Factors controlling Arctic denitrification in cold winters of the 1990s

Abstract

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 "closedflow 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.

Large nitric acid trihydrate particles and denitrification caused by mountain waves in the Arctic stratosphere

“…The contour of the polar vortex edge (indicated by the black line) is concentric with the NAT region at this time and continues to be so for several days. As demonstrated by Mann et al [2002, 2003], this concentricity indicates that there is a large region of cold closed flow, hence the sedimenting NAT particle trajectories will tend to remain in the growth region for several days and there will be efficient denitrification if there are sufficient mother clouds to act as a source of sedimenting NAT.…”

“…When used for “full chemistry” denitrification studies (such as that of Davies et al [2005]), the CTM is run with a 41‐tracer chemistry scheme. However, to reduce computation time in these high‐resolution simulations, a reduced four‐tracer scheme was used as also used by Carslaw et al [2002], Mann et al [2002] and Mann et al [2003]. The four tracers are NO y , passive NO y , H 2 O and aerosol H 2 SO 4 (required for the STS calculation).…”

Atmospheric impacts of the strongest known solar particle storm of 775 AD

“…Because of the relatively soft spectrum (even for such an extreme event), SEPs produce NO x mostly in the polar stratosphere, from where it takes, depending on meteorological conditions20, several months to reach the troposphere. SEP-induced nitrate production in the troposphere would have a direct effect on ice core concentrations, but its fraction is very small (only several percent).…”

A closer look at Arctic ozone loss and polar stratospheric clouds

“…In the vertical, the effect of the decreasing number density with altitude is offset by the increasing Cl y mixing ratios, and in the few winters where denitrification occurs the increased ozone loss at the denitrified altitudes is offset by the decreased ozone loss at the lower altitudes where renitrification takes place. The baroclinicity of the vortex affects the degree of denitrification (Mann et al, 2003): this would affect our conclusions about denitrification if the cold region were sufficiently deep that the renitrification occurred below the region of ozone loss. However this is very rare in the Arctic and has probably not occurred even in the years of extensive denitrification.…”