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

• Published in: Geophysical research letters 2000-06, Vol.27 (11), p.1651-1654
• Main Authors: Creasey, David J., Heard, Dwayne E., Lee, James D.
• Format: Article
• Language: English
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/1999GL011014

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, HO2 and RO2 field instruments that employ photolysis of water vapour as a radical source, and O2 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 O2 was found to vary with the O2 column and the choice and operating current of the lamp, reinforcing the requirement that practitioners of OH, HO2 and RO2 field measurements who calibrate by this method should measure effective O2 cross‐sections at frequent intervals under their particular field conditions.