Present-day variability of background ozone in the lower troposphere

There is a substantial background of ozone (O-3) present in the lower tropo sphere in the Northern Hemisphere that has both a stratospheric and photoch emical tropospheric origin. Levels of hourly averaged O-3 concentrations in the range 0.04-0.08 ppm are often measured as part of the "background O-3" burden. Stratospheric processes play a significant role in defining these background O-3 concentrations. In order to better understand the frequency, spatial, and temporal characteristics of this background O-3 burden, we ha ve analyzed hourly average O-3 concentrations greater than or equal to0.05 and greater than or equal to0.06 ppm that were experienced during the photo chemically quiescent months in the winter and spring at several rural sites across southern Canada, the northern United States, and northern Europe. O ur results were mostly consistent and indicated that hourly average O-3 con centrations greater than or equal to0.05 and greater than or equal to0.06 p pm occur frequently during the winter and spring months. Most occurrences w ere during April and May but sometimes as late as June. In some, but not al l, of the cases that were studied, a plausible explanation for the higher O -3 values was the presence of upper tropospheric and stratospheric air that was transported down to the surface. For purposes of making the analysis m ore straightforward, this study was limited to the more northerly latitudes . It might be expected that a significant contribution from natural sources is also important in producing O-3 levels in the 0.05-0.06 ppm or higher r ange at more southerly locations where stratospheric intrusions also have a n important impact on the tropospheric O-3 distribution.