Measurement-based J(NO2) sensitivity in a cloudless atmosphere under low aerosol loading and high solar zenith angle conditions

The comparison between measured and simulated photodissociation frequencies of NO2, J(NO2), in a cloudless atmosphere in a recent paper by Früh et al., 2000 (Journal of Geophysical Research 105, 9843–9857) revealed an overestimation of J(NO2) near ground level by model calculations compared with measurements and an underestimation in the upper part of the aerosol layer. A possible reason for the disagreement is the changing sun position during the vertical ascent. To resolve this problem we carried out a sensitivity study varying the solar zenith angle of 74° by 1.4° (which corresponds to the change of sun position during the vertical flight patterns). This results in a considerable deviation of J(NO2) of about 10%. Further sensitivity studies on J(NO2) have been done. These include realistic variations in ground albedo, humidity and aerosol properties. A variation in ground albedo from the measured value of AG=0.023 (292–420 nm wavelength) to AG=0 and AG=0.05, respectively, resulted in an average J(NO2) reduction and enhancement of only 2% near ground level with a slight decrease with increasing altitude. Furthermore, we compared simulations based on different relative humidity profiles with results from a dry atmosphere. Compared to the dry case the deviations of J(NO2) were considerable (5–16%) although the measured aerosol concentration was very low. Moreover, we doubled the aerosol particle concentration. The maximum J(NO2) deviations were in the same order of magnitude as for the relative humidity (5–16%). These changes are in the range of measurement uncertainty of J(NO2).