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

Abstract

The relationship between O 3 and NO x (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 NO x is greater for lower NO x . Model calculations without nonmethane hydrocarbons (NMHC) tend to underestimate the O 3 production rate at NO x higher than 1.5 parts per billion by volume and show the opposite dependence on NO x . 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 NO x measurements in the central and eastern United States the daily O 3 increase in summer also agrees with the O 3 and NO x 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 NO x is known. The dependence of O 3 production rate on NO x 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 NO x and NMHC is estimated. This photochemical production is roughly 20 times the average cross‐tropopause O 3 flux. Production of O 3 from NO x 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 NO x 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 NO x level. However, because of longer NO x lifetime in the winter, the integrated O 3 production over the lifetime of NO x may be comparable to the summer value. Moreover, because the natural NO x 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.