NEHA December 2024 Journal of Environmental Health

(Langley/min), daily maximum temperature (°F), daily noon relative humidity (%), and daily noon wind speed (mi/hr). Descriptive statistics, scatterplots, rank correlations, and MDA8 exceedances of 0.070 ppm were com- puted to evaluate the data. Rank correlations were used because the data were non-nor- mally distributed. Results and Discussion Overall, the annual number and annual average of MDA8 ozone concentrations above 0.070 ppm both trended downward, with some notable anomalies (Table 1). For example, 2005, 2011, and 2021–2023 had relatively high numbers of exceedances and annual MDA8 averages considering the over- all temporal trend (Table 1). Further, 2005 and 2011 had relatively high solar radiation, high air temperature, and low relative humid- ity (Table 1). Moreover, 2005 had relatively high NO x and low wind speed, which also tended to produce higher ozone concentra- tions (Table 1). Similarly, 2021–2023 had rel- atively high solar radiation and temperature (Table 1). In contrast, 2010 and 2014 had fewer MDA8 ozone exceedances and lower annual MDA8 averages, coinciding with lower solar radiation, lower temperature, and (for 2014) higher wind speed (Table 1). Daily time series show variable concentra- tions at a finer temporal scale (Figures 2–7). Overall, MDA8 ozone concentrations trended downward ( p < .01) but with strong seasonal- ity or tendency for higher concentrations in summer months (Figure 2). Relatively large numbers of exceedances occurred early in the time series. Thereafter, concentrations tended to decline overall, with a clear upward depar- ture from that trend late in the data record (Figure 2). Overall decreasing MDA8 ozone concen- trations—attributed to improved emissions control technology—are consistent with prior studies in the U.S. (Simon et al., 2015). In addition to MDA8 ozone concentrations, both daily average NO x (Figure 3) and noon rela- tive humidity (Figure 6) trended significantly downward ( p < .01). Despite sharp increases in population and vehicle miles traveled in the study area, NO x concentrations decreased from 2001–2023. This study also found that NO x concentrations tended to be higher in winter months, whereas relative humidity showed less seasonality but was somewhat

FIGURE 4

Plot of Noon Solar Radiation Over Time

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6

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 5

Plot of Maximum Daily Temperature Over Time

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