Reactive nitrogen partitioning fuels contribution of Canadian wildfire plumes to U.S. ozone air quality

Accurately quantifying wildfire impacts on ozone air quality is challenging due to complex physical and chemical processes in wildfire smoke. Here we use measurements from the 2018 WE-CAN aircraft campaign to parameterize emissions of reactive nitrogen (NOy) from wildfires into PAN (37%), NO3- (27%), and NO (36%) in a global chemistry-climate model with 13 km horizontal resolution over the contiguous US. The NOy partitioning, compared with emitting all NOy as NO, reduces model ozone bias in near-fire smoke plumes sampled by the aircraft but significantly enhances ozone downwind when Canadian smoke plumes reach cities in Washington state, Utah, Colorado, and Texas. Using multi-platform observations, we identify the smoke-influenced days with daily maximum 8-h average (MDA8) ozone of 70-85 ppbv in Spokane, Salt Lake City, Denver and Dallas. On these days, mixing of wildfire smoke into urban pollution enhance simulated MDA8 ozone by 10–20 ppbv.