Collisional effects in laser detection of tropospheric OH

摘要

The hydroxyl radical plays a key role in the photochemistry of the troposphere, and its measurement can form a crucial test of models of that chemistry. However, the OH concentration is often sub-part-per- trillion, so its measurement demands highly sensitive and interference- free techniques. The method of laser-induced fluorescence can meet these requirements, if properly designed to avoid interference due to OH generated by laser photolytic processes. Quantitative laser detection of OH requires knowledge of collisional processes affecting the measurement and these interference effects. Collisions induce quenching of the excited A-state, and vibrational and rotational energy transfer in the excited and ground states. The state-specific nature of these processed is important, particularly the rotational level dependence of quenching and vibrational transfer in excited OH. The temperature dependence is also important: quenching cross sections increase sharply with decreasing temperature. Laboratory experiments investigating these collisional processes are described and their implications for laser detection of tropospheric OH are discussed.