Effects of lightning on reactive nitrogen and nitrogen reservoir species in the troposphere

The impact of lightning on tropospheric reactive nitrogen NOx (NO + NO2) an d nitrogen reservoir species (HNO3, peroxyacetyl nitrate (PAN), N2O5, and H NO4) has been evaluated using a global chemical/transport model. Comparison of calculations made with and without lightning show that lightning has a significant effect on the nitrogen species on a global scale, resulting in significant enhancements of NOx HNO3 and PAN over the no lightning case. Of the nitrogen species, HNO3 is influenced the most, comprising approximatel y 60 to 80% of the total increase in the nitrogen species concentration. Th e increase in PAN accounts for approximately 20 to 30% of the nitrogen enha ncement by lightning in the middle troposphere. In the lower troposphere of the tropics, NOx is rapidly converted into HNO3 due to the high OH concent ration in this region. As a result, the enhancement in NOx from direct ligh tning emission is limited primarily to the upper troposphere. The conversio n between NO, and less reactive nitrogen species (PAN and HNO3) also plays an important role in affecting NOx, especially over the oceans where lightn ing activity is low. The model results suggest that recycling of NOx from t he lightning-enhanced PAN and HNO3 produces 2 to 10 parts per trillion by v olume (pptv) increases in NOx over the oceans in the lower troposphere of t he tropics. The enhancement of NOx over the oceans can partially explain th e observations of NOx (mixing ratios of typically 10-50 pptv) over the trop ical oceans, which are characterized by higher concentrations than would be expected from direct transport of NOx from the continents.