Particulate Nitrate Photolysis as a Possible Driver of Rising Tropospheric Ozone

Tropospheric ozone is an air pollutant and a greenhouse gas whose anthropogenic production is limited principally by the supply of nitrogen oxides (NO x ) from combustion. Tropospheric ozone in the northern hemisphere has been rising despite the flattening of NO x emissions in recent decades. Here we propose that this sustained increase could result from the photolysis of nitrate particles (pNO 3 − ) to regenerate NO x . Including pNO 3 − photolysis in the GEOS‐Chem atmospheric chemistry model improves the consistency with ozone observations. Our simulations show that pNO 3 − concentrations have increased since the 1960s because of rising ammonia and falling SO 2 emissions, augmenting the increase in ozone in the northern extratropics by about 50% to better match the observed ozone trend. pNO 3 − will likely continue to increase through 2050, which would drive a continued increase in ozone even as NO x emissions decrease. More work is needed to better understand the mechanism and rates of pNO 3 − photolysis.