HR: 17:15h
AN: A12F-06    [PDF]
TI: Turbulent coagulation of cloud droplets - Impact on sulfate production in clouds
AU: * Riemer, N
EM: nsriemer@ucdavis.edu
AF: Department of Mechanical and Aeronautical Engineering, One Shields Avenue, Davis, CA 95616 United States
AU: Wexler, A S
EM: aswexler@ucdavis.edu
AF: Department of Mechanical and Aeronautical Engineering, One Shields Avenue, Davis, CA 95616 United States
AU: Wexler, A S
EM: aswexler@ucdavis.edu
AF: Department of Civil and Environmental Engineering, One Shields Avenue, Davis, CA 95616 United States
AU: Wexler, A S
EM: aswexler@ucdavis.edu
AF: Department of Land, Air and Water Resources, One Shields Avenue, Davis, CA 95616 United States
AB: Clouds represent a major environment for atmospheric chemistry and measurements have shown that cloud droplets of different size can differ substantially in their composition and pH value. Mixing of droplets of different sizes, however, as occurs when cloud droplets collide and coalesce, averages differences in their composition. This clearly impacts the chemical processes in clouds, such as the sulfate production and other important aqueous phase reactions, and eventually also affects the aerosol phase when the cloud drops evaporates. Laboratory and numerical work have shown that the velocity and spatial distributions of particles may be modified significantly in a turbulent flow field. Although there is a general agreement in current literature that turbulence enhances the collision frequency of cloud droplets, this process is not yet well understood and therefore ignored in most current cloud models. This study therefore addresses the question of how the collision rate of cloud droplets is enhanced due to turbulence in the cloud and how this process impacts the in-cloud chemistry. Since experimental data of the droplet-turbulence interactions is difficult to obtain, modeling studies are an important tool for investigation. Recently, direct numerical simulations (DNS) have been carried out to obtain expressions for the coagulation kernels due to the turbulent flow. Important mechanisms of the turbulence-droplet interaction can be identified. The key mechanisms are that: (1) coagulation rates should be enhanced due to local concentration increases for particles with response times on the order of the Kolmorgorov scale, (2) particle inertia leads to relative velocities and less correlated velocity directions and hence to higher collision rates, (3) wind field shear produces collisions between particles even with the same inertia. For this study, a box model is developed to carry out simulations of cloud droplet evolution incorporating the effects mentioned above and the results from the DNS. On the basis of the resulting droplet distribution the in-cloud chemistry is calculated focusing on the conversion of SO2 to sulfate. The individual processes are quantified and their importance for the sulfate production is assessed for different atmospheric conditions.
DE: 0305 Aerosols and particles (0345, 4801)
DE: 0320 Cloud physics and chemistry
SC: Atmospheric Sciences [A]
MN: 2003 Fall Meeting