A Three-dimensional Modeling Study of the Effects of Solid-phase Hydrometeor- Chemical Interactions in Cumulonimbus Clouds on Tropospheric Chemical Distributions

Clouds can significantly impact trace chemical distributions in the troposphere and the chemical composition of precipitation. Clouds affect chemical distributions through convection of air and hydrometeors containing trace chemicals. They also provide a multiphase environment for chemical phase changes and reaction. Due to the large vertical motions and turbulence of strong convective cloud systems, they can transport and mix trace chemicals between the atmospheric boundary layer and upper troposphere. If strong enough, they can also penetrate into the lower stratosphere. Hydrometeors in these clouds provide surfaces for chemical phase changes. They also act as condensed phase reactors for chemical reactions. Finally, hydrometeors serve as conduits for chemical transport from the atmosphere to the ground through scavenging and precipitation. Hence, storm clouds can impact acidic precipitation, urban pollutant dispersal, and concentrations of ozone and other atmospheric cleansing species (such as odd hydrogen radicals) in the upper troposphere and lower stratosphere, Due to the variety of effects of convective clouds on surface and atmospheric chemistry, there has been considerable effort to understand the complex processes involved in these systems, including modeling studies. In these studies, interactions of volatile chemicals with iceand mixed-phase hydrometeors (cloud ice, snow, graupel, and hail) are often excluded or limited due to their complexity as well as lack of theoretical understanding. Modeling studies which have included representations of interactions of ice-phase cloud hydrometeors with volatile chemicals have found that they may significantly impact chemical distribution and deposition (Audiffren et al., 1999; Chen and Lamb, 1990; Cho et al., 1989; Rutledge et al., 1986; Wang and Chang, 1993). However, the available studies use differing representations of these interactions, yield varying results, and give no consistent picture of the effects of these interactions on chemistry. A systematic investigation of the range of potential impacts of these interactions on chemical distributions and deposition is still lacking. In this work, we discuss the state of current knowledge on the interactions between solid-phase cloud hydrometeors and chemicals. Using modeling simulations, we also investigate the effects of these interactions on tropospheric chemistry and chemical deposition for the case of a thunderstorm. We use a three-dimensional dynamical, microphysical, and chemical model. Interactions considered include sorption of chemicals from the gas phase, entrapment of chemicals in the solid-phase during liquid hydrometeor freezing, and chemical reactions in / on solid-phase hydrometeors. 2. ICEAND MIXED-PHASE CHEMISTRY