Direct satellite observation of lightning-produced NO<sub>x</sub>

Abstract

Lightning is an important source of NO_x in the free troposphere, especially in the tropics, with high impact on ozone production. However, estimates of lightning NO_x (LNO_x) production efficiency (LNO_x per flash) are still quite uncertain. <br><br> In this study we present a systematic analysis of NO₂ column densities from SCIAMACHY measurements over active thunderstorms, as detected by the World-Wide Lightning Location Network (WWLLN), where the WWLLN detection efficiency was estimated using the flash climatology of the satellite lightning sensors LIS/OTD. Only events with high lightning activity are considered, where corrected WWLLN flash rate densities inside the satellite pixel within the last hour are above 1 /km²/h. For typical SCIAMACHY ground pixels of 30×60 km², this threshold corresponds to 1800 flashes over the last hour, which, for literature estimates of lightning NO_x production, should result in clearly enhanced NO₂ column densities. <br><br> From 2004–2008, we find 287 coincidences of SCIAMACHY measurements and high WWLLN flash rate densities. For some of these events, a clear enhancement of column densities of NO₂ could be observed, indeed. But overall, the measured column densities are below the expected values by more than one order of magnitude, and in most of the cases, no enhanced NO₂ could be found at all. <br><br> Our results are in contradiction to the currently accepted range of LNO_x production per flash of 15 (2–40)&times;10²⁵ molec/flash. This probably partly results from the specific conditions for the events under investigation, i.e. events of high lightning activity in the morning (local time) and mostly (for 162 out of 287 events) over ocean. <br><br> Within the detected coincidences, the highest NO₂ column densities were observed around the US Eastcoast. This might be partly due to interference with ground sources of NO_x being uplifted by the convective systems. However, it could also indicate that flashes in this region are particularly productive. <br><br> We conclude that current estimates of LNO_x production might be biased high for two reasons. First, we observe a high variability of NO₂ for coincident lightning events. This high variability can easily cause a publication bias, since studies reporting on high NO_x production have likely been published, while studies finding no or low amounts of NO_x might have been rejected as errorneous or not significant. Second, many estimates of LNO_x production in literature have been performed over the US, which is probably not representative for global lightning

Midlatitude Lightning NOx Production Efficiency Inferred From OMI and WWLLN Data

“…Pickering et al (2016) imposed a minimum threshold of 1,000 fl/hr in 1°× 1°grid boxes and obtained a relatively low PE of 80 mol/fl. The 287 cases examined by Beirle et al (2010) were restricted to rates equivalent to a threshold of 9,000 fl/hr. They found an LNO x -flash correlation coefficient of only 0.04 and PEs less than 15 mol/fl in approximately half of their examined cases.…”

“…Retrieved LNO x * is inversely proportional to the air mass factor A LNOx in equation (1). Beirle et al (2009Beirle et al ( , 2010 estimated AMFs for a variety of storm environments and found that a fixed average AMF of 0.46 was adequate for their LNO x retrievals from Scanning Imaging Absorption Cartography NO 2 data. We find this value reasonable, since substitution of the Beirle et al AMF decreased the present PE by only 13%.…”

“…Ambient tropospheric NO x must also be distinguished from the fresh LNO x signal. The use of cloudy scenes limits contamination of the lightning signal by NO x in the lower troposphere with a likely anthropogenic source (Allen et al, 2019;Beirle et al, 2010;Marais et al, 2018;Pickering et al, 2016). High flash-rate restrictions have also been imposed to ensure that lightning NO x is the predominant component of the satellite signal and that the background contribution is minor (Beirle et al, 2010;Pickering et al, 2016).…”

“…The use of cloudy scenes limits contamination of the lightning signal by NO x in the lower troposphere with a likely anthropogenic source (Allen et al, 2019;Beirle et al, 2010;Marais et al, 2018;Pickering et al, 2016). High flash-rate restrictions have also been imposed to ensure that lightning NO x is the predominant component of the satellite signal and that the background contribution is minor (Beirle et al, 2010;Pickering et al, 2016). Approaches for estimating the background have been described by Bucsela et al (2010), Pickering et al (2016), and Allen et al (2019).…”

“…Over the same region, using Global Ozone Monitoring Experiment data, Beirle et al (2006) derived a similar value of 90 (range of 32 to 240) mol/fl. However, Beirle et al (2010) obtained a nearly null result in their Scanning Imaging Absorption Cartography-based study of 287 convective events, with no detectable LNO x enhancement over most storms and overall negligible correlation with flash counts. Allen et al (2019) used updated versions of the Pickering et al (2016) data sets, similar to those of the present study to estimate a PE value of 170 ± 100 mol/fl in the tropics.…”