Stratospheric and tropospheric NO2 variability on the diurnal and annual scale: a combined retrieval from ENVISAT/SCIAMACHY and solar FTIR at the Permanent Ground-Truthing Facility Zugspitze/Garmisch

[摘要] Columnar NO₂ retrievals from solar FTIR measurements at the Zugspitze(47.42° N, 10.98° E, 2964 m a.s.l.), Germany were investigatedsynergistically with columnar NO₂ retrieved from SCIAMACHY data by theUniversity of Bremen scientific algorithm UB1.5 for the time span July 2002-October2004. A new concept to match FTIR data to the time of satelliteoverpass makes use of the NO₂ daytime increasing rate retrieved fromthe FTIR data set itself [+1.02(6)E+14 cm^-2/h]. This measured increasingrate shows no significant seasonal variation. SCIAMACHY data within a 200-kmradius around Zugspitze were considered, and a pollution-clearingscheme was developed to select only pixels corresponding to clean background(free) tropospheric conditions, and exclude local pollution hot spots. The resultingdifference between SCIAMACHY and FTIR columns (without correctingfor the different sensitivities of the instruments) varies between0.60-1.24E+15 cm^-2 with an average of 0.83E+15 cm^-2. Aday-to-day scatter of daily means of ≈7-10% could be retrieved in mutualagreement from FTIR and SCIAMACHY. Both data sets are showing sufficientprecisions to make this assessment. Analysis of the averaging kernels givesproof that at high-mountain-site FTIR is a highly accurate measure for the purestratospheric column, while SCIAMACHY shows significant troposphericsensitivity. Based on this finding, we set up a combined a posterioriFTIR-SCIAMACHY retrieval for tropospheric NO₂, based upon the averagingkernels. It yields an annual cycle of the clean background (free)tropospheric column (<10 km) with variations between 0.75-1.54E+15 cm^-2,an average of 1.09E+15 cm^-2, and an intermediate phasebetween that of the well known boundary layer and stratospheric annualcycles. The outcome is a concept for an integrated global observing systemfor tropospheric NO₂ that comprises DOAS nadir satellite measurementsand a set of latitudinally distributed mountain-site or clean-air FTIRstations.

[学科分类] 大气科学