Tropospheric ozone rise in the northern midlatitudes increasingly driven by nitrate particles

Tropospheric ozone is a major air pollutant and greenhouse gas whose anthropogenic production is limited principally by the supply of nitrogen oxides (NOx) from combustion. The background ozone concentration in the extratropical northern hemisphere has been increasing at 5% per decade since the 1950s, a trend that has continued into the 21st century despite the leveling off of NOx emissions. Here we explain this sustained increase by the photolysis of nitrate aerosol particles (pNO3-) to regenerate NOx. pNO3- forms from thermodynamic gas-aerosol partitioning of nitric acid, a product of NOx oxidation, at low aerosol acidity. Including pNO3- photolysis in the GEOS-Chem global atmospheric chemistry model increases ozone in the extratropical northern hemisphere by up to 10 ppbv, improving the consistency with observations from ozonesondes and aircraft. pNO3- concentrations in the northern extratropics have likely been increasing over the past several decades because of decreasing aerosol acidity driven by falling anthropogenic SO2 and rising anthropogenic ammonia emissions. We find in the model that this has augmented the increase in ozone in the northern extratropics by about 50% to better match the observed ozone trend. pNO3- is likely to continue to increase through 2050 and drive an increase in ozone. Further work is needed to better determine the rates and mechanism of pNO3- photolysis.