![]() ![]() The statistical model accounts for these effects by adjusting the ozone trend lines upward in 2009 and downward in 2012. ![]() in 2012 contributed to increased ozone formation. in the summer of 2009 contributed to decreased ozone formation, while above average temperatures and below average humidity in the Central and Eastern U.S. The solid lines represent ozone levels anticipated under weather conditions observed in a typical year and serve as a more accurate assessment of the trends in ozone due to changes in precursor emissions.įor example, below average temperatures and above average humidity in the Eastern U.S. The dotted lines show the observed trends measured at long-term monitoring sites, while the solid lines show the underlying trends after removing the effects due to weather. The graph below shows the national trends in the average (black lines), 90th percentile (blue lines) and 98th percentile (red lines) of the May-September daily maximum 8-hour ozone concentrations from 2000 to 2021 at monitoring stations across the contiguous U.S. EPA uses a statistical model to adjust for the variability in seasonal ozone concentrations due to weather to provide a more accurate assessment of the underlying trend in ozone caused by emissions. Conversely, ozone production is more limited when it is cloudy, cool, rainy, or windy. Ozone is more readily formed on warm, sunny days when the air is stagnant. Variations in weather conditions play an important role in determining ozone concentrations. ![]()
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