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Surface ozone (O3) levels in Southeast Michigan (SEMI) exceeds U.S. National Ambient Air Quality Standards (NAAQS), posing risks to human health and agroecosystems. SEMI, a relatively small region in the state of Michigan, contains a majority of anthropogenic emission sources and more than half of the state’s population, and is also prone to long-range and transboundary pollutant transport. Understanding the physical and chemical drivers of elevated O3 through detailed and innovative modeling studies are crucial to address the issue. In this study, we explore the distribution of O3 and its precursors (e.g., NOx & VOCs) over SEMI for the summer of 2021 using the 3-D chemistry-climate model, MUSICAv0 (Multi-Scale Infrastructure for Chemistry and Aerosols, Version 0). Model simulations are evaluated with Michigan-Ontario Ozone Source Experiment (MOOSE) field campaign measurements. A finer horizontal resolution of ~7 km x 7km in MUSICAv0 was developed over Michigan to better understand the local-scale impacts of chemical and dynamic complexity existing in SEMI. MUSICAv0 with the refined model grid shows excellent skill in capturing diurnal variations of temperature and O3, but shows larger variations for nitrogen dioxide (NO2). The MUSICAv0 results for NOx and its oxidation products (e.g., HNO3) were improved by applying a diurnal cycle to anthropogenic nitric oxide (NO) emissions, as global models generally do not include diurnal variation of emissions. The source attribution of O3 in SEMI is also quantified using a carbon monoxide (CO) tagging method. Optimization of a regionally-refined, coupled model such as MUSICAv0, through resolution and emission modeling studies, have significant implications for air quality projects at the local-scale and the design of effective surface O3 mitigation strategies. 

The summertime surface ozone (O3) concentrations over Southeast Michigan (SEMI) often exceed 70 ppbv. However, the associated O3 formation regime is still not well known. In this study, we examined the chemical drivers of O3 exceedances in SEMI, based on the Michigan-Ontario Ozone Source Experiment (MOOSE) field campaign during the period of May 20 – June 30, 2021. We employed a zero-dimensional (0-D) box model, which was constrained by measurements of meteorology and trace gas concentrations during MOOSE. Our model simulations demonstrated that the formaldehyde to nitrogen dioxide ratio (HCHO/NO2) for the transition between the VOC- and NOx-limited O3 production regimes was 3.0 ± 0.3 (mean ± 1σ) in SEMI. The midday (12:00-16:00) averaged HCHO/NO2 ratio during MOOSE was 1.62 ± 1.03, suggesting that O3 production in SEMI was likely limited by VOC emissions. Our study has significant implications for air quality policy and the design of effective O3 pollution control strategies through ground-based HCHO/NO2 measurements and model simulations.