The importance of NO x production by lightning in the tropics

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.