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The MAXDOAS instrument and measurements

All stations of the BREDOM network are equipped with Multi Axis Differential Optical Absorption Spectroscopy (MAXDOAS) instruments. These instruments are basically UV/visible spectrometers observing scattered light in different viewing directions.

Instrument

The requirements for a MAXDOAS instrument are

• spectral resolution of 0.4 - 1.5 nm depending on target species
• spectral range from 300 - 700 nm depending on target species
• low polarisation sensitivity or active azimuth tracking to avoid artefacts from the changing polarisation of scattered light
• high sensitivity for twilight measurements
• large dynamic range for measurements around noon
• high stability for long-term measurements
• regular calibration measurements for data consistency
• fully automated operation for long-term measurements
• zenith-sky and horizon viewing modes for stratospheric and tropospheric measurements

The Bremen MAXDOAS instrument consists of a grating spectrometer equipped with a cooled CCD detector and a separate telescope unit connected to the main instrument via a quartz fibre bundle. The spectrometer is temperature stabilized to avoid wavelength drifts. Although the CCD used is a 2-dimensional detector, it is operated in full vertical binning for optimal signal to noise. The quartz fibre bundle efficiently depolarizes the incoming light and also provides flexibility for instrument set-up. The telescope unit (see figure below) has two viewing ports, one to the zenith and one to the horizon. The viewing direction is selected via a motorized mirror. In the lower part of the housing, two calibration lamps (a HgCd line lamp and a white light source) are integrated for calibration measurements. To limit the field of view of the instrument, a small lens is installed in front of the quartz fibre bundle. To prevent direct sunlight from entering the telescope, additional shades are mounted on both the zenith and the horizon ports (not shown in the figure).

During operation the instrument scans sequentially several viewing directions from the horizon to the zenith, taking about 1 minute of measurements in each direction. The angle selection depends on location and the focus of the measurements and integration times increase towards twilight. At night, calibration measurements (line lamp for wavelength calibration, white light source for throughput monitoring, dark measurements for dark current characterization) are taken. As the calibration unit can be closed by a shutter, the calibration is also possible during polar day.

To optimize spectral coverage and resolution, most BREDOM stations are equipped with two spectrometers, one for the UV and one for the visible part of the spectrum. The instruments share the telescope unit through a quartz fibre bundle that splits into two ends on the spectrometer side.

Stratospheric Observations (zenith-sky DOAS)

For stratospheric observations, usually the zenith viewing direction is used. If the species of interest is known to be negligible in the troposphere, any viewing direction can be used and in fact measurements towards the bright part of the sky have been used in some studies of e.g. OClO in Antarctica to improve signal. The sensitivity of the instrument is largest at twilight as a result of the long light path in the stratosphere. This is illustrated in the figure below, where a simplified light path is shown at high and low sun.

In the real atmosphere, the situation is more complex as a result of the curvature of the atmosphere, refraction and the effects of multiple scattering. However, the basic idea remains valid, and measurements of stratospheric constituents are therefore based on comparison of data taken at dawn and dusk with a reference taken around noon. If the instrument is stable enough, a background spectrum from another day can be used to improve consistency or signal (important in high altitudes where the range of solar zenith angles (SZA) available is small during some times of the year).

An interesting aspect of the above sketch is that the light path in the lower troposphere (red) is the same for measurements at high and low sun. Thus, a constant tropospheric contribution will cancel when comparing noon and twilight measurements of the same day, making the measurements even more adequate for stratospheric research. However, if the tropospheric concentrations vary over the day or if the tropospheric light path changes e.g. due to clouds, the stratospheric measurements can be affected.

While sensitivity is highest at twilight, this time of measurement is not always desirable for other reasons. Many atmospheric species of interest undergo rapid photochemistry, and concentrations can change strongly during twilight making interpretation difficult. On the other hand, in combination with model calculations, such changes can provide additional information on the chemistry of the absorber. For satellite validation, the quantity needed is the column at time of overpass, and this is often not at twilight. For such cases, the measurements must either be interpolated (possibly using a chemical model) or a reduced sensitivity of the measurement at higher sun has to be accepted.

Tropospheric Observations (MAXDOAS)

For tropospheric observations, the MAXDOAS instruments use the horizon viewing measurement directions. As illustrated in the figure below, the light path through the upper atmosphere (dark blue, above the scattering point) does not depend on viewing direction, while in the lowest atmospheric layers, the light path (dark red) increases as the viewing direction approaches the horizon.

The length of the light path in the lowest layers depends on geometry (the elevation angle used) but also on the mean free path of the photon. In the sketch, the scattering point is above the surface layer and the light path is determined by geometry only. If scattering probability increases e.g. at higher aerosol loading or for measurements in the UV (more Rayleigh scattering), the last scattering point is closer to the instrument and the light path for the lowest viewing directions reduces. In the extreme case (fog, snow fall), there is no difference in where the instrument is pointing. An important boundary condition for the interpretation of the measurements is the assumption, that horizontal gradients can be neglected. In real measurements, this is not always the case and results can become ambiguous with respect to whether a signal variation results from vertical or from horizontal changes.

As the vertical sensitivity is a function of elevation angle, the combination of all measurements can be used to retrieve vertical profiles of absorber concentrations. The vertical resolution of such profiles depends on SZA, aerosol loading and surface albedo but is in the order of 3 - 5 layers for the lower troposphere. For such an inversion, a good estimate of the aerosol optical depth and vertical distribution is needed. This can be retrieved from measurements of species with well known vertical distribution such as O₂ or O₄, and in fact aerosol optical depth is another output of the inversion algorithm. If data are taken not only for different elevation angles, but also at various azimuth angles, information on the aerosol phase function and thus aerosol composition can also be obtained.

For satellite validation, the tropospheric column is often the quantity of interest. This can be determined either by integrating the retrieved profile or - more quickly - by using a viewing direction of 30° or 45° elevation where the last scattering point is almost always above the layer with high concentrations. 

web master:  folkard@iup.physik.uni-bremen.de      Last Update: 22-06-11