Gas concentrations are measured using passive sampling techniques, which rely on both chemical and physical processes: a chemical reaction and laminar diffuse process, respectively. Impregnated filters are used to collect species-specific samples of SO2, HNO3, NH3, NO2 and O3, and the the rate at which a particular gas species diffuses into the sampler depends on the diffusion coefficient of that gas. The passive samplers used were developed at the Laboratoire d’Aérologie according to the DEBITS procedures and based on the work of Martin Ferm [1991, 1994]. They have been tested and validated at the IDAF stations [Al Ourabi and Lacaux, 1999; Lacaux, 1999] as well as in other tropical and subtropical regions [Ferm and Rodhe, 1997; World Meteorological Organization (WMO), 1997; Carmichael et al., 1996].
The figure below presents the typical design of an IDAF passive sampler.
Diffusive samplers have many advantages in the field: they do not require calibration, sampling tubing, electricity or on-site technicians. In addition, the samplers are small, light, re-usable, cost-efficient and soundless.
Samplers are installed at all stations, mesh-side down, on the underside of a plastic support plate to avoid direct impact from wind and splashing from precipitation. The support is attached to a pole at a height of between 1.5-2m for savannah sites and at 3m at the forest sites. The samplers are left in position for one month, until collection and replacement. All samplers are exposed in pairs so as to ensure reproducibility of results and to reduce data loss if a particular sampler suffers interference.
Technical preparation of passive samplers
The Laboratoire d’Aérologie prepares and analyses all passive samplers.
- The passive samplers are supplied ready for use in a sealed container and loaded with a filter treated for measurement of a particular gaseous species. They are sent to the station with a technical sheet, which is filled in by the field technician and includes information such as installation date, removal date, temperature during exposition, etc.
- After collection, samples are sent back to the Laboratoire d’Aérologie where they are refrigerated until analysis.
- The exposed filters are removed from the sampler and extracted in de-ionised water using a Coulter accuvette and ultrasonic bath (for 15 minutes). Samples are then analysed using ion chromatography to determine the gas concentrations.
- Thereafter, all reusable parts of the sampler (caps, body, mesh) are cleaned three times using de-ionised water (10 minutes in ultrasonic bath) and then dried on absorbent paper under a hood.
- Sampler filters (cellulose Whatman 40) are cut to fit and submitted to three washes with de-ionised water in an ultrasonic bath and a further fourth wash with methanol. After drying in a stove at 50°C, the filters are impregnated with different coating solutions for 2 hours and then dried again.
- The sampler is then reassembled under a hood and reloaded with a fresh filter, resealed, ready for use in a plastic flask and then in a plastic bag and returned via post to the site.
Calculation of gas concentrations
The concentration of the net flux of the gas through the sampler itself is calculated using Fick’s first law of diffusion [Ferm, 2001]. The average concentration, Cavg in µmol/m3, is calculated using the following equation:
where X (in µmol) is the amount of gas trapped on the filter after exposure time t (in seconds), D (in m2/s-1) is the diffusion coefficient of the gas in air, L (in m) the diffusive path length and A (in m2) the cross-sectional area of the diffusion path.
Reproducibility and detection limits
|Gas Species||Reproducibility (%)||Detection limit (ppbv)|