Carboxylic acids and carbonyl compounds in southern California clouds and fogs

Formaldehyde, formate, and acetate have been determined in fog and cloudwater from several areas of southern California. Up to 190 mu M formate and acetate was seen in high pH fog samples from the San Joaquin Valley. Formaldehyde concentrations as high as 500 mu M 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 mu M formate and acetate. The highest concentrations were observed in the samples with highest pH. Formaldehyde at Riverside was as high as 380 mu M. Average formate and acetate concentrations in intercepted stratus clouds from the Santa Barbara Channel coast and San Pedro Hill were 20-60 mu M and 10-30 mu M, respectively. Formaldehyde concentrations in stratus clouds were 10-20 mu M. 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 aqueous-phase 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.