Stratospheric NO2 climatological trend at northern mid-latitudes from 8 years of ground based observations at Mt. Cimone Station

In this paper we report a climatological analysis of nitrogen dioxide total column amounts at Mt. Cimone station 44^° 11´ N, 10^° 42´ E, 2165 m asl). Eight years of measurements obtained with the Gas Absorption Spectrometer Correlating Optical Differences (GASCOD) UV-Vis spectrometer are presented and a method based on the power spectrum analysis is proposed to investigate the nitrogen dioxide long term trend. This kind of diagnostic approach uses the geophysical phenomena (predictor) Identification in the frequency space to filter the time series from the natural induced oscillations. The advantage with respect to a statistical approach based on the correlation study between measurements and geophysical predictors is that only the identified phenomena are removed from the trend. On the other hand the effect of data missing in the filtering should be discussed and quantified in order to avoid misunderstanding in the results interpretation. Results show that a 7% increase per decade in the total column NO₂ is present at northern mid-latitudes.