The AC2 group develops and uses several instruments to make measurements of atmospheric trace gas species . On this page is detailed a brief description of these instruments.
DOAS - Differential Optical Absorption Spectroscopy
Noxon [1975] was the first to analyse light measured in the field with the differential optical absorption spectroscopy (DOAS) method, here of scattered sunlight, although the name was given to this technique only later by Platt et al. [1979]. Due to the numerous processes a photon can undergo in the atmosphere (e.g scattering), the absolute light intensity of a recorded spectrum cannot easily be simulated. The lack of this information is overcome by investigating only the narrowband absorption features to quantify concentrations of trace gases. This is expressed by the term 'differential' in DOAS. In general, this technique can be applied to a variety of target species, light sources and platforms as well as observation geometries: Occultation measurements of the sun, moon and stars. But also artificial light sources can be used in the field in the so-called long-path DOAS technique (see below) or for cavity-enhanced measurements where the light path is folded several times with the help of a set of highly reflective mirrors. This technique can as well be applied in laboratory studies. Scattered sun-light can be observed from the ground, from aircrafts, and from satellite platforms. With balloon-borne instruments, usually occultation measurements are performed. Target species include OH, HO2, HONO, HCHO, CHOCHO, O3, NO2, NO3, ClO, OClO, IO, OIO, I2, and BrO. In practice, concentrations can be retrieved from the measured spectra using a modified Lambert-Beer's law. For that, the absorption cross sections off all relevant absorbers in a certain wavelength interval are simultaneously fitted to the measured optical density after a polynomial has been subtracted to account for broad-band features.
We run two different DOAS systems for field measurements: a long-path DOAS and a multi-axis DOAS. Both systems have been used in several campaigns [e.g. Plane and Smith, 1995; Saiz-Lopez and Plane, 2004; Allan et al., 2001] which will be further described below. In general, our group focusses on the chemistry of halogen species in the marine boundary layer and it's effects on ozone.
The DOAS instruments at AC2 group and their deployment in future field campaigns are part of a collaboration with Prof. John Plane, University of Leeds.
LP-DOAS
The long-path differential optical absorption spectroscopy (LP-DOAS) instrument is an active measurement system that uses a high pressure Xenon arc lamp as light source. This lamp is placed at the focal point of a Newtonian telescope that acts as transmitter and receiver unit at the same time. The collimated light beam is directed towards a corner cube array reflector (accuracy < 5 arc seconds) which is placed several kilometres away from the source. The reflected beam is then focused onto an optic quartz fibre bundle which feeds the light into a Czerny Turner spectrometer with a focal length of 0.5 m. The refraction grating disperses the light onto the chip of a cooled charge-coupled device (CCD) detector. The spectrometer is temperature-stabilised to avoid wavelength drifts with changes of the ambient temperature. The spectral resolution of this system obtained from the full-width half-maximum of a representative line is 0.3 nm.
MAX-DOAS
The multi-axis DOAS is an advancement to the classical zenith-sky scattered-sunlight spectroscopy method [e.g Solomon et al., 1987; Perliski and Solomon, 1993]. Whereas zenith measurements are most sensitive towards stratospheric absorbers, pointing the telescope to the horizon significantly enhances the light path in the lower troposphere while not effecting the stratospheric path. This is illustrated in the following figure. The absorption by a certain molecule along the light path can be converted into a slant column density by spectral de-convolution as described above.
The Leeds MAX-DOAS system utilises an optical fibre bundle mounted to a stepper motor. This enables us to flexibly scan the horizon under different elevation angles. A lens is placed in front of the glass fibre to limit the field of view to 1 degree. The same spectrometer/detector system is used here as for the LP-DOAS. By combining the slant column densities from several lines of sight including the zenith, the concentration of an absorber in the boundary layer can be obtained by simulating the light path through the atmosphere with a radiative transfer model taking into account multiple scattering and the correct treatment of the aerosol load. For the interpretation of the measurements, a programme has been developed to retrieve profiles. The radiative transfer model SCIATRAN, e.g. Rozonov et al. [2005], in combination with a retrieval based on the method of optimal estimation is used for that [Rodgers, 2000].
Resonance and Off-resonance Fluorescence by Lamp Excitation
The detection of gas phase atomic species in the atmosphere requires in situ techniques with an excellent detection limit (pptv level) and a compact, portable and robust set-up. The techniques based on atomic resonance fluorescence excited by microwave or radiofrequency discharge lamps offer all these advantages, as compared e.g. to the more sensitive but heavier, more complex laser induced fluorescence approaches. Discharge lamps have been already utilized in the past for the detection of stratospheric halogen species [Anderson, 1977; Anderson, et al., 1978; Anderson, et al., 1980; Brune and Anderson, 1986; Brune, et al., 1989]. However, the potential of the lamp based techniques in the boundary layer has not been fully explored [Avallone, et al., 2003; Bale, et al., 2008], as a result of the technical challenges posed by collisional quenching, background signal and statistical noise caused by scattered light and the requirement of a accurate calibration of the fluorescence signal.
At AC2 we have initiated the construction of a new line of machines based on atomic resonance fluorescence detection of halogen atoms. The first prototype [Gómez Martín et al., 2010] is devoted to the detection of atomic iodine. Our concept adds the capability of measuring simultaneously off-resonance fluorescence from molecular iodine, I2. The core of the instrument is an expansion chamber (<50 Torr) where atoms and molecules are excited by a radiofrequency discharge lamp built at the IAPS Latvia [Gross et al. 2000] and florescence is collected at right angles by high sensitivity counting photon modules (Perkin Elmer). Full automation and housekeeping are provided by an IGI-Systems interface. Calibration of the molecular signal is achieved by using an absorption technique (see below), whereas the atomic signal is calibrated by the photolysis of known amounts of molecular iodine. Detection limits are about 5 pptv for integration times of 3 minutes.
IBBCEAS - Incoherent Broad Band Cavity Enhanced Absorption Spectroscopy
The IBBCEAS technique has created great expectations in the atmospheric measurement community in the last few years due to its exceptional sensitivity and the simplicity of its design as compared to white cell-type absorption set-ups [Vaughan, et al., 2008; Washenfelder, et al., 2008]. However, its use in the field has similar drawbacks than other optical cavity-based techniques [Bitter, et al., 2005; Wada, et al., 2007].
At AC2 we have built an IBBCEAS device comprising a 1.6m long optical cavity, a broad band 100 W Xe lamp, a grating spectrometer and a CCD camera. We use regularly this set-up in the lab for calibration of the I2 fluorescence signal detected by ROFLEX. However we plan facing the technical challenges and further develop the IBBCEAS set-up into a independent field detector in the short future. With the current combination of filters and mirrors [Vaughan, et al., 2008] we would add to our suite of field instruments the capability of measuring in situ iodine dioxide (OIO). Small variations of the set up would similarly allow us measuring other trace species like IO and NO3.