Precision of the radiochemical OH measurement method

Twenty-eight radiochemical ¹⁴C tracer measurement of tropospheric hydroxyl radical (OH) concentrations were obtained at a rural site near Washington State University, Pullman, WA (117°W, 47°N). Diurnal OH concentration variations were observed for the five days between 9 August and 14 August 1990. These data made it possible to estimate the midday precision and detection limit of the radiochemical OH measurement method. Experiments performed at a peak O₃ photolysis rate J (O(¹D)) of (3.0±0.2) × 10⁻⁵ s⁻¹ yielded a mean midday OH concentration of (5.6±0.1 (1σ)) × 10⁶ cm⁻³. Other data put an upper bound of 16% on fluctuations of instrument sensitivity. Low-light or nighttime background OH concentrations were less than (2.6±2) × 10⁵ cm⁻³. A lower detection limit of 10⁵ cm⁻³ was obtained when extra care was taken with the low-level ¹⁴C counting procedure.