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 OH radicals and the photolysis of nitrogen dioxide, which is the primary step in ozone production are essential quantities for atmospheric chemistry calculations. As part of the EU-funded project Scientific Training and Access to Aircraft for Atmospheric Research Throughout Europe (STAAARTE) measurements of the photolysis rate coefficients for ozone and nitrogen dioxide were performed on board the research aircraft FALCON 20-E5 of DLR (German Aerospace Center, Oberpfaffenhofen, Germany). Additional measurements of basic meteorological parameters, global irradiance, trace gas concentrations and particles were conducted. The influence of clouds on the vertical profile of the photolysis rate coefficients was analyzed in combination with radiative transfer model calculations. Compared to clear-sky values the actinic flux is generally lower below clouds, but shows an increase inside and an enhancement above clouds. This work presents some of the results from the flight experiments that were performed in the summer 1998 over southern Austria.