A space-based perspective of trends in air quality in major cities in the UK and India

Air pollution is one of the leading global causes of premature mortality, necessitating routine monitoring of air quality in cities where most (55%) people now reside. Surface monitors are sparse and costly to operate, whereas satellites provide global coverage of a multitude of pollutants spanning more than 2 decades. Here we make use of the dynamic range of satellite products to understand long-term changes in air quality in target cities in the UK (London and Birmingham) and India (Kanpur and Delhi). These include nitrogen dioxide (NO₂) from OMI for 2005-2018, formaldehyde (HCHO) from OMI for 2005-2016 to monitor non-methane volatile organic compounds (NMVOCs), ammonia (NH₃) from IASI for 2008-2017 and aerosol optical depth (AOD) from MODIS for 2005-2018 to monitor PM_2.5. Where surface observations are available (almost exclusively the UK), we first evaluate the ability of the satellite observations to reproduce variability in surface air pollution. We find temporal consistency for most pollutants (R >= 0.5), with the exception of MODIS AOD and surface PM_2.5 (R = 0.3), but the decline in AOD (3.0% a^-1) and surface PM_2.5 (2.8% a^-1), so far only evaluated for London, is similar. Inconsistencies result from seasonal variability in the planetary boundary layer, differences in sampling footprint between the satellite and surface monitors, and interferences in the surface measurements (as is the case for NO₂). We find a decrease in all pollutants in Birmingham and London and an increase in all pollutants in Delhi and Kanpur, over the analysis period, but not all trends are significant. Birmingham and London NO₂ both declined by 2.5% a^-1, whereas Delhi NO₂ increased by 2.0% a^-1, so that by the end of 2018 Delhi and London have the same tropospheric column concentrations of NO₂. Only Delhi exhibits a significant NMVOCs trend (increase) of 1.8% a^-1. NH₃ trends are not significant in any of the four cities, consistent with bottom-up inventories and lack of direct controls on emissions of this pollutant, mostly from agriculture. These data show no evidence of air quality improvements in Delhi, despite rollout of strict controls on industry and vehicles.