The importance of NOx production by lightning in the tropics

The importance of NOx production by lightning in the tropics has been assessed by using satellite lightning measurements from the Lightning Image Sensor (LIS). The lightning data from LIS over the period of 1998–2000 are analyzed and partitioned based on the latitude to obtain the numbers of cloud-to-ground (CG) and cloud-to-cloud (intracloud or IC) flashes. The average annual lightning counts over the 3-yr period are 3.0×108 CG flashes and 1.1×109 IC flashes between 35°N and 35°S. The resulting lightning distributions are employed to calculate the production of NOx. We obtain a lightning NOx production of 6.3 Tg N yr−1 over this latitudinal region, using representative production values of 6.7×1026 and 6.7×1025 NO molecules for each CG and IC flash, respectively. NOx production by lightning varies slightly on a seasonal basis in accordance with the lightning distribution, with the maximum production occurring in the months of September, October, and November. The geographical and seasonal production of NOx by lightning is compared to NOx emissions from other sources (i.e., from anthropogenic activity, biomass burning, and soil emissions). The results indicate that production of NOx by tropical lightning is significant throughout the year. Lightning accounts for almost all of the NOx emitted over the oceans and 50–90% of NOx emitted over some continental areas on a seasonal basis. On the annual basis, the contribution of lightning to total NOx production is 23% in the tropics, and globally the lightning NOx production occurs predominately in this region. The uncertainty in estimating NOx production by lightning over the tropics is discussed.