Ground-based NO2 and O3 analysis at Monte Cimone station during 1995-1996:: A case study for spring 1995 NO2 concentration profiles

A UV/Vis DOAS spectrometer (GASCOD, Gas Analyzer Spectroemter Correlating Optical Differences) was installed at Monte Cimone station in 1993 and since then it has been measuring zenith scattered solar radiation at sunset and sunrise. During 1995 it was possible to investigate two spectral regions, about 50 nm width, centered at 365 nm and 436 nm while later we only have measurements at 436 nn available. The spectra obtained during the 1995-96 period have been processed with DOAS technique to obtain column amounts of NO2 and O-3. The seasonal and diurnal variation of the NO:! column amounts is shown with a summer maximum (about 1.2x10(17) mol.cm(-2) for p.m. value and 6x10(16) mol.cm(-2) for a.m.) and winter minimum (about 2x10(16) mol.cm(-2) for a.m, and 5x10(16) mol.cm(-2) for p.m.). An anomalous spring increase in p.m. NO2 value during 1995 is investigated through a vertical distribution analysis. The gas profile is retrieved through a Chahine inversion algorithm applied to the slant columns measured at different solar zenith angle. In fact the air mass factor variation with solar zenith angle can be used to extract information about the gas concentration at each atmospheric layers. A consistent and frequent tropospheric increase in NO2 a.m. concentration is evident. The method and the results obtained are discussed.