Development of a versatile portable MAX-DOAS instrument

Multi-AXis Differential Optical Absorption Spectroscopy has become a versatile and mature measurement technique for determining various components of the Earth’s atmosphere, e.g. O₃, SO₂, NO₂ properties. Since the concentration of these trace gases might strongly vary locally and in time, easy in-situ measurements with a mobile device are highly desirable.

We are currently developing a portable MAX-DOAS instrument setup consisting of three small telescopes with a diameter of 50mm. Each of these telescopes is equipped with an individual fiber-fed low-resolution spectrograph (Stellarnet Blue Wave devices, 2048 pixel CCD) to enable simultaneous measurements ranging from 300 to 1000nm in one shot. The entire wavelength range is therefore covered by three spectral arms: (a) The UV arm, equipped with a Stellarnet BLUE-Wave UV2 spectrograph ranging from 300 to 500nm; (b) the VIS arm consisting of a NIR4 device (500…700nm), and (c) The NIR arm, based on a Stellarnet NIR2 ranging from 600 to 1000nm. All spectrographs are fed with wavelength-optimised fibers and equipped with the smallest possible slit (14 µm slit width) to maximise the throughput and the spectral resolving power (λ-dispersion UVB + VIS: 0.2nm; NIR: 0.4nm).

The three telescopes are aligned in parallel and installed on a small astronomical azimuthal mount (Skywatcher AZ-EQ5, powered by a mobile 12V Lithium-Polymer battery) to enable measurements in all directions. The mount control software will be based either on the ASCOM or the INDILIB platform. For the control of the spectrographs we use the programme SpectraWiz (by Stellarnet). As DOAS analysis software we have chosen QDOAS, provided by the Royal Belgian Institute for Space Aeronomy. All software is freely available and is installed on a Dell Latitude 5450 Rugged laptop, which is optimised for outdoor applications.

The chosen setup enables a mobile usage easily transportable by a small car. Since the development is currently ongoing, especially with respect to the automation of the measurements and the data processing, we report on the status of the project in this presentation.