mast) Agra, India

Talbot et al., 1988 Grosjean, 1989 Hartmann et al., 1991 Lawrence and Koutrakis, 1994 Schultz Tokos et al., 1992 Norton, 1992 Present work

x--~80°C

~

C

N

Ihi °

w

Bomhay~ •

> 5 0 x 105

[]

10 - 12 x 105

I"1

5-10x

O

>lxl05

D

Thar desert

[]

Areas with significant industrial activity

105

-

260N

Fig. 2. Map of Agra showing the location of major towns within a radius of about 200 km of it. The population of major towns is indicated in the legend and areas with significant industrial activity are shaded. The uneven lines represent state boundaries.

Formic and acetic acid levels in the vapour phase II Acetic acid 13 Formic acid

1 o

~o

illl N

NE

E

SE

S

SW

W

NW

Wind direction

Fig. 3. Mean concentrations of formic and acetic acid in the atmosphere at Dayalbagh as a function of wind direction. by polluted air from the city of Agra when winds blow from the south. O n a regional level, within a radius of 200 k m of Agra are several towns to its north, east a n d s o u t h (Fig. 2). M a n y of these are associated with significant levels of a n t h r o p o g e n i c as well as industrial activity a n d m a y also c o n t r i b u t e to formic a n d acetic acids at this site. Figure 3 shows the variation in m e a n c o n c e n t r a t i o n of formic a n d acetic acids with regard to wind direction. The lowest m e a n levels of b o t h acids (1.1 a n d 0.7 ppbv, acetic a n d formic acid, respectively) were observed from the west c o m p a r e d to the other directions which indicates t h a t d u r i n g this study it was the cleanest.

CONCLUSIONS The values o b t a i n e d at o u r site indicate that Agra, in terms of formic a n d acetic acid concentrations, is semi-rural in c h a r a c t e r but is u n d e r the influence of pollution from the city a n d some regional level sources. In the samples collected, yearly acetic acid averaged to 1.7 p p b v while the m e a n yearly formic acid mixing ratio was 1.3 ppbv. An e x a m i n a t i o n of daily trends shows a higher F / A ratio d u r i n g the day as c o m p a r e d to night. The d a t a collected in this study show no seasonal variations in the m e a n levels of formic a n d acetic acids. Associations with wind direction suggest the influence of a n t h r o p o g e n i c emissions from regional as well as local sources o n levels observed at Dayalbagh. The west sector m a y be identified as a clean air sector c o m p a r e d to the rest. Acknowledgements---We thank Prof. Satya Prakash, Head of the Department of Chemistry, Dayalbagh Educational Institute, Agra, for the laboratory facilities. We are grateful to the Council for Scientific and Industrial Research and the Department of Science and Technology, New Delhi, and to Mr. V. P. Aggrawal of AGI, Nunhai for fabricating the mist chamber.

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REFERENCES

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