NMOC, ozone, and organic aerosol in the southeastern United States, 1999–2007: 2. Ozone trends and sensitivity to NMOC emissions in Atlanta, Georgia

Non-methane organic carbon (NMOC) measurements made in Atlanta, Georgia from 1999–2007 are used with nitrogen oxide (NOx or NOy) and ozone (O3) data to investigate relationships between O3 precursors and peak 8-hour O3 concentrations in the city. Data from a WNW-to-ENE transect of sites illustrate that the mean urban peak 8-hour O3 excess constitutes about 20% of the peak 8-hour O3 measured at the area-wide maximum O3 site when air-mass movement is from the northwest quadrant; local influence is potentially greater on days with more stagnation or recirculation. The peak 8-hour O3 concentrations in Atlanta increase as (1) surface temperature (T), ambient NMOC and NOy concentrations, and previous-day peak O3 concentrations increase, and as (2) relative humidity, surface wind speeds, and ratios of NMOC-to-NOy decrease. An observation-based statistical model is introduced to relate area-wide peak 8-hour O3 concentrations to ambient NMOC and NOy concentrations, while accounting for the non-linear dependences of peak 8-hour O3 concentrations on meteorological factors. On the majority of days when the area-wide peak 8-hour O3 exceeds 75 ppbv, meteorologically-adjusted peak 8-hour O3 concentrations increase as ambient NMOC concentrations increase (NMOC sensitive) and ambient NOy concentrations decrease. This result contrasts with regional conditions in which O3 formation appears to be NOx-sensitive in character. The results offer observationally-based information of relevance to O3 management strategies in the Atlanta area, potentially contributing to “weight-of-evidence” assessments.