Download Measuring Ozone Using Differential absorption

Results obtained with the Tropospheric Ozone DIAL System Using a
YAG Laser and Raman Cells (A53Q – 0439)
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J. T. Sullivan ([email protected]) , T. J. McGee , G. K. Sumnicht
1. Department of Atmospheric Physics, University of Maryland Baltimore County (UMBC), Baltimore, MD, United States.
2. Code 614.0, NASA GSFC, Greenbelt, MD. 3. Science Systems and Applications, Inc., Lanham, Md.
Significance of Tropospheric Ozone
Left: Laser emits the fundamental wavelength of 266.0 nm.
The beam is then steered and focused into the center of each
Raman cell filled with a Raman active gas (H2 or D2). A
phenomenon known as Stimulated Raman Scattering occurs
(Table 1. for more info), shifting the wavelength to 299 or 289
nm. A large 45 cm telescope collects the free troposphere
signal and two smaller 2.5 cm telescopes collect the near
surface signal. The optics module package houses the narrow
band interference filters for the PMTs and the chopper helps
eliminate saturation of the PMTs.
Bottom: View from inside the 40’ trailer. Right: Outside, hatch
doors open for transmission into the atmosphere.
 Contribution to global warming from the preindustrial era to the present is
regarded as the third most important, following those of carbon dioxide (CO2)
and methane (CH4) (IPCC [2007]).
 Ozone near the surface is harmful to humans and vegetation
 Satellite observations of surface ozone are very difficult to obtain due to the
optically thick stratospheric ozone layer strongly attenuating the signal
 Sending balloon-borne instruments through the atmosphere is helpful, but not
on the continuous scale that is necessary to fully characterize tropospheric
ozone
 Tropospheric ozone is created by complex (non-linear) interactions with NOx
and Volatile Organic Compounds (VOC) in the presence of near ultraviolet
sunlight.
Wavelength Importance
“on”
Simulation
Retrieval (Elastic)
 Using ozonesonde data from Discover – AQ (July 2011) and assuming a Rayleigh atmosphere it was possible to
simulate a return signal from a “mid-summer high ozone” day.
 The molecular number density of ozone (ppb) is shown below as function
of discrete range bins and physical quantities. This is derived directly from
the elastic lidar equation.
 Assuming pulse energy, constant aerosol extinction value, and ozone absorption cross sections. Also, there is an
additional 1% noise and the assumption of negligible background light at these two wavelengths.
 Results suggest the system is capable of retrieving up to 12 km. Calculated ozone plot agrees well with sonde.
“off”
 There are small (<10% ppb)
corrections for the spectral
dependence of molecular
(Rayleigh) and aerosol extinction
coefficients.
 Also, interfering gases may need
to be corrected for.
Validation and Long Term
 There are currently in situ surface ozone measurements hourly (EPA
AirNOW sites) (http://airnow.gov/)
 Below 285 nm ozone is overly absorbent and attenuates photons rapidly
making it very difficult to obtain a necessary return signal.
 Above 305 nm solar contamination becomes a strong factor in addition to
other trace gases beginning to emerge with similar extinction values
 Use Stimulated Raman Scattering (SRS) to obtain wavelengths shown in
Table 1. Denoted as “on” (more absorbing) or “off” (less absorbing).
 Ozonesonde launches occasionally (Howard U. Beltsville, MD, possibly
UMBC)
 This system will be the first to make long term ozone profile measurements
in the Washington, DC - Baltimore area. (Daily, Weekly, Monthly, Seasonal
Trends)
 NASA has funded the ground based Tropospheric Ozone Lidar Network
(TOLNET) which currently consists of five stations across the US.
(http://www-air.larc.nasa.gov/missions/TOLNet/index.html)
References
 Bass, A., and R. Paur, UV absorption cross sections for ozone: the temperature dependence, J. Photochem, 17,141, 1981.
 Haner, D., and I. McDermid, Stimulated raman shifting of the nd:yag fourth harmonic, Quantum Electronics, IEEE Journal of, 26 (7), 1990.
 Kuang, S., J. Burris, M. Newchurch, S. Johnson, and S. Long, Differential absorption lidar to measure sub-hourly variation of tropospheric ozone profiles,
Geo-science and Remote Sensing, IEEE Transactions on, 49 (1), 557 {571, doi:10.1109/TGRS.2010.2054834, 2011.
 IPCC, Climate Change 2007 - Fourth Assessment Report of the IPCC, Climate Change 2007, Cambridge University Press, 2007.