Comparison of observed and simulated NO2 photodissociation frequencies in a cloudless atmosphere and in continental boundary layer clouds

To validate radiative transfer models for the actinic flux and photodissociation frequencies, it is essential to compare model results with observed data. For this purpose, we used in situ vertical profile measurements of thermodynamic, aerosol particle, and cloud drop properties in order to compare simulated photodissociation frequencies of NO2, J(NO2), with respective measurements. Two case studies in cloudless and four cases in cloudy conditions were investigated. We found that the deviations between the simulated J(NO2) profiles and the measured data are less than 10% in all cases which is in the range given by the measurement uncertainties. For quantifying the sensitivity of the simulated J(NO2) to the different input parameters the impact of experimental uncertainties on the measurement of aerosol and drop microphysical parameters was investigated. In cloudless conditions the largest effect on the modeled J(NO2) is due to the uncertainty of the aerosol volume (about 2% at ground level). In cloudy atmosphere the uncertainty of the observed liquid water content (about 20% at ground level) is of largest importance. Furthermore, the influence of different quantum yield and absorption cross section spectra of NO2 was examined: The different spectra result in changes of +/-5% for the computed J(NO2). Additionally, we investigated the effect of different solution methods for the radiative transfer equation on J(NO2). The delta-four-stream method was shown to be a very fast and accurate technique with errors of less than 2%. On the other hand, the delta-two-stream method may cause relative errors of up to 10 to 20%.