Tropospheric Chemistry


Lightning discharges are a major source of nitrogen oxide gases called NOx. The two primary gases (NO and NO2) are formed during the lightning discharge when the air is heated to 30,000 degrees inside the lightning channel. Since air is made up of approximately 80% N2 and 20% O2, these molecules breakup into nitrogen and oxygen atoms. When the channel expands outwards and cools, new compounds form as a result of the nitrogen and oxygen atoms recombining. The amount of NOx gases formed is related to the rate of cooling of the channel. There is a debate among scientists working in this field as to the relative efficiency of different types of lightning flashes (intracloud versus cloud-to-ground) to produce NOx. Besides the hot lightning channel, it is also likely that NOx is produced outside the channel, within the region of high electric fields surrounding the channel.

The rapid expansion of the channel due to the immense heating also results in a shock wave that produces a sonic boom that we hear as thunder. Since the boom arrives at our ears from each part of the lightning channel at a slightly different time, we actually hear a rumble and not a single boom (as is common when jet aircraft cross the sound barrier).

These NOx gases react with other gases in the atmosphere resulting in the formation of ozone (O3). Ozone in the lower atmosphere where we live, and where the lightning occurs is "bad" ozone since not only is it toxic to humans and plants, but it absorbs heat from the earth's surface, acting as a greenhouse gas, contributing to the warming of the atmosphere. The "good" ozone is the ozone in the stratosphere (above 20km altitude) that protects us from harmful ultraviolet radiation from the sun. There is convincing evidence that "bad" ozone is increasing in concentration, while "good" ozone is decreasing. My interests in the area of tropospheric chemistry are primarily with the bad ozone produced as a result of lightning activity.

Lightning is not the only source of NOx in the atmosphere. In fact, there are many sources of NOx, with the anthropogenic burning of fossil fuels being the main contributor to NOx concentrations in the atmosphere. However, lightning is the largest natural source (~12 Tg N/yr), and perhaps the largest source overall in the upper parts of the troposphere where changes in ozone concentrations are very important in the study of future climate change.  Global maps of the January and July climatologies of lightning-produced nitrogen oxides (NOx) are shown below.  These results are now available on the Global Emissions Inverntory Activity (GEIA) data base.



In recent years researchers have started to wonder whether sprites and upper atmospheric discharges are also a major source of NOx.  Since sprites occur in the stratosphere and mesosphere, with blue jets moving through the ozone layer, could sprites have an impact on the ozone layer?  This is still a topic of research.

References:

    Price, C., J. Penner and M. Prather, 1997: NOx From Lightning, Part I: Global Distribution Based on Lightning Physics, J. Geophys. Res., 102, 5929-5941.
    Price, C., J. Penner and M. Prather, 1997: NOx From Lightning, Part II: Using the Global Atmospheric Electric Circuit, J. Geophys. Res., 102, 5943-5951.
    Penner, J.E., D.J. Bergmann, J.J. Walton, D. Kinnison, M.J. Prather, D. Rotman, C. Price, K. Pickering and S. Baughcum, 1998: An evaluation of upper tropospheric NOx with two models, J. Geophys. Res., 103, 22097-22113.
    Shindell, D. T., J. L. Grenfell, D. Rind, C. Price and V. Grewe, 2001:  Chemistry-climate interactions in the GISS GCM. Part 1: Tropospheric chemistry model description and ecaluation, J. Geophys. Res., 106 (D8), 8047-75.
    Price, C., 2002: Lightning, Production of Nitric Oxide, in Encyclopedia of Atmospheric Science, eds. J.R. Holton, J. Pyle and J.A. Curry, Academic Press, USA.
    Ojelede, M., H. Annegarn, C. Price and M. Keen, 2008:  Lightning-produced NOx (LNOx) budget over the Highveld region in Southern Africa, Atmos. Environ., 44(22), 5706-5714, doi: 10.1016/jatmosenv.2007.12.072.

Related Sites:

     Tropospheric Ozone
     Lightning-NOx