Absorption cross‐section measurements of water vapour and oxygen at 185 nm. Implications for the calibration of field instruments to measure OH, HO 2 and RO 2 radicals

Absorption cross sections for oxygen and water vapour were measured using a low‐pressure mercury lamp optically filtered to isolate the emission close to 184.9 nm. The cross‐sections were determined for conditions typically used in the calibration of OH, HO 2 and RO 2 field instruments that employ photolysis of water vapour as a radical source, and O 2 absorption as the actinometer for lamp flux. For water vapour, an absorption cross‐section of 7.22 (± 0.22) × 10 −20 cm² molecule −1 was determined at 25°C, in excellent agreement with recent studies, but ∼30% higher than the recommended value. The absorption cross‐section for O 2 was found to vary with the O 2 column and the choice and operating current of the lamp, reinforcing the requirement that practitioners of OH, HO 2 and RO 2 field measurements who calibrate by this method should measure effective O 2 cross‐sections at frequent intervals under their particular field conditions.