ADVANCEMENT OF THE SCIENCE
lower in summer months (Figure 6). Further, NO x emissions in Texas decreased from 1,937 thousand tons in 2000 to 881 thousand tons in 2020; over that same period, VOC emis- sions increased from 1,341 thousand tons to 1,645 thousand tons (U.S. EPA, 2024b). These findings suggest that slower progress toward controlling VOC emissions could be limiting ozone reductions. In contrast, data from the daily time series showed significant upward trends for noon solar radiation ( p < .05; Figure 4), maximum temperature ( p < .01; Figure 5), and noon wind speed ( p < .01; Figure 7). Both solar radiation and temperature showed strong seasonality, with higher values in summer months and especially high values late in the data record. While average wind speed trended downward, stronger winds at the upper part of the distribution occurred early in the time series (Figure 7). Overall, wind speed tended toward slightly higher values in the winter months. MDA8 ozone significantly correlated ( p < .01) with each variable studied, from strongest to weakest (correlation coefficient in paren- theses): noon solar radiation (+0.712), maxi- mum temperature (+0.681), noon relative humidity (-0.472), noon wind speed (-0.102), and average NO x (+0.060). Strong associa- tions among MDA8 ozone levels, maximum temperature, and relative humidity were also observed in previous studies. Using data from 2000 to 2016 for the contiguous U.S., Wells et al. (2021) found that midday relative humid- ity and daily maximum temperature were the most important meteorological predictors of MDA8 ozone levels. Previously, Camalier et al. (2007) found strong associations between MDA8 ozone levels, relative humidity, and temperature in a study of 39 urban areas of the Eastern U.S. using data collected between 1997 and 2005. Further, Sadeghi et al. (2022) used a convolutional neural network to evalu- ate hourly ozone concentrations between 2000 and 2019, concluding that tempera- ture and specific humidity contributed most to MDA8 ozone levels over Dallas, and solar radiation strongly affected MDA8 variation over West Texas. Conclusion The objective of this study was to compile and evaluate long-term trends in ground- level ozone concentrations, meteorologi-
FIGURE 6
Plot of Noon Relative Humidity Over Time
100
90
80
70
60
50
40
30
20
10
0
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 7
Plot of Noon Wind Speed Over Time
35
30
25
20
15
10
5
0
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.
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Volume 87 • Number 5
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