Stratospheric NO2 trends over the high mountain Ottavio Vittori station, Italy

This paper presents an evaluation of the nitrogen dioxide (NO 2 ) trend inferred from UV-visible zenith-sky measurements performed at the 'Ottavio Vittori' ground-based research station (www.isac.cnr.it/cimone) at Monte Cimone (44.18 o N, 10.7 o E, 2165 m above sea level (a.s.l.)). Experimental data series are obtained from the measured spectra adopting a retrieval procedure according to differential optical absorption spectroscopy (DOAS) methodology. Data series cover the period from 1993 to 2009, with limited interruptions due to technical maintenance of the instrument or poor meteorological conditions. The NO 2 trends are calculated with a linear regression model considering different factors assumed to contribute to NO 2 atmospheric content. Before running the model, a procedure for data quality control was applied to reduce or exclude the influence of measurements affected by poor meteorological conditions or the vertical transport of polluted air masses from the lower troposphere. The negative trends obtained for sunrise (a.m.) and sunset (p.m.) NO 2 measurements are in good agreement with previous studies accomplished for ground-based stations in the northern hemisphere.