Estimation of the Natural Background of Ozone Present at Surface Rural Locations

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Abstract
The natural background in the ozone concentration at rural locations in the United States and western Europe has been estimated by use of several approaches. The approaches utilized include the following: (1) historical trends in ozone concentration measurements, (2) recent ozone measurements at remote sites, (3) use of tracers of air originating in the stratosphere or upper troposphere and (4) results from applications of tropospheric photochemical models. While each of these approaches has its own limitations it appears that the natural background of ozone during the warmer months of the year is in the range of 10 to 20 ppb. Most of the ozone originating in the lower stratosphere or upper troposphere is lost by chemical or physical removal processes as well as undergoing dilution by air in the lower troposphere before reaching ground level rural locations. Lower tropospheric photochemical processes, those below 5 km, are likely to account for most of the ozone measured at rural locations during the warmer months of the year. A key aspect to improved quantitation of the contributions from lower tropospheric photochemical processes to ozone concentrations continues to be more extensive atmospheric measurements of the distribution of reactive species of nitrogen. The emission densities of anthropogenic sources of NOx are known to be highly variable over populated areas of continents as well as between continental areas and the oceans. The emission densities of biogenic sources of NOx are small, likely to be highly variable, but poorly quantitated. These wide variations indicate the need for use of three dimensional tropospheric photochemical models over large continental regions. Available results do indicate higher efficiencies for ozone formation at lower NOx concentrations, especially below 1 ppb.