Local and Regional Contributions to Tropospheric Ozone Concentrations

TL;DR

This study analyzes regional and local contributions to tropospheric ozone in Utah's counties, revealing a shift from VOC-limited to more NOx-limited conditions over ten years, which informs targeted pollution mitigation strategies.

Contribution

It provides a detailed analysis of regional versus local ozone contributions and the changing oxidative regime in Utah over a decade, aiding in effective air quality management.

Findings

Most counties shifted from VOC-limited to NOx-limited or transitional states between 2012 and 2022.

Local ozone contributions increased notably in Cache and Salt Lake Counties.

Regional contributions to ozone decreased in several counties, indicating changing pollution dynamics.

Abstract

The Wasatch Front in Utah, USA, is currently a non-attainment area for ozone according to the Environmental Protection Agency's (EPA) National Ambient Air Quality Standards (NAAQS). Nitrogen oxides ($\mathrm{NO_x = NO_2 + NO}$) and volatile organic compounds (VOCs), in the presence of sunlight, lead to ozone formation in the troposphere. When the rate of oxidant production, defined as the sum of $\mathrm{O_3}$ and $\mathrm{NO_2}$, is faster than the rate of $\mathrm{NO_x}$ production, a region is said to be $\mathrm{NO_x}$limited, and ozone formation will be limited by the concentration of $\mathrm{NO_x}$ species in the region. The inverse of this situation makes the region VOC-limited. Knowing whether a region is $\mathrm{NO_x}$-limited or VOC-limited can aid in generating effective mitigation strategies. Understanding the background or regional contributions to ozone in a region, whether from the transport of precursors or of ozone, provides information about the lower limit for ozone concentrations that a region can achieve through regulation of local precursors. In this paper, measured oxidant and $\mathrm{NO_x}$ concentrations are analyzed from 14 counties in the state of Utah to calculate the regional and local contributions to ozone for each region. This analysis is used to determine the nature of the atmosphere in each county by identifying whether the region is VOC or $\mathrm{NO_x}$-limited. Furthermore, this analysis is performed for each county for the years 2012 and 2022 to assess changes in the oxidative nature and quantify regional and local contributions to ozone over a 10-year period. All studied counties--except for Washington County--in Utah were found to be VOC-limited in 2012. This shifted in 2022, with most counties being either in a transitional state or $\mathrm{NO_x}$-limited. Local contributions to ozone increased in two major counties, Cache and Salt Lake Counties, but decreased in Carbon, Davis, Duchesne, Uinta, Utah, Washington, and Weber Counties. Generally, the regional contributions to oxidant concentrations decreased across the state. A summertime spike in both regional and local contributions to oxidants was observed. Smoke from wildfires was found to increase regional contributions to oxidants and shift the local regime to be more $\mathrm{NO_x}$-limited.