Midlatitude Lightning NO_x Production Efficiency Inferred From OMI and WWLLN Data

Abstract

Oxides of nitrogen are critical trace gases in the troposphere and are precursors for nitrate aerosol and ozone, which is an important pollutant and greenhouse gas. Lightning is the major source of NOx (NO + NO2) in the middle to upper troposphere. We estimate the production efficiency (PE) of lightning NOx (LNOx) using satellite data from the Ozone Monitoring Instrument and the ground‐based World Wide Lightning Location Network in three northern midlatitudes, primarily continental regions that include much of North America, Europe, and East Asia. Data were obtained over five boreal summers, 2007–2011, and comprise the largest number of midlatitude convective events to date for estimating the LNOx PE with satellite NO2 and ground‐based lightning measurements. In contrast to some previous studies, the algorithm assumes no minimum flash‐rate threshold and estimates freshly produced LNOx by subtracting a background of aged NOx estimated from the Ozone Monitoring Instrument data set itself. We infer an average value of 180 ± 100 moles LNOx produced per lightning flash. We also show evidence of a dependence of PE on lightning flash rate and find an approximate empirical power function relating moles LNOx to flashes. PE decreases by an order of magnitude for a 2 orders of magnitude increase in flash rate. This phenomenon has not been reported in previous satellite LNOx studies but is consistent with ground‐based observations suggesting an inverse relationship between flash rate and size.

Observing U.S. Regional Variability in Lightning NO2 Production Rates

“…To test the sensitivity of our results to this stroke rate threshold, we repeated our full analysis at minimum stroke rates spanning 1–10,000 strokes·cell −1 ·hr −1 (Figure a). In agreement with Bucsela et al (), we observed a decreasing trend in LNO 2 with increasing stroke frequency that was accompanied by an increasing trend in R 2 . For example, increasing the minimum stroke rate to 5,000 strokes·cell −1 ·hr −1 decreased total LNO 2 by 13% and increased R 2 to 0.6 (from 0.54).…”

“…At a higher minimum stroke rate of 10,000 strokes·cell −1 ·hr −1 , total LNO 2 decreased by 31%, and R 2 increased to 0.66. As a consequence of focusing on higher stroke rate storms, our absolute LNO 2 production efficiencies yielded lower LNO 2 per flash compared to those that would be derived using a lower threshold and should be interpreted in the context of the LNO x dependence on stroke frequency (Bucsela et al, ). Critical to this work, we also observed a dependence on minimum stroke rate for the CG/IC LNO 2 , which was largely caused by variability in CG LNO 2 (Figure ), which we report in section and discuss in section .…”

“…Because recent work has demonstrated that LNO x production rates vary with flash rate, such that LNO x decreases with increasing storm flash frequency (Bucsela et al, ), LNO x comparisons between studies are complicated as results often do not report storm flash rates. With no flash frequency threshold, Bucsela et al () found an LNO x rate of 180 ± 100‐mol NO x ·flash −1 . Under the same conditions (minimum flash rate = 1), we computed LNO x equal to 36‐mol NO x ·flash −1 .…”

“…Intercept values were larger than in Pickering et al (), which were stated to be near 0; however, their region of study was the Gulf of Mexico, where surface NO 2 emissions were minimal. Cloud‐resolving models and aircraft studies over rural areas found that 83–90% of NO x within the anvil of a storm was produced by lightning (DeCaria et al, ; DeCaria et al, ); as a result, we considered errors in the tropospheric background NO 2 column to be ±20% (Bucsela et al, ). We used ENTLN current errors of ±15% (Zhu et al, ), determined for negative polarity CG strokes.…”

“…To distinguish the LNO x signal from UT background NO 2 (NO 2,background ) “noise,” Pickering et al () used a flash frequency threshold of 1,000 flashes·cell −1 ·hr −1 . However, Bucsela et al () recently demonstrated that LNO x production efficiency varied with storm flash rate such that lower flash rates yielded higher LNO x , suggesting that LNO x estimates were highly sensitive to the application of a minimum flash number. That said, without a flash frequency threshold, there was no guarantee that grids used in the analysis were even electrically active, and, under this condition, the R 2 of the NO 2 (0) and strokes correlation across CONUS was ~0.…”