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Abstract:
The production of NOx by lightning over the contiguous United States has
been evaluated by using combined ground-based and satellite lightning
measurements. The lightning data from the National Lightning Detection
Network (NLDN) over the period of 1995-1999, along with a ratio of
intracloud (IC) to cloud-to-ground (CG) flashes derived in conjunction
with satellite lightning measurements from the Optical Transient
Detector (OTD), are analyzed to obtain 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 for IC flashes. The resulting
lightning distributions are employed to calculate the production of NOx
assuming a NO production rate of 6.7 x 10(26) molecules 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,
July, and August) and in the Southeast. CG flashes produce more NOx than
IC flashes 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 from other sources (i.e., from anthropogenic, soil, and
biomass-burning emissions). The results indicate that regional emissions
of NOx by lightning can be significant in the summertime and may play a
critical role in ozone formation in the free troposphere. Our estimate
of NOx emission by lightning over 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 are significantly lower than
previous estimates.
