Until June 1998, 24-hour aerosol samples were taken every Wednesday and Saturday from 0:01 to 23:59 local time. On June 22, 1998, the sampling schedule was changed to one-in-six-days in order to allign the GAViM sampling with other Canadian air-quality sampling programs like NAPS (Canadian National Air Pollution Surveillance) or IADN (Integrated Atmospheric Deposition Network). Samples are now collected from 10 AM Local Standard Time to 10 AM following day.
Air particulates are collected on Gelman Teflo (TM) filters 25 mm in diameter with 3 um pores in a sampler built to IMPROVE Module A specifications. The sampler has a particle size cut of 2.5 micron at 22.9 L/min. The particulate load collected on our remote sites ranges from several tens to several hundreds of ug per sample. The pictures on right compare a blank and an exposed Teflo TM filter with approximately 300 micrograms of fine air particulates.
Gravimetric Mass Analysis
Measured variable: PM2.5
A computer controlled Mettler MT5 microbalance is used to weigh the Teflon filters (total mass around 40mg) with 1ug precision and about 2ug reproducibility. The average particulate loading is around 200ug.
Laser Integrating Plate Method (LIPM)
Measured variable: atmospheric absorption coefficient on particles babs and, in turn, concentration of light-absorbing (elemental) carbon
Method: The LIPM measures the ratio of the transmittance of a filter before sampling to that after sampling. Although determination of the absorption coefficient of the collected aerosol is straightforward and precise (within a few percent), subsequent calculations of particle babs in the atmosphere are difficult. Shadowing effect, which increases with heavier particulate loading, is a major source of error. Once the babs is known, the amount of soot carbon can be estimated.
Experimental setup: An aerosol filter sandwiched between a collimator and a diffuser is illuminated by a laser beam. The opal diffuser integrates the transmitted light over a wide angle so that the scattering properties of particles are less important. Light transmitted through the filter is then detected by a sample photodiode. Simultaneously, the reference photodiode monitors the primary laser beam and an electronic system compensates for beam intensity discrepancies.
Particle Induced X-Ray Emission (PIXE)
Measured variables: air concentrations of elements with atomic number from Z=11 to 83 (sodium to bismuth)
Method: X-ray emission occurs when an electron is removed from one of the inner electron shells of an atom (during collision with a projectile) and the vacancy is subsequently filled by another electron falling from a higher shell. The characteristic x-ray photons are then collected. A typical PIXE spectrum of an aerosol target is shown below. The minimum detection limit of PIXE analysis of environmental samples is in the range of ng/m3, depending on the trace element and sample composition.
The new Guelph target chamber simultaneously utilizes two x- ray detectors, one for light elements (sodium to chlorine) and another for heavy elements (potassium to bismuth). This setup increases sensitivity for elements with low atomic number (sodium, silicon) and provides a crosscheck on the analysis. The chamber allows fully automatic analysis of aerosol filters mounted in slide frames.
Proton Elastic Scattering Analysis (PESA)
Measured variable: hydrogen, which is a tracer of organic matter
Method: Projectiles scattered on target atoms into the forward angles are detected by a particle detector. Energy of these scattered projectiles depends on the atomic number of target atoms. Because blank Teflon filters contain virtually no hydrogen, all counts in the hydrogen peak in the forward-scattered spectrum then correspond to hydrogen atoms in the collected aerosol. PESA is employed simultaneously with PIXE. PESA sensitivity for hydrogen is in the ng/m3 range.
More detailed description of the analytical facilities can be found in (GWP) 2 M.Sc. thesis by Zdenek Nejedly. A limited description, concerned mainly with aspects of Proton Induced X-ray Emission analysis has been published in Nucl. Instr. & Meth.