Abstract. Air quality networks in cities can be costly and inconsistent and typically
monitor a few pollutants. Space-based instruments provide global coverage
spanning more than a decade to determine trends in air quality, augmenting
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 PM2.5. 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 rural sites in the UK (R = 0.5–0.7) but not
between AOD and surface PM2.5 (R < 0.4). MODIS AOD is
consistent with AERONET at sites in the UK and India (R ≥ 0.8) and
reproduces a significant decline in surface PM2.5 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. Trends of all pollutants are positive in Delhi, suggesting no air
quality improvements there, despite the roll-out of controls on industrial and
transport sectors. Kanpur, identified by the WHO as the most polluted city
in 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 controls
on 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) increase
in reactive NMVOCs in London. The cause for this rapid increase is
uncertain but may reflect the increased contribution of oxygenated volatile organic compounds (VOCs) from
household products, the food and beverage industry, and domestic wood
burning, with implications for the formation of ozone in a VOC-limited city.