A one-dimensional photochemical model for cloud-topped boundary layers is developed which includes detailed descriptions of gas-phase and aqueous-phase chemistry, and of the radiation field in and below cloud. The model is used to interpret the accumulation of pollutants observed over Bakersfield, California, during a wintertime stagnation episode with low stratus. The main features of the observations are well simulated; in particular, sulfate accumulates progressively over the course of the episode due to sustained aqueous-phase oxidation of SO2 in the stratus cloud. The major source of sulfate is the reaction S(IV)+Fe(III), provided that this reaction proceeds by a non radical mechanism in which Fe(III) is not reduced. A radical mechanism with SO-3 and Fe(II) as immediate products would quench sulfate production because of depletion of Fe(III). The model results suggest that the non radical mechanism is more consistent with observations, although this result follows from the absence of a rapid Fe(II) oxidation pathway in the model. Even with the non-radical mechanis most of the soluble iron is present as FE(II) because Fe(III) is rapidly reduced by O-2. The S(IV)+Fe(III) reacton provides the principal source of H2O2 in the model; photochemical production of H2O2 from HO2 or O2(-I) is slow because HO2 is depleted by high levels of NOx. The aqueous-phase reaction S(IV)+OH initiates a radical-assisted S(IV) oxidation chain but we find that the chain is not propagated due to efficient termination by SO-4+Cl- followed by Cl+H2O. A major uncertainty attached to that result is that the reactivities of S(IV)-carbonyl adducts with radical oxidants are unknown. The chain could be efficiently propagated, with high sulfate yields, if the S(IV)-carbonyl adducts were involved in chain propagation.

A remarkable feature of the observations, which is well reproduced by the model, is the close balance between total atmospheric concentrations of acids and bases. We argue that this balance reflects the control of sulfate production by NH3, which follows from the pH dependence of the S(IV)+Fe(III) reaction. Such a balance should be a general charateristic of polluted environments where aqueous-phase oxidation of SO2 is the main source of acidity. At night, the acidity of the cloud approaches a steady state between NH3 emissions and H2SO4 production by the S(IV)+Fe(III) reaction. A steady state analysis suggests that +> at night should be proportional to (ESO2/ENH3)1/2 where ESO2 and ENH3 are emission rates of SO2 and NH3, respectively. From this analysis it appears that cloud water pH values below 3 are unlikely to occur in the Bakersfield atmosphere during the nighttime hours. Very high acidities could, however, be achieved in the daytime because of photochemical acid production by the gas-phase reactions NO2+OH and SO2+OH. ¿ American Geophysical Union 1989