High Sensitivity HNO3 Monitor using Continuous Wave Quantum Cascade Laser IR Absorption

Gas phase nitric acid (HNO3) is an important aerosol precursor that is coupled to the removal of natural and anthropogenic NOx from the atmosphere. HNO3 is highly soluble and is extremely effective at increasing the cloud droplet number density, and has been implicated in the reduction of average cloud droplet size which increases cloud albedo. It can also decrease precipitation and increase the effective cloud coverage. Direct measurements of HNO3 that are accurate, interference free, sensitive and rapid are needed to support particulate nitrate aerosol studies. The proposed instrument development will produce a robust approach for measuring HNO3 using newly developed infrared quantum cascade lasers. This will provide a major advancement over current commercial instrument capabilities. This technology will yield a sensitive, selective measurement of this important aerosol precursor in measurement campaigns and at remote clean air monitoring stations. A prototype instrument using state of art quantum cascade lasers was used for simultaneous detection of both HNO3 and NH3 at the targeted sensitivity of 30 ppt (parts-per- trillion) in one second and averaging to less than 10 ppt in 100s. The inlet effects for sampling and particle separation were assessed, and the design improvements for Phase II were initiated. The results are extremely promising and are among the highest precision yet achieved for HNO3. Design improvements in optics and electronics will lead to the production of a more robust and sensitive instrument. Improved design of the gas phase inlet will minimize surface reactivity and improve gas-particle separation. The resulting instrument will be applied to both laboratory and field measurements in conjunction with an aerosol mass spectrometer (AMS) to evaluate particle formation mechanisms from aerosol precursors. Commercial Applications and Other Benefits: The proposed instrument will provide the capability to measure atmospheric abundances of HNO3 in real time in order to better assess nitrogen oxide chemistry resulting in tropospheric ozone production and subsequent aerosol and cloud modifications. Improved understanding of aerosol and cloud properties will improve our understanding of global climate change and help evaluate mitigation strategies. There is a worldwide market for such measurement systems in the environmental and aerosol research communities.