Recent satellite-based trends of tropospheric nitrogen dioxide over large urban agglomerations worldwide

Abstract. Trends in tropospheric nitrogen dioxide (NO₂) columns over 66 large urban agglomerations worldwide have been computed using data from the SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY) instrument onboard the Envisat platform for the period August 2002 to March 2012. A seasonal model including a linear trend was fitted to the satellite-based time series over each site. The results indicate distinct spatial patterns in trends. While agglomerations in Europe, North America, and some locations in East Asia/Oceania show decreasing tropospheric NO₂ levels on the order of −5% yr⁻¹, rapidly increasing levels of tropospheric NO₂ are found for agglomerations in large parts of Asia, Africa, and South America. The site with the most rapidly increasing absolute levels of tropospheric NO₂ was found to be Tianjin in China with a trend of 3.04 (±0.47) × 10¹⁵ molecules cm⁻²yr⁻¹, whereas the site with the most rapidly increasing relative trend was Kabul in Afghanistan with 14.3 (±2.2) % yr⁻¹. In total, 34 sites exhibited increasing trends of tropospheric NO₂ throughout the study period, 24 of which were found to be statistically significant. A total of 32 sites showed decreasing levels of tropospheric NO₂ during the study period, of which 20 sites did so at statistically significant magnitudes. Overall, going beyond the relatively small set of megacities investigated previously, this study provides the first consistent analysis of recent changes in tropospheric NO₂ levels over most large urban agglomerations worldwide, and indicates that changes in urban NO₂ levels are subject to substantial regional differences as well as influenced by economic and demographic factors.