Quantitative comparison of measured and simulated O4 absorption for one day with extremely low aerosol load over the tropical Atlantic

Measurements of the atmospheric absorption of the oxygen dimer O₄ are often used to characterize the atmospheric light paths, e.g. to derive properties of clouds and aerosols. Some recent studies indicated discrepancies between measurements and simulations of the atmospheric O₄ absorption, while others found exact quantitative agreement. One difficulty in these studies was to correctly represent the aerosol properties in the radiative transfer simulations, e.g. due to lack of information about the vertical profiles or scattering properties.

In this study we investigate MAX-DOAS measurements of the atmospheric O₄ absorption during a ship cruise in April and May 2019 over the tropical Atlantic. The elevation angle of the instruments telescope is automatically stabilized in order to compensate the motion of the sea. We select measurements on one day (2 May 2019) with extremely low aerosol optical depth (between about 0.03 and 0.05 at 360 nm). For such conditions the atmospheric scattering processes are dominated by Rayleigh scattering on air molecules.

Besides the MAX-DOAS measurements, also measurements by a ceilometer and sun photometer are available, which are used to constrain the atmospheric aerosol properties. The radiative transfer simulations are carried out with the full spherical radiative transfer model MCARTIM.