Performance modeling and validation of a chirped pulse quantum cascade laser for open-path ambient gas monitoring

The quantum cascade laser (QCL) has been successfully used to measure atmospheric trace-gas concentrations at part per billion by volume (ppbv) in portion of the mid-infrared (mid-IR) region (3-5, 8-10 mum). In this proposal, we explore the feasibility of using a time chirped pulsed QCL to simultaneously measure ozone (O3) and ammonia (NH3) which are two important gases in ground level air because of their adverse impact on human, and the role they play in climate change. Based on careful optimization of the spectral absorption features of O3 and NH 3 as well as interfering gases, we present a numerical simulation approach for a compact open-path system design that can detect O3 and NH 3 concentrations to 1% ambient concentration levels.;The Quantum Cascade Laser Open-path System (QCLOPS) we are optimizing uses a single distributed feedback (DFB) QCL operating at 9.6 mum wavelength (1045 cm-1 in wavenumber and turning range of 2 cm-1 ), which corresponds to the best absorption lines of O3, and NH3 in the presence of interfering gases H20 and CO2. Based on accurate measured chirp responses, both the spectral sampling and the resolution are derived and used to simulate the convolved spectral response for all ambient gases calculated from the monochromatic spectral responses from GENSPECT (a line-by-line molecular transition simulation code). This allows us to estimate the required SNR needed per pulse to reach the needed trace gas detection levels for ambient urban conditions using linear least square analysis.;Once the per-pulse SNR requirements are determined, an open-path system model (OPSM), which evaluates the laser-matter interactions in the atmosphere including scattering, geometric and turbulence losses as well as the details of the light source, remote target and receiver is proposed and used to determine the feasibility of measuring the ambient species. In particular, the model is used to assess the measurement duration needed to obtain specified retrieval thresholds as a function of the path length and realistic laser power and noise sources. In particular, we find that for path lengths as short as 100 meters, 1% ambient concentrations can be measured with durations < 1min while 1% ambient levels can be realistically done in < 1s at path lengths between 1km and 6km.;Due to unavailability of QCL at the optimized wavelength, the approach was applied instead to an equivalent system which was readily available. This prototype instrument, called QCLOPS-CCNY, uses a 5.3 mum QCL instead of 9.6 mum QCL. While this system is not designed for Ozone-Ammonia, it is suitable for measurements of water vapor which also provides us with a means of validation against Meteorological surface measurements as well as complementary open-path FTIR measurements. Preliminary measurements for links of L=120 m were made that validate many facets of the simulation model including the convolved trace gas absorption spectrum, inversion algorithm and the link-budget analysis including both the detector and path noises. Comparisons with both FTIR and ambient MET models show retrieval performances < 2% providing confidence that our Ozone-Ammonia estimates are reasonable.