Polar stratospheric cloud formation and odd nitrogen chemistry in simulated antarctic conditions

Abstract

Results are presented for the simulation of polar stratospheric cloud growth obtained with a two-dimensional model which includes chemistry and a microphysics code to take into acount condensation and coagulation of particles. Under cooling conditions of the region between 60S latitude and the Pole and 12 to 20 km height, both nitric-acid trihydrate and pure water ice are observed to condense. Condensation takes place on preexisting sulfate aerosol particles calculated with the same model. Extinctions ranging from 10−8 to 10−5 cm−1 are obtained in the cooling region depending on whether only HNO3·3H2O condenses out or also pure water ice, and if coagulation is included. Characteristic growth times of 1 day are obtained and the effect on nitric acid and water vapour is to reduce their mixing ratios at the saturation values. Between 60S and 90S the column density of nitric acid is halved. No effects could be evaluated on the NO x concentration because all the model runs are made during winter darkness. The resulting aerosol size distribution is monodisperse when only nitric-acid trihydrate condenses and is bimodal when also water ice is formed. In this case the size distribution is centred around 1 μm.