Absorption cross-section measurements of water vapour and oxygen at 185 nm. Implications for the calibration of field instruments to measure OH, HO2 and RO2 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 u sed in the calibration of OH HO2, and RO2 field instruments that employ pho tolysis of water vapour as a radical source, and O-2 absorption as the acti nometer for lamp nux. For water vapour, an absorption cross-section of 7.22 (+/- 0.22) x 10(-20) cm(2) molecule(-1) was determined at 25 degrees C, in excellent agreement with recent studies, but similar to 30% higher than th e 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, rein forcing the requirement that practitioners of OH HO2 and RO2 field measurem ents who calibrate by this method should measure effective O-2 cross-sectio ns at frequent intervals under their particular field conditions.