Ammonia is one of the most important gas-phase precursors of ammonium aerosols, which are harmful to the public health. The regions most polluted by ammonium salt particles typically have strong local ammonia sources along with other sources of particles. A continuing need exists for field research into several aspects of ammonia in the environment, including: 1) accurate determination of emission rates from actual major sources like large feedlots; 2) rate of deposition to the surface near the source; and 3) vertical profiles of ammonia and temperature and their impact on particle formation. A coherent CO₂ lidar using the differential absorption (DIAL) method can provide unique and valuable information in such studies of ammonia.

Past research (Zhao et al. 2002) demonstrated good accuracy in measurement of integrated line-of-sight concentrations and fluxes of a plume from a point source released from an ammonia tank. Ammonia only weakly absorbed the laser radiation at the 10.228 µm-wavelength [ 10R(14) line of ¹²C¹⁶O₂ ], so only high concentrations of ammonia could be detected. This laser line is suitable only for measurement of ammonia close to strong sources. We attempted to operate with a more strongly absorbing line at 9.220 µm [ 9R(30) line of ¹²C¹⁶O₂ ] identified in Zhao (2000) to enable ambient ammonia measurements. Unfortunately, small laser gain and greater system losses severely limited the pulse energy, and we were unsuccessful at this more sensitive wavelength.

We have reexamined the potential for ammonia DIAL measurements with CO2 lidar with improved technology and found it promising. Rare isotopes of CO2 were evaluated and a line found with good DIAL characteristics not far from the peak of the laser gain curve at 9.219 µm [ 9R(24) line ] for the ¹⁶O¹²C¹⁷O isotope. Improvements in laser technology have also widened the range of possibilities. We have already operated this lidar with another rare isotope (¹²C¹⁸O₂) for Doppler measurements, which is encouraging for DIAL measurements with a rare isotope. Alternatively, a different type of CO₂ laser with higher pulse rate, shorter pulses, and higher average power is commercially available, factors which would improve the DIAL accuracy and spatial resolution. This paper will describe the expected performance for ammonia measurements from these systems. It will also summarize their capability for simultaneous Doppler wind observations which enable measurement of ammonia fluxes and determination of emission rates with a single remote sensing instrument. The performance parameters will be placed in context of the research scenarios mentioned above to illustrate the extent and quality of data that appear feasible.

References:

Zhao, Y., 2000: Line-pair selections for remote sensing of atmospheric ammonia by use of a coherent CO2 differential absorption lidar system. Appl. Opt., 39, 997-1007.

Zhao, Y., W. A. Brewer, W. L. Eberhard and R. J. Alvarez, 2002: Lidar measurement of ammonia concentrations and fluxes in a plume from a point source. J. Atmos. Oceanic Technol., 19, 1928-1938.