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Effects of clouds on aerosol and chemical species processing, production, and distribution in the boundary layer and upper troposphere




Zhang, Yiping, author

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Clouds play important roles in boundary layer and tropospheric aerosol and chemical processes. This work addresses the aerosol and chemical species processing, production, and distribution through two important types of clouds: convective and stratocumulus clouds. A modeling study of the effects of convection on the transformation and redistribution of chemical species and evolution and redistribution of aerosol particles in the troposphere is presented. A two-mode, two-moment aerosol evolution model is coupled with a two­ dimensional, mixed-phase, two-moment microphysics, Eulerian cloud model and a sulfate cloud chemistry model [Kreidenweis et al., 1997; Taylor et al., 1997; Zhang et al., 1998] to examine the new particle formation mechanism and the importance of different pathways for aqueous sulfate production. In the simulations, the complexation of CH20 with S(IV) is found to be of minor importance in most of the model cloud, compared with the oxidation of S(IV) by H202 and 03, while Fe (III)-catalyzed oxidation plays an important role in aqueous sulfate production. Significant S02 is convectively transported to the mid-to­ upper troposphere, where it is oxidized to gas-phase H2S04. After cloud processing, cloud condensation nuclei (CCN) particles are removed by precipitation and graupel to form a CCN-depleted region above cloud top and in the cold and humidified cloud outflow region. The new particle formation in the mid- to upper- troposphere interacts with cloud processing and transport of chemical species and aerosol particles and produces a peak of small particle concentration in the outflow region. The model results suggest that both small aerosols and aerosol precursors can be transported into the mid- to upper- troposphere by convective clouds, affecting vertical profiles of aerosol concentrations. The sensitivity of the S(VI) and aerosol production, S02 and aerosol redistribution to variations in the initial chemical and aerosol conditions and several model parameters are also examined. A trajectory ensemble model (TEM) is used to investigate stratocumulus processing of gases and C CN in the boundary layer. The fully coupled aqueous chemistry/ cloud microphysics model (Feingold et al., 1998; Zhang et al., 1998] is driven by a set of boundary layer parcel trajectories derived from a large eddy simulation model to study the effects of variations in the initial chemical fields and initial aerosol number concentration on chemical heterogeneity, broadening of the CCN and drop spectra, effective drop radius, and differences in the overall fractional conversion between the TEM and a single parcel experiencing mean conditions in a stratocumulus-capped marine boundary layer. It is found that the TEM offers a more representative method of describing the stratocumulus processing of aerosol and gases than does a single parcel model. In the base case simulation, the 03 oxidation rate averaged over all parcels is larger than the H202 oxidation rate, whereas the volume-mean cloudwater pH might suggest that H202 oxidation dominates. The liquid water-weighted pH generally increases with increasing drop size, to a peak pH. The drop size at this peak corresponds to the minimum in S(VI) concentration and is located near the mode of the drop mass distribution. However, the pH dependence on drop size at larger cloud drop sizes is affected by the initial chemical conditions. Aqueous chemistry contributes to the broadening of the drop size distribution, but the magnitude of the broadening depends on the initial aerosol and chemical conditions. In cases where more mass is added onto large particles in the tail of the initial CCN spectrum, the broadening of the drop spectrum is most evident, and may even trigger the collision coalescence process and drizzle formation in stratocumulus clouds. The change in initial CCN number concentration has the most prominent effect on the effective drop radius.


September 1998.
Also issued as Yiping Zhang's dissertation (Ph.D.) -- Colorado State University, 1998.

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Cloud physics
Convection (Meteorology)
Boundary layer (Meteorology)
Upper atmosphere
Aerosols -- Measurement


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