Airborne chemical measurements in nighttime stratus clouds in the Los Angeles Basin

An aircraft was used to collect cloudwater, aerosol, and gas samples and to make air quality and meteorological measurements in stratus clouds during five sampling sessions in the Los Angeles Basin between November 1981 and June 1985. The clouds form at night in a thin layer, so most samples were collected at altitudes between 600 and 750 m msl. There were strong vertical gradients in the air quality, but the cloudwater composition was horizontally quite uniform and the pH was typically just above 3.0. H2O2 was found in all cloudwater samples analyzed for this species. No analyses were performed for organic species other than aldehydes. All data below are averages for the May–June 1985 samples and except for S(IV) are similar to earlier data. The average mass composition of the species measured in the cloudwater was 48% NO3−, 23% SO42−, 7.6% NH4+ , 6.9% Cl−, 5.4% Na, 2.7% H2O2,1.3% Ca, 1.2% HCHO, 0.8% Mg, 0.7% H+, 0.5% K, 0.5% Pb, 0.4% Fe, 0.3% CH3CHO, 0.03% S(IV), and 0.02% Mn. Stated as equivalents, the cations averaged 42% H+, 27% NH4+, 20% Na+, and 11% other metals, and the anions averaged 51% NO3− , 32% SO42−, and 17% Cl−. Cloudwater species concentrations multiplied by the cloud liquid water content gave mean ambient concentrations of 22 μgm−3 NO3−, 11 μgm−3 SO42−, and 3.4 μgm−3 NH4+. It was estimated that if enough SO2 to consume the H2O2 were made available, the SO42− concentrations would increase by an average of 5.2 μgm−3 or an average percentage increase of 65%. In 1983–1985, the conversion of SO2 to sulfate was essentially complete and the conversion of NOx to nitrate was typically half to three-quarters complete in the clouds. Flights which followed air parcels showed that vertical mixing and removal processes such as droplet settling were important. Droplets can evaporate below the clouds and form large aerosol particles during the night and morning. Cloudwater composition and droplet size data were used to estimate size distributions and composition of the aerosol formed when the clouds evaporate. Volatilization of the HNO3 would leave an aerosol with an average of 7 μg m−3 NO3 and 11 μg m−3 SO42− with most of the aerosol mass in 0.4–1.0 um diameter size range.