Evaluation of 15 years of modeled atmospheric oxidized nitrogen compounds across the contiguous United States

Atmospheric nitrogen oxide and nitrogen dioxide (NO + NO2, together termed as NOX) estimates from annual photochemical simulations for years 2002-2016 are compared to surface network measurements of NOX and total gas-phase-oxidized reactive nitrogen (NOY) to evaluate the Community Multiscale Air Quality (CMAQ) modeling system performance by U.S. region, season, and time of day. In addition, aircraft measurements from 2011 Deriving Information on Surface Conditions from Column and Vertically Resolved Observations Relevant to Air Quality are used to evaluate how emissions, chemical mechanism, and measurement uncertainty each contribute to the overall model performance. We show distinct seasonal and time-of-day patterns in NOX performance. Summertime NOX is overpredicted with bimodal peaks in bias during early morning and evening hours and persisting overnight. The summertime morning NOX bias dropped from between 28% and 57% for earlier years (2002-2012) to between -2% and 7% for later years (2013-2016). Summer daytime NOX tends to be unbiased or underpredicted. In winter, the evening NOX overpredictions remain, but NOX is unbiased or underpredicted overnight, in the morning, and during the day. NOX overpredictions are most pronounced in the Midwestern and Southern United States with Western regions having more of a tendency toward model underpredictions of NOX. Modeled NOX performance has improved substantially over time, reflecting updates to the emission inputs and the CMAQ air quality model. Model performance improvements are largest for years simulated with CMAQv5.1 or later and for emission inventory years 2014 and later, coinciding with reduced onroad NOX emissions from vehicles with newer emission control technologies and improved treatment of chemistry, deposition, and vertical mixing in CMAQ. Our findings suggest that emissions temporalization of specific mobile source sectors have a small impact on model performance, while chemistry updates improve predictions of NOY but do not improve summertime NOX bias in the Baltimore/DC area. Sensitivity runs performed for different locations across the country suggest that the improvement in summer NOX performance can be attributed to updates in vertical mixing incorporated in CMAQv5.1.