Aircraft instrument design for in-situ tropospheric OH measurements by laser-induced fluorescence at low pressures

The hydroxyl radical (OH) is important for many processes involved in tropospheric chemistry. For instance, it initiates the photochemical degradation of gases that cause global climate change, such as methane and the chlorofluorocarbon substitutes (HCFCs). Because of its reactivity, its abundances are less than 0.1 pptv. Thus, OH has been very difficult to measure accurately, despite its importance. Techniques have evolved, however, so that good measurements of tropospheric OH abundances are now possible. One of these techniques that is adaptable to aircraft measurements is the laser induced fluorescence detection of the OH radical in a detection chamber at low pressures, a technique first developed by R. J. O'Brien and T. M. Hard. Because OH is both excited and detected in the A²Σ⁺(υ′ = 0) → X²II(υ″ = 0) transition near 308 nm (approximately 10 milliwatts at 10 kHz repetition rate), significant laser generation of OH and interference signals from other trace gases are avoided. The current ground-based instrument, which can be readily adapted to aircraft, can detect OH abundances of 1.4 × 10⁵ OH molecules cm^-3 with S/N = 2 in 30 seconds, and 5 × 10⁴ cm^-3 in five minutes.