Climatology of ozone exceedences in the Atlanta metropolitan area: 1- hour vs 8-hour standard and the role of plume recirculation air pollution episodes

This work compares the relative frequencies of and meteorological conditions most conducive to exceedences of the current 1-h/120-ppbv national ambient air quality standard for ozone and the proposed revised standard based on daily maximum 8-h averages. Chemical and meteorological data gathered from 1987 through 1993 in Atlanta, GA, are used to determine the daily maximum 1-h and 8-h average ozone mixing ratios at each local monitoring site, during the ozone season for each year. The nature of the highest among the maxima and the concurrent local meteorological conditions are examined. We find a high degree of correlation between the daily maximum 1-h and 8-h averages (R2 = 0.92) in Atlanta, with an 8-h daily maximum concentration of 98 ppbv being climatologically equivalent to a 1-h daily maxima of 120 ppbv. Over the 7-year period investigated, exceedences of 8-h- averaged concentrations of 70, 80, and 90 ppbv were about 5, 3, and 2 times more frequent in Atlanta than exceedences of the current 1-h/120-ppbv standard. Meteorological conditions that fostered Atlanta's most severe 1-h and 8-h maximum ozone pollution events over the period were quite similar. The majority of the most extreme 1-h and 8-h events were associated with multiple-day episodes with very stagnant meteorological conditions and a recirculation and recooking of Atlanta's plume on consecutive days. These events, called 'plume recirculation' episodes, are distinct from other multiple-day episodes that were characterized by fairly steady winds, little or no recirculation, and more modest O3 concentrations. Plume recirculation episodes are among Atlanta's most severe O3 exceedences and therefore were chosen by the State of Georgia for urban airshed modeling in its most recent Stats Implementation Plan (SIP) and will likely remain the focus in future SiPs even if a new 8-h standard is promulgated.

Duke Scholars

Author William Chameides Earth and Climate Sciences