Article révisé par les pairs
Résumé : Abstract. Air quality networks in cities can be costly and inconsistent and typicallymonitor a few pollutants. Space-based instruments provide global coveragespanning more than a decade to determine trends in air quality, augmentingsurface 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 ResolutionImaging Spectroradiometer (MODIS) for PM2.5. We assess the skill ofthese products at reproducing monthly variability in surface concentrationsof air pollutants where available. We find temporal consistency betweencolumn and surface NO2 in cities in the UK and India (R = 0.5–0.7)and NH3 at two of three rural sites in the UK (R = 0.5–0.7) but notbetween AOD and surface PM2.5 (R < 0.4). MODIS AOD isconsistent with AERONET at sites in the UK and India (R ≥ 0.8) andreproduces a significant decline in surface PM2.5 in London (2.7 % a−1) and Birmingham (3.7 % a−1) since 2009. We derive long-termtrends in the four cities for 2005–2018 from OMI and MODIS and for 2008–2018from IASI. Trends of all pollutants are positive in Delhi, suggesting no airquality improvements there, despite the roll-out of controls on industrial andtransport sectors. Kanpur, identified by the WHO as the most polluted cityin the world in 2018, experiences a significant and substantial (3.1 % a−1) increase in PM2.5. The decline of NO2, NH3, and PM2.5 in London and Birmingham is likely due in large part to emissions controlson vehicles. Trends are significant only for NO2 and PM2.5.Reactive NMVOCs decline in Birmingham, but the trend is not significant.There is a recent (2012–2018) steep (> 9 % a−1) increasein reactive NMVOCs in London. The cause for this rapid increase isuncertain but may reflect the increased contribution of oxygenated volatile organic compounds (VOCs) fromhousehold products, the food and beverage industry, and domestic woodburning, with implications for the formation of ozone in a VOC-limited city.

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