Chemistry of a polluted cloudy boundary layer

Abstract

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 SO₃⁻ 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 mechanism, most of the soluble iron is present as Fe(II) because Fe(III) is rapidly reduced by O₂⁻. The S(IV) + Fe(III) reaction 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 NO_x. 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 SO₄⁻ + 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 characteristic 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 [H⁺] at night should be proportional to (E_SO2/E_NH3)^1/2 where E_SO2 and E_NH3 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.