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

The impact of lightning on tropospheric reactive nitrogen NOx (NO + NO2) and nitrogen reservoir species (HNO3, peroxyacetyl nitrate (PAN), N2O5, and HNO4) 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 approximately 60 to 80% of the total increase in the nitrogen species concentration. The increase in PAN accounts for approximately 20 to 30% of the nitrogen enhancement by lightning in the middle troposphere. In the lower troposphere of the tropics, NOx is rapidly converted into HNO3 due to the high OH concentration in this region. As a result, the enhancement in NOx from direct lightning emission is limited primarily to the upper troposphere. The conversion between NO, and less reactive nitrogen species (PAN and HNO3) also plays an important role in affecting NOx, especially over the oceans where lightning activity is low. The model results suggest that recycling of NOx from the lightning-enhanced PAN and HNO3 produces 2 to 10 parts per trillion by volume (pptv) increases in NOx over the oceans in the lower troposphere of the tropics. The enhancement of NOx over the oceans can partially explain the observations of NOx (mixing ratios of typically 10-50 pptv) over the tropical oceans, which are characterized by higher concentrations than would be expected from direct transport of NOx from the continents.