A laser-induced fluorescence instrument for detecting tropospheric OH and HO²: Characteristics and calibration

Measurement capability for the detection of atmospheric OH and HO₂ has been developed at the Pennsylvania State University over the last decade. The instrument is used in two forms: an aircraft configuration, Airborne Tropospheric Hydrogen Oxides Sensor (ATHOS), and the configuration used on towers, Ground-based Tropospheric Hydrogen Oxides Sensor (GTHOS). The instrument uses ultraviolet laser induced fluorescence (LIF) to detect OH in air that is pulled by a vacuum pump through a small inlet into a low-pressure detection chamber; HO₂ is detected by reacting it with NO to form OH, which is detected by LIF in a second detection chamber. In the calibration, equal amounts of OH and HO₂ ranging from 0.15 pptv to 100 pptv are produced via photolysis of water vapor by the 185 nm emission from a low-pressure Hg lamp. Estimated absolute uncertainty at the 2σ confidence level is ±32% for both OH and HO₂. The dependence of the instrument detection sensitivity has been quantified for changes in ambient water vapor, pressure, laser power, and the flow velocity of ambient air past the inlet. During the last 7 years, the instrument has been deployed in multi-investigator intensive field studies 5 times on the NASA DC-8 aircraft and 8 times on ground-based towers. The descriptions in this manuscript detail our cumulative wisdom of the instrumental response and calibration techniques developed over this time.