Analysis of variations and trends of the NO2 slant column abundance obtained by DOAS measurements at Stara Zagora and at NDACC European mid-latitude stations in comparison with subtropical stations

Abstract

Daily time series of the NO2 slant column amounts obtained by the GASCOD-BG instrument at Stara Zagora and by instruments at European NDACC stations near 40°N and of two subtropical stations are analyzed. Monthly averages are determined after the rejection of the extreme values. The series are homogenized, based on linear regression between neighbor stations, by interpolation and by filling data gaps with seasonal means.To determine the linear trend a multiple linear regression model is used, including different impact factors as the solar activity, the aerosol loading, the El Nino impact. The QBO effect on the NO2 variation is also checked. With small exceptions the examined data show no significant trends. A significant solar activity impact on NO2 was determined only for the station at Mauna Loa. An influence of the south oscillation index was established for the Izaña station. The significances of the impacts are tested, taking into account the auto-correlation of the NO2 residuals.The density variations of the stratospheric NО2 can change the ozone concentration, which in turn influences the radiative balance in the stratosphere and troposphere. This makes the NO2 trend analysis important for the global climate change study.