The satellite data revolution: How new satellite instruments can provide better estimates of NOx pollution

Abstract

Measurements from satellite instruments orbiting Earth can be useful in quantifying the abundance of air pollution in locations with few or no monitors. Even in the United States, a country with 327 million people, there are fewer than 500 active NO2 air quality monitors, representing ~1 monitor per 1 million residents. The typical downside of satellite instruments is that they acquire measurements of gaseous pollutants at spatial extents analogous to entire city sizes (>20 x 20 km2). However more recently, an instrument launched by the European Space Agency (TROPOMI), is able to quantify NO2 air pollution at sub-city spatial resolution (3.5 x 5.5 km2) and with enhanced sensitivity. In this work, we use high-resolution data from TROPOMI to generate 2019 annual human exposure estimates to NO2. This is accomplished by merging the satellite data with other variables, which are highly correlated with the spatial heterogeneities of NO2, such as roadway density, population density, and atmospheric boundary layer depth. This new exposure estimate, at 1 x 1 km2 spatial resolution, is developed by training to the existing network of ground measurements, where the quantities of NO2 are well-known, and then is applied to the United States as a test-bed (R2 > 0.6), with further goals to apply globally. In an additional step, we also quantify NOx emission rates from power plants and large cities, by identifying the source of the pollution using the satellite spatial maps and then tracking the plume decay over time. Using these two complementary methods, we better understand the sources of NO2 pollution and the human exposure to it. These new NO2 exposure estimates can be used in epidemiological studies to explore the relationship between NO2 and health outcomes, and in health impact assessments to estimate the health benefits of emission regulations over time.