Ozone production in the rural troposphere and the implications for regional and global ozone distributions

Abstract

The relationship between O 3 and NOx (NO + NO 2) which was measured during summer and winter periods at Niwot Ridge, Colorado, has been analyzed and compared to model calculations. Both model calculations and observations show that the daily O 3 production per unit of NOx is greater for lower NOx. Model calculations without nonmethane hydrocarbons (NMHC) tend to underestimate the O 3 production rate at NOx higher than 1.5 parts per billion by volume and show the opposite dependence on NOx. The model calculations with NMHC are consistent with the observed data in this regime and demonstrate the importance of NMHC chemistry in the O 3 production. In addition, at eight other rural stations with concurrent O 3 and NOx measurements in the central and eastern United States the daily O 3 increase in summer also agrees with the O 3 and NOx relationship predicted by the model. The consistency of the observed and model‐calculated daily summer O 3 increase implies that the average O 3 production in rural areas can be predicted if NOx is known. The dependence of O 3 production rate on NOx deduced in this study provides the basis for a crude estimate of the total O 3 production. For the United States an average summer column O 3 production of about 1×10 12 Cm−2 S−1 from anthropogenically emitted NOx and NMHC is estimated. This photochemical production is roughly 20 times the average cross‐tropopause O 3 flux. Production of O 3 from NOx that is emitted from natural sources in the United States is estimated to range from 1.9×10 11 to 12×10 11 cm−2 s−1, which is somewhat smaller than ozone production from anthropogenic NOx sources. Extrapolation to the entire northern hemisphere shows that in the summer, 3 times as much O 3 is generated from natural precursors as those of anthropogenic origin. The winter daily O 3 production rate was found to be about 10% of the summer value at the same NOx level. However, because of longer NOx lifetime in the winter, the integrated O 3 production over the lifetime of NOx may be comparable to the summer value. Moreover, because the natural NOx sources are substantially smaller in the winter, the wintertime O 3 budget in the northern hemisphere should be dominated by ozone production from anthropogenic ozone precursors. The photochemical lifetime of O 3 in the winter in the mid‐latitude is approximately 200 days. We propose that this long lifetime allows anthropogenically produced O 3 to accumulate and contribute substantially to the observed spring maximum that is usually attributed to stratospheric intrusion. Furthermore, the anthropogenic O 3 may be transported not only zonally but also to lower latitudes. Thus the long‐term interannual increase in O 3, observed in the winter and spring seasons at Mauna Loa, may be due to the same anthropogenic influences as the similar winter trend observed at Hohenpeissenberg, Germany.