NEHA December 2024 Journal of Environmental Health

ADVANCEMENT OF THE SCIENCE

tion and therefore remove less ozone (Huang et al., 2016; Kavassalis & Murphy, 2017).

FIGURE 2

Study Area The study area was the Dallas-Fort Worth region of the Texas Commission on Environ- mental Quality (TCEQ) State Implementa- tion Plan, which covers 10 counties in North Central Texas (Figure 1). In 2020, the study area had a population of approximately 7.5 million people. According to the U.S. Cen- sus Bureau (2024), the area’s most populous counties are Dallas (2.6 million), Tarrant (2.1 million), Collin (1.1 million), and Den- ton (900,000). This area has sustained sharp increases in population and vehicle miles traveled in recent decades (Texas Department of Transportation, 2024), yet MDA8 ozone levels have trended downward with reduced NO x emissions (TCEQ, 2024). Dallas-Fort Worth—the second most pop- ulous metropolitan area in the Sunbelt region of the U.S.—is prone to high ozone concen- trations, especially between May and Octo- ber (Cox & Chu, 1996; Sather & Cavender, 2012). Precursor gases from mobile anthro- pogenic sources contribute a greater amount than stationary sources to ozone levels in Dal- las-Fort Worth (Luria et al., 2008). Approxi- mately 90% of NO x in Dallas-Fort Worth comes from cars, trucks, and off-road mobile sources (TCEQ, 2024). Ozone and precursor gases also move into Dallas-Fort Worth from sources elsewhere in Texas and neighboring states (Kemball-Cook et al., 2009; Kim et al., 2009). In the summer, sustained southeast- erly winds have been shown to cause distinct clusters of high ozone concentrations at the northern perimeter of the Dallas-Fort Worth metropolitan area (Hudak, 2014). Methods Daily measurements of ozone, NO x , and meteorological variables were compiled from Station C56 (at the Denton Airport South) in the TCEQ monitoring network during Janu- ary 1, 2001–December 31, 2023 (Figure 1). This station is generally downwind of the Dallas-Fort Worth metropolitan core—espe- cially in the summer—and historically has produced high ozone concentrations in the study area. Several variables were compiled, includ- ing MDA8 ozone levels (ppm), daily aver- age of NO x (ppb), daily noon solar radiation

Plot of Maximum Daily 8-Hour Average (MDA8) Ozone Levels Over Time

0.12

0.11

0.1

0.09

0.08

0.07

0.06

0.05

0.04

0.03

0.02

0.01

1/1/2001

1/1/2005

1/1/2009

1/1/2013

1/1/2017

1/1/2021

Date (Month/Day/Year)

Note. The dashed yellow line represents the trend line.

FIGURE 3

Plot of Daily Average Nitrogen Oxides (NO x ) Levels Over Time