Observing decoupling processes of NO2 pollution and GDP growth based on satellite observations for Los Angeles and Tokyo

Nitrogen dioxide (NO2) pollution is mainly caused by anthropogenic processes such as burning of fossil fuels. Due to its severe impact on health as well as the environment in general, it is important to monitor the amount of NO2 pollution especially in areas with a high population density. Therefore, the aim of this study is to investigate to what extent spatiotemporal fluctuations in the tropospheric NO2 column density can map variations in economic output. To do so, we analyzed satellite based tropospheric NO2 column observations obtained from the ERS-2, ENVISAT, MetOp-A and MetOp-B satellite missions covering the period from 1996 to 2017 for Tokyo, Japan and Los Angeles, United States. Within our studies, a harmonic analysis was carried out in order to exclude meteorological influences. Afterwards, the NO2 time series were further investigated through a wavelet analysis method to characterize the fluctuations, that is the temporal variability of the NO2. These fluctuations have been shown to be a particularly sensitive measure of the change in NO2 pollution. We further use the gross domestic product (GDP) for the metropolitan areas as an indicator for economic performance. The results for the study area of Los Angeles shows a substantial reduction in NO2 variability starting in 2007. The NO2 variability for the period during the global financial crisis (December 2007–April 2012), dropped by around 77% in comparison to the previous period (January 1997–November 2007). However, a second period from May 2012 to December 2016 presents a further reduction of the NO2 variability of around 82% in contrast to the period from 2007 to 2012. Contrary to this, continuous economic growth can be observed during the second period. A similar picture emerges for the metropolitan region of Tokyo. A significant decline in NO2 variability of around 73% can be detected from October 2003 to January 2012. Additionally, a second period with a strong decline in NO2 variability of around 80% can be identified from February 2012 to December 2016. A plunge in GDP during 2008 reflects that year's global financial crisis, but cannot independently explain the following sharp decline in NO2 variability. Those results suggest that Tokyo as well as Los Angeles managed to substantially decouple its NO2 pollution from the economic growth due to strict air quality policies.