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Formaldehyde, formate, and acetate have been determined in fog and cloudwater from several areas of southern California. Up to 190 pM formate and acetate was seen in high pH fog samples from the San Joaquin Valley. Formaldehyde concentrations as high as 500 pM were observed there. Organic acid concentrations were much lower at an acidic site along the margin of the valley. Fog from Riverside, CA had maximum concentrations of 1500 and 500 pM formate and acetate. The highest concentrations were observed in the samples with highest pH. Formaldehyde at Riverside was as high as 380 pM. Average formate and acetate concentrations in intercepted stratus clouds from the Santa Barbara Channel coast and San Pedro Hill were 20-60 pM and 1Ck30 pM, respectively. Formaldehyde concentrations in stratus clouds were 1&20 pM. Because the lifetime of a cloud or fog droplet is usually much longer than the characteristic time for interfacial mass transport, hydration or dissociation, droplets can be assumed to be in equilibrium with their surroundings. The product of aqueousphase concentration and liquid water content in the cloud (fog) is nearly always much less than the predicted equilibrium partial pressure. Only the high pH fogs from the San Joaquin Valley have the potential to deplete the gas-phase reservoir of organic acid.

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Cloudwater was sampled during the fall of 1985 and the spring of 1986 in Sequoia National Park in the southern Sierra Nevada Mountains of California. Concentrations of major species in the cloudwater vary widely both within one cloudwater impaction event and from one event to another. The pH values of the samples range from 4.4 to .j. 7. Organic acids are seen to be important contributors to the chemical composition of the cloudwater at this site. The advance of cold fronts seems to lead to higher aerosol and gas phase concentrations in the Park than would be seen under normal mountain-valley circulations. particularly during the night and morning hours. The arrival of these increased concentrations prior to and during cloud impaction on the mountain slopes leads to higher cloudwater concentrations than would otherwise be expected. Estimates of annual deposition rates of N0 3-, so 4 2-, NH 4 +, and H+ due to cloudwater impaction are comparable to those measured from rainfall.