Regional estimates of lightning production of nitrogen oxides

Abstract

Summertime distribution of lightning over the United States and the potential importance of lightning‐generated NOx (NO + NO2) was investigated by using data from the National Lightning Detection Network (NLDN) for June, July, and August 1989 through 1992. The data were compiled and gridded to yield hourly and monthly flash densities. Without correcting the data for the network's detection efficiency, on the average, 10 million flashes occur over the United States each summer with 2.6 strokes occurring per flash. The densest concentration of flashes is over the Southeast. In 1989 the summertime lightning activity (9.4 million flashes) accounted for 70% of the annual flashes. To investigate the regional characteristics of lightning, the data were also compiled for the eastern United States and a smaller subdomain of the southeastern United States. NOx production rates of 0.36×1026, 4×1026, and 30×1026 molecules/flash were chosen to represent the low, median, and high end of estimates suggested by different investigators. Using these three production rates and hourly gridded flash densities, lightning‐generated NOx emissions were calculated. These estimates were compared to anthropogenic emissions derived from the 1985 National Acid Precipitation Assessment Program (NAPAP) inventory. Based on the high production rate, NOx emissions produced by lightning are comparable to monthly anthropogenic NOx emissions in the Southeast during the summer. Even for the low production rate, hourly emissions of lightning‐produced NOx frequently exceed anthropogenic emissions, with the highest frequencies in the Southeast. These results suggest that estimates of lightning‐generated NOx in the rural southeastern United States are not negligible and that this natural source of NOx could play a significant role in summertime tropospheric ozone production in the Southeast. Given the importance of NOx in ozone photochemistry, especially in NOx‐limited regimes, this natural source cannot be dismissed relative to anthropogenic sources even though the uncertainties in actual emission factors are large.