Air pollution takes a holiday: the July 4th effect

Observed ozone concentrations in Atlanta, GA over the American Independence Day holiday (July 4th) are lower than would otherwise be expected for early July. These lower concentrations are noteworthy because they occur when ozone pollution is near its seasonal mid-summer maximum. Using a regression model to account for ozone variability due to meteorological influences, peak 1-hour ozone concentrations at one long-standing site in the Atlanta metro area averaged 10 ppbv less on July 5th than can be accounted for by meteorology alone. The implication is that smog causing emissions from commercial, industrial, and mobile sources during the holiday are significantly different (i.e. lower) which leads to improved air quality outcomes. The holiday provides a natural experiment for assessing the impact of reduced emissions on air pollutant concentrations. Such results can aid air quality managers in finding innovative strategies to control urban smog.