Nitrate, Nitric Acid, and Ammonia in the Upper Midwest—Measurements and Reactive Production Pathways

Episodic wintertime particle pollution by ammonium nitrate is an important air quality concern across the Midwest U.S. From December 2008 to March 2009, an intensive monitoring campaign was conducted including continuous measurements of ammonia, ammonium, nitrate, nitric acid at an urban site (Milwaukee, Wisconsin), and an upwind rural site. The monitoring indicated numerous short duration episodes of enhanced total nitrate over the Upper Midwest, that were subsequently investigated by chemical transport modeling. Understanding and accurately forecasting PM2.5 episodes is complicated by multiple pathways for aerosol nitrate formation, each with uncertain rate parameters. 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 PM2.5 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. 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.