Cloud water chemistry in Sequoia National Park

Interception of cloudwater by forests in the Sierra Nevada Mountains may contribute significantly to acidic deposition in the region. Cloudwater sampled in Sequoia National Park had pH values ranging from 4.4 to 5.7. The advance of cold fronts into the Park appears to lead to higher aerosol and gas phase concentrations than are seen under normal mountain-valley circulations, producing higher cloud-water concentrations than might otherwise be expected. Estimates of annual deposition rates of NO_3^−, SO_4^(2−), NH_4^+ and H^+ due to cloudwater impaction are comparable to those measured in precipitation

Chemical composition of coastal stratus clouds: Dependence on droplet size and distance from the coast

The aerosol at elevated sites in the South Coast Air Basin in California is a mixture of sea salt and pollution-derived secondary aerosol. The influence of sea salt declines with increasing distance from the coast. Nitric acid appears to react with the NaCl in sea salt aerosol to release HCl_(g) and form NaNO_3 in the aerosol. At inland sites, aerosol concentrations differ during periods of onshore and offshore flow. The highest concentrations were observed during the day when the onshore flow transported pollutants to the sites, while lower concentrations were observed at night when drainage flows from nearby mountains influenced the sites. Variations, in liquid water content are a major influence on cloudwater ion concentrations. Comparisons of the ionic concentrations in two size-segregated fractions of cloudwater collected during several sampling intervals suggest that there is a large difference between the average composition of the smaller droplets and that of the larger droplets. The concentrations of Na^+, Ca^(2+) and Mg^(2+) in the large-droplet fraction were observed to be higher than in the small-droplet fraction, while the concentrations of SO_4^(2−), NO_3^−, NH_4^+ and H^+ were higher in the small-droplet fraction. Chloride concentrations were nearly equal in both fractions. Differences in the composition of size-fractionated cloudwater samples suggest that large droplets are formed on sea salt and soil dust, which are large aerosol, and small droplets are formed on small secondary aerosol composed primarily of (NH_4)_2SO_4 and NH_4NO_3. The concentrations of several components that exist partly in the gas phase (e.g. Cl^−, HCOOH and CH_3COOH) appear to be independent of droplet size

Fogwater chemistry at Riverside, California

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