Aerosol–Cloud Interactions in a Mesoscale Model. Part II: Sensitivity to Aqueous-Phase Chemistry

Abstract

The feedbacks between aerosols, cloud microphysics, and cloud chemistry are investigated in a mesoscale model. A simple bulk aqueous-phase sulfur chemistry scheme was fully coupled to the existing aerosol and microphysics schemes. The representation of aerosol and microphysics follows the explicit bulk double-moment approach. A case of summertime stratocumulus cloud system is simulated at high resolution (3-km grid spacing), and the evolution of an observed continental aerosol spectrum that changes during the course of the simulation as a result of cloud processing is examined. The results demonstrate that the bulk approach to the aerosol and droplet spectra correctly represents the feedbacks in the coupled system. The simulations capture the characteristic bimodal aerosol size spectrum resulting from cloud processing, with the first mode consisting of particles that did not participate as cloud condensation nuclei and the second mode, in the region of 0.08?0.12-?m radii, comprising the particles that were affected by processing. New information is revealed about the impact of the two main processing pathways and about the spatial distribution of the processed aerosol. One cycle of physical processing produced a relatively modest impact of 3%?5% on the processed particle mean radius of the order that was comparable to the impact of chemical processing, while continuous physical recycling produced a much larger impact as high as 30%?50%. A strong constraint on the chemical processing was found to be the initial chemistry input and the assumption of bulk chemical composition. Simple tests with a more slowly depleting primary oxidant (H2O2) and including the droplet chemical heterogeneity effect favor stronger sulfate production, by, respectively, the H2O2 and O3 oxidation reaction, and both show a larger impact on the processed particle mean radius of similar magnitude, 10%?20%. Spatially, the impact of processing is found initially in the downdraft regions below cloud and at later times at substantial distances downwind. It is shown that cloud processing can either enhance or suppress the number of activated drops in subsequent cycles.