Chemical transfer to ice-containing cumulonimbus cloud hydrometeors and its effects on tropospheric chemical distributions

Thunderstorms can significantly impact chemical distributions in the troposphere through redistribution of air and hydrometeors containing trace chemicals, and by providing a multi-phase environment for chemical phase changes and reactions. Interactions between ice-containing cloud hydrometeors and chemicals, and their effects on tropospheric chemistry, are not well understood. In this work, we 1) develop a theoretical framework for chemical transfer to / from ice-containing cloud hydrometeors, 2) apply this theory to elucidate available laboratory data, and 3) present modeling results investigating the effects of these interactions on tropospheric chemical distribution and deposition in cumulonimbus clouds. Interactions considered include chemical transfer between the gas-phase and ice-containing hydrometeors, and entrapment / release of chemicals in / from ice-containing hydrometeors during freezing. For modeling simulations we use the COMMAS model, a three dimensional, time varying, nonhydrostatic cloud model. Microphysical processes are represented with a bulk-water parameterizations including three ice-containing hydrometeor species (cloud ice, snow, and hail). Data from the STERAO/Deep Convection thunderstorm that occurred in northeastern Colorado on July 10, 1996, are used for chemical and meteorological initialization of the simulations. Current modeling results indicate that entrapment in the ice-phase during hydrometeor freezing may have significant impacts on chemical spatial and phase distributions and the flux of chemical mass into the upper troposphere and to the ground. Allowing entrapment in ice-containing hydrometeors leads to losses of mass of moderately soluble chemical species (Henry's constants greater than 10^5 M/atm) from the cloud anvil region and increases of mass deposited to the ground, in comparison with allowing species to degas during hydrometeor freezing. Chemical transfer to hail and precipitation of hail and rain (formed from melting hail) are largely responsible for these differences.