Mechanisms driving altitude- and latitude-dependent air quality variations from high-altitude NOx emissions

The environmental impact of nitrogen oxide (NOx) emissions varies with emission altitude and latitude. NOx emissions from subsonic aviation (9-12 km) contribute to net global ozone formation, whereas those from supersonic aircraft (above 14 km) lead to net global ozone depletion. However, the effects of NOx emission altitude on surface air quality remain understudied. We evaluate how NOx emissions at different altitudes (8-22 km) and latitudes influence near-surface concentrations of two known air pollutants: ozone and fine particulate matter (PM2.5). Using the global chemical transport model GEOS-Chem, we find that NOx emissions of 1 Tg N yr-1 at 8-10 km (30-60°N) increase surface ozone (population-weighted) by 0.52 ppb and surface PM2.5 by 35 ng m-3, whereas emissions at 20-22 km reduce surface ozone by 1.73 ppb and increase surface PM2.5 by 310 ng m-3; this is nine times the PM2.5 increase per unit NOx from lower-altitude emissions. These effects stem from altitude-dependent mechanisms: at lower altitudes typical of subsonic aviation, NOx emissions increase upper tropospheric ozone which leads to enhanced surface ozone and nitrate aerosol. However, when emitted at higher altitudes NOx instead depletes ozone, permitting more ultraviolet light to reach the troposphere which boosts OH production and accelerates production of sulfate aerosol while destroying near-surface ozone. Our findings suggest that NOx emissions from high-altitude sources including supersonic aircraft may not only contribute to stratospheric ozone depletion but also cause larger changes (albeit of mixed sign) in surface air quality than subsonic aviation per unit of NOx emitted.