Impact of polar stratospheric cloud particle composition, number density, and lifetime on denitrification

One-dimensional simulations of polar stratospheric clouds (PSCs) composed of nitric acid trihydrate (NAT) and nitric acid dihydrate (NAD) particles are used to investigate vertical redistribution of nitric acid by clouds with these compositions. We find that for typical Arctic PSC lifetimes of a few days or less the net depletion of HNO3 is well below 50%, regardless of the NAT or NAD particle number density. With cloud lifetimes of a week or longer, greater than 50% denitrification is possible even with relatively low NAT particle number densities (as low as 1 x 10(-3) cm(-3)). Denitrification is most efficient with NAT particle number densities of about 3 x 10(-3) cm(-3) to 10(-2) cm(-3). For a given temperature history the degree of denitrification by NAD clouds is much less severe than that by NAT clouds. The clouds also cause a local maximum in HNO3 mixing ratios at the cloud base where the cloud particles sublimate. Our simulations suggest that repeated short-lived PSC events do not build up the denitrification beyond about 50%; however, the depth of the denitrified layer increases with each successive PSC event.