Multi Component Vertical Gradient Measurements by WRD and SJAC Sampling as in GRAEGOR and GRAHAM
Rene P. Otjes
Air Quality and Climate Change,
ECN (Energy Research Centre of the Netherlands),
Wet Rotating Denuders (WRD) and Steam Jet Aerosol Collectors (SJAC) have been used for over a decade now in on-line measurement of water-soluble gases and aerosols. By applying multiple sampling units it became possible to perform vertical gradient measurements. This method is used in the GRAHAM, an analyzer for ammonia. The GRAHAM consists of three WRD’s combined with a gas diffusion conductivity cell. In laboratory studies the relative standard deviation for the three channels was 0.34% when exposed to an ammonia calibration gas of 8 microgram/m3. The detection limit is 0.02 microgram/m3.
A recent development is the GRAEGOR. Apart from ammonia this monitor measures nitric acid, hydrochloric acid, sulfur dioxide and the aerosol-components sulfate, nitrate, chloride, and ammonium. The GRAEGOR uses two sampling units. Each WRD is followed by a SJAC for collection and analysis of the aerosols. There is an option to also analyze the cations with cation-chromatography. The average relative standard deviation is 2% for the components mentioned, at a typical concentration of 1 ug m-3. The detection limits were similar to that of the GRAHAM, viz., 0.01-0.03 ug m-3.
The instruments have been successfully deployed for gas flux measurements. For aerosols, which usually have a 10 times lower deposition velocity, gradients were harder to resolve. Gradients by GRAEGOR have been detected at a fertilized field fetch showing both heterogeneous particle gas-phase reactions as well as particle deposition. GRAHAM detected gradients above a non-fertilized grassland. A flux precision of better than 1 ng/m2/s was established.