National Ambient Air Quality Objectives For Particulate Matter - Executive Summary

Aesthetic Effects

Increases in the amount of fine particulate and gases in the atmosphere are associated with reductions in the ability of the human eye to see through the atmosphere or identify an object at a distance. Decreased visual range in parks and wilderness areas has become a concern as a result of the potential losses of tourism. The public may also consider visibility to be an indicator of overall air quality. Hence reduced visual range is perceived to indicate poor air quality, and consequently, quality of life in general may be negatively impacted.

Visual range (or conversely, light extinction) is a function of the light scattering and light absorption properties of gas molecules and particles. Fine particles ( PM_2.5), and submicron particles in particular in the size range of 0.3-0.7 µm, are most effective in reducing visibility. Reduced visibility is usually more a function of light scattering than light absorption, except where there are particles (e.g. elemental carbon, otherwise known as "soot") or gases (e.g. NO₂) present in the atmosphere that are particularly effective at absorbing light.

It has been determined that there is a linear relationship between b_scat, a measure of light scattering, and fine particle mass, with ²/g. ^bscat/fine mass = 3.1 m b_scat can also be related mathematically to visual range. Thus, visual range may be determined directly from fine particle mass if these values are known, or as a function of PM₁₀ mass concentrations, if the ratios of PM_2.5/ PM₁₀ are known. Coarse particles can also disrupt visibility but are typically one-half to one-third as effective as fine particles. In some areas, where coarse particle concentrations are unusually high, they may make significant contributions to reduced visibility.

Particle light extinction is modified by relative humidity. When relative humidity exceeds 70%, light scattering efficiencies begin to increase, as a function of increased water uptake by the particles. As humidity increases to 95% or more, water soluble components of the fine aerosol can swell to seven times their dry radius, dramatically increasing scattering efficiency. The effect of humidity is also very dependent on chemical and microphysical variables. Components of fine particles, such as sulphates (ammonium sulphates), nitrates (ammonium nitrate), organics, elemental carbon and soil dust, will vary in their ability to absorb water. The hygroscopic (water attracting) fraction of the aerosol will affect visibility to a greater extent in regions of Canada with higher relative humidity.

A number of studies have examined the relative contributions of different chemical species to visibility reductions. Typically, sulphates are the largest contributor to visibility reduction. Nitrates are also important to visibility reduction but are less efficient per unit mass than sulphates in light scattering. Even where fine mass has been shown to be dominated by a secondary organic component, sulphate and ammonium nitrate dominated light scattering (e.g. Pacific '93 field study in the Fraser Valley). Particle-phase organic compounds are relatively inefficient contributors to light scattering. Elemental (or black) carbon contributes to light absorption, as previously mentioned, and to a small extent, light scattering.

There is relatively little information on natural visual ranges in Canada. Estimates of visual range from three sites in different regions of Canada relatively unimpacted by anthropogenic PM were obtained using nephelometer measurements of light scattering (b_scat), and mathematical relationships between b and visual range (b_scat/b_ext = 0.9; VR = 3.91/b_ext) with the followingresults: Waterton, Alberta - 210-350 km; Egbert, Ontario - 86-120 km; St. Andrews, N.B. - 185-210 km). PM_2.5 levels were then calculated using the relationship ²/g (see above) to yield the b_scat/ PM_2.5 = 3.1 m following estimates of background PM_2.5: Waterton - 3.2-5.5 µg/m³; Egbert - 9.7-13.0 µg/m³; St Andrews -5.5-6.1 µg/m³. Southeastern Canada (SE Ontario) clearly has lower visibility and higher background PM_2.5 levels than do either western or eastern Canada. A 10% increase in PM_2.5 from background is used to define the level above which effects on visual range are noticeable. Thus in western and eastern Canada, using the upper ranges for background PM_2.5, impacts on visual range 3 would be observed when PM_2.5 levels exceed 6-7 µg/m and for southeastern Canada, 14 µg/m³.

Visual range at urban sites can be estimated from known concentrations of PM_2.5 and PM₁₀ and from the mathematical relationships between PM_2.5 and visual range (see above). On this basis, visual range at the NAPS urban sites has been estimated to range from 34 km at a site in Montréal to 73 km in St. John, New Brunswick. In general, urban sites in Ontario and Québec have lower estimated visual ranges (approx. 35-50 km) than do urban sites in either eastern or western Canada (approx. 45-70 km.). Likewise, visual range at the NAPS rural sites can be estimated, with the following results: Kejimkujik VR = 136 km; Sutton VR = 132 km; Egbert VR = 93 km. These values are based on average 24 hour PM_2.5 (or PM₁₀) levels, and thus represent average visibilities. In fact, visual ranges have been shown to vary with the season, as would be expected with changing PM concentrations and humidity levels.

In addition to the regional differences in visibility, there are regional differences in the public's response to changes in air quality. Differences in public perception of unacceptable visibility may be related to the nature of environments and vistas viewed, as the public may be less willing to accept degradation of a wilderness area than an urban environment. In a public perception study in Denver, Co. (U.S.), acceptable visibility as measured by visual range was approximately 50 km. Acceptable visibilities or acceptable changes in visibility in different regions of Canada still need to be determined.

As noted previously, a noticeable change in visibility is expected for a 10% change in fine particulate levels. Therefore, where fine particulate loadings are lower (e.g. rural areas), there will be a larger change in visual range for incremental increases in PM_2.5 than in (urban) areas with higher particle loadings. Given that mean PM_2.5 levels at NAPS sites across Canada range from about 10-20 µg/m³ , a 10% change in PM_2.5 levels (a noticeable change in visibility), corresponds to a 1-2 µg/m³ increase in PM levels. From observed relationships between PM_2.5 and PM₁₀ levels at Canadian rural and urban sites, a noticeable change in visibility would be expected to occur for a 1-2 µg/m³ increase in PM₁₀ levels at rural sites and a 2-5 µg/m³ increase in PM₁₀ at urban sites.

Recognizing the paucity and variability of data on natural background PM_2.5 levels and that it is not possible to define a single background concentration of PM_2.5 for all of Canada, it is recommended that no Reference Level for PM_2.5 based on visibility be identified at this time. However, it is emphasized that incremental impacts on visual range can be expected to occur when ambient PM_2.5 concentrations rise by 10% over existing concentrations. Given that natural visual range estimates for different regions of Canada are in the range of 86 to 350 km (see previous section), and that these visibility measurements give estimated natural levels of PM_2.5 from 5.5 to 13 µg/m³ (upper estimates), (with western Canada and eastern Canada having natural levels of ³, and southeastern Canada PM_2.5 around 5.5 to 6 µg/m³), it is having higher PM_2.5 levels of around 13 µg/m conservatively concluded that the level above which effects may be demonstrated on visual range as a function of PM_2.5 loadings within Canada is 6 to 14 µg/m Any additional particle loading above these levels may be considered to reduce visual range. Since it is recognized that fine particles, in particular those in the 0.3-0.7µm size range, are responsible for reduced visibility, no Reference Level for PM₁₀ is identified.