Fogwater chemistry at Riverside, California

Abstract

Fog, aerosol, and gas samples were collected during the winter of 1986 at Riverside, California. The dominant components of the aerosol were NH_4^+, NO_3^−, and SO_(42−). Gaseous NH_3 was frequently present at levels equal to or exceeding the aerosol NH_4^+. Maximum level were 3800, 3100, 690 and 4540 neq m^(−3) for NH_4^+, NO_3^(2−) and NH_(3(g)), respectively. The fogwater collected at Riverside had very high concentrations, particularly of the major aerosol components. Maximum concentrations were 26,000 29,000 and 6200 μM for NH_4^+, NO_3^− and SO_4^(2−), respectively. pH values in fogwater ranged from 2.3 to 5.7. Formate and acetate concentrations as high as 1500 and 580 μM, respectively, were measured. The maximum CH_2O concentration was 380 μM. Glyoxal and methylglyoxal were found in all the samples; their maximum concentrations were 280 and 120 μM, respectively. Comparison of fogwater and aerosol concentrations indicates that scavenging of precursor aerosol by fog droplets under the conditions at Riverside is less than 100% efficient. The chemistry at Riverside is controlled by the balance between HNO_3 production from NO_x emitted throughout the Los Angeles basin and NH3 emitted from dairy cattle feedlots just west of Riverside. The balance is controlled by local mixing. Acid fogs result at Riverside when drainage flows from the surrounding mountains isolate the site from the NH_3 source. Continued formation of HNO_(3(g)) in this air mass eventually depletes the residual NH_(3(g)). A simple box model that includes deposition, fog scavenging, and dilution is used to assess the effect of curtailing the dairy cattle feedlot operations. The calculations suggest that the resulting reduction of NH_3 levels would decrease the total NO_3^− in the atmosphere, but nearly all remaining NO_3^− would exist as HNO_3. Fogwater in the basin would be uniformly acidic.

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