The importance of NO x production by lightning in the tropics

Abstract

The importance of NO x 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 3.0×10 8 CG flashes and 1.1×10 9 IC flashes between 35°N and 35°S. The resulting lightning distributions are employed to calculate the production of NO x . We obtain a lightning NO x production of 6.3 Tg N yr −1 over this latitudinal region, using representative production values of 6.7×10 26 and 6.7×10 25 NO molecules for each CG and IC flash, respectively. NO x 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 NO x by lightning is compared to NO x emissions from other sources (i.e., from anthropogenic activity, biomass burning, and soil emissions). The results indicate that production of NO x by tropical lightning is significant throughout the year. Lightning accounts for almost all of the NO x emitted over the oceans and 50–90% of NO x emitted over some continental areas on a seasonal basis. On the annual basis, the contribution of lightning to total NO x production is 23% in the tropics, and globally the lightning NO x production occurs predominately in this region. The uncertainty in estimating NO x production by lightning over the tropics is discussed.