Is a scaling factor required to obtain closure between measured and modelled atmospheric O4 absorptions? – A case study for two days during the MADCAT campaign

In this study the consistency between MAX-DOAS measurements and radiative transfer simulations of the atmospheric O4 absorption is investigated on two mainly clear days during the MAD-CAT campaign in Mainz, Germany, in Summer 2013. In recent years several studies indicated that measurements and radiative transfer simulations of the atmospheric O4 absorption can only be brought into agreement if a so-called scaling factor (4 absorption. However, many studies, in particular based on direct sun light measurements, came to the opposite conclusion, that there is no need for a scaling factor. Up to now, there is no explanation for the observed discrepancies between measurements and simulations. Previous studies infered the need for a scaling factor from the comparison of the aerosol optical depth derived from MAX-DOAS O4 measurements with that derived from coincident sun photometer measurements. In this study a different approach is chosen: the measured O4 absorption at 360 nm is directly compared to the O4 absorption obtained from radiative transfer simulations. The atmospheric conditions used as input for the radiative transfer simulations were taken from independent data sets, in particular from sun photometer and ceilometer measurements at the measurement site. The comparisons are performed for two selected clear days with similar aerosol optical depth but very different aerosol properties. For both days not only the O4 absorptions are compared, but also all relevant error sources of the spectral analysis, the radiative transfer simulations as well as the extraction of the input parameters used for the radiative transfer simulations are quantified. One important result obtained from the analysis of synthetic spectra is that the O4 absorptions derived from the spectral analysis agree within 1 % with the corresponding radiative transfer simulations. The performed tests and sensitivity studies might be useful for the analysis and interpretation of O4 MAX-DOAS measurements in future studies. Different comparison results are found for both days: On 18 June, measurements and simulations agree within their (rather large) errors (the ratio of simulated and measured O4 absorptions is found to be 1.01 ± 0.16). In contrast, on 8 July measurements and simulations significantly disagree: For the middle period of that day the ratio of simulated and measured O4 absorptions is found to be 0.71 ± 0.12, which differs significantly from unity. Thus for that day a scaling factor is needed to bring measurements and simulations into agreement. One possible reason for the comparison results on 18 June is the rather large aerosol extinction (and its large uncertainty) close to the surface, which has a large effect on the radiative transfer simulations. Besides the inconsistent comparison results for both days, also no explanation for a O4 scaling factor could be derived in this study. Thus similar, but more extended future studies should be performed, which preferably include more measurement days, more instruments and should be supported by more detailed independent aerosol measurements. Also additional wavelengths should be included. The MAX-DOAS measurements collected during the recent CINDI-2 campaign are probably well suited for that purpose.