CONCENTRATIONS OF MONOCARBOXYLIC AND DICARBOXYLIC ACIDS ... · the benzyl ester derivatives ... and there is no reported measurements on the distri- ... Carboxylic acids and aldehydes

Pergamon Atmospheric Environment Vol. 30, No. 7, pp. 1035 1052, 1996 Copyright © 1996 Elsevier Science Ltd

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CONCENTRATIONS OF MONOCARBOXYLIC AND DICARBOXYLIC ACIDS AND ALDEHYDES IN SOUTHERN CALIFORNIA WET PRECIPITATIONS: COMPARISON OF

URBAN AND NONURBAN SAMPLES AND COMPOSITIONAL CHANGES DURING SCAVENGING

K I M I T A K A K A W A M U R A , * S P E N C E R S T E I N B E R G t

a n d I S A A C R. K A P L A N Institute of Geophysics and Planetary Physics, University of California at Los Angeles, Los Angeles,

CA 90024, U.S.A.

(First received 15 May 1995 and in final Jbrm 16 September 1995)

Abstract - Rain and snow samples collected at nine southern California sites and time series rain samples obtained during 13 rain events were studied for their content of polar organic compounds, including C1--(79 monocarboxylic acids, C2 Cto ~,og-dicarboxylic acids, C1--C2 aldehydes and C2 C3 c~-dicarbonyls. Formic (0.1 33/IM), acetic (0.3-24/~M), oxalic (0.2 28/tM), succinic (0.03 7.3/~M) and malonic (0.01 5.5 #M) acids, in addition to formaldehyde (0.3 37 #M), are the dominant species. The concentrations of the monocarboxylic and dicarboxylic acids in bulk rain were inversely proportional to the amount of rainfall, however, those of aldehydes remained almost unchanged. The relative abundances of the major compounds are similar among the samples and no significant trend was found to differentiate urban and nonurban samples. However, some minor dissolved components showed different distributions; e.g. phthalic acid/benzoic acid ratios and 2-dicarbonyl/aldehyde ratios for urban samples are higher than those of nonurban samples. The monocarboxylic and dicarboxylic acids in the bulk rainwaters were found to constitute 11-44% of the measured organic and inorganic anions and are important contributors to the lowering of pH in Los Angeles rain samples. During wet precipitation events, the rain fluxes of the polar organic compound classes, generally decreased as a function of time, suggesting a rapid removal from tlhe air. The concentration ratios of monoacids to aldehydes, diacids to aldehydes and formic acid to acetic acid showed a decrease during early stages of precipitation, suggesting a preferential scavenging of carboxylic acids over aldehydes and CI acid over C2 acid.

Key word index: Monoacids, diacids, aldehydes, rainwater, snow, time series rain, rain acidity, pH.

INTRODUCTION

A homologous series of monocarboxylic acids (CI-C~o) in rainwater samples were first identified in New York by gas chromatography (GC) employing the benzyl ester derivatives (Galloway et al., 1976). This study which reported concentrations ranging from 1.1 to 16.5/~M, concluded that organic acids make negligible contribution to the acidity of rainwater. The same authors reached a similar conclusion from further studies at Ithaca, New York and Hub- bard Brook, New Hampshire (Likens et al., 1983). By

* Present address: Department of Chemistry, Faculty of Science, Tokyo Metropolitan University, 1-1 Minamiohsawa, Hachioji, Tokyo 192-03, Japan. Fax No. + 81-426-77-2525. E-mail: [email protected].

t Present address: Department of Chemistry, University of Nevada Las Vegas, 4505 Maryland Parkway, Las Vegas, NV 89154-4003, U.S.A.

AE 30:7-C

contrast, other investigations on organic acids in rainwater in remote sites using ion chromatography in- dicated that formic and acetic acids are important chemical species in lowering the pH of rain (Galloway et al., 1982; Keene et al., 1983; Andreae et al., 1988, 1990). Formic and acetic acids have been extensively measured in urban (Dawson et al., 1980; Dawson and Farmer, 1988; Vainiotalo et al., 1983; Puxbaum et al., 1988; Talbot et al., 1988; Winiwarter et al., 1988; Grosjean, 1992; Schultz-Tokos et al., 1992; Willey and Wilson, 1993) and remote (Li and Winchester, 1989; Servant et al., 1991; Norton, 1992; Har tmann et aL, 1991; Helas et al., 1992) atmospheres, however, concentration data of higher monocarboxylic acids ( ~> C3) have rarely been reported.

Recently, homologous series of monocarboxylic acids (C1-C10) dominated by formic and acetic acids were detected in Los Angeles rainwater at a concentration range of 5.1-91/~M using a capillary GC and GC-mass spectrometry employing p-bromophenacyl

1035

1036 K. KAWAMURA et al.

esters (Kawamura and Kaplan, 1984). Their concentrations were found 40-300 times higher than those of long-chain monocarboxylic acids (C~o-C3o) (Kawamura and Kaplan, 1984). Furthermore, a homologous series of low molecular weight dicarboxylic acids (Cz--Cl0) dominated by oxalic acid have also been detected using capillary GC in the same rainwater samples at concentrations of 2.9 51#M (Kawamura et al., 1985b). Oxalic acid has been reported in the rain and snow samples from Colorado Mountains up to 3 #M by using ion chromatography (Norton et al., 1983), however, longer chain diacids have not been reported by the method. These low molecular weight monocarboxylic and dicarboxylic acids are assumed to be produced directly either from incomplete combustion of fossil fuels (Kawamura et al., 1985a; Kawamura and Kaplan, 1987; Grosjean, 1989) and/or by photo-oxidation of biogenic and anthropogenic organic compounds (Norton et al., 1983; Dawson and Farmer, 1988; Talbot et al., 1988; Gros- jean, 1989, 1992; Satsumabayashi et al., 1990; Servant et al., 1991; Kawamura et al., 1995). Only few studies have been reported on these compounds in rainwaters and there is no reported measurements on the distributions and correlation of both monocarboxylic and dicarboxylic acids together with volatile dissolved aldehydes.

In this study, we report on 14 wet precipitation samples collected in southern California (9 sites), from

urban and nonurban areas, to compare the molecular distributions of a homologous series of aliphatic monocarboxylic acids (C1-C9) and :~,~o-dicarboxylic acids (C2-Clo), as well as aromatic acids and aldehydes (formaldehyde, acetaldehyde, glyoxat and methylglyoxal). Inorganic species such as nitrate and sulfate as well as pH were also measured for a limited number of samples collected separately. We also collected 46 time series rain samples during 13 rain events to better understand compositional changes and wet scavenging processes of the polar organic compounds.

EXPERIMENTAL

Eleven bulk rain samples were collected through a steel collector with surface area of 0.22 m 2 (Kawamura and Ka- plan, 1984) and in pre-cleaned glass carboy each containing 50 mg HgCI2 and subsequently stored with HgC12 at 4(;' in pre-cleaned 4 f brown glass bottles until analysis was per- formed. Three snow samples were collected in an aluminum jar with a surface area of 0.05 m 2 or collected directly from snow layers. The samples were allowed to thaw at ambient temperature in the laboratory and then stored in a pre-cleaned amber glass bottle (4 [) with HgC12 at 4C. It is necessary to preserve the wet precipitation samples with HgCI2 or some other metabolic retardant, otherwise, organic acids in the samples are subjected to fast biological degradation (Herlihy et al., 1987; Kawamura and Kaplan, 1990).

Figure 1 shows a map of southern California, including the nine sampling sites and major highway and freeway

M t . P i n o s

Wri

3abrie"~tns. Snow V a l l e y

0 I p

San Bernardino Mtns.

~¢7 0 i f t O

LOS " Angeles

O0 0@0

5 0 krn C or ,o n a : ~

USE Del Mar ":

anta Catalina Island

Fig. 1. Sampling sites for rain and snow collection in southern California with major freeway and highway systems.

Concentrations of mono- and dicarboxylic acids 1037

systems. Los Angeles rain samples were collected on the roof of the Geology Building (six floors) or University Research Library Building (five floors) on the UCLA north campus. The Ojai rain sample was collected at ground level near the Ojai fire station. The Santa Catalina rain was collected near the pier at the University of Southern California (USC) Marine Science Center on Santa Catalina Island. The Duarte sample was collected on the roof of a three storey building at the City of Hope Hospital. The Corona del Mar sample was collected on the Kirchoff Lab Building, Depart- ment of Biology, California Institute of Technology. The Valencia rain was collected on the roof of a five storey building on the California Institute of Arts campus. Samples of recently fallen snow were collected at ground level of Mt Pinos, Snow Valley and Wrightwood in San Gabriel and San Bernardino Mountains. Table 1 gives information on the date, sampling location, precipitation amounts for the bulk wet samples studied.

For inorganic components and pH, rainwater was collected separately using a polyethylene collector (surface area: 0.071 m 2) on UCLA campus (Geology Building) during the same wet precipitation events. The samples were analyzed by Global Geochemistry Corporation, Canoga Park, California (Brewer et al., 1983).

Time series rainwaters (46 samples) were collected during 13 wet precipitation events from the spring of 1982 to the winter of 1983 on the roof of the Geology Building on UCLA campus. Duration of sample collection generally ranged from one to a few hours; however, the duration exceeded 10 h for some samples. Table 2 gives information on the time series rain samples. Sample collection and storage were the same as described above.

The analytical procedures for organic species are detailed elsewhere and are only briefly described below. A homologous series of monocarboxylic acids (C1 C9) including benzoic acid were determined on 10-50 ml samples by a capillary GC method using p-bromophenacyl esters (Kawamura and Kaplan, 19841. A homologous series of aliphatic dicarboxylic acids (C2 C1o) and aromatic (phthalic) diacids were determined on 10 ~ 50 ml samples by a capillary GC method, employing dibutyl esters (Kawamura et al., 1985b). Al- dehydes (C1 C,,) and :t-dicarbonyls (C2 C3) were determined on 1 ml samples by a HPLC method without pre- concentration using 2,4-dinitrophenylhydrazone (DNPH) derivatives (Steinberg and Kaplan, 1984). Analytical pre- cision was within 15%. Recoveries of authentic standards including monocarboxylic and dicarboxylic acids and aldehydes were better than 73% (Kawamura and Kaplan, 1984; Steinberg and Kaplan, 1984; Kawamura et al., 1985b).

The data reported here were corrected for procedural blanks. Measurement of the samples was completed in 1985.

It should be noted that a previously used GC method (methanol extraction and methyl ester derivation) has been shown to suffer from serious underestimation of oxalic acid determination (Kawamura and Ikushima, 1993).

RESULTS

Carboxylic acids and aldehydes in bulk wet precipitations from Los Angeles and vicinities

Table 3 presents concent ra t ions for al iphatic Co-C9 monocarboxyl ic acids, benzoic acid, al iphatic C2-C 10 dicarboxylic acids, phthal ic diacids, C~ C2 aldehydes and C2-Cs ct-dicarbonyls in the 14 wet precipi ta t ion samples collected at nine sites in southern California. Figure 2 gives ca rbon-cha in length dis t r ibut ions of homologous series of monocarboxyl ic and dicarboxylic acids in selected ra inwater samples collected during two precipi tat ion events.

Stra ight-chain C1-C9 monocarboxyl ic acids as well as isobutyric acid and benzoic acid were detected in the samples. As shown in Fig. 2, formic and acetic acids are dominan t species, followed by propanoic acid. The abundance of Ca acid is generally less than 10% of C1 or C2 acid. This t rend was found in all the rain and snow samples studied. The abundance of individual aliphatic monocarboxyl ic acids generally decreases with an increase in ca rbon-cha in length up to Cs. However, C6, Ca, and C9 acids are often more a b u n d a n t than C5, C7, and C8 acids, respectively (Table 2 and Fig. 2). Al though benzoic acid is generally a minor species, this a romat ic acid was the fourth most a b u n d a n t acid in the samples collected at two locat ions (Corona del Mar and Duarte , see Fig. 2).

The total concent ra t ions of C~-C9 aliphatic monoacids ranged from 0.61 to 24.3 # M in the wet precipita t ion samples (Table 3). The moun ta in snow samples showed concent ra t ions less than 1.4 #M (av. 1.1/~M), whereas uban Los Angeles samples showed generally

Table 1. Information on the bulk rain and snow samples collected from different locations of southern California

Sample Type of Type of Precipitation Duration no. Date, month, year Location location sample (mm} (h)

1 11 Nov. 1982 Mt. Pinos N 2 20 Nov. 1982 Snow Valley N 3 27 Jan. 1983 Ojai N 4 4 Feb. 1983 Wrightwood N 5 6 Apr. 1984 Santa Catalina N 6 19 Apr. 1984 Valencia N 7 19 Apr. 1984 Duarte U 8 19 Apr. 1984 Corona del Mar U 9 21 Dec. 1982 Los Angeles (UCLA, URL) U

10 27 Jan. 1983 Los Angeles (UCLA, URL) U 11 2 Feb. 1983 Los Angeles (UCLA, URL) U 12 6 Apr. 1984 Los Angeles (UCLA, Geology) U 13 19 Apr. 1984 Los Angeles (UCLA, Geology) U 14 6 Jun. 1984 Ls Angeles (UCLA, Geology) U

Snow Snow Rain Snow Rain Rain Rain Rain Rain Rain Rain Rain Rain Rain

40

1.6 0.27 4.5 1.8

19 59 32

7.7 1.8 0.61

24 11 15 11 17 1.2

Note: For locations, see Fig. 1. URL: University Research Library Building of UCLA, Geology: Geology Building of UCLA, N: nonurban, U: urban.


Table 2. Time series rainwater samples collected in west Los Angeles during 13 rain events

Event Rain collection/ Precipitation Duration Rainfall intensity no. Date time (mm) (h) (mmh 1)

1 11-12 Mar. 1982 1st 1100-1335 7.6 2.6 2.9 2nd 1335 1505 6.1 1.5 4.1 3rd 1505 1625 3.0 1.3 2.3 4th 1625--1900 6.1 2.6 2.3 5th 1900-2100 7.6 2.0 3.8 6th 2100-1900 0.8 12.0 0.07

2 8 Sep. 1982 1st 1500-2020 2.7 5.3 0.51 2nd 2020-2400 0.4 3.7 0.11

3 9 10 Nov. 1982 1st 013043430* 4.0 3.0 1.3 2nd 0930-1245 6.4 3.3 1.9 3rd 1245-1600" 4.5 3.3 1.4 4th 1945--2325 2.7 3.7 (173 5th 2325~)940 14.5 10.3 1.4

4 21 22 Dec. 1982 1st 1930-0915 4.5 13.8 0.3 2nd 0915-1230 6.8 3.3 2.1 3rd 1230-1500 3.9 2.5 1.6 4th 1500-2000 3.4 5.0 I).68

5 2 3 Feb. 1983 1st 0800-1430 13.6 6.5 2.1 2nd 1430-1700 9.1 2.5 36 3rd 1700-0800 6.2 15.0 (i),41 4th 0800-1200 0.76 4.0 0.19

6 17-18 Apr. 1983 1st 1440-1730 1.6 2.8 0,57 2nd 17304)000 8.6 6.5 1.3 3rd 010)04)930 11.4 9.5 1.2

7 19-20 Apr. 1983 1st 1800--2400 7.3 6.0 1.2 2nd 240043730 5,8 7.5 0.77 3rd 1730-1240 4,2 52 0.81

8 18-19 Aug. 1983 1st 1030-1130' 0.2 1.0 0.20 2nd 1730-O920 18.2 15.8 1.2

9 20 Sep. 1983 1st 1530-1630 0.39 1.0 0.39 2nd 1630-1930 0.23 3.0 0.08

10 30 Sep-I Oct. 1983 1st 1000-1800 5.8 8.0 (3.7 2nd 1800-0700 31.8 13.0 2.4

11 4-5 Oct. 1983 1st 0630-1900" 0.15 2.5 0.06 2nd 0000-0600 6.1 6.0 I.I)

12 9 Dec. 1983 1st 1115---1215 2.3 1.0 2.3 2rid 1215-1315 5.3 1.0 5.3 3rd 1315--1430 1.7 1.3 1.3 4th 1430-1530 2.1 1.0 2. I

13 24~26 Dec. 1983 1st 1445--1745 12.3 3.0 4.1 2nd 1745-2045 2.7 3.0 0.90 3rd 2045-0015 1.2 3.5 0.34 4th 0015-O915 34.1 9.0 3.8 5th 0915-2015 0.61 11.0 0.06 6th 2015-0500 0.61 4~8 0.13

* Break of rain.

much higher concent ra t ions (3.7-24.3/~M, av. 11.0 /~M). However in n o n u r b a n samples (Santa Cata l ina and Valencia), the organic acid concent ra t ions are unexpectedly high (7.3-20.6/~M), and similar to the concent ra t ion range in u rban rain. The benzoic acid concent ra t ion range is 0.04).51/~M, which is < 4 % of the total al iphatic monoacids . The concent ra t ions of formic and acetic acids in the Los Angeles rain samples are generally less than those reported for fog and cloud samples collected from Los Angeles basin (Erey et al., 1993), a l though organic acids > C3 have not been reported in fog or cloud samples.

Straight-chain C2~C1o and b ranched chain C 4 ~ 6 dicarboxylic acids as well as a romat ic diacids (phthalic acid and methylphthal ic acid) were detected

in the precipi tat ion samples (Table 3). Unsa tu ra ted diacids are also present in the samples, including maleic, fumaric, methylmaleic and dimethylmaleic acids. The chain- length dis t r ibut ions of dicarboxylic acids are shown in Fig. 2. Oxalic acid, which is the shortest chain (Cz) diacid, is always the most abun- dan t diacid species, followed by succinic acid (C4). The th i rd group includes malonic (C3) , maleic (C#), methylmaleic (uiCs), methylsuccinic (iC~), glutaric (C5) and phthal ic acids. Fumar ic (C4), methylmalonic (iC4) , 2 -methylg lu tar ic (iC6), 3-methylglutaric (aiC6) and s t ra ight-chain C7 C1o diacids are minor species. Interestingly, azelaic acid (C9 diacid) is relatively a b u n d a n t compared to Cs and C1o diacids (e.g. Fig. 2), suggesting that this acid is part ly produced by the


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Fig. 2. Relative abundance of individual monocarboxylic and dicarboxylic acids in rainwater samples collected in

southern California sites during two rain events.

photochemical oxidation of biogenic unsaturated fatty acids containing a double bond at C-9 position (Kawamura and Gagosian, 1987).

Total concentrations of aliphatic Cz-Cto diacids ranged from 0.41 to 25/~M in the bulk precipitation samples (Table 3). These values are comparable to those of monocarboxylic acids. The diacid concentrations among different locations show a trend similar to monoacids; that is, they are low (0.87-1.4 ~M, av. 1.2 ~tM) in snow samples collected in mountain areas, whereas they are high (1.8-12/~M, av. 4.9 #M) in urban Los Angeles samples, and very high (11-25 ~M)

in the Santa Catalina, Valencia and Corona del Mar samples. Very high concentrations in the latter three samples are associated with very weak rainfall intensity (0.27-1.8 ram, see Table 1). Concentrations of the aromatic, phthalic acids ranged from 0.03 to 0.87 #M (av. 0.30/~M), which correspond to 1.8-22% (av. 6.7%) of total aliphatic diacids. They are relatively abundant in urban Los Angeles rain samples (0.22- 0.67/~M, av. 0.37 #M), and show low abundance in mountain snows (0.04-0.05 #M), but display relatively high abundance (0.03-0.87/~M, av. 0.36/~M) in rural rain samples (e.g. Santa Catalina). Very high concentration of oxalic acid up to 16.5/~M in bulk rainwater is almost equal to its concentration in fog water samples (up to 16.8/~M) collected from the South Coast Air basin (Erey et al., 1993). Very high concentrations of oxalic acid (17-28/~M, Table 5) were also observed in time series rainwater samples, whose rainfall inten- sities were very weak (0.084).9 mmh i , i.e. # 9 rain event, Table 2).

Four aldehydes are generally present in the bulk precipitation samples: formaldehyde, acetaldehyde, glyoxal and methylglyoxal (Table 3). By contrast, propanal was rarely detected, and other aldehydes were not detected in the wet precipitation samples analyzed. Formaldehyde is present as the dominant species in all the rain and snow samples and its concentration ranges from 0.3 to 8.2/~M (av. 2.7/~M). Acetaldehyde concentrations are less than 0.7 #M (av. 0.30/~M), and are about one-tenth of formaldehyde concentrations. Glyoxal and methylglyoxal are minor species and their concentration ranges are 0.0~0.40 /~M (av. 0.30 #M) and 0.04).60 #M (av. 0.18 I~M), respectively. These aldehyde concentrations are low in the mountain samples (0.46 2.6/~M), whereas they are relatively high in Los Angeles and Duarte samples (1.9-9.7~M). However, nonurban samples (Santa Catalina and Valencia) showed relatively high (5.2 5.9 k~M) concentrations of aldehydes (Table 3), thus correlating with monocarboxylic and dicarboxylic acids. With the exception of one sample from Wright- wood, concentration of the aldehydes in the wet precipitation samples are always less than those of monocarboxylic and dicarboxylic acids.

Carboxylic acids and aldehydes in time series rain samples

Concentration of C~-C9 monocarboxylic acids and Cz-Clo dicarboxylic acids in rainwater samples collected sequentially during wet precipitation events is given in Tables 4 and 5, respectively. Their concentrations are generally similar to those found in the bulk rainwaters as described above. However, the relative abundance changes as a function of time during wet scavenging and their concentrations sometimes fluctuate abruptly during precipitation events depending on rainfall intensity.

Total concentrations of C1-C9 monoacids fluctuated in the range from 0.8 59 #M (Table 4, Fig. 3al and show that concentrations of monoacids generally


Table 4. Concentrations (pM) of monocarboxylic acids in time series rainwater samples collected in west Los Angeles during 13 rain events

Rain event Rain no. CI C2 C3 iC4 C4 Cs C6 Cv C8 C9 C1-C9 Benz Total

1 1st 7.75 16.88 0 .34 0 .03 0 .07 0 . 0 4 0 .12 0 .05 0 .03 0 . 0 3 25.34 0.05 25.39 2nd 1,99 1.86 0 .10 0 .01 0.02 0 .01 0 .02 0.01 0 .01 0.02 4.03 0.01 4.04 3rd 3,43 3.47 0 .18 0 . 0 2 0 .03 0 .01 0 .03 0 ,01 0 .01 0.02 7.21 0.02 '7.23 4th 17,54 5.42 0 .65 0 .04 0 .12 0 .05 0 .09 0 .04 0 .05 0 . 0 2 24.01 0.10 24.10 5th 5.35 5.84 5.36 0 .03 0 .07 0 .03 0 .06 0 ,02 0 .04 0 . 0 2 11.82 0.03 11.85 6th 7.72 24.64 0 .94 0 .08 0 .18 0 .08 0 .18 0 ,04 0 .05 0 . 0 2 33.93 0.09 34.03

2 1st 26 .60 14.80 0 .87 0 .09 0 .15 0 .06 0.10 0 ,05 0 .07 0 . 0 5 42.83 0.13 42.96 2nd 33.60 23.10 1.34 0 .18 0 . 2 6 0 .10 0 .16 0 .08 0 .08 0 . 0 5 58.94 0.15 59.08

3 1st 6.09 4.61 0 .39 0 .05 0 .10 0 .05 0 .06 0 .03 0 . 0 4 0 . 0 6 11.46 0.07 11.53 2nd 2.36 2.19 0 .16 0 .02 0 .03 0.02 0 .03 0 .01 0 .02 0.03 4.87 0.03 4.90 3rd 1.37 1.39 0.11 0 .01 0 . 0 2 0 .01 0 .02 0 .01 0 .01 0.02 2.96 0.01 2,98 4th 2.73 3.28 0 .22 0 .02 0 . 0 4 0 .02 0 .03 0 .02 0 .02 0.11 6.48 0.02 6.50 5th 0.42 0 .58 0.09 tr 0.01 tr 0.01 nd nd nd 1.10 nd IAO

4 1st 11.60 5.06 0 .37 0 .03 0 .07 0 .03 0 .04 0 .02 0 .05 0 . 0 2 17.29 0.06 17.35 2nd 3.36 2 .10 0 .14 0 .01 0 . 0 2 0.01 0 .03 0 .01 0 . 0 2 0.03 5.73 0,02 575 3rd 1.79 1.36 0 .09 0 .01 0 . 0 2 0 .01 0 .02 0 .01 0 .01 0.04 3.34 0.01 3.35 4th 0.43 1.00 0 .07 0 .01 0 .01 0 .01 0 . 0 2 0 .01 0 .01 0.03 1.59 0.01 1.60

5 1st 1.07 2.76 0 .25 0 .02 0 .05 0 .02 0 .02 0 .01 0 .01 0.03 4.24 0.02 4.26 2nd 0.19 0 .79 0 .07 0 .00 0.01 tr 0.01 nd nd nd 1.06 nd 1.06 3rd 0.34 0.81 0.09 nd nd nd 0.01 nd nd nd 1.24 nd 1.24 4th 5.86 6.18 0 .49 0 .03 0 .07 0 .02 0 .03 0 .01 0 .01 0 . 0 2 12.73 0.03 12.76

6 1st 12.00 16.40 1.87 0 .23 0 .43 0 . 1 7 0 .22 0 .10 0 .07 0 . 1 0 31.59 0.17 31.76 2nd 0.82 0 .99 0.11 0 .01 0.01 nd nd nd nd nd 1.4 nd 1.94 3rd 0.33 0.51 0.05 nd nd nd nd nd nd nd 0.89 nd 0.89

7 1st 3.24 2 .26 0 .19 0 .01 0.02 nd nd nd nd nd 5.72 0.02 5.74 2nd 0.89 0.91 0.07 nd 0.01 nd nd nd nd nd 1.88 nd 1.88 3rd 3.37 3.58 0.30 0 .02 0 .05 0 .02 0 .03 0 .01 0 .01 0.01 7.39 0.02 7.41

8 1st 20 .20 12.50 0.91 0 .09 0 .19 0 .08 0 .19 0 .06 0 .08 0 . 1 3 34.44 0.17 34.61 2nd 6.06 5.23 0 .27 0 .02 0 .04 0 .02 0 ,03 0 .01 0 .02 0 . 0 2 11.72 0.03 Jll.75

9 Ist 28 .00 14.80 1.26 0 .17 0.30 0 .14 0 .20 0.11 0 .15 0 . 0 8 45.21 0.19 ,15.39 2nd 10.30 7.43 0 .66 0 .06 0 .12 0 .05 0 .07 0.04 0.03 nd 18.76 0.06 18.82

10 Ist 5.45 4 .92 0 .40 0 .04 0 .07 0 .03 0 .05 0 .01 0 .02 0.01 11.00 0.03 11.03 2nd 1.11 1.21 0.10 nd 0.11 0 .01 0 .02 0 .01 0 .01 0.01 2.59 0.01 2.60

11 1st 33 .80 21.00 2 .57 0 .49 0 .67 0 .28 0 .40 0 .15 0 .14 0 . 0 5 59.56 0.26 :59.82 2nd 7.06 5.59 0 .44 0 .04 0 .08 0 .03 0 .04 0 .01 0 .01 0.01 13.31 0.03 13.34

12 1st 4.74 5.10 0 .46 0 .04 0 .11 0 .04 0 .05 0 .02 0 .02 0 . 0 2 10.60 0.04 10.63 2nd 0.60 1.04 0 .12 0 .01 0 .02 0 .01 0 .01 0 .01 0.01 nd 1.81 0.01 1.82 3rd 0.90 0 .47 0 .05 0 .01 0 .01 0 .00 0.01 0 .00 0.00 nd 1.46 0.01 1.47 4th 1.11 1.31 0.12 0 .01 0 .02 0 .01 0 .02 0.00 0.00 nd 2.60 0.01 2.61

13 1st 1.55 1.24 0 .10 0.01 0 . 0 2 0 .01 0 .01 0 .00 0 .00 0.01 2,94 0.01 2.95 2nd 3.00 3.29 0 .22 0 .02 0 .04 0 .01 0 . 0 2 0 .01 0 .01 0.01 6.61 0.02 6.63 3rd 3.!6 5.00 0 .27 0 .02 0.04 0.01 0 .03 0 .01 0 .01 0.01 8.56 0.02 8.57 4th 1.67 1.04 0 .08 0 .01 0 .01 0 .01 0.01 nd nd nd 2.82 0.01 2.83 5th 1.91 4.87 0 .23 0 .03 0 .06 0 .02 0 .04 0.01 0 .01 0.03 7.21 0.01 7.22 6th 8.61 8.11 0 .45 0 .04 0 .08 0 .04 0 .08 0 .03 0 .02 0 . 0 3 17.49 0.04 17.53

Note: C,: monocarboxylic acid with n-carbon numbers, iC4: isobutyric acid, Benz: benzoic acid, nd: not detected, tr: trace.

decrease as a function of time, indicating that carboxylic acids are scavenged early in the wet precipitation event by raindrops. However, the concentrat ions did not always continue to decrease with time: they sometimes increased in the middle or late stage of

precipitation events (Fig. 3a). In the time series precipitation samples, oxalic acid

comprised 30-74% (av. 52 _+ 11%) of the total diacid concentration. Phthalic, malonic, maleic, fumaric, methylmaleic and glutaric acids constitute a less abundant group (Table 5). Aromatic (phthalic) acids are about 10% of the aliphatic diacids. Although the relative abundances of individual diacids did not markedly change during precipitation events, their

individual concentrat ions fluctuated over an order of magnitude during some rain events. Generally, their concentrat ions decreased from the first to second rain collections. However, higher concentrat ions occa- sionally appeared during a prolonged rain event (Fig.

3b), similar to the patterns for monoacids (Fig. 3a). Formaldehyde was detected as the most abundant

aldehyde in all the time series rainwaters and comprised 54-96% (av. 78 + 10%)of total concentrat ions of aldehydes detected. Acetaldehyde, glyoxal and methylglyoxal and methylglyoxal are minor constitu- ents, whereas propanal and butanal were seldom detected. Figure 3c illustrates that fluctuations of total C~-C , aldehyde concentrat ions in the rainwaters as

Tab

le

5.

Con

cent

ratio

ns

(PM

) of

dic

arbo

xylic

ac

ids

in

time

seri

es

rain

wat

er

sam

ples

co

llect

ed

in w

est

Los

A

ngel

es

duri

ng

13 r

ain

even

ts

Rai

n R

ain

even

t no

. C

2 C

3 iC

, M

C

, iC

, F

mM

C

, dm

M

iC,

aiC

, C

6 C

, C

g C

,, (Z

-C,,

Ph

4-m

Ph

Tot

al

Ph

Tot

al

1st

0.98

0.

07

nd

0.13

0.

28

0.10

0.

04

0.12

0.

09

0.06

0.

03

0.05

0.

03

0.02

0.

06

0.04

2.

07

0.19

2n

d 0.

36

0.02

nd

0.

02

0.09

0.

03

0.03

0.

04

0.02

0.

05

nd

nd

0.01

nd

0.

01

nd

0.68

0.

05

3rd

0.28

0.

01

nd

0.02

0.

05

0.01

0.

01

0.03

0.

02

0.01

0.

01

nd

0.01

nd

0.

01

nd

0.45

0.

04

4th

0.64

0.

04

nd

0.08

0.

24

0.08

0.

07

0.08

0.

09

0.03

0.

03

0.02

0.

01

nd

0.01

nd

1.

42

0.21

5t

h 0.

70

0.13

nd

0.

10

0.24

0.

08

0.04

0.

10

0.09

0.

04

0.02

0.

02

0.02

nd

0.

01

nd

1.59

0.

16

6th

1.50

0.

09

nd

0.10

0.

26

0.10

0.

10

0.11

0.

09

0.04

0.

03

0.02

0.

02

nd

0.02

nd

2.

49

0.19

1s

t 1.

54

0.15

nd

0.

13

0.33

0.

10

0.05

0.

11

0.10

0.

03

0.03

0.

04

0.03

0.

01

0.03

0.

01

2.69

0.

26

2nd

0.77

0.

08

nd

0.04

0.

17

0.04

0.

04

0.04

0.

04

0.01

0.

01

0.01

0.

01

0.01

0.

02

0.00

1.

30

0.12

3r

d 0.

70

0.08

nd

0.

03

0.15

0.

03

0.03

0.

03

0.04

0.

01

0.01

0.

01

0.02

0.

01

0.03

0.

01

1.17

0.

09

4th

0.79

0.

13

nd

0.06

0.

27

0.09

0.

06

0.08

0.

10

0.03

0.

03

0.02

0.

03

nd

0.02

nd

1.

69

0.09

5t

h 0.

27

0.04

nd

0.

02

0.08

0.

02

0.02

0.

02

0.02

0.

01

0.01

0.

01

0.01

nd

0.

00

nd

0.52

0.

05

1st

25.3

0 4.

73

0.85

1.

77

5.36

1.

73

2.08

0.

74

2.38

nd

nd

nd

1.

22

nd

nd

nd

46.1

6 1.

24

2nd

2.78

0.

54

nd

nd

0.31

nd

nd

nd

0.

11

nd

nd

nd

nd

nd

nd

nd

3.74

0.

14

1st

28.3

0 5.

55

0.76

1.

37

7.34

1.

78

0.73

0.

62

1.81

nd

nd

nd

0.

46

0.16

0.

19

nd

49.0

7 1.

87

2nd

17.2

0 4.

79

0.82

1.

42

4.30

1.

11

1.64

0.

88

1.20

nd

nd

nd

0.

47

0.24

0.

09

nd

34.1

6 1.

07

1st

0.63

0.

11

nd

0.10

0.

21

0.08

0.

02

0.10

0.

08

0.10

nd

0.

07

0.03

nd

0.

02

nd

1.54

0.

1 I

2nd

0.58

0.

09

nd

0.13

0.

38

0.10

0.

08

0.13

0.

10

0.08

0.

05

0.11

0.

07

nd

0.03

nd

1.

91

0.15

3r

d 1.

24

0.35

nd

0.

17

0.64

0.

26

0.15

0.

22

0.22

nd

nd

nd

0.

16

nd

0.17

nd

3.

57

0.21

4t

h 0.

18

0.04

nd

0.

14

0.06

0.

02

0.02

0.

03

0.03

0.

02

nd

0.02

0.

01

nd

nd

nd

0.56

0.

04

5th

1.82

0.

08

nd

0.03

0.

23

0.06

0.

04

0.05

0.

05

0.05

0.

01

0.02

0.

02

nd

0.03

nd

2.

48

0.09

6t

h 2.

83

0.32

nd

0.

18

0.79

0.

23

0.12

0.

15

0.22

0.

06

0.07

0.

07

0.09

nd

0.

1 1

nd

5.22

0.

14

0.03

0.

01

0.26

nd

0.

35

0.16

0.

03

0.05

0.23

0.

05

0.05

0.

24

0.19

0.

24

0.31

0.

14

0.11

0.

12

0.06

1.

50

0.14

2.

22

1.23

0.

14

0.20

0.

31

0.05

0.

11

0.18

2.30

0.

73

0.50

1.

65

1.78

2.73

?

2.99

1.

44

F

1.28

1.

80

S z 0.

58

47.6

6 E

3.

88

1 51

.29

a 35

.39

--

1.68

2.

11

3.88

0.

61

2.59

5.

41

Not

e:

C,:

dica

rbox

ylic

ac

id

with

n-

carb

on

num

bers

. 1:

iso.

ai

: an

teis

o,

M-

mal

eic

acid

. m

M:

met

hylm

alei

c ac

id.

dmM

: di

met

hylm

alei

c ac

id,

Ph:

phth

alic

ac

id.

4-m

ph:

4-m

ethy

lpht

halic

ac

id,

nd:

not

dete

cted

, tr

: tr

ace.


60

4O

1 20

o

50

~ 40

°1 30

o

0 t - -

40 t

t

1

(a) Monoacids

\ ° I2 ii2i; !!i . . . . . " S : ; T o

× , ) \ , / \ . / , / .B

(b) Diacids

\ \ \

(c) Aldehydes / ,/

/ /

20 ": . / It. Io - . /

1 2 3 4 5 6

Time Series Rain Samples

Fig. 3. Concentrations of (a) C1-C~ monocarboxylic acids, (b) C2 Clo dicarboxylic acids and (c) C1 C4 aldehydes in time series rain samples collected during 13 rain events in

west Los Angeles.

a function of time show a pattern similar to the monocarboxylic acids, although a decrease in the concentration from the first to second rain is not so abrupt as monoacids.

D I S C U S S I O N

Oriyin o f polar or,qanic compounds in wet precipi ta t ions

Motor exhaust emissions with concentrations ca. 100 times higher than those in average Los Angeles atmosphere (Kawamura et al., 1985a; Kawamura et al., in preparation) have been proposed as a source for atmospheric organic acids. The distributions of monoacids in gasoline engine emissions were charac- terized by a predominance of acetic acid followed by formic acid (Ct/C2 ratio; 0.11-0.83, av. 0 .37_ 0.26) and higher acids are less abundant. However, the formic/acetic ratios in the wet precipitation samples (0.30-2.3, av. 1.0 + 0.52, Table 3) reach values signifi-

cantly higher than those found in the motor exhaust (Kawamura et al., 1985a). There is an additional source most probably by photochemical production of monocarboxylic acids which significantly contrib- utes to the atmosphere (e.g. Tuazon et al., 1978, 1981; Niki et al., 1983; Dumdei and O'Brien, 1984).

Based on computer simulation models, Chameides and Davis (1983) proposed that aqueous-phase OH radical reactions are important in the production of formic acid from formaldehyde in clouds. On the other hand, organic acids including formic acid are major degradation products of phenol by ozone in the aqueous phase (Yamamoto et al., 1979). Phenols are directly emitted to the atmosphere from automobiles and other combustion sources (Graedel et al., 1986) and are abundant in the urban atmosphere (Leuen- berger et al., 1985).

Biogenic emission of volatile acids and biomass burning over central Amazonia has been considered as an important contributor of atmospheric formic and acetic acids (Talbot et al., 1988, 1990), where industrial or vehicular sources are not important. Graedel and Eisner (1988) proposed that atmospheric formic acid may also be liberated by formicine ants in continental tropospheres.

Individual diacid species as well as a homologous series of dicarboxylic acids (C2-C1 o) have been detected in the atmosphere (O'Brien et al., 1975; Grosjean et al., 1978; Appel et al., 1979; Yokouchi and Ambe, 1986; Kawamura and Kaplan, 1987; Ludwig and Klemm, 1988; Willey and Wilson, 1993; Kawamura and Ikushima, 1993; Kawamura and Usukura, 1993; Kawamura et al., 1995) and also in rainwater (Kawamura et al., 1985b; Steinberg et al., 1985; Sem- pere and Kawamura, 1994). These diacids may originate from different sources by several pathways. Schuetzle et al. (1975) and Grosjean et al. (1978)considered that C3-C6 diacids can be produced by photo- oxidation of cyclic olefins. Norton et al. (19831 proposed that oxalic acid is produced by the photo-oxidation of ~-dicarbonyls (glyoxal and methylglyoxal), which are major degradation products of toluene in smog chamber experiments (Nojima et al., 1974; Hoshino et al., 1978; Takagi et al., 1980). Yokouchi and Ambe (1986) and Kawamura and Gagosian (1987) proposed that C9 diacid originates from biogenic unsaturated fatty acids containing a double bond at C-9 position by photo-oxidation. Maleic and methylmaleic acids in rainwater could be produced by photo-oxidation of aromatic structures such as benzene, toluene, xylene and phenols (Kawamura et al., 1985b; Sempere and Kawamura, 1994). Atmospheric reactions of both anthropogenic and biogenic organic compounds are probably important sources for the dicarboxylic acids in wet precipitations.

In addition, direct emission of diacids from motor vehicles to the atmosphere are also important. We found that C2-C~o aliphatic diacids are present in gasoline and diesel engine exhausts and that C4 maleic (cis) acid is much more abundant than C,~


fumaric (trans) acid (Kawamura and Kaplan, 1987). These results probably indicate that the cis configura- tion of the diacids originate from the conjugated double bonds of aromatic hydrocarbons by oxidative degradation during incomplete combustion process. Aromatic diacids (phthalic acids), which are present in wet precipitation samples, have also been found in motor exhaust (Kawamura and Kaplan, 1987), and are widely distributed industrial products (e.g. plas- ticizers) as well as photochemical reactions by the oxidation of polycyclic hydrocarbons such as naph- thalene (Kawamura and Ikushima, 1993).

Biogenic sources may also contribute to the diacid distribution of wet precipitation, because some diacids such as oxalic, fumaric and succinic acids are important metabolic intermediates and are present in soils, but no direct correlation of these diacids to biogenic emission has yet been shown,

Short-chain aldehydes (C1-C4), dominated by formaldehyde and acetaldehyde, have been reported in the Los Angeles atmosphere (Grosjean, 1982; Gros- jean et al., 1983; Grosjean and Fung, 1984). The C1 and C2 aldehyde concentrations are comparable in the atmosphere. These aldehydes originate from direct motor exhaust emission (Kuwata et al., 1979; Johnson et al., 1981; Lipari and Swarin, 1985), atmospheric ozone/OH radical-olefin reactions (Niki et al., 1983), as well as plant emission (Nicholas, 1973). However, in the wet precipitation samples, formaldehyde is always more abundant than acetaldehyde, with C~/C2 ratio of 9.9 + 7.0 (Table 3). A significant difference in the aldehyde distributions between air and rain indicates apparent fractionation of aldehydes during precipitation events, resulting from (a) selective scavenging of C~ over C2 aldehyde, and/or (b) selective loss of C2 aldehyde by oxidation to acetic acid (C2). The former is very likely because Henry's Law constant for C~ aldehyde ( 3 . 5 x l 0 3 m o l / - ~ a t m -~) is greater than that of Ca aldehyde (11.5 m o l / - ~ atm l ) (Okita et al., 1983). Formaldehyde could be preferentially stabilized relative to acetaldehyde by hydration (Grosjean and Wright, 1983) and by forming a hydroxymethanesulfonate adduct with HSO;- more easily than acetaldehyde in wet precipitation (Munger et al., 1986).

Factors controlling concentrations of organic acids and aldehydes in collected rainwater

The Los Angeles and Duarte sites are located in polluted areas, whereas Santa Catalina Island, Ojai, Valencia and Corona del Mar are further from major pollution sources (see Fig. 1). Therefore, the polar organic pollutants would be more abundant in wet precipitation samples collected at Los Angeles and Duarte than at the other sampling sites. However, the concentrations of monoacids, diacids and aldehydes in Los Angeles and Duarte samples were not always higher than those in the rural or semi-rural samples collected during the same rain storm events (samples No. 5 and 12 for the 6 April 1984 event and samples

No. 6-8 and 13 for the 19 April 1984 event, see Table 3). This suggests that air mass containing organic pollutants can be transported from urban to non-urban areas (sometimes over the ocean) and that they may also be produced in the air during transport. The concentration of polar compounds in wet samples is probably controlled by the content of the pollutants in the atmosphere, their solubility and also by total precipitation amounts. Because short-chain organic acids and, to a lesser extent, aldehydes are mainly water soluble, they are effectively scavenged during the early part of a rain event and then subsequently are diluted by additional precipitation. In fact, on 6 April 1984, the precipitation at Los Angeles was 7.7 ram, whereas that of Santa Catalina Island was only 1.6 mm, which could explain why the latter rain is more concentrated (Table 3). A similar trend is observed for the 19 April 1984 rain events at different sites (see Table 3l.

Figure 4 shows changes in the concentration of C1-C9 monoacids, C2-Clo diacids and C1 C4 aldehydes in Los Angeles bulk rain samples as a function of the precipitation amounts. Concentrations of monocarboxylic and dicarboxylic acids generally increase with a decreasing precipitation, which suggests that total rainfall intensity imposes a control on the concentration of these carboxylic acids in collected rainwater. Because low molecular weight monocarboxylic acids are mostly present in gas phase (Kawamura et al., 1985a; Norton, 1992) and are very water soluble, they should be scavenged from the atmosphere into rain droplets during an early stage of each rain event, being consistent with rain flux results for time series samples (Fig. 5). Their concentrations will then decrease with subsequent precipitation un- less there is a significant input of these compounds to the atmosphere during the rain event being consistent with rain flux results for time series samples (Fig. 5). On the other hand, diacids which are also water soluble are mostly present as particles in the atmosphere (Kawamura and Kaplan, 1987). Thus. they are

3 0

2:5

20 C 0

m

1 0 0 e~

8 ,

0

0 . 1

Monoaclds Dlac x

Aldehydes ~ . . . . . . . ~ _ . . . ~ _

. . . . . . . . . . . . . . . . J . . . . .

1 1 0 100

PreclpitaUon (ram)

Fig. 4. Changes in the concentration of low molecular weight monocarboxylic acids (C~ Co), dicarboxylic acids (C2-~1o) and aldehydes (C1-C4) in Los Angeles rainwaters

as a function of precipitation amounts.


also effectively scavenged (below-cloud) at an early stage of rain events or scavenged during cloud droplet formation (in-cloud scavenging).

In contrast, the aldehyde concentrations show a trend different from monocarboxylic and dicarboxylic acids: they do not show a significant increase with decreasing precipitation amounts among the six rain events in Los Angeles (Fig. 4). One possible explanation is that aldehydes are not scavenged as rapidly as carboxylic acids, because of limited water solubility and higher vapor pressure. In fact, Henry's Law constants for aldehydes (C1:3.5 x 103 tool ~ 1 atm 1, Ca: 11.5 mol / 1 atm 1, Okita et al., 1983) are lower than those of monocarboxylic acids (C1:5.6 x 103 mol E 1 atm ', C2:8.8 x 103 m o l [ - 1 a tm- 1, Keene and Gal- loway, 1986). It is likely that the total atmospheric concentrations of C1 and C2 aldehydes does not change significantly during the same rain event. Alter- natively, the aldehydes may be depleted in the rainwater samples collected due to rapid oxidation to acids in the aqueous phase.

Difference in the carboxylic acid distributions between urban and nonurban wet precipitation

The differences in the distributions of monocarboxylic and dicarboxylic acids in samples collected at urban and nonurban areas may reflect the differences in sources and transport mechanisms. The Los Angeles and Duarte rain (six samples) showed that monoacid concentrations are greater than diacid concentrations, whereas the converse was observed in nonurban samples. This effect may result from photochemical production of diacids during transport of polluted air from urban to nonurban areas (e.g. transport from Los Angeles to Santa Catalina Island). However, a few rainwater samples collected in Los Angeles during the summer season showed the predominance of diacids (for example, monoacids: 34.41~M and diacids: 46.2/2M for 18 August 1983 rain, Tables 4 and 5), suggesting that photochemical production of diacids is significantly enhanced during summer time. Enhanced photochemical production of dicarboxylic acids has been reported in the urban Tokyo atmosphere during the summer season (Kawamura and Ikushima, 1993) and in the arctic atmosphere at polar sunrise (Kawamura et al., 1995).

Phthalic acids are much more abundant (2~,0 times) than benzoic acid in all the samples studied, except for the rain from Corona del Mar site, which is located on the Pacific Ocean southeast of Los Angeles about 30 km south of any industrial sources and not immediately close to a large highway. Although aromatic diacids are always less abundant than aliphatic diacids, the aromatic/aliphatic concentration ratios varied widely (0.01-0.22) among the samples studied (Table 6). The ratios for urban area samples are higher (0.0564).22, av. 0.11 _ 0.06) than those for nonurban samples (0.0099-0.054, av. 0.026 + 0.015). This is consistent with the view that phthalic acids are derived from anthropogenic origin.

30

25

20

15

10

5

0 - -

25

~ 20

E

"5 E

~ s

0 - -

25

2O

15

10

5

0

(a) M o n o a c i d s ---a-.- #2 - - , ~ - - #7 1

1 " -~ " #3 - - t , - - #9

[~ - - + ' " #5 -,~,'"" #10

"',, ~ ---~-'- #12 ",,,, . . . .

(b) Diac tds

r - - T - - - - ~ " ~

(c) Aldehydes

• . . . . . ~ :

1 2 3 4 5

Time Series Rain Samples

Fig. 5. Rain fluxes of (a) C1 C9 monocarboxylic acids, (b) C2 C10 dicarboxylic acids and (c) CI-C4 aldehydes in time series rain samples collected during wet precipitation events

in west Los Angeles.

~-Dicarbonyls (glyoxal and methylglyoxal) are photo-oxidation products of aromatic structures (Nojima et al., 1974; Hoshino et al., 1978; Takagi et al., 1980; Bandow et al., 1985). Therefore, 0t-dicarbonyl/ monoaldehyde ratios (ratios of concentrations of glyoxal and methylglyoxal to those of formaldehyde and acetaldehyde) may be indicators of photo-oxidation. Such ratios ranged in 0-0.19 (av. 0.10 + 0.06) in urban Los Angeles and Duarte samples and is 0-0.04 (av. 0.016 + 0.02) in nonurban samples (Table 6). These results indicate that the ct-dicarbonyl/monoaldehyde ratio is a potential index of photo-oxidation of aromatic (anthropogenic) hydrocarbons in the atmosphere. It will be of special interest to measure this ratio in areas where forest fire or agricultural burning frequently occur and polynuclear aromatic hydrocarbons are common gaseous products of combustion.

Cis-unsaturated diacids (e.g. maleic) originate mostly from incomplete combustion and/or photo- oxidation of anthropogenic aromatic hydrocarbons whereas trans (fumaric) diacid is mainly of biogenic origin (Kawamura et al., 1985b). Hence, the cis/trans


E

E O

O

e~

e-

-d

O ¢.~

"a

O

e-

",2,

~o

~ -

• "2 i

O

o~5 m

.o. I C : ~ O

O ~ e , l

~ o ~ ~.~

. ~ .,'~ , - -

~'-~ N , x : I

ratio (ratio of concentration of cis-diacids to that of fumaric acid) is also a possible indicator of anthropogenic/biogenic sources. The ratios for urban samples are 2.3-6.6 (av. 3.9 + 1.2) whereas those for nonurban samples are 1.6-4.5 (av. 3.1 ± 1.21.

Total deposition and flux ojmonocarboxylic and dicarboxylic acids and aldehydes

Table 7 presents total wet deposition and flux for low molecular weight monocarboxylic and dicarboxylic acids and aldehydes for l0 rain events. Their total depositions fluctuate as follows: 6-150 pmol m- 2 for monoacids. 7 110/~mol m 2 for diacids, 1-110/~mol m 2 for aldehydes and 0-12pmolm 2 for ~-dicarbonyls. Deposition fluxes of aldehydes are contolled by total rainfall amounts, solubility and rates of photochemical conversion, whereas carboxylic acids may be more influenced by solubility in rainwaters and dry deposition•

Fluxes of the polar organic compounds were calculated as 2 17pmolm-2h i for monoacids, 1 11 ~lmolm 2h ~ for diacids, 0 .4d0pmolm 2h ~ for aldehydes and 0.02-1pmolm 2h 1 for ~-dicarbonyls. The total mass flux of these water soluble organic compounds presented by a unit of p g m - 2 h ~ ranges from150 to1400 #g m Zh ~.Itis important to note that total mass fluxes of these low molecular weight (water soluble) organic compounds are two orders of magnitude higher than those (13-25 pg m- 2 h ~) of higher molecular weight (solvent extractable) organic compounds including nor- real alkanes, polynuclear aromatic hydrocarbons, and fatty acids, etc. reported in Los Angeles rainwaters (Kawamura and Kaplan, 1986), although rainfall in- tensities were not constant during the wet precipitation events.

Contribution q[monocarboxylic and dicarbo.x ylic acids to rain acidity

To compare the contribution of H~ from carho- xylic acids with those of inorganic acids in the wet precipitation samples, nitrate, sulfate and pH were also determined in the rain samples collected during the same rain events (Table 8). Nitrate and sulfate concentrations were 8.5-116 and 4.7-44/~M, respectively. Nitrate concentrations are consistently higher than sulfate concentrations in Los Angeles rainwater. due to the contribution of NO~ from automobile emissions (Ellis et at., 1984). Although nitric and sul- furic acids are the principal contributors to H + ion in acid rain, total concentration of organic acids is about one third that of the inorganic acids. In this study, the ratios of organic acid concentrations to total (organic + inorganic) acid concentrations [OA/(OA + 1A)]

are calculated to be in the range 11-44% with an average of 30 + l 1% (Table 8).

The effect of organic acids on the pH of rainwater was estimated for the six samples, where sulfate and nitrate data were obtained (Table 8) by the following procedure. The rainwater samples were considered to


Table 7. Total wet deposition (D) and flux (F) of low molecular weight polar organic compounds for some rain events in southern California

Deposition (#mol m- 2) and flux (b~mol m 2 h - 1 }

Cl C9 monoacids C2 Clo diacids C1 C2 aldehydes C2-C3 :~-dicarbonyls Sample Duration no. th) D F D F D F D F

3 11 94 8 110 10 24 2 0 (} 5 6.0 12 2 17 3 9 2 0.3 0.05 6 4.0 6 2 7 2 1 0.4 0 0 7 4.0 67 17 23 6 39 10 5 I 8 4.0 3t 8 42 11 10 3 0.4 0.1 9 24 150 6 53 2 62 3 11 0.5

10 11.0 140 13 110 10 12 1 11 8.5 120 14 70 g 63 7 7 I 12 11.0 52 5 30 3 13 1 2 0.2 13 8.0 26 3 9 1 4 0.5 0.2 0.02 14 1.0 15 15 8 8

be produced from a "recipe" consisting of the most concentrated organic acids, HNO3 and H2SO4. This mixture was considered to have been titrated with base (NaOH for convenience) to the observed (measured) pH. Our calculation also considers the rain to be in equilibrium with atmospheric CO2 (10 3.5 atm). The following acids were considered in the calculation: formic, acetic, oxalic, malonic, succinic, methylsuccinic, maleic and phthalic. The N a O H calculated to have been added to the acid mixture would be equivalent to the sum of all of the positive charge, from basic cations (Na, Ca, Mg, etc.), in a real rain sample.

Using mass balance, the amount of N a O H ([NaOH],} that needs to be added to the sample is given by

[NaOH] , = 2[HzSO4] , + [HNO3] t - [H +]

+ Kw/'[H * ] + K~. 1KhPcof [H + ]

-4- 2K~. 1 K¢. : KhPco: / [H + ] 2

+ ~ [HA~],K,/ '([H +] + K~)

+ ~ [H2Dj]t(Kj.1 [H+] + 2Ki,1 Kj.2}/([H+]2

+ K j . 1 [ H ~ ] +Kj .~Kj ,z )

where the subscript t indicates total analytical concentrations; Kw is the autoionization constant of water (10 14); K~ is the ionization constant of the mono- protic acid H & ; K j. ~ and K j. 2 are the first and second ionization constants of the dicarboxylic acids; and K~.I (10 ~,35) and Kc,2 {10 -10"33) are the first and

second ionization constants of carbonic acid, while Kh (10 ~ ~ ) is the Henry's Law constant for carbon dioxide (Stumm and Morgan, 1981). The organic acid ionization constants used in the calculation are listed in Table A1. Once this N a O H concentration was

estimated, the pH of the rainwater in the absence of organic acids was calculated by removing the organic acids from the recipe. It was assumed that the recipe now consisted of the previously calculated N a O H and the "observed" HzSO 4 and HNO3. The system was still considered to be in equilibrium with atmospheric C O > This organic acid free pH was calculated by solving the following mass balance equation for [H + ]:

[H+] - Kw/[H+] - K c , ~ K h P c o j [ H ~]

- 2Kc. 1Kc.2 Kh Pco2/[H +]2

= [HNO3] -+ 2 [ H 2 S O 4 ] t - [NaOH] , .

The organic acid free pH and difference between this calculated pH and the original observed pH would provide a measurement of the effect of organic acids on the pH of rainwater. The results of this calculation are reported in Table 8. The calculation indicates that organic acids do lower the pH of the rainwater (from 0.1 to 1.9 pH units). This calculation also indicates that most of the %trong acids" are "titrated" with NaOH. The approximate balance of strong acid anions and cations in southern California rain is consistent with the observations of Sakugawa et al. (1993).

Although fog water generally contains more carboxylic acids than rainwater does (Kawamura and Ka- plan, 1984), relative concentrations of organic acids to inorganic acids are lower in fog water than :rainwater. For example, fog waters (10 samples) collected in June 1983 at Henninger Flats in southern California gave an average OA/(OA + IA) ratio of ca. 6% (Kawamura et al., in preparation); five times lower than that ( 3 0 ° ) for average rainwater samples calculated in this study (Table 8). These resul~ts indicate that there is a significant difference in the scavenging of atmospheric pollutants by fog water and rainwater. Fog water scavenges gas and particles near ground level, whereas rainwater scavenges them from the


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atmosphere over a long air column from upper troposphere to ground level. This may be due to the fact that HNO3, a dominant species of IA, is immediately produced by photochemical oxidation near ground level. In the upper levels of the troposphere, organic acids are likely accumulated or generated more efficiently than inorganic acids probably as a result of vertical upward transport of gaseous hydrocarbons and their subsequent photochemical oxidation to carboxylic acids.

Another explanation is that, oxalic acid, a major contributor to the atmospheric [H +] with a K~ = 5.9 x 10 2 and K 2 = 6.4x 10 -5 is present in the atmo-

sphere largely as particles (Kawamura and Kaplan, 1987), and may be more efficiently scavenged by rain drops than by fog droplets. It is important to note that oxalic acid concentrations are less than 10% of formic plus acetic acid concentrations in fog waters (Kawamura et al., in preparation), whereas the former are more than 30% of formic plus acetic acids in the rainwaters of Los Angeles (see Table 3) and sometimes oxalic acid is more abundant than formic acid. Oxalic acid may be largely produced at the upper levels of the troposphere by photochemical oxidations of gaseous aromatic hydrocarbons and their derivatives.

Preferential scavenging of formic acid over jbrmalde- hyde during wet precipitation

Formic acid to formaldehyde ratios decrease from the first to second rain collection and continue to decrease in a prolonged rain except for a few events (Fig. 6). The results are consistent with the Henry's law constants (H): where H for formic acid (C~: 5.6 x 103 mol d- ~ atm 1, Keene and Galloway, 1986) is greater than that for formaldehyde (C1:3.5 x 103 mold latin -1, Okita et al., 1983). The above dis- cussion supports a preferential scavenging of formic acid over formaldehyde. In contrast, acetic acid to acetaldehyde ratios did not clearly decrease with time, although about half of the rain events showed a decrease in acetic acid to acetaldehyde ratios from the first to second rain collection. This may suggest a possible oxidation of acetaldehyde to acetic acid in rain droplets.

The concentrations of dicarboxylic acids relative to aldehydes also show a decrease during wet precipitation. This also suggests that during early stages of wet precipitation dicarboxylic acids are depleted in the atmosphere in comparison to aldehydes due to a preferential wet scavenging of the diacids. However, the concentrations of diacids relative to monoacids do not show a clear trend, suggesting that no significant selectivity occurs between (gaseous) monoacids and (particulate) diacids during wet scavenging.

Possible production ojformic acid during wet precipitation

Figure 7 plots concentration ratios of formic (C~) acid to acetic (C2) acid as a function of time during 13 rain events. Except for one (# 1, 3/11-12/82), all the


5 I I - O - #1 ~ - - - # 6 ~C---#11 -"El--'#2 " -~ ' "#7 ~ " - # 1 2 ' " . , . .o_ 4 "'D ~ - # 3 "--41-'-#8 - - B " - # 1 3

[X~ - -X- - #4 + # 9

3 ~ '%"N ~ ' - # 5 . . . . . #10

0 - - - - , ~r 7 , ,

1 2 3 4 5 6 Time Series Rain Samples

Fig. 6. Changes in the concentration ratios of formic acid over formaldehyde in time series rain samples collected dur-

ing wet precipitation events in west Los Angeles•

I1:

0-- #1 ~" #2

- -~ ~ #3 )<-" #4

#5

'i 0 - - -

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-.A- "#10 / \ ! \

× ' " " ' . / / \ \

l - - - - I I I T - - I

1 2 3 4 5 6 Time Series Rain Samples

Fig. 7. Changes in the concentration ratios of formic acid over acetic acid in time series rain samples collected during

13 rain events in west Los Angeles.

rain events show that the C 1 / C 2 ratios decrease from the first to second rain collection. These results suggest a preferential removal of Ca over C2 acid during wet scavenging processes. Since both acids are mostly present in gaseous phase in the ambient air (Kawamura et al., 1985a), removal occurred during the partitioning between gas and dissolved phases. However, the data (Fig. 7) are not consistent with Henry's law constants. Based on Henry's law, acetic acid should not be preferentially dissolved. One possible explanation is that formic acid may be produced in the rain waters by the oxidation, aqueous phase OH radical oxidation of aldehyde to formic acid in clouds (Chameides and Davis, 1983).

Although Ca/C2 acid ratios display an initial decrease, in prolonged rain events the ratios increase in the middle or late stage of several events (e.g. # 1, # 5, # 13, see Fig. 7). Similar fluctuation patterns were also observed for monocarboxylic acid/aldehyde ratios for the same events. These patterns can be explained by transient inputs of formaldehyde into the atmosphere. Episodic inputs of anthropogenic chemical species to explain the fluctuation, were supported by the detec- tion of hydrocarbons, phenols and solvent extractable organic compounds of anthropogenic origin in the

same samples ( # 1 rain series, Kawamura and Ka- plan, 1986).

S U M M A R Y A N D C O N C L U S I O N

(1) Ca-C9 monocarboxylic acids, C2-Clo dicarboxylic acids, Ca-C2 aldehydes, C2-C3 ~-dicarbonyls and benzoic and phthalic acids have been studied in the urban and nonurban rain and snow samples collected from various fractions in southern California, as well as in time series rainwater samples collected in west Los Angeles. Formic, acetic and oxalic acids and formaldehyde are the dominant polar corapounds identified, whereas the higher molecular weight species are less abundant on a molar level. Whereas diacids primarily exist in a particulate form in the atmosphere, the others are present as gases. These gaseous compounds are considered to mainly originate from the transport of automobile emissions and photochemical oxidations of anthropogenic organic matter in the atmosphere. The origin of aliphatic dicarboxylic acids is uncertain.

(2) The concentration ranges are 0.6--24 ~M for monocarboxylic acids, 0.9-25 #M for dicarboxylic acids, 0.5-8.7/~M for monoaldehydes and 0.0-1.0 I~M for ~-dicarbonyls. There is no clear trend in the total concentration of these compounds among urban and nonurban rain samples. The concentrations of organic acids were highest when total precipitation volume was low, however, aldehyde concentrations were independent of rainwater volume.

(3) Although concentration of polar compound classes is generally similar among the wet precipitation samples collected in urban and nonurban samples, differences exist in the distribution patterns. Monoacids are more abundant than diacids in urban areas and vice versa in nonurban area, except for one Los Angeles summer rain. Aromatic/aliphatic ratios for diacids and ~-dicarbonyl/monoaldehyde ratios are higher in urban samples than in nonurban samples, reflecting a strong vehicular source for the aromatic acids.

(4) Concentrations of monocarboxylic and dicarboxylic acids were found to be within an order of magnitude of nitrate and sulfate acids in Los Angeles rainwater. This study demonstrates thal organic acids, especially oxalic, formic, and acetic acids, can contribute significantly to lowering the pH of urban rainwater (up to 1.9 pH units).

(5) Time series rainwater samples collected during 13 precipitation events showed that the fluxes of the mono- and dicarboxylic acids generally decreased as a function of time, suggesting that these acids are significantly scavenged at early stages of precipitation.

(6) During wet scavenging, concentration ratios of monoacids/aldehydes and diacids/aldehydes were found to decrease with time, suggesting an early preferential scavenging of carboxylic acids over aldehydes.


(7) The t ime series da ta are fur ther suggest ive of formic acid fo rma t ion dur ing the progress of a rain s torm, p r o b a b l y f rom p h o t o c h e m i c a l ox ida t ion of formaldehyde .

Acknowled.qements We thank R. Brewer and S. Shepard, Global Geochemistry Corporation, Canoga Park, Califor- nia, for their help in collecting some precipitation samples and in analyzing inorganic species. We also appreciate the helpful comments of two anonymous reviewers. This study was financially supported by U.S. Environmental Protection Agency. Although the information in this document has been funded wholly or in part by the Agency under assistance agreement number CR 807864-01 to NCITR at UCLA, it does not necessarily reflect the views of the Agency and no official endorsement should be inferred.

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AE 30:7-D

1052 K. K A W A M U R A et al.

APPENDIX

Table A1. Dissociat ion constants (2YC) of organic acids

Acids KI K2

Formic 1,77 x 10- 4 Acetic 1,76 x 10 5 Oxalic 5,90 × 10 -2 6.40 × 10-5 Malonic 1.49 >( 1 0 3 2.03 X 10- ~' Maleic 1A2 × 10-2 8.57 x 10- 7 Succinic 6,89 × 10 5 2.47 × 10 ~ Methylsuccinic 7.40 × 10- 5 2.30 × 10- 6 o-Phthal ic 1,30 × 10 -3 3.09 x 10 -6

D a t a f rom Weas t (1983).

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CONCENTRATIONS OF MONOCARBOXYLIC AND DICARBOXYLIC ACIDS ... · the benzyl ester derivatives ... and there is no reported measurements on the distri- ... Carboxylic acids and aldehydes