OH Field Measurements: A Critical Input into Model Calculations on Atmospheric Chemistry

As part of a European program, a field campaign was carried out in May 1994 at the Izana Station, Tenerife, Canary Islands, to study the influence of European emissions on the oxidizing capacity of a clean tropospheric environment. This paper contains daytime measurements covering the OH chemistry. The OH measurements are taken with the Multipass Optical Absorption Spectroscopy (MOAS) instrument, which is based on the differential absorption technique. The main parameters controlling the OH concentration were monitored. Calculations from a box model are used to compare its results with the measurements. The comparison shows good agreement in the OH daily variations. The measured OH in absolute terms, however, always has lower values than the calculated ones, indicating that an overall OH sink is missing. This sink term is composed of a continuous background, most probably caused by biogenic emissions and temporal variations from pollutional events superimposed. The photolysis of formaldehyde contributes about 10% to the total OH production, demonstrating its importance in the chemistry of the atmosphere.