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Abstract

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.