Nitrogen Oxides Emissions, Chemistry, Deposition, and Export Over the Northeast United States During the WINTER Aircraft Campaign

We examine the distribution and fate of nitrogen oxides (NO_x) in the lower troposphere over the Northeast United States (NE US) using aircraft observations from the Wintertime INvestigation of Transport, Emissions, and Reactivity (WINTER) campaign in February–March 2015, as well as the GEOS-Chem chemical transport model and concurrent ground-based observations. We find that the National Emission Inventory from the U.S. Environmental Protection Agency is consistent with WINTER observations of total reactive nitrogen (^TNO_y) to within 10% on average, in contrast to the significant overestimate reported in past studies under warmer conditions. Updates to the dry deposition scheme and dinitrogen pentoxide (N₂O₅) reactive uptake probability, ɣ(N₂O₅), result in an improved simulation of gas-phase nitric acid (HNO₃) and submicron particulate nitrate (pNO₃ ⁻), reducing the longstanding factor of 2–3 overestimate in wintertime HNO₃ + pNO₃ ⁻ to a 50% positive bias. We find a NO_x lifetime against chemical loss and deposition of 22 hr in the lower troposphere over the NE US. Chemical loss of NO_x is dominated by N₂O₅ hydrolysis (58% of loss) and reaction with OH (33%), while 7% of NO_x leads to the production of organic nitrates. Wet and dry deposition account for 55% and 45% of ^TNO_y deposition over land, respectively. We estimate that 42% of the NO_x emitted is exported from the NE US boundary layer during winter, mostly in the form of HNO₃ + pNO₃ ⁻ (40%) and NO_x (38%).