Wet deposition and related atmospheric chemistry in the São Paulo metropolis, Brazil: Part 2—contribution of formic and acetic acids

Wet-only deposition samples were collected at a site in the urban area of the São Paulo metropolis between February (end of the rainy summer) and October (beginning of spring) 2000, an atypical period due to rainfall 40% below the 30-year average. The majority ions in rainwater were measured by capillary zone electrophoresis with contactless conductivity detection, CZE–CCD, applied for the first time to the organic anions acetate and formate. The volume weight mean (VWM) concentrations of the majority anions NO3−, SO42− and Cl− were, respectively, 15.6, 9.5 and 4.7 μmoll−1. The VWM concentration of HCOO−t, (HCOO−+HCOOH) was 17.0 μmoll−1, about twice the 8.9 μmoll−1 of CH3COO−t. The VWM concentration of free H+ was low (16.9 μmoll−1), corresponding to pH 4.77. This denotes the relevance of species like ammonia, analyzed as NH4+ (VWM=27.9 μmoll−1), and calcium carbonate (VWM=5.3 μmoll−1 Ca2+) as partial neutralizers of the acidity. By hypothetically assuming that H+ is the only counterion of the non-sea-salt fraction of the dissociated anions, their contribution to the total potential acidity would decrease in the following order: sulfate (29%), formate (29%), nitrate (26%), acetate (15%) and chloride (1%). The 44% potential participation of the carboxylic acids reveals their importance to the acidity of São Pauloʹs rainwater during the study period. Direct vehicular emission of lower carboxylic acids and aldehydes (in particular, acetic acid and acetaldehyde) is singularly high in the metropolis due to the extensive use of ethanol and gasohol (containing 20% of ethanol) as fuels of the light fleet of 5.5 million cars; in addition, regional atmospheric conditions favor the photochemical formation of the acids, since concentrations of ozone and aldehydes are high and solar irradiation is intense at the 23°34′S latitude. The presence of higher concentrations of HCOOH than CH3COOH indicates a prevalence of its photochemical production by H2CO oxidation in the atmosphere.