Long-term trends in air quality in major cities in the UK and India: A view from space

Abstract. Air quality networks in cities are costly, inconsistent, and only monitor a few pollutants. Space-based instruments provide global coverage spanning more than a decade to determine trends in air quality and address deficiencies in surface networks. Here we target cities in the UK (London and Birmingham) and India (Delhi and Kanpur) and use observations of nitrogen dioxide (NO₂) from the Ozone Monitoring Instrument (OMI), ammonia (NH₃) from the Infrared Atmospheric Sounding Interferometer (IASI), formaldehyde (HCHO) from OMI as a proxy for non-methane volatile organic compounds (NMVOCs), and aerosol optical depth (AOD) from the Moderate Resolution Imaging Spectroradiometer (MODIS) for PM_2.5. We assess the skill of these products at reproducing monthly variability in surface concentrations of air pollutants where available. We find temporal consistency between column and surface NO₂ in cities in the UK and India (R = 0.5–0.7) and NH₃ at two of three UK supersites (R = 0.5–0.7), but not between AOD and surface PM_2.5 (R < 0.4). MODIS AOD is consistent with AERONET at sites in the UK and India (R > = 0.8) and reproduces significant decline in surface PM_2.5 in London (2.7 % a⁻¹) and Birmingham (3.7 % a⁻¹) since 2009. We derive long-term trends in the four cities for 2005–2018 from OMI and MODIS and for 2008–2018 from IASI. Concentrations of all pollutants increase in Delhi, suggesting no air quality improvements there, despite rollout of controls on industrial and transport sectors. Kanpur experiences a significant and substantial (3.1 % a⁻¹) increase in PM_2.5. Concentrations of NO₂, NH₃ and PM_2.5 decline in London and Birmingham likely due in large part to emissions controls on vehicles. Trends are significant only for NO₂ and PM_2.5. Reactive NMVOCs decline in Birmingham, but the trend is not significant, and there is a recent (2012–2018) steep (> 9 % a⁻¹) increase in reactive NMVOCs in London. This may reflect increased contribution of oxygenated VOCs from household products, the food and beverage industry, and domestic wood burning, with implications for formation of ozone in a VOC-limited city.