Field measurements of atmospheric photolysis frequencies for O3, NO2, HCHO, CH3CHO, H2O2, and HONO by UV spectroradiometry

A calibrated spectroradiometer was used for the measurement of spectra of the absolute actinic flux F-lambda(lambda) during the POPCORN field campaign in Pennewitt (53.8 degrees N, 11.7 degrees E, sea level) in August 1994. The obtained set of actinic flux spectra was used to determine the photolysis frequencies J((OD)-D-1), J(NO2), J(HCHO), J(H2O2), J(HONO), and J(CH3CHO), using molecular photodissociation data from literature. The accuracy of the actinic flux measurement was about +/-5%. The accuracy of the photolysis frequency determination is limited by the uncertainties of the molecular absorption cross section and quantum yield data. A good agreement within the experimental uncertainties was found in comparison with measurements of J((OD)-D-1) and J(NO2) by filterradiometer which were calibrated absolutely against chemical actinometer. A comparison of this work's photolysis frequency measurements at 40 degrees solar zenith angle with respective measured and modeled data from the literature also shows good agreement for most of the processes considered in this work. However, in the case of J(NO2) data reported in the literature as a function of solar zenith angle differences up to a factor of 1.6 with respect to this work's J(NO2) data are observed. Since this is far beyond the estimated experimental uncertainties, other atmospheric variables, such as aerosols, seem to affect J(NO2) to an extent that is underestimated by now and make indirect comparisons of J(NO2) measurements difficult.