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| Titel |
Satellite observations of lightning-generated NOx in volcanic eruption clouds |
| VerfasserIn |
Simon Carn, Nickolay Krotkov, Ken Pickering, Dale Allen, Eric Bucsela |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250129483
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| Publikation (Nr.) |
 EGU/EGU2016-9606.pdf |
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| Zusammenfassung |
| The generation of NO2 by lightning flashes is known to be an important source of NOx in the free troposphere, particularly in the tropics, with implications for ozone production. Although UV-visible satellite observations of lightning-generated NOx (LNOx) in thunderstorms have been previously reported, here we present the first satellite observations of LNOx generated by lightning in explosive volcanic eruption clouds (vLNOx) from the Ozone Monitoring Instrument (OMI) aboard NASA’s Aura satellite. To date we have identified vLNOx in operational OMI NO2 measurements (OMNO2) during the high-latitude eruptions of Okmok (Aleutian Is; July 2008), Kasatochi (Aleutian Is; August 2008), Redoubt (Alaska; March 2009) and Grimsvötn (Iceland; May 2011), although analysis of OMNO2 data for other eruptions is underway. We use World Wide Lightning Location Network (WWLLN) observations to verify the occurrence and location of lightning flashes in the volcanic eruption clouds. All the vLNOx anomalies are associated with strong UV Aerosol Index (UVAI) signals due to volcanic ash. Preliminary analysis shows that the maximum vLNOx column detected by OMI decreases linearly with time since eruption, and suggests that the vLNOx signal is transient and can be detected up to ~5-6 hours after an eruption. Detection of vLNOx is hence only possible for eruptions occurring a few hours before the daytime OMI overpass. Based on the number of lightning flashes detected by WWLLN in each eruption cloud, we also estimate the vLNOx production efficiency (moles vLNOx per flash). Preliminary estimates for the 2008 Kasatochi eruption suggest that this is significantly higher than the production efficiency in thunderstorms, but may be biased high due to the low detection efficiency of WWLLN (<10-50% of flashes detected over most regions). The measured vLNOx columns also require adjustment using an algorithm designed to retrieve LNOx from OMI, which takes the total OMI slant column NO2 and removes the stratospheric contribution and tropospheric NO2 background and applies an appropriate air mass factor to convert the slant column LNO2 to a vertical column of LNOx. However, OMI measurements of LNOx in thunderstorms suggest that any NOx below the cloud optical centroid pressure (OCP; ~350-500 hPa) is not detected. We speculate that the OCP may be lower (i.e., at higher altitude) in fresh volcanic clouds due to higher optical depths. The observation of vLNOx in volcanic clouds is significant since it implies active convection and plume electrification close to the satellite overpass time, with implications for aviation hazards due to volcanic ash. Furthermore, the vLNOx observations may provide information on air entrainment in volcanic eruption columns, which is required for some volcanic ash dispersion models. Although vLNOx is undoubtedly a very minor fraction of global LNOx production, explosive volcanic eruptions may inject NOx into the stratosphere where it has implications for ozone chemistry. |
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