Factors controlling Arctic denitrification in cold winters of the 1990s G.W. Mann, S. Davies, K. S. Carslaw, and M. P. Chipperfield Institute for Atmospheric Science, School of the Environment, University of Leeds, Leeds, UK
Abstract. Denitrification of the Arctic winter stratosphere has been calculated
using a 3-D microphysical model for the winters 1994/95, 1995/96, 1996/97 and 1999/2000. Denitrification is assumed to occur through
the 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 in
1999/2000 also occurred in the other cold winters. The results show that winter 1999/2000 had conditions that were optimum for
denitrification by large NAT particles, which are a deep concentric NAT area and vortex. Under these conditions, NAT particles can
circulate in the NAT-supersaturated air for several days, reaching several
micrometres in radius and leading to a high downward flux of nitric acid. The other winters had shorter periods with optimum conditions
for denitrification. However, we find that NAT particles could have grown to large sizes in all of these winters and could have caused
significant denitrification. We define the quantity "closed-flow area'' (the fraction of the NAT area in which air parcel trajectories
can form closed loops) and show that it is a very useful indicator of possible denitrification. We find that even with a constant NAT
nucleation rate throughout the NAT area, the average NAT number concentration and size can vary by up to a factor of 10 in response to
this meteorological quantity. These changes in particle properties account for a high degree of variability in denitrification between
the different winters. This large meteorologically induced variability in denitrification rate needs to be compared with that which could
arise from a variable nucleation rate of NAT particles, which remains an uncertain quantity in models. Sensitivity studies show that although
denitrification was likely approaching asymptotic minimum values throughout much of the 1999/2000 vortex, decreases in the volume-averaged
nucleation rate would have substantially reduced the denitrification.
Citation: Mann, G.W., Davies, S., Carslaw, K. S., and Chipperfield, M. P.: Factors controlling Arctic denitrification in cold winters of the 1990s, Atmos. Chem. Phys., 3, 403-416, doi:10.5194/acp-3-403-2003, 2003.