National Ambient Air Quality Objectives For Particulate Matter - Executive Summary

Ambient Monitoring

Measurements of particulate matter for the purpose of current compliance monitoring are generally expressed in terms of mass. Mass measurements may be made directly or indirectly. Direct (or manual) measurements of PM concentrations in the ambient air are made by collecting particles on a pre-weighed filter over a specified period of time, weighing the soiled filter, and then dividing the gain in mass by the volume of air sampled. Samples are typically collected for a 24-hour period, once every six days, as in the National Air Pollution Surveillance (NAPS) network. Different sampling periods and frequencies may be used where required, although the fact that the filters are collected manually is a constraint on the operation of these samplers. Indirect measurements are made using parameters other than mass that can then be converted to units of mass concentration based on known relationships between the two parameters.

Historically, the standard manual PM monitor in Canada was the hi-vol sampler, deriving its name from the fact that it draws large volumes of air through the filter upon which the particulate matter is retained. Originally used for the sampling of TSP, a modification of this technology was required to accommodate monitoring of PM₁₀. A specially designed sampling inlet was added that removes particles >10 µm so that particles in the PM₁₀ size range are selectively retained on the filter. This type of sampler is therefore called a size selective inlet (SSI) hi-vol sampler.

The dichotomous sampler was the original sampler for inhalable particulates ( PM₁₀), dividing particles into two size classes, as its name suggests. The sampler operates at a comparatively low sampling rate relative to the hi-vol samplers. The sample is first drawn through a selective inlet restricted to particles -10 µm. Subsequently, particles are fractionated into a fine fraction (-2.5 µm) and a coarse fraction (>2.5-10 µm) which are collected on separate filters for measurement and analysis. Hence dichot samplers enable measurements of PM₁₀ (fine plus coarse fractions), PM_2.5 (fine fraction) and the coarse fraction. The partisolsampler is another low-volume manual sampler used to monitor PM₁₀. It can be fitted with TSP, PM₁₀, or PM_2.5 inlet heads. Other fine particle samplers include a variety of inertial impactors and cyclone samplers that are fitted with inlets designed to capture particles -2.5 µm in diameter.

The inlets of these manual PM samplers are designed with specified 50% cut points (D₅₀), which are defined as the particle aerodynamic diameter at which 50% of the particles pass through the inlet and 50% are rejected. The 50% cut points are accurate only at specified flow rates and therefore, the degree to which air flow through the sampler can be controlled is an important design feature of the instrument. Of concern with respect to PM₁₀ samplers is that the 50% cut point (at approx. 10 µm) occurs near the maximum of particle mass distributions. Therefore, even slight differences in the cut points of different instruments may result in differences in PM mass measurements. This is less of a concern with PM_2.5 monitors since the 2.5 µm cut point occurs near a minimum in the particle mass distribution.

The comparability of PM mass measurements made with different samplers is an issue of concern. Differences may arise from cut point biases, differences in maintenance regimes that in turn affect operation of the instrument and other factors. In the national network, both dichot and SSI hi-vol samplers are co-located at five sites: Saint John, Ottawa, Edmonton, Vancouver Rocky Point Park and Vancouver West 10th Ave. Based on samples collected between 1984-1993, mean ratios of [ PM ] measured with the dichot samplers to PM measured with hi-vol samplers ranged from 0.93 at sites in Edmonton and Saint John to 1.17 at the Vancouver West 10th Ave. site. Excellent correlation was found between the samplers (0.84-0.95). Relatively few inter-comparison studies have been conducted to date for PM_2.5 samplers.

A variety of analytical techniques are available for determining concentrations of inorganic and organic compounds from mass filter specimens of particulate matter. Some of these are non-destructible methodologies that leave the filter intact, enabling further chemical analyses; others are destructive of the filter. While these techniques are routinely used to help assess particle composition at sites across Canada, for the kind of detailed chemical analyses required for source apportionment studies (the attribution of PM at a site to specific sources), specialized particle sampling systems are necessary. One example of such a specialized particle sampler is the IMPROVE sampler, which has been used in intensive studies carried out at few urban and rural sites across Canada to study visual range and PM composition.

Indirect measurements of particulate matter have historically been made using two methodologies: the British Smoke Shade (BSS) sampler and the AISI sampler, which measures Coefficient of Haze (CoH). Both the BSS and the CoH techniques are based on optical properties of particles and are most sensitive to the sooty components of PM that fall in the size range of approximately -3.5-4.5 µm in diameter. The Beta Attenuation monitor, also known as the beta-gauge monitor, has been used in Europe and Japan for several years. Mass determination is based on the attenuation that a beta-ray particle undergoes as it passes through an exposed filter. The beta attenuation monitor can provide hourly PM concentrations, but is a very expensive instrument and has a number of operational constraints.

In contrast to the techniques described above, development of the Tapered Element Oscillation Microbalance (TEOM) offers the opportunity for much less labour intensive continuous measurement of PM concentrations; The TEOM sampler operates 24 hours a day and is automated. The TEOM operates on the principle that PM accumulations on a filter will result in changes to the oscillation frequency of a specially designed tube attached to the filter. Based upon the direct relationship between PM mass and oscillation frequency, the instrument's microprocessor computes the total mass accumulation on the filter, as well as the mass concentrations and mass rate, in real time. TEOM samplers can be fitted with either a PM₁₀ or PM_2.5 sampling inlet, but not both at once.

As with manual samplers, issues of comparability of measurements have arisen. Some data from co-located TEOMS and PM₁₀ hi-vols have shown good agreement between the two samplers, with correlation coefficients of 0.977 for 24 hour averages. However, other data seem to indicate that TEOM data are consistently lower than data from manual samplers, a difference that has been linked at least partially to the high sampling temperature used in TEOMS with a corresponding loss through volatilization of some volatile compounds, an issue of particular concern when sampling for PM_2.5.

Spatial Representativeness

In developing a monitoring network, selection of monitoring sites must be based on siting criteria that reflect the purpose of the data collection exercise and that will minimize undue bias in the resulting measurements. Nonetheless, individual sites are unique with respect to sensor location, surrounding structures, land use patterns, local meteorology etc. all of which will influence ambient pollution levels. As a result, it is recognized that monitoring data from fixed samplers may not portray pollution levels representative of the entire community. Furthermore, fixed monitors do not accurately reflect pollution levels that individuals may be exposed to (see section below on Human Exposure Assessment). To better characterize individual exposures, Personal Exposure Monitors (PEMS), sampling devices worn on the body, have been developed. Due to the nature of their application, however, the design of PEMS must meet a number of challenging criteria: low noise, light weight, portability, rugged design, ease of operation, adequate battery lifetime to meet sampling requirements and comparability with fixed site monitors.