South Pole NOx chemistry:: an assessment of factors controlling variability and absolute levels

Several groups have now shown that snow covered polar areas can lead to the release of NO, to the atmosphere as a result of the UV photolysis of nitrate ions. Here we focus on a detailed examination of the NO observations recorded at South Pole (SP). Topics explored include: (1) why SP NOx levels greatly exceed those at other polar sites; (2) what processes are responsible for the observed large day to day NO concentration shifts at SP; and (3) possible explanations for the large variability in NO seen between SP studies in 1998 and 2000. As discussed in the main body of the text, the answer to all three questions lies in the uniqueness of the summertime SP environment. Among these characteristics is the presence of a large plateau region just to the east of SP. This region defines one of the world's largest air drainage fields, being nearly 1000 kin across and having elevation of similar to 3 km. In addition, summertime SP surface temperatures typically do not exceed -25degrees C, leading to frequent cases where strong near surface temperature inversions occur. It experiences 24 h of continuous sunlight, giving rise to non-stop photochemical reactions both within the snowpack and in the atmosphere. The latter chemistry is unique at SP in that increasing levels of NOx lead to an enhanced lifetime for NOx, thereby producing non-linear increases in NOx. In addition, the rapid atmospheric oxidation of NOx, in conjunction with very rapid dry deposition of the products (HNO3 and HO2NO2), results in a very efficient recycling of NOx back to the snowpack. Details concerning these unique SP characteristics and the extension of these findings to the greater plateau region are discussed. Finally, the relationship of NOx recycling and total nitrogen deposition to the plateau is explored. (C) 2004 Elsevier Ltd. All rights reserved.