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Open Access
Warmer Air Disrupts Downward Trend in Ozone Concentrations in North Central Texas, United States
Paul F. Hudak, PhD Department of Geography and the Environment, University of North Texas
Abstract Tropospheric ozone concentrations have trended downward in many U.S. cities since 2000. In Texas, however, concentra- tions and regulatory exceedances abruptly rose in Dallas-Fort Worth af- ter 2020. To explore this anomaly, the following metrics were compiled for each day from January 1, 2001–December 31, 2023: maximum daily 8-hr average (MDA8) ozone concentrations, number of days with MDA8 ozone concentrations exceeding 0.070 ppm, average nitrogen oxides (NOx, ppb), and meteorological variables. Measurements were taken at a moni- toring station in northern Dallas-Fort Worth. Levels of MDA8 ozone most strongly correlated with noon solar radiation (positive), followed by maxi- mum temperature (positive), noon relative humidity (negative), noon wind speed (negative), and average NOx (positive). After a long-term decline from 2000 to 2020, MDA8 ozone concentrations and regulatory exceed- ances sharply increased, a trend associated with increased solar radiation and air temperatures in the study area. Results suggest that warmer at- mospheric conditions associated with global warming are also increasing ground-level ozone concentrations in the study area.
FIGURE 1
Map of Study Area With County Boundaries and Monitoring Station C56
DENTON
C56 +
COLLIN
WISE
ROCKWALL
DALLAS
TARRANT
PARKER
KAUFMAN
N
ELLIS
JOHNSON
0
20 km
Texas
0 200 km N
Keywords: ozone, meteorology, emissions, Dallas-Fort Worth, Texas
Note: The plus sign (+) indicates the location of the monitoring station.
Introduction Air pollution has improved in parts of the world but remains a pervasive and challeng- ing environmental problem. Contaminated air from natural and human sources harms both plants and animals. Air pollutants also aect climate, for example, by interacting with solar radiation and outgoing heat. More- over, a warming climate can lead to higher ozone concentrations. To protect human and environmental health, the U.S. Environmen- tal Protection Agency (U.S. EPA) regulates
ozone and five other common air pollutants: particulate matter, sulfur dioxide, nitrogen dioxide, lead, and carbon monoxide. Ozone levels are considered unhealthy when the 8-hr average exceeds 0.070 ppm (U.S. Environmental Protection Agency [U.S. EPA], 2024a). Despite increases in popula- tion and vehicle trac, ozone levels have trended downward in Dallas-Fort Worth in recent decades following emission reduction programs (Hudak, 2022; Sather & Caven- der, 2012; Texas Commission on Environ-
mental Quality [TCEQ], 2024). Maximum daily average 8-hr (MDA8) ozone exceed- ances, however, abruptly increased in recent years. In 2022 from January to mid-July, for example, air monitors in Texas registered double the number of unhealthy ozone days compared with 2021 and most days since 2012 (Douglas, 2022). The Dallas- Fort Worth area had 48 ozone alert days in 2022, which represents the most alerts since 2012. Reasons for this increase, however, remain unclear (Samsel, 2023). The objec-
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Volume 87 • Number 5
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