Wet deposition of low molecular weight mono- and di-carboxylic acids, aldehydes and inorganic species in Los Angeles

About 60 rainwater samples were collected at west Los Angeles, California i n 1981-1984 and were analyzed for C-1-C-9 monocarboxylic acids (0.33-79 muM , average (av.) 13 +/- 15 muM), C-2-C-10 dicarboxylic acids (2.9.51 muM, av . 7.5 +/- 14 muM) and C-1-C-4 aldehydes (0.85-28 muM, av. 9.2 +/- 11 muM). Distributions of monocarboxylic acids show a predominance of formic (averag e concentration: 6.5 muM) and acetic (av. 5.6 muM) acids followed by propio nic acid (av. 0.44 muM). Oxalic acid is the dominant diacid (av. 3.9 muM) f ollowed by succinic acid (av. 1.0 muM). Formaldehyde (av. 6.9 muM) is the d ominant aldehyde, with the next most abundant, acetaldehyde, being minor (a v. 0.65 muM). For select rain samples described in this paper, were found t o comprise monocarboxylic acids 0.9-12.3% (av. 4.4 +/-3.4%). diacids compri se 1.2-9.5% (av. 4.2 +/-3.3%) and aldehydes comprise 0.2-6.2% (av. 2.1 +/-2 .2%) of total organic carbon (TOC, 2.0-18.6 mg Cl-1; av. 9.8 +/-5.4 mg Cl-1 ). Annual rain fluxes of monocarboxylic acids and aldehydes during 1982-198 3 were calculated to be 0.24 and 0.11 gm(-2) yr(-1), respectively, with an annual estimated wet deposition in the Los Angeles Basin of 3120 and 1430 t ons, respectively. These fluxes are equivalent to 2500 times of the acids a nd 2.5 times of the aldehydes emitted from automobile exhausts in the Los A ngeles air basin. This comparison suggests that major portions of the carbo xylic acids detected in the rain are not directly emitted from auto-exhaust s, but are most likely produced in the atmosphere by gaseous and/or aqueous phase photo-induced reactions. (C) 2001 Published by Elsevier Science Ltd.