Does cloud processing of aerosol enhance droplet concentrations?

In this paper we explore phase space for cloud processing of cloud condensa tion nuclei (CCN) via heterogeneous chemistry. A range of input CCN size sp ectra, parameterized as lognormal distributions, are used as input to a par cel model driven along trajectories derived from a large-eddy simulation of the stratocumulus-capped marine boundary layer. A simple sulfate chemistry model is coupled to the microphysical model. Gas phase concentrations of S O2, O-3, H2O5, and NH3 are varied so as to generate one case for which SO2 processing is dominated by oxidation via O-3 and another case for which pro cessing is dominated by oxidation via H2O2. The processed aerosol spectra a re then used as input to an adiabatic parcel model that predicts the drop c oncentration for a given updraft velocity. Comparisons are made between pre dictions of drop concentration based on input of aerosol spectra that have experienced processing and an equivalent set that has not experienced proce ssing. It is shown that for both of the chemical processing scenarios, hete rogeneous processing can either enhance or suppress the number of drops act ivated in the subsequent cloud cycle, depending on the input CCN distributi on and the magnitude of the updraft. Enhancement of drop concentration occu rs in cases where the subsequent cloud cycle has low vertical velocity. A r eduction in drop number occurs when updraft velocities in the subsequent cl oud cycle are high. The size of the smallest CCN size category activated in the subsequent cloud cycle, relative to that experienced in the original c loud cycle, is important in determining the change in number of activated d rops. By applying probability distribution functions of the vertical veloci ty, we calculate that,on average, drop concentrations are likely to be enha nced by between 10% and 20% for the cases examined. Nevertheless, the poten tial for both positive and negative changes in drop concentration due to cl oud processing may complicate predictions of the indirect effect of aerosol s on climate.