Supersaturation, dehydration, and denitrification in Arctic cirrus

[摘要] A polar cirrus case study is discussed with the help of a one-dimensional model withexplicit aerosol and ice microphysics. It is demonstrated that continuous cooling ofair in regions with small amounts of ice and slow ice deposition rates of water vapordrives significant in-cloud supersaturations over ice, with potentially importantconsequences for heterogeneous halogen activation. Radiatively important cloud propertiessuch as ice crystal size distributions are investigated, showing the presence of highnumber concentrations of small crystals in the cloud top region at the tropopause, broadbut highly variable size spectra in the cloud interior, and mostly large crystals at thecloud base. It is found that weakly forced Arctic cirrostratus are highly efficient atdehydrating upper tropospheric air. Estimating nitric acid uptake in cirrus with anunprecedented treatment of diffusion-limited trapping in growing ice crystals suggeststhat such clouds could also denitrify upper tropospheric air masses efficiently, but acloser comparison to suitable observations is needed to draw a definite conclusion onthis point. It is also shown that low temperatures, high ice supersaturations, and theabsence of ice above but close to the cloud top region cause efficient uptake of nitricacid in background aerosol particles.

[学科分类] 大气科学