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

► NO2 time series of 4 mid-latitude stations and 2 subtropical ones are analyzed. ► The series are homogenized, based on linear regression. ► For determination of trends a multiple linear regression model is used. ► Not significant trends are found. ► A solar activity influences on NO2 was found only for the Mauna Loa station.