Abstract
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.
Highlights
More than half of the world’s population lives in urban areas
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
There is a strong link between trends in satellitebased NO2 columns and changes in nitrogen oxide (NOx) emissions, which in turn have an important effect on the quality of the results provided by modeling systems for atmospheric composition such as the Copernicus Atmospheric Monitoring Service
Summary
More than half of the world’s population lives in urban areas. Megacities, i.e., cities with more than 10 million inhabitants, as well as other large urban agglomerations have seen rapid growth over the last decades and this trend is expected to continue in the near future (United Nations, 2012a). The number of megacities is expected to rise from currently 23 to a total of 37 megacities in the year 2025 (Zhu et al, 2012) Such large urban agglomerations exhibit intensive human activities and high levels of energy consumption. Nitrogen dioxide (NO2) is one of the major air pollutants and is strongly related to population density (Lamsal et al, 2013) As such it is a significant environmental issue in many large urban agglomerations (Schneider and van der A, 2012; Liu and Zhu, 2013; Hilboll et al, 2013; Guerreiro et al, 2014). Going beyond trends determined over actual megacities as done in previous studies (Schneider and van der A, 2012; Hilboll et al, 2013), we present here the first detailed satellite-based global analysis of tropospheric NO2 trends for a significantly expanded set of of large urban agglomerations worldwide.
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