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

The impact of lightning on tropospheric reactive nitrogen NO_x (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 NO_x, 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, NO_x is rapidly converted into HNO₃ due to the high OH concentration in this region. As a result, the enhancement in NO_x from direct lightning emission is limited primarily to the upper troposphere. The conversion between NO_x and less reactive nitrogen species (PAN and HNO₃) also plays an important role in affecting NO_x, especially over the oceans where lightning activity is low. The model results suggest that recycling of NO_x from the lightning-enhanced PAN and HNO₃ produces 2 to 10 parts per trillion by volume (pptv) increases in NO_x over the oceans in the lower troposphere of the tropics. The enhancement of NO_x over the oceans can partially explain the observations of NO_x (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 NO_x from the continents.