Measurements and modeling of photolysis rates during the Photochemical Activity and Ultraviolet Radiation (PAUR) II campaign

[1] In this paper we compare radiative transfer model calculations of the actinic flux in the UV spectral region with airborne measurements of the actinic flux, obtained during the Photochemical Activity and Ultraviolet Radiation (PAUR) campaign, which took place in the Aegean Sea, Greece, in June 1996, in order to assess the accuracy of the model in calculating photolysis rates, when the model input parameters are well defined from measurements. The model can simulate the total actinic flux (4 pi sr) in the UV-A region with an accuracy of 5% for all altitudes (0.1-12 km) in the cloud free troposphere, while in the UV-B the impact of the vertical distribution of ozone and aerosol can lead to differences, of about 5-10% at low altitudes up to 20% at higher altitudes. Next, the photolysis rates of J((OD)-D-1) and J(NO2) measured during the PAUR 2, in Crete, Greece (35.5degreesN, 23.8degreesE), in May 1999, at two altitudes (Gerani 30 m and Prases 1000 m), are compared with the respective model calculations in order to examine the effect of the alternating Sahara dust/maritime aerosol environments imposed to these photolysis rates, as well as to examine their differences due to the altitude difference. It is shown that high levels of tropospheric ozone and absorbing aerosols can cause a decrease in the photolysis rates of ozone near the surface, even under conditions of reduced total ozone content. This fact indicates that tropospheric ozone can be disproportionately important as a filter against UV-B radiation when most scattering of the radiation by air molecules and dust occurs in the troposphere. This behavior is not simulated accurately using a radiative transfer model constrained by observations of ozone and aerosol optical depths. The differences of the of the photolysis rates between the two altitudes as determined by the model and by the measurements, differ significantly for the Sahara dust event, indicating that during this event in the boundary layer, there is a mixture of desert and nonabsorbing aerosols.