Direct satellite observation of lightning-produced NOx 1Max-Planck-Institut für Chemie, Mainz, Germany
24 Nov 2010
2Institut für Physik der Atmosphäre, Deutsches Zentrum für Luft- und Raumfahrt (DLR), Oberpfaffenhofen, Germany
Received: 09 July 2010 – Published in Atmos. Chem. Phys. Discuss.: 02 August 2010 Abstract. Lightning is an important source of NOx in the free
troposphere, especially in the tropics, with strong impact on ozone
production. However, estimates of lightning NOx (LNOx)
production efficiency (LNOx per flash) are still quite
Revised: 11 November 2010 – Accepted: 18 November 2010 – Published: 24 November 2010
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
NOx production, should result in clearly enhanced NO2 column densities.
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.
Our results are in contradiction to the currently accepted range of
LNOx production per flash of 15
(2–40)×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.
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 NOx being
uplifted by the convective systems. However, it could also indicate
that flashes in this region are particularly productive.
We conclude that current estimates of LNOx 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
NOx production have likely been published, while studies
finding no or low amounts of NOx might have been rejected as
errorneous or not significant. Second, many estimates of LNOx
production in literature have been performed over the US, which is
probably not representative for global lightning.
Citation: Beirle, S., Huntrieser, H., and Wagner, T.: Direct satellite observation of lightning-produced NOx, Atmos. Chem. Phys., 10, 10965-10986, doi:10.5194/acp-10-10965-2010, 2010.