Aircraft measurements of photolysis rate coefficients for ozone and nitrogen dioxide under cloudy conditions

Atmospheric chemistry is mainly driven by photodissociation of trace gases. Especially the photolysis of ozone, which is a major source of OI-I radica ls and the photolysis of nitrogen dioxide, which is the primary step in ozo ne production are essential quantities for atmospheric chemistry calculatio ns. As part of the EU-funded project Scientific Training and Access to Airc raft for Atmospheric Research Throughout Europe (STAAARTE) measurements of the photolysis rate coefficients for ozone and nitrogen dioxide were perfor med on board the research aircraft FALCON 20-E5 of DLR (German Aerospace Ce nter, Oberpfaffenhofen, Germany). Additional measurements of basic meteorol ogical parameters, global irradiance, trace gas concentrations and particle s were conducted. The influence of clouds on the vertical profile of the ph otolysis rate coefficients was analyzed in combination with radiative trans fer model calculations. Compared to clear-sky values the actinic flux is ge nerally lower below clouds, but shows an increase inside and an enhancement above clouds. This work presents some of the results from the flight exper iments that were performed in the summer 1998 over southern Austria. (C) 20 00 Elsevier Science Ltd. All rights reserved.