FREQUENTLY ASKED QUESTIONS
GLOSSARY
Light extinction coefficient: A measure of the amount of light attenuated by scattering or absorption as the light traverses the atmosphere, hence it is a measure of atmospheric transparency. It has units of inverse length, and is the sum of four components: light scattering by gas molecules and particles and light absorption by gas molecules and particles.
Meteorological range: The furthest distance at which a black object silhouetted against a sky would be visible assuming a 2% threshold value for distinction of an object from the background.
Particles/Aerosols: Any solid or liquid in the atmosphere with diameter between approximately 0.002 and 100 µm.
Primary particles: Particles emitted directly into the atmosphere.
PM10: Particles of diameter 10 µm. Also referred to as 'inhalable particles'.
PM2.5: Particles of diameters 2.5 µm (fine particles). Typically, dry fine (PM2.5) particle mass is dominated by; sulphates, organics, ammonium nitrate, soil, and elemental carbon.
Receptor oriented source apportionment: Techniques used to identify the sources (origins) of particles. These techniques rely on the principle of defining patterns within ambient samples of chemically speciated particulate matter to infer source types.
Secondary particles: Particles formed within the atmosphere from gaseous precursors.
Source oriented source apportionment: Relate emissions to ambient concentrations using numerical models which simulate the physical and chemical transport and transformations.
TSP: Total suspended particles. The total atmospheric aerosol burden.
Visibility: The distance at which the contrast of the target is equal to the threshold contrast value for the human eye.
Visual range: The greatest distance at which a black target can be perceived against the horizon.
Units:
µm: micro-meter (micron) (10-6 m-1)
µg m-3: micro-gram per meter cubed.
ng: nano-gram (10-9 grams).
BACKGROUND INFORMATION
What is visual air quality?
Humans see an object as a result of light reflected from the object (we actually see the
object in terms of a contrast against the background, so a black target silhouetted against a light
coloured sky is more visible than a light target against a light sky). As light from the object
traverses the atmosphere some portion of the beam is scattered away from the observers view or
absorbed by particles and gases in the atmosphere. Equally, light from other "sources" can be
scattered into the observers line of sight and therefore reduce the perceived contrast of the
viewed object. Hence it can be shown that visibility (the ability of the observer to see an object)
will decrease as the amount of particles and gases in the atmosphere increases (resulting in
decreasing atmospheric transparency). Visibility is often defined within air pollution literature as
the distance at which the contrast of a visual target against the background (usually the sky) is
equal to the threshold contrast value for the human eye.
(1) i.e. Visual Range (VR) = ln(C(0)/Cd)/bext
where: bext = light extinction coefficient which is a measure of atmospheric transparency.
C(0) = objects inherent contrast (i.e. contrast at zero distance)
Cd = threshold for detection of the objects contrast.
Note: Using this definition, the visual range is the distance at which the target is just visible.
The extinction coefficient, bext is given by;
(2) bext = bsp + bsg + bap + bag
where bsp, and bsg are the scattering coefficients by particles and gases, respectively,
and bap and bag the absorption coefficients for particles and gases, respectively.
In general, light scattering by particles is the largest of the terms in (2). Scattering of light by particles is most efficient for small particles of diameter approximately equal to the wavelength of the incident light. Given that sunlight has its maximum intensity at around 0.3 *10-6 m-1, this is the diameter of particles that most efficiently scatters light and reduces visibility.
Visibility perception is more complex than simply calculating visual range, and human perception of visibility conditions includes the effects of atmospheric constituents on the ability of an observer to see colour, texture, and form of both near and distant vistas, and the scene characteristics. Protection of visibility/visual range/Visual Air Quality (VAQ) is difficult to regulate because unlike other atmospheric pollutants, it involves physiological parameters of human perception. As Saxena and Weintraub (1990) state; "This perception involves the combination of the following: (1) the thermal structure and water vapor gradients in the atmosphere, (2) ambient concentration, chemical composition, and size distribution of particulates and gases, (3) illumination of the target and sight-path associated with cloud cover, (4) the physical characteristics of the target itself, and (5) the disposition of the observer."
What are atmospheric aerosols?
The relative abundance of particles in these size ranges; TSP, PM10 and PM2.5 is highly variable
in space and time, and has a profound influence upon the potential health and visibility impacts
of a given aerosol mass. What are typical background concentrations of atmospheric aerosols?
What was REVEAL?
How do you use the REVEAL II database? (an introduction to the basics)
How were the data in the data base quality checked?
Which other resources have been developed to aid use and interpretation of the REVEAL
II data?
What was the PM2.5 'made of' (i.e. what was the average composition of the fine aerosol)?
How much of the sulphate aerosol might be marine derived?
Which aerosol components were primarily responsible for the visibility impairment?
What are the major sources of PM2.5?
"Atmospheric particles" and "atmospheric aerosols" are terms used to describe both
solid and liquid matter suspended in the atmosphere. Elevated particle concentrations in the
atmosphere are responsible for health impacts, and degradation of visual air quality.
Atmospheric aerosols may be emitted directly into the atmosphere (primary particles), or may
form within the atmosphere from gases (secondary particles). Typically secondary aerosols have
relatively small diameters and so are efficient at scattering light, are easily inhaled into the
respiratory tract and have long atmospheric lifetimes. Three categories of atmospheric particles
are defined based on particle size diameter and effects of the particles;
Under the NAPAP research effort Trijonis (1990) developed estimates of natural
background annual average aerosol concentrations for the western United States for coarse
particles (2.5-10 µm) and fine aerosols (<2.5 µm) of 1.5 and 3 µg m-3, respectively.
REVEAL II AND THE REVEAL II DATABASE
REVEAL (Regional Visibility Experimental Assessment in the Lower Fraser Valley)
was a field campaign conducted during July and August 1993 to characterize summertime
visibility and ambient aerosol loadings in southwestern British Columbia (B.C.).
It was assumed that all
have been subject to rigorous quality control at source (confirmed by agencies supplying the
data) and so no further quality assurance was performed on this data and no QA codes have been
assigned. The data collected under the REVEAL II monitoring campaign was rigorously assessed
for quality and codes applied which indicate the level of data quality of each data point. No data
have been removed from the database, but data which are considered suspicious or dubious
quality or which lie outside the expected range of data values have been assigned the appropriate
QA codes (which are described in Pryor and Barthelmie 1996).
A World Wide Web site has been designed which summarizes the REVEAL II data,
analyses and results. This WWW site may be accessed from: http://php.indiana.edu/~spryor.
ANALYSIS AND INTERPRETATION OF DATA COLLECTED DURING REVEAL II
Inorganic aerosol species dominated measured fine mass during REVEAL II at both CLBR and
CHIL but organic aerosols are a consistently high proportion of measured mass.
Using algorithms from the IMPROVE network and ambient data from the IMPROVE samplers
deployed at Clearbrook and Chilliwack it was shown that a maximum of up to 39 and 34% of
the total S recorded at CLBR and CHIL (respectively) is of marine origin.
Reconstructed light scattering budgets indicate inorganic aerosol species dominated light
scattering at both CLBR and CHIL. Days with high light scattering were characterized by higher
nitrate and organic aerosol concentrations suggesting that these aerosol components may be the
primary cause of variability of visibility conditions.
Average particle light scattering efficiencies are higher at CLBR than CHIL. This indicates that
on a per unit mass basis aerosols at CLBR degrade visibility more efficiently than those
sampled at CHIL. This reflects differing aerosol composition and is an explanation of the
observation that meteorological range is lower at CLBR despite lower average fine mass.
Receptor modeling to identify sources of ambient aerosols indicates the following components
dominate the sources of fine mass at the two receptor sites:
CLBR: Mobile emissions, secondary aerosols, primary geologic materials, industrial emissions
and a marine contribution (listed in terms of average contribution to measured mass).
CHIL: Mobile emissions, secondary aerosols, primary diesel emissions, primary geologic
materials, a marine contribution and a local component (dominated by iron) (listed in
terms of average contribution to measured mass).
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