Using high-resolution TROPOMI NO2 columns to assess health disparities in NO2 exposure across London

Long-term exposure to air pollution is a major global public health threat. However, the health burden due to air pollution exposure is unequal. In cities, ethnic minorities experience the most severe exposures and adverse health outcomes to air pollution, particularly from traffic. Exposure to traffic-related air pollution is associated with childhood-onset asthma and adult premature mortality. Contemporary data of the location and size of the disparities to determine and address inequities in exposure and health burdens is not readily available for all cities. Here we address this dataset by deriving surface concentrations of nitrogen dioxide (NO₂) as proxy for traffic-related air pollution for London at fine spatial scales (~400 m). This is achieved by oversampling 5 years of TROPOspheric Monitoring Instrument (TROPOMI) tropospheric NO₂ column data from its nadir resolution of ~5.6 km x 3.5 km. We then subtract off a uniform free-tropospheric NO₂ column of 50 pptv determined from cloud-sliced vertically resolved TROPOMI data to isolate the boundary layer. These boundary layer columns are then converted to surface concentrations using an exponential relationship between the TROPOMI boundary layer column and midday mean in-situ network measurements. Midday concentrations are converted to 24-hour concentrations using a midday-to-24h ratio of 1.30, which is calculated from surface network sites in the Greater London Area (GLA). The TROPOMI-derived 24-hour mean surface NO₂ concentrations reproduce the NO₂ observed by the surface network. We use 2021 census data at the finest resolution for three out of five high-level ethnic groups defined by the UK census: Asian, Black and White. To calculate the health burden, we use borough level baseline mortality rates, the finest scale available, which range between 44 and 2818 per 100,000, and an exposure response coefficient of 1.023 per 10 µg m^-3 annual mean NO₂ as recommended by the UK’s Committee on the Medical Effects of Air Pollutants (COMEAP). We find premature mortality rates due to traffic-related pollution exposure for each ethnic group exhibit similar spatial variation across the city, with highest mortality in central and central northwest London. The average GLA premature mortality rate is worst for the Black population (59 per 100,000), then the Asian population (57 per 100,000) and least severe for the White population (56 per 100,000), reflecting the ethnic injustices in air pollution exposure. Work is underway to identify specific census-tract areas with the most severe disparities that require immediate regulatory actions and to quantify the disparities in childhood asthma instances across the city. We aim to expand this investigation to four other major UK cities with the greatest traffic-related pollution emissions, according to the UK National Atmospheric Emission Inventory (NAEI). These are Birmingham, Leeds, Manchester and Glasgow.