Publication Abstracts

Jacob et al. 1989

Jacob, D.J., E.W. Gottlieb, and M. Prather, 1989: Chemistry of a polluted cloudy boundary layer. J. Geophys. Res., 94, 12975-13002, doi:10.1029/JD094iD10p12975.

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 over Bakersfield, California, during a wintertime stagnation episode with low stratus. The main features of the observation 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 mode; 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 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 (ESO2/ENH3)1/2 where ESO2 and ENH3 are emission rates os 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 nighttime hours. Very high acidities could, however, be achieved in the daytime becuase of photochemical acid production by the gas-phase reactions NO2 + OH and SO2 + OH.

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BibTeX Citation

  author={Jacob, D. J. and Gottlieb, E. W. and Prather, M.},
  title={Chemistry of a polluted cloudy boundary layer},
  journal={Journal of Geophysical Research},

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RIS Citation

ID  - ja03000u
AU  - Jacob, D. J.
AU  - Gottlieb, E. W.
AU  - Prather, M.
PY  - 1989
TI  - Chemistry of a polluted cloudy boundary layer
JA  - J. Geophys. Res.
JO  - Journal of Geophysical Research
VL  - 94
SP  - 12975
EP  - 13002
DO  - 10.1029/JD094iD10p12975
ER  -

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