MAX-DOAS spectra from the Southern Ocean made using an Envimes instrument aboard the Aurora Australis

Brief description

Spectra: one binary file per spectrum. Spectra can be processed using DOASIS or QDOAS software. Spectrum files are saved in folders numbered by date.
Daily log files: for spectra (extra geometric information as well as latitude, longitude, solar zenith angle) and temperature (instrument, internal and external temperature measurements).
Accelerometer: One ascii file per day with pitch, roll and yaw euler angles as the columns
Images: taken by a small camera, co-directional with the MAX-DOAS, for context of broad light conditions (i.e. checking sunny/cloudy weather)
Calibration files: Binary and text files for dark current, offset, slit function shape and wavelength calibrations

Lineage

Progress Code: completed

Statement: The MAX-DOAS technique relies on precise knowledge of the viewing geometry. This is an inherent challenge of MAX-DOAS measurements on a moving vessel. The Envimes MAX-DOAS instrument used in this work has in-built elevation angle compensation firmware which for slowly varying pitch/roll of the ship works well, however in rough seas this is not always the case.
Recommendation for data analysis: The spectral log files contain ‘start elevation angle’ and ‘end elevation angle’ fields. This can be used to calculate a difference whose absolute value should not be greater than 0.2 for confidence in the the measured viewing geometry.

Notes

Purpose
This dataset comprises chiefly of solar scattered light spectra (level zero product) collected using a MAX-DOAS instrument. The MAX (Mult-Axis) refers to the fact that spectra are collected in a ‘set’ of elevation angle scans (-3, -2, -1, 0, 1, 2, 3, 5, 10, 20, 40, 90 deg). The DOAS (differential optical absorption spectroscopy) refers to the analysis method required for analysis of the scattered light spectra. Briefly, analysis of these spectra relies on fitting absorption cross sections of trace gases of interest to determine the amount of trace gas integrated along the scattered light path, for each elevation angle in each set, using an appropriate fitting algorithm. This gives the slant column density for each trace gas at each elevation angle (level one product). The differential slant column density (dSCD) is determined by taking the ratio of low elevation angle scans with zenith scans, producing tropospheric specific information. Typically some atmospheric corrections are required to be included in the analysis including a polynomial, which accounts for broadband absorption and scattering processes, and corrections for the filling in of solar Fraunhofer lines (the Ring effect). By inverting measured dSCDs, and dSCDS modelled using a radiative transfer model, vertical profile information for trace gases can be calculated using MAX-DOAS observations. Finally, since knowing the elevation angle precisely is crucial to retrieval of vertical profile information from MAX-DOAS measurements, this dataset also includes Euler angles measured using a co-located accelerometer which may, if necessary, be used to correct MAX-DOAS elevation angles for the pitch and roll of the ship in the Southern Ocean. MAX-DOAS instrument specifications ================================ Two ultra-low straylight 75mm Avantes spectrometers: UV: 295 – 450nm, 100 μm m slit (from fibre), 0.6nm resolution, Hamamatsu backthinned detector optimized for UV, Schott BG3 filter. Visible: 430 – 565nm, 100 μ m slit (from fibre), 0.6nm resolution, Sony 2048L detector Typical instrumental stray light less than 0.05% , RMS of 1 · 10−4 (vis) and 2 · 10−4 (uv) for 1000 scans around noon. Spectrometers temperature stabilized at a fixed temperature (20 C) with a deviation of less than 0.05 C Elevation angle accuracy less than 0.1 Telescope field of view (opening angle) less than 0.3