Factors controlling Arctic denitrification in cold winters of the 1990s

[摘要] Denitrification of the Arctic winter stratosphere has been calculatedusing a 3-D microphysical model for the winters 1994/95, 1995/96, 1996/97 and 1999/2000.Denitrification is assumed to occur throughthe sedimentation of low number concentrations of large nitric acid trihydrate (NAT) particles (as inferred by e.g. Fahey et al., 2001).We examine whether the meteorological conditions that allowed particles to grow to the very large sizes observed in1999/2000 also occurred in the other cold winters. The results show that winter 1999/2000 had conditions that were optimum fordenitrification by large NAT particles, which are a deep concentric NAT area and vortex. Under these conditions, NAT particles cancirculate in the NAT-supersaturated air for several days, reaching severalmicrometres in radius and leading to a high downward flux of nitric acid. The other winters had shorter periods with optimum conditionsfor denitrification. However, we find that NAT particles could have grown to large sizes in all of these winters and could have causedsignificant denitrification. We define the quantity "closed-flow area'' (the fraction of the NAT area in which air parcel trajectoriescan form closed loops) and show that it is a very useful indicator of possible denitrification.We find that even with a constant NATnucleation rate throughout the NAT area, the average NAT number concentration and size can vary by up to a factor of 10 in response tothis meteorological quantity. These changes in particle properties account for a high degree of variability in denitrification betweenthe different winters. This large meteorologically induced variability in denitrification rate needs to be compared with that which couldarise from a variable nucleation rate of NAT particles, which remains an uncertain quantity in models. Sensitivity studies show that althoughdenitrification was likely approaching asymptotic minimum values throughout much of the 1999/2000 vortex, decreases in the volume-averagednucleation rate would have substantially reduced the denitrification.

[学科分类] 大气科学