Viral Shah, Christoph Keller, K. Emma Knowland, Amy Christiansen, Lu Hu, Haolin Wang, Xiao Lu, Becky Alexander, Daniel J. Jacob

Tropospheric ozone is an air pollutant and a greenhouse gas whose anthropogenic production is limited principally by the supply of nitrogen oxides (NOx) from combustion. Tropospheric ozone in the northern hemisphere has been rising despite the flattening of NOx emissions in recent decades. Here we propose that this sustained increase could result from the photolysis of nitrate particles (pNO3-) to regenerate NOx. Including pNO3- photolysis in the GEOS-Chem atmospheric chemistry model improves the consistency with ozone observations. Our simulations show that pNO3- concentrations have increased since the 1960s because of rising ammonia and falling SO2 emissions, augmenting the increase in ozone in the northern extratropics by about 50% to better match the observed ozone trend. pNO3- will likely continue to increase through 2050, which would drive a continued increase in ozone even as NOx emissions decrease. More work is needed to better understand the mechanism and rates of pNO3- photolysis.