Articles | Volume 15, issue 3
Research article
03 Feb 2015
Research article |  | 03 Feb 2015

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

P. Schneider, W. A. Lahoz, and R. van der A

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.

Short summary
We use a homogeneous 10-year record of satellite data to study recent trends in NO2 over the world's major urban agglomerations. The results indicate distinct spatial patterns in trends, with moderate but consistent reductions in NO2 throughout most developed countries and rapid increases of up to 15 % per year over many sites in Asia, Africa, and South America. We also show links between urban NO2 trends and economic as well as demographic factors, and how the latter drive regional differences.
Final-revised paper