Assessment of Nocturnal NOx Heterogeneous Reaction Mechanisms in the Community Multiscale Air Quality (CMAQ) Model

Abstract

AbstractNitrogen oxides (NOx) have adverse human health impacts and play a central role in the production of ozone and PM2.5. Nighttime heterogeneous chemistry regulates the nocturnal reservoirs and sinks of NOx such as N2O5 removal and ClNO2 production. However, existing parameterizations of nocturnal NOx heterogeneous chemistry in air quality models do not capture the variability in observations. Here, we implemented for the first time in the Community Multiscale Air Quality (CMAQ) model the Gaston N2O5 uptake (γ(N2O5)) mechanism that accounts for the role of particulate organic matter in regulating N2O5 uptake and the Staudt ClNO2 yield (Φ(ClNO2)) mechanism that includes the role of reactive solutes in suppressing ClNO2 production. With the Gaston and Staudt parameterizations, the coarse mode contributed modestly to N2O5 loss (17.2%) but significantly to ClNO2 production (60.3%), highlighting the impact of coarse mode chemistry. The Gaston γ(N2O5) parameterization in the fine mode increased agreement between modeled N2O5 concentration and observations (RMSEnew = 0.37ppb) compared to the model default (RMSEdefault = 0.43ppb). The Gaston γ(N2O5) parameterization was overall biased low due to underestimates in modeled particle oxygen to carbon ratio (O:C). The Staudt Φ(ClNO2) parameterization resulted in further underestimation (NMBnew = −73.7%) compared to the model default (NMBdefault = −37.9%) because of underestimation of fine mode particle chloride concentration. We expect that the updated parameterizations may more accurately capture the mean state and variability in γ(N2O5) and Φ(ClNO2) under conditions where model particulate O:C and chloride are better represented.