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Large-scale upper tropospheric pollution observed by MIPAS HCN and C2H6 global distributions

by N Glatthor, T von Clarmann, G P Stiller, B Funke, M E Koukouli, H Fischer, U Grabowski, M Hoepfner, S Kellmann, A Linden show all authors
ATMOSPHERIC CHEMISTRY AND PHYSICS ()

Abstract

We present global upper tropospheric HCN and C2H6 amounts derived from\nMIPAS/ENVISAT limb emission spectra. HCN and C2H6 are retrieved in the\nspectral regions 715.5-782.7 cm(-1) and 811.5-835.7 cm(-1),\nrespectively. The datasets consist of 54 days between September 2003 and\nMarch 2004. This period covers the peak and decline of the southern\nhemispheric biomass burning period and some months thereafter. HCN is a\nnearly unambiguous tracer of biomass burning with an assumed\ntropospheric lifetime of several months. Indeed, the most significant\nfeature in the MIPAS HCN dataset is an upper tropospheric plume of\nenhanced values caused by southern hemispheric biomass burning, which in\nSeptember and October 2003 extended from tropical South America over\nAfrica, Australia to the Southern Pacific. The spatial extent of this\nplume agrees well with the MOPITT CO distribution of September 2003.\nFurther there is good agreement with the shapes and mixing ratios of the\nsouthern hemispheric HCN and C2H6 fields measured by the ACE experiment\nbetween September and November 2005. The MIPAS HCN plume extended from\nthe lowermost observation height of 8 km up to about 16 km altitude,\nwith maximum values of 500-600 pptv in October 2003. It was still\nclearly visible in December 2003, but had strongly decreased by March\n2004, confirming the assumed tropospheric lifetime. The main sources of\nC2H6 are production and transmission of fossil fuels, followed by\nbiofuel use and biomass burning. The C2H6 distribution also clearly\nreflected the southern hemispheric biomass burning plume and its\nseasonal variation, with maximum amounts of 600-700 pptv. Generally\nthere was good spatial overlap between the southern hemispheric\ndistributions of both pollution tracers, except for the region between\nPeru and the mid-Pacific. Here C(2)H(6)was considerably enhanced,\nwhereas the HCN amounts were low. Backward trajectory calculations\nsuggested that industrial pollution was responsible for the elevated\nC2H6 concentration in these particular air masses.\nExcept for the Asian monsoon anticyclone in September 2003, there were\nonly comparably small regions of enhanced HCN in the Northern\nHemisphere. However, C2H6 showed an equally strong northern hemispheric\nsignal between the equator and low midlatitudes, persisting over the\nwhole observation period. Backward trajectory calculations for air\nmasses from this region also point to industrial sources of this\npollution. Generally, C2H6/HCN ratios between 1 and 1.5 indicate biomass\nburning and ratios larger than 1.5 industrial pollution. However, in\nMarch 2004 ratios of up to 2 were also found in some regions of the\nformer southern biomass burning plume.

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