Instrument Support Level 3
Aero-Laser CO Monitor
The AL5001 from Aero-Laser is a fast carbon monoxide (CO) monitor with a unique sensitivity below 1ppb (parts per billion). The detection of CO is based on a VUV-fluorimetry, employing the excitation of CO at 150nm. This method combines a high sensitivity with high selectivity and is linear from 1ppb to 100 ppm.
The AL5001 is widely used in remote research stations like in Antarctica, on research ships and aircrafts, where very low detection limits, high sensitivity, fast response, high temperature stability, low zero drift and a huge linearity range is required.
The excitation light is generated by a cw-CO2 resonance lamp, which needs only a low maintenance. A monochromator unit, using dielectric mirrors, filters out the VUV from the lamp’s spectrum. The fluorescence light in the wavelength range between 160 nm and 190 nm is detected by a VUV sensitive photomultiplier followed by a fast counter. The basic principle is described in detail by Gerbig et al. . The AL5001 calibrates within minutes, using only a low amount of calibration gas and an in-built zero gas source. The calibration procedure is fully automatic and can be scheduled in custom-set time intervals. The gas concentration is displayed in real time and is logged via a standard RS-232 interface. The instrument is rugged and designed for field campaigns in rough environment, as well as for laboratory applications.
Parameter/diagnostic information (e.g. of pressures, flows, temperatures) are logged simultaneously with the measurement data allowing fast troubleshooting of problems.
One second data from the instrument is averaged over a minute and this data is available through the British Atmospheric Data Centre (BADC) for use either as supporting data to interpret other measurements or inputted into atmospheric models over a range of timescales
 C. Gerbig, S. Schmitgen, D. Kley, A. Volz-Thomas, K. Dewey, D. Haaks, An improved fast-response vacuum-UV resonance fluorescence CO instrument, J. Geophys. Res. 104 D1 (1999) 1699
The instrument consists of a resonance CO2-lamp excited by a RF discharge, an optical filter for selection of the appropriate wavelength interval around 150 nm, and a fluorescence chamber.
The emission of the lamp is collimated within the optical filter to a parallel beam of 150 nm light, using off-axis parabola and dielectric mirrors. The VUV-light is then focused into the fluorimeter chamber, where the fluorescence from CO is viewed at right angles by a VUV-sensitive photomultiplier (PMT) with Suprasil pre-optics. The dielectric mirrors in the parallel part of the light beam, provide the spectral band path (bandwidth of ~10 nm full width at half maximum (FWHM) at approximately 150 nm).
The optical filter is continuously flushed with dry N2 (purity of 6.0 and filtered to compress organic substances) which is necessary to avoid absorption of radiation in the Schumann-Runge continuum of molecular oxygen and from impurities, in particular CO. This only requires a flow of ca. 35 ml/min N2 since the volume is very small.
The optical filter housing is made from Al covered with a black coating for reduction of stray light. The air is vented into the fluorimeter chamber, which is connected to an external vacuum pump at the end of the light trap and at the PMT flange, in order to provide a well-defined air flow. A pressure sensor and a electrically driven dosing valve stabilize the pressure and the air flow through the cell.
MACC (Monitoring Atmospheric Composition and Climate) Analysis
Near real-time hourly averaged carbon monoxide data (within 3 days) is submitted to the MACC project where it is used to evaluate the global model performance. Modelled gas concentrations are validated with the surface in-situ measurements.
In order to compare modelled and observed values, modelled concentrations of CO (and O3) at the stations’ location are interpolated linearly from the model gridded data in the horizontal domain. In the vertical, modelled gas concentrations are extracted at the model level that matches the Global Atmospheric Watch (GAW) stations’ real altitude. This is equivalent to matching the mean pressure of model level and the ground station.
The comparison of surface in-situ data with global model output comprises several issues of representativeness: In the horizontal scale, the estimation of errors is rather uncomplicated. In the vertical scale, however, representative matching of measured and modelled values remains critical. Especially for mountainous stations, the chosen model level (matching the GAW station’s real altitude) may lack topographic effects. Generally, stations in flat terrain are less critical.
For evaluation of model performance, scores for the displayed period are printed on top of each figure.
Currently, the online validation service operates with 12 GAW stations, which are delivering hourly data of tropospheric CO and O3. The data are preliminary and have not been extensively quality checked.
Calibration of CO is performed with reference to a single gas standard containing a known amount of CO. It is well understood that the instrument behaves linearly.
Calibrations are performed every 9 hours and the data is applied to the subsequent 9 hours of measurements. The data is quality assured and if step changes are observed in the concentration data (e.g. due to humidity effects) then data is recalibrated or rejected accordingly. Instrument parameters (temperatures, flows, and gas pressures) are logged alongside those of the concentrations and used to diagnose problems. Instrument sensitivity is monitored and when it drops below 15Hz/ppbV a thorough cleaning of the instrument is performed.
The carbon monoxide measurements are fully traceable to the international Global Atmospheric Watch (GAW) scale (currently NOAA/ESRL WMO-2000). This means that the gas calibration standards used to calculate the final concentrations are referenced to a common standard held at the NOAA/ESRL laboratory. To ensure this is upheld, the designated world calibration centre for Carbon Monoxide: WCC-Empa, carries out regular audits of the measurements using travelling standards.
For more information please see the report from the most recent of these audits.
Data from instruments that are part of a laboratory do not archive their data to CEDA and so do not have to conform to any AMOF standard.
Data can be provided for the period of time that the user has accessed the facility.