Is a scaling factor required to obtain closure between measured and modelled atmospheric O4 absorptions? An assessment of uncertainties of measurements and radiative transfer simulations for 2 selected days during the MAD-CAT campaign
Authors: Wagner Thomas , Beirle Steffen , Benavent Nuria , Bösch Tim , Chan Ka Lok , Donner Sebastian , Dörner Steffen , Fayt Caroline , Frieß Udo , García-Nieto David , Gielen Clio , González-Bartolome David , Gomez Laura , Hendrick François , Henzing Bas , Jin Jun Li , Lampel Johannes , Ma Jianzhong , Mies Kornelia , Navarro Mónica , Peters Enno , Pinardi Gaia , Puentedura Olga , Puķīte Janis , Remmers Julia , Richter Andreas , Saiz-Lopez Alfonso , Shaiganfar Reza , Sihler Holger , Van Roozendael Michel , Wang Yang , Yela Margarita 
Published: May 2019

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.