Variability of active chlorine in the lowermost Arctic stratosphere

Abstract

[1] We examine the variability of ClO in the Arctic upper troposphere and lowermost stratosphere (UTLS) during the winter of 1999-2000. Data are binned relative to NO y , a species that is a proxy for photochemical age and a photochemical source of NO x . Enhancements in the [ClO]/[Cl y ] ratio relative to values expected from gas-phase chemistry alone were observed throughout the region and were largest in the coldest sampled regions, where T < 208 K. At low NO y values, where particles containing NO y and water were often detected, twilight ClO abundances in the afternoon were nearly a factor of 3 larger than those in the morning. At higher NO y values, where much lower particle surface areas were measured, ClO abundances in morning twilight were somewhat larger than those in the afternoon. These observations are consistent with a daytime mechanism of rapid heterogeneous activation of inorganic chlorine in particle-rich, low-NO y regions, with slower deactivation in relatively particle-poor, higher-NO y regions of the lowermost stratosphere. While the data clearly show widespread chlorine activation, knowledge of the precise value of the [ClO]/[Cl y ] ratio is limited because of the lack of available data on inorganic chlorine species, notably HCl, believed to be the dominant reservoir of inorganic chlorine at these altitudes.

Evidence for heterogeneous chlorine activation in the tropical UTLS

“…While changes in transport in the lowermost stratosphere could explain at least part of the observed trend (Fusco and Salby, 1999;Salby and Callaghan, 2004;Hood and Soukharev, 2005), chemical ozone loss due to halogen chemistry is also thought to play a significant role (Salawitch et al, 2005;Solomon et al, 1997). Besides higher than previously suggested and still slightly increasing amounts of bromine in this region (Dorf et al, 2006), catalytic ozone destruction by ClO produced as a result of heterogeneous chlorine activation on sulphate aerosol and on ice-particles in cirrus clouds has been suggested (Borrmann et al, 1996Bregman et al, 2002;Keim et al, 1996;Solomon et al, 1997;Thornton et al, 2003Thornton et al, , 2005.…”

“…This seems quite realistic, because the low temperatures prevalent in the TTL will lead to relatively high heterogeneous reaction rates. Indeed, the highest degree of chlorine activation -given by the x(ClO)/x(Cl y ) ratio following Thornton et al (2003) -was found in air masses with low ozone but also very low temperatures (Fig. 4), where heterogeneous reactions are most likely to play a relevant role.…”

“…Therefore, we estimate Cl y from observed O 3 adopting the approach by Thornton et al (2005). To derive a correlation representative for the region and time of our measurements, we used ACE-FTS (Atmospheric Chemistry Experiment -Fourier Transform Spectrometer, Bernath et al, 2005) Marcy et al (2004).…”

“…Tropical Cirrus clouds are particularly widespread and often optically thin (Wang et al, 1996). For mid and high latitudes a number of observations of ClO near the tropopause exist and have been linked to heterogeneous activation on cirrus or background aerosol (Thornton et al, 2003;Borrmann et al, 1997;Keim et al, 1996;Stroh et al, 2004). Here we present observations of ClO from the TROCCINOX (Tropical Convection, Cirrus, and Nitrogen Oxides Experiment) and the SCOUT-O 3 (Stratospheric-Climate links with emphasis On the Upper Troposphere and lower stratosphere) aircraft campaigns, which represent the first in-situ measurements of enhanced ClO in the UTLS in the tropics.…”

Evaluation of stratospheric chlorine chemistry for the Arctic spring 2005 using modelled and measured OClO column densities

“…This could potentially increase the OClO columns. Thornton et al (2003Thornton et al ( , 2005 report measurements of ClO in the lowermost stratosphere at high latitude regions. They suggest heterogeneous chlorine activation on cold particulate surfaces as water-rich background aerosols containing sulphate and nitrate.…”