Results obtained with the Tropospheric Ozone DIAL System Using a YAG Laser and Raman Cells (A53Q – 0439)

J. T. Sullivan (jsull1@umbc.edu)1,2, T. J. McGee2, G. K. Sumnicht2,3

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 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.

“on”

“off”

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).

Wavelength Importance

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.

Retrieval (Elastic) 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.

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

References

There are currently in situ surface ozone measurements hourly (EPA AirNOW sites) (http://airnow.gov/)

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)

Simulation 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.

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.

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.