Testing our understanding of Arctic denitrification using MIPAS-E satellite measurements in winter 2002/3

Abstract

Abstract. Observations of gas-phase HNO3 and N2O in the polar stratosphere from the Michelson Interferometer for Passive Atmospheric Sounding aboard the ENVISAT satellite (MIPAS-E) were made during the cold Arctic winter of 2002/3. Vortex temperatures were unusually low in early winter and remained favourable for polar stratospheric cloud formation and denitrification until mid-January. MIPAS-E observations provide the first dataset with sufficient coverage of the polar vortex in mid-winter which enables a reasonable estimate of the timing of onset and spatial distribution of denitrification of the Arctic lower stratosphere to be performed. We use the observations from MIPAS-E to test the evolution of denitrification in the DLAPSE (Denitrification by Lagrangian Particle Sedimentation) microphysical denitrification model coupled to the SLIMCAT chemical transport model. In addition, the predicted denitrification from a simple equilibrium nitric acid trihydrate-based scheme is also compared with MIPAS-E. Modelled denitrification is compared with in-vortex NOy and N2O observations from the balloon-borne MarkIV interferometer in mid-December. Denitrification was clearly observed by MIPAS-E in mid-December 2002 and reached 80% in the core of the vortex by early January 2003. The DLAPSE model is broadly able to capture both the timing of onset and the spatial distribution of the observed denitrification. A simple thermodynamic equilibrium scheme is able to reproduce the observed denitrification in the core of the vortex but overestimates denitrification closer to the vortex edge. This study also suggests that the onset of denitrification in simple thermodynamic schemes may be earlier than in the MIPAS-E observations.

Polar stratospheric cloud microphysics and chemistry

“…However, observations of NAT PSCs in situations where this cannot have occurred suggest that this mechanism is not the only source of NAT-rock particles in the stratosphere . A number of studies of NAT formation during the winter of 2002/2003 have suggested that observed NAT particle numbers (Larsen et al, 2004;Voigt et al, 2005) and levels of denitrification (GrooX et al, 2005;Davies et al, 2006) can be reproduced using a simple flat nucleation rate of NAT particles, at temperatures below T NAT . The nucleation rates determined are 25 Â 10 À6 (Larsen et al, 2004), 8 Â 10 À6 , 7:8 Â 10 À6 (GrooX et al, 2005), and 12 Â 10 À6 particles cm À3 h À1 (Davies et al, 2006).…”

“…A number of studies of NAT formation during the winter of 2002/2003 have suggested that observed NAT particle numbers (Larsen et al, 2004;Voigt et al, 2005) and levels of denitrification (GrooX et al, 2005;Davies et al, 2006) can be reproduced using a simple flat nucleation rate of NAT particles, at temperatures below T NAT . The nucleation rates determined are 25 Â 10 À6 (Larsen et al, 2004), 8 Â 10 À6 , 7:8 Â 10 À6 (GrooX et al, 2005), and 12 Â 10 À6 particles cm À3 h À1 (Davies et al, 2006). Modelling studies of denitrification during previous Arctic winters have determined optimal nucleation rates of a similar magnitude: 2:9 Â 10 À6 particles cm À3 h À1 for the winter 1999/2000 and 12 Â 10 À6 particles cm À3 h À1 for the winter 1994/1995 .…”

MIPAS: an instrument for atmospheric and climate research

“…11). Operationally retrieved HNO 3 as a proxy for NO y within the polar vortex has been used for the validation of the denitrification scheme included in SLIMCAT (Davies et al, 2006). It was possible to show, for the first time, that a sedimentation model, allowing the growth of nitric acid trihydrate (NAT) "rocks", could reproduce both the timing and spatial distribution of denitrification in the polar vortex.…”

A 3D-CTM with detailed online PSC-microphysics: analysis of the Antarctic winter 2003 by comparison with satellite observations

“…An approximative value for denitrification as observed by MIPAS can then be calculated as the difference between the observed HNO 3 and NO y as derived from the observed N 2 O (Davies et al, 2006).…”

“…This scheme is coupled to the SLIMCAT 3-D chemical transport model (CTM) (Chipperfield, 1999). This approach has been successful in determining the magnitude and spatial distribution of denitrification in several Arctic winters (Mann et al, 2003;Davies et al, 2006). A recent version of the CLaMS model also incorporates this DLAPSE scheme (Grooß et al, 2005).…”