Global peroxyacetyl nitrate (PAN) retrieval in the upper troposphere from limb emission spectra of the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS)

摘要

We use limb emission spectra of the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS)onboard the ENVIronmental SATellite (ENVISAT) to derive the first global distribution of peroxyacetylnitrate (PAN) in the upper troposphere. PAN is generated in tropospheric air masses polluted by fuelcombustion or biomass burning and acts as a reservoir and carrier of NO_x in the cold freetroposphere. PAN exhibits continuum-like broadband structures in the mid-infrared region and wasretrieved in a contiguous analysis window covering the wavenumber region 775–800 cm−1.The interfering species CCl₄, HCFC-22, H₂O, ClONO₂, CH₃CCl₃ and C₂H₂ were fittedalong with PAN, whereas pre-fitted profiles were used to model the contribution of other contaminantslike ozone. Sensitivity tests consisting in retrieval without consideration of PAN demonstratedthe existence of PAN signatures in MIPAS spectra obtained in polluted air masses. The analysed datasetconsists of 10 days between 4 October and 1 December 2003. This period covers the end of the biomass burningseason in South America and South and East Africa, in which generally large amounts of pollutantsare produced and distributed over wide areas of the southern hemispheric free troposphere. Indeed,elevated PAN amounts of 200–700 pptv were measured in a large plume extending from Brasilover the Southern Atlantic, Central and South Africa, the South Indian Ocean as far as Australia ataltitudes between 8 and 16 km. Enhanced PAN values were also found in a much more restricted area betweennorthern subtropical Africa and India. The most significant northern midlatitude PAN signal was detectedin an area at 8 km altitude extending from China into the Chinese Sea. The average mid and highlatitude PAN amounts found at 8 km were around 125 pptv in the northern, but only between 50 and 75 pptvin the southern hemisphere. The PAN distribution found in the southern hemispheric tropics and subtropicsis highly correlated with the jointly fitted acetylene (C₂H₂), which is another pollutantproduced by biomass burning, and agrees reasonably well with the CO plume detected during end of September 2003at the 275 hPa level (~10 km) by the Measurement of Pollution in the Troposphere (MOPITT)instrument on the Terra satellite. Similar southern hemispheric PAN amounts were also observed byprevious airborne measurements performed in September/October 1992 and 1996 above the South Atlanticand the South Pacific, respectively.