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. In this study the consistency between MAX-DOAS measurements and radiativetransfer simulations of the atmospheric O4 absorption is investigatedon 2 mainly cloud-free days during the MAD-CAT campaign in Mainz, Germany,in summer 2013. In recent years several studies indicated that measurementsand radiative transfer simulations of the atmospheric O4 absorption canonly be brought into agreement if a so-called scaling factor (&lt;1)is applied to the measured O4 absorption. However, many studies,including those based on direct sunlight measurements, came to the oppositeconclusion, that there is no need for a scaling factor. Up to now, there isno broad consensus for an explanation of the observed discrepancies betweenmeasurements and simulations. Previous studies inferred the need for ascaling factor from the comparison of the aerosol optical depths derived fromMAX-DOAS O4 measurements with that derived from coincident sunphotometer measurements. In this study a different approach is chosen: themeasured O4 absorption at 360 nm is directly compared to the O4absorption obtained from radiative transfer simulations. The atmosphericconditions used as input for the radiative transfer simulations were takenfrom independent data sets, in particular from sun photometer and ceilometermeasurements at the measurement site. This study has three main goals: firstall relevant error sources of the spectral analysis, the radiative transfersimulations and the extraction of the input parameters used for theradiative transfer simulations are quantified. One important result obtainedfrom the analysis of synthetic spectra is that the O4 absorptionsderived from the spectral analysis agree within 1 % with the correspondingradiative transfer simulations at 360 nm. Based on the results fromsensitivity studies, recommendations for optimised settings for the spectralanalysis and radiative transfer simulations are given. Second, the measuredand simulated results are compared for 2 selected cloud-free days withsimilar aerosol optical depths but very different aerosol properties. On 18 June,measurements and simulations agree within their (rather large)uncertainties (the ratio of simulated and measured O4 absorptions isfound to be 1.01±0.16). In contrast, on 8 July measurements andsimulations significantly disagree: for the middle period of that day theratio of simulated and measured O4 absorptions is found to be 0.82±0.10,which differs significantly from unity. Thus, for that day ascaling factor is needed to bring measurements and simulations intoagreement. Third, recommendations for further intercomparison exercises arederived. One important recommendation for future studies is that aerosolprofile data should be measured at the same wavelengths as the MAX-DOASmeasurements. Also, the altitude range without profile information close tothe ground should be minimised and detailed information on the aerosoloptical and/or microphysical properties should be collected and used. The results for both days are inconsistent, and no explanation for a O4scaling factor could be derived in this study. Thus, similar but moreextended future studies should be performed, including more measurementdays and more instruments. Also, additional wavelengths should be included.

atmospheric measurement techniques

Is a scaling factor required to obtain closure between measured and modelled atmospheric O&amp;lt;sub&amp;gt;4&amp;lt;/sub&amp;gt; absorptions? An assessment of uncertainties of measurements and radiative transfer simulations for 2 selected days during the MAD-CAT campaign