MAX‐DOAS O4 measurements: A new technique to derive information on atmospheric aerosols—Principles and information content

Abstract

Multi AXis Differential Optical Absorption Spectroscopy (MAX‐DOAS) observations of the oxygen dimer O4 which can serve as a new method for the determination of atmospheric aerosol properties are presented. Like established methods, e.g., Sun radiometer and LIDAR measurements, MAX‐DOAS O4 observations determine optical properties of aerosol under atmospheric conditions (not dried). However, the novel technique has two major advantages: It utilizes differential O4 absorption structures and thus does not require absolute radiometric calibration. In addition, O4 observations using this method provide a new kind of information: since the atmospheric O4 profile depends strongly on altitude, they can yield information on the atmospheric light path distribution and in particular on the atmospheric aerosol profile. From O4 observations during clear days and from atmospheric radiative transfer modeling, we conclude that our new method is especially sensitive to the aerosol extinction close to the ground. In addition, O4 observations using this method yield information on the penetration depth of the incident direct solar radiation. O4 observations at different azimuth angles can also provide information on the aerosol scattering phase function. We found that MAX‐DOAS O4 observations are a very sensitive method: even aerosol extinction below 0.001 could be detected. In addition to the O4 absorptions we also investigated the magnitude of the Ring effect and the (relative) intensity. Both quantities yield valuable further information on atmospheric aerosols. From the simultaneous analysis of the observed O4 absorption and the measured intensity, in particular, information on the absorbing properties of the aerosols might be derived. The aerosol information derived from MAX‐DOAS observations can be used for the quantitative analysis of various trace gases also analyzed from the measured spectra.