Modeling and assessing the impact of nocturnal NOx chemistry on air quality

Abstract

Nitrogen oxides (NOx ≡ NO + NO2) are air pollutants in the United States because they have direct human health impacts and play a central role in the production of secondary pollutants like ozone and particle pollution. Air quality models are a powerful tool to understand the complex chemistry that affects NOx, but model representations of nocturnal NOx chemical mechanisms cannot capture the full variability seen in observations because of simplifying assumptions. My dissertation assesses the impact of these assumptions on nighttime NOx chemistry and daytime air quality in models. First, I updated the nocturnal heterogeneous chemical mechanisms for N2O5 uptake and ClNO2 yield in the EPA Community Multiscale Air Quality (CMAQ) model. The new mechanisms are more representative of the chemistry of real particles because they account for the role of particulate organic matter in regulating N2O5 uptake and the role of reactive solutes in suppressing ClNO2 production. Following the implementation of the two updated heterogeneous chemistry mechanisms, I calculate the contribution of different nocturnal loss pathways to the NOx budget. Nocturnal loss pathways of NOx reservoir species impact daytime concentrations of NO2, so it is important to understand the role of nocturnal loss pathways, such as N2O5 uptake, in local air quality. By changing the model representation of heterogeneous N2O5 uptake, early morning NO2 concentrations fluctuated by 7.3% in January. Finally, I assessed the sensitivity of CMAQ to representations of sea spray particle emissions to highlighting the impact these particles have on heterogeneous chemistry. Although sea spray aerosol particles affect heterogeneous chemistry, their emission in chemical transport models excludes the smallest particle sizes. By changing the sea spray emissions in CMAQ, I increased particle surface area over 200% in the smallest particle size. The changes to modal surface area resulted in a similar impact to heterogeneous loss of N2O5 to the particle phase. This dissertation highlights the interconnected nature of nocturnal NOx chemistry and the complex impact on daytime air quality. It contributes to the efforts to improve the CMAQ model and its representations of NOx chemistry. Through my dissertation, I show that updating chemical and emission mechanisms in the CMAQ model have implications for nocturnal heterogeneous loss and therefore daytime air quality.