The impact of man-made and natural NOx emissions on upper tropospheric ozone: A two-dimensional model study

The impact of various sources of NOx on the upper tropospheric ozone production has been studied using a comprehensive two-dimensional zonally averaged chemistry/transport model. Three sources of NOx in the upper troposphere were examined in the study: NOx produced by lightning, NOx (and NOy) brought to the upper troposphere from the planetary boundary layer by rapid vertical transport processes, and NOx emitted from aircraft. The strength of the lightning source was assumed to be 3 TgN yr(-1), the average injection of NOx in the troposphere by rapid vertical transport was about 0.7 TgN yr(-1), and the total release of NOx from aircraft was 0.58 TgN yr(-1). For comparison, the various sources at the ground total 34 TgN yr(-1) of NOx. Using these emissions the model estimated an average tropospheric ozone production of 5.2 x 10(10) molecules cm(-2) s(-1) over the Southern Hemisphere and 14.1 x 10(10) molecules cm(-2) s(-1) over the Northern Hemisphere, and a flux of ozone from the stratosphere of 4.1 x 10(10) molecules cm(-2) s(-1). A sensitivity experiment indicated that the ozone yield per NOx molecule in the upper troposphere is more than 5 times that near the surface. Considering only the upper troposphere (above 490 hPa) the NOx from aircraft released there resulted in an additional ozone production of 37 molecules of O-3 per molecule of NOx. The tropospheric average based on all NOx sources was, for comparison, 10 molecules of O-3 per molecule of NOx. It was more surprising to discover that the increased upper tropospheric ozone production resulting from all three NOx sources was accompanied by a reduced ozone production in the lower troposphere. The sources therefore showed only a small influence on the total ozone production. Nevertheless, the impact of all three sources on the ozone concentration in the upper troposphere was quite significant. The effect of both aircraft NOx emissions and rapid vertical transport was strongest in the upper troposphere in the 30-60 degrees N latitude zone where the impact of each on the NOx concentration was in the range 5-40% and the impact on the O-3 concentration was in the range 5-10%. In the same area the impact of lightning NOx on the NOx and O-3 concentrations was in the range 20-50% and 10-20%, respectively. The above numbers are based on July conditions and indicate that about one-half of the ozone produced chemically in the upper troposphere at northern mid-latitudes is due to man-made emissions. At other latitudes lightning NOx appears to be the dominating upper tropospheric ozone precursor.