Abstract
Abstract. In this study the consistency between MAX-DOAS measurements and radiative transfer simulations of the atmospheric O4 absorption is investigated on 2 mainly cloud-free 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 (<1) is applied to the measured O4 absorption. However, many studies, including those based on direct sunlight measurements, came to the opposite conclusion, that there is no need for a scaling factor. Up to now, there is no broad consensus for an explanation of the observed discrepancies between measurements and simulations. Previous studies inferred the need for a scaling factor from the comparison of the aerosol optical depths 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. This study has three main goals: first all relevant error sources of the spectral analysis, the radiative transfer simulations and 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 at 360 nm. Based on the results from sensitivity studies, recommendations for optimised settings for the spectral analysis and radiative transfer simulations are given. Second, the measured and simulated results are compared for 2 selected cloud-free days with similar 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 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 0.82±0.10, which differs significantly from unity. Thus, for that day a scaling factor is needed to bring measurements and simulations into agreement. Third, recommendations for further intercomparison exercises are derived. One important recommendation for future studies is that aerosol profile data should be measured at the same wavelengths as the MAX-DOAS measurements. Also, the altitude range without profile information close to the ground should be minimised and detailed information on the aerosol optical and/or microphysical properties should be collected and used. The results for both days are inconsistent, and no explanation for a O4 scaling factor could be derived in this study. Thus, similar but more extended future studies should be performed, including more measurement days and more instruments. Also, additional wavelengths should be included.
Highlights
Observations of the atmospheric absorption of the oxygen collision complex (O2)2 are often used to derive information about atmospheric light paths from remote-sensing measurements of scattered sunlight
In addition to air mass factors (AMFs) and differential AMFs (dAMFs), synthetic spectra were simulated. They are analysed in the same way as the measured spectra, which allows the investigation of two important aspects: 1. The derived O4 dAMFs from the synthetic spectra can be compared to the O4 dAMFs obtained directly from the radiative simulations at one wavelength using the same settings
For most of the day the ratio is very close to unity, indicating that for solar zenith angle (SZA) < 75◦ the O4 (d)AMFs obtained from the spectral analysis are almost identical to the O4 directly obtained from the radiative transfer simulations
Summary
Observations of the atmospheric absorption of the oxygen collision complex (O2) (in the following referred to as O4; see Greenblatt et al, 1990) are often used to derive information about atmospheric light paths from remote-sensing measurements of scattered sunlight (for example made from ground, satellite, balloon or airplane). About 10 years ago, Wagner et al (2009) suggested applying a scaling factor (SF < 1) to the O4 SCDs derived from MAX-DOAS measurements at 360 nm in Milan in order to achieve agreement with forward model simulations. We follow a different approach: to Ortega et al (2016) we directly compare the measured O4 SCDs with the corresponding SCDs derived with a forward model (consisting of a radiative transfer model and assumptions of the state of the atmosphere). For this comparison, atmospheric conditions which are well characterised by independent measurements are chosen.
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have