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The discovery of the very acidic nature of fog and clouds has created much interest in sampling, analysing, and elucidating the chemistry of fog, principally because an understanding of the chemical transformations leading to acid fog may provide important clues to the origin of acid rain. Recently, the knowledge of the chemistry of fog has expanded to include carbonyl compounds, volatile organic acids, and alkyl sulphonates. We have discovered that a variety of pesticides and their toxic alteration products are present in fog, and that they occasionally reach high concentrations relative to reported rainwater concentrations. In our experiments, we were able to measure the air-water distribution coefficients of pesticides between the liquid fog and the interstitial gas phase. These measurements reveal that some chemicals are enriched several thousandfold in the suspended liquid fog droplets compared to equilibrium distributions expected from Henry's Law coefficients for pure aqueous solutions.

The vertical fluxes of alachlor, atrazine, simazine, and toxaphene were measured by air-sampling and aerodynamic measurements over a 24-day period after surface application to a fallow soil in eastern Maryland. The triazines were applied at 1.68 kg/ha as a wettable powder formulation and alachlor and toxaphene at 2.24 and 2.52 kg/ha, respectively, as emulsifiable concentrates. Calculated volatilization losses in the first 21 days were 780 g/ha toxaphene, 420 g/ha alachlor, 40 g/ha atrazine, and 21 g/ha simazine. Daily losses varied with soil moisture content, alachlor and toxaphene volatilization being reduced as the surface soil layers became dry. Daily volatilization patterns of atrazine and simazine indicated that some wind erosion of wettable powder formulation occurred as the surface soil dried, but the amounts transported were small. Volatilization losses of triazines were much smaller than disappearance by chemical degradation. A simple empirical equation was shown to yield estimated volatilization rates that were within about a factor of 10 of field-measured rates for six of eight compounds whose vapor pressures spanned a range of 104.

We measured the distribution of four organophosphorus insecticides (diazinon, parathion, chlorpyrifos, and methidathion) between the droplet and air phases during six fog events. We also measured the distribution of their oxon transformation products, which result from the oxidative conversion of the parent organophoshorothioate to the corresponding organophosphate. We found up to 60 Mg/L for the total of the four parent insecticides, and up to 75 ug/L for the total of their oxons in the fogwater. In agreement with earlier studies, nearly all the compounds exhibited aqueous-phase concentrations much higher than would be expected from measured vapor concentrations and Henry's law. Even though high concentrations and high enrichments were found in the aqueous phase, for most of the compounds the largest proportion was present in the interstitial air phase, either as vapor or adsorbed to aerosol particles. Only very small amounts (<1%) of any of the compounds were found associated with particles within the fog droplets.

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