Modeled aerosol nitrate formation pathways during wintertime in the great lakes region of North America

Episodic wintertime particle pollution by ammonium nitrate is an important air quality concern across the Midwest U.S. Understanding and accurately forecasting PM_2.5 episodes are complicated by multiple pathways for aerosol nitrate formation, each with uncertain rate parameters. Here, the Community Multiscale Air Quality model (CMAQ) simulated regional atmospheric nitrate budgets during the 2009 LADCO Winter Nitrate Study, using integrated process rate (IPR) and integrated reaction rate (IRR) tools to quantify relevant processes. Total nitrate production contributing to PM_2.5 episodes is a regional phenomenon, with peak production over the Ohio River Valley and southern Great Lakes. Total nitrate production in the lower troposphere is attributed to three pathways, with 57% from heterogeneous conversion of N₂O₅, 28% from the reaction of OH and NO₂, and 15% from homogeneous conversion of N₂O₅. TNO₃ formation rates varied day-to-day and on synoptic timescales. Rate-limited production does not follow urban-rural gradients and NO_x emissions due, to counterbalancing of urban enhancement in daytime HNO₃ production with nocturnal reductions. Concentrations of HNO₃ and N₂O₅ and nighttime TNO₃ formation rates have maxima aloft (100–500 m), leading to net total nitrate vertical flux during episodes, with substantial vertical gradients in nitrate partitioning. Uncertainties in all three pathways are relevant to wintertime aerosol modeling and highlight the importance of interacting transport and chemistry processes during ammonium nitrate episodes, as well as the need for additional constraint on the system through field and laboratory experiments.