Seasonal and global NOx production by lightning estimated from the Optical Transient Detector (OTD)

Abstract

The Optical Transient Detector (OTD) lightning data for the 12-month period of 1996 are used to estimate the seasonal and global distributions of lightning-produced NOx. The relatively small viewing footprint and the low detection efficiency of the OTD sensor and other difficulties require extrapolations of the OTD data to the actual global flash distributions. Furthermore, available measurements for the ratios of intracloud (IC) to cloud-to-ground (CG) flashes have been used to partition lightning counts for IC versus CG flashes from the OTD observations. The resulting lightning distributions are then used to calculate the global and seasonal production of NOx, assuming a NO production rate of 6.2×1025 molecules for each CG flash and 8.7×1024 molecules for each IC flash. Consequently, we find that CG flashes produce more NOx than IC flashes despite fewer CG flashes by a factor of 3 or more. NOxproduction by lightning varies seasonally in accordance with the global lightning distribution, with the maximum production occurring in the Northern Hemisphere in the local summer. The latitudinal distribution of NOx production exhibits a strong seasonal variation outside the tropics with the production occurring mainly in the summer hemisphere, whereas in the tropics the production is high throughout the year. The annual contribution to NOx production by lightning is higher in the Northern Hemisphere than that in the Southern Hemisphere.