Global trends in tropospheric NO2 observed from space

Here we present results of a global trend analysis using nearly a decade of NO₂ observations acquired by the SCIA-MACHY (SCanning Imaging Absorption spectroMeter for Atmospheric CartograpHY) instrument onboard the Envisat satellite platform. Monthly average tropospheric NO₂ column data was acquired for the period between August 2002 and August 2011. A trend analysis was subsequently performed by fitting a statistical model including a seasonal cycle and linear trend to the time series extracted at each grid cell. The linear trend component and the trend uncertainty were then mapped spatially at both regional and global scales. The results show that spatially contiguous areas of significantly increasing NO₂ levels are found primarily in Eastern China. In addition, many urban agglomerations in Asia and the Middle East similarly exhibit significantly increasing trends, with Dhaka in Bangladesh being the megacity with the most rapid relative increase during the study period. In contrast, significantly decreasing trends in NO₂ levels exist over large parts of Europe and the Eastern United States. The satellite-derived time series were further analysed with respect to identification of the impact of the 2008/2009 economic crisis. European trends obtained from the satellite analysis are also compared with corresponding trends computed using data of the EMEP model, as well as with NO₂ trends calculated from station observations throughout Europe.