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Ambient air pollution is responsible for 4-9 million premature deaths worldwide each year. Routine ground-based monitoring of air quality in cities is sparse and expensive and only includes a handful of pollutants. Most health risk assessment models are derived with limited health outcomes and cover a narrow range (2.4-35 µg m) of fine particulate (PM) concentrations. Satellites provide daily global coverage of a dynamic range of pollutants for more than a decade and there are updated health risk assessment models that account for the increasing number of health outcomes that have been associated with air pollution and that cover a wider exposure range than previous models. In this work, the skill of satellite observations at reproducing variability in surface air quality in the UK and Indian cities was assessed. Temporal consistency (R>0.5) occurred between space-based and surface observations of nitrogen dioxide (NO) and ammonia (NH), whereas measurements of aerosol optical depth (AOD) have weak month-to-month variability (R<0.4) with surface PM, but do replicate long term trends in PM. This provided the confidence to use satellite observations to determine recent (2000s 2010s) long-term trends in NO, NH, formaldehyde (HCHO) as a marker for reactive non-methane volatile organic compounds (NMVOCs), and AOD as a marker for PM in London and Birmingham in the UK, and Delhi and Kanpur in India. Trends in most pollutants declined in UK cities because of successful control on vehicular emissions but increased in Indian cities despite recent pollution control measures. These validated satellite observations were then used to quantify long-term trends in air quality …