NOx production by lightning over the continental United States

The production of NOx by lightning over the contiguous United States has be en evaluated by using combined ground-based and satellite lightning measure ments. The lightning data from the National Lightning Detection Network (NL DN) over the period of 1995-1999, along with a ratio of intracloud (IC) to cloud-to-ground (CG) flashes derived in conjunction with satellite lightnin g measurements from the Optical Transient Detector (OTD), are analyzed to o btain the number of CG and IC flashes. The average annual lightning counts over the 5-year period are about 23 million for CG flashes and 55 million f or IC flashes. The resulting lightning distributions are employed to calcul ate the production of NOx assuming a NO production rate of 6.7 x 10(26) mol ecules for each CG flash and 6.7 x 10(21) molecules for each IC flash. NOx production by lightning varies seasonally in accordance with the lightning distribution, with the maximum production occurring in the summer (June, Ju ly, and August) and in the Southeast. CG flashes produce more NOx than IC f lashes despite fewer CG flashes by a factor of 2 or more. The geographical and seasonal production of NOx by lightning is compared to NOx emissions fr om other sources (i.e., from anthropogenic, soil, and biomass-burning emiss ions). The results indicate that regional emissions of NOx by lightning can be significant in the summertime and may play a critical role in ozone for mation in the free troposphere. Our estimate of NOx emission by lightning o ver the United States would decrease by an order of magnitude if we use the production rates from a recent laboratory study [Wang et al., 1998] that a re significantly lower than previous estimates.