ENVIRONMENTAL EFFECTS OF PARTICULATE MATTER

Reduction of fine PM levels quantifiably improves visibility and reduces ozone levels, but the effects on levels of acid deposition are unclear.. The mounting anecdotal evidence of its h

S ECTION 5: E NVIRONMENTAL E FFECTS OF P ARTICULATE

This section with the environmental effects of particulate matter We begin by recapitulatingsome basic information regarding particulate matter composition and then define the scope of this review The main environmental effects of PM are expected to be those from acid deposi- tion, impaired visibility, and ozone Reduction of fine PM levels quantifiably improves

visibility and reduces ozone levels, but the effects on levels of acid deposition are unclear Paradoxically, the effects of acid deposition are the most well-studied In addition, the effects

of acid deposition are widespread, affecting urban area, rural landscapes and natural

ecosystems, while problems associated with visibility and ozone are more likely to be

localized The difficulty in assessing

the economic costs and benefits to the environment of reducing particulate matter concentrations lies in the uncertainty of dose-response pathways as well as the difficulty of assigning economic value to ecological systems, services and processes

pollution

threat to human health The effect of air pollution on ecology, however, was not yet considered

a serious issue Having been first documented in England at the end of the 19th century, acid rain and its ecological effects became regional issues in northwestern Europe and in the

northeastern United States in the late 1960s The mounting anecdotal evidence of its harmful

effects on aquatic and terrestrial ecosystems launched acid rain as perhaps the first air

pollution threat to the environment to the environment to receive international attention

Before we continue, we would like to define two basic terms In the literature, there is oftensome confusion over the use and meaning of environmental versus ecological effects

Generally, environment refers to all aspects of one’s surroundings, both living and non-living Ecological effects are strictly limited to living organisms and the interactions between them In

this section, we use environment to include urban landscapes as well as agricultural, rural and

natural areas Our discussion of ecological effects refers only to “natural areas”, e.g

non-urban, non- agricultural systems

As reviewed in Section 1, there are two major categories of particulate matter (PM), PM10 and

PM2.5 Particulate matter is composed of a mixture of particles directly emitted into the air and

particles formed in the air from the chemical transformation of gaseous pollutants (secondaryparticles) The principle types of directly emitted particles are soil-related particles and organic and elemental carbon particles from the combustion of fossil fuels and biomass materials Sec-

ondary particles are primarily ammonium sulfate and nitrate formed in the air from gaseous

emissions of sulfur dioxide (SO2) and oxides of nitrogen (NOx) reacting with ammonia (NH3) (EPA, 1997a) In eastern urban areas, almost 50% of ambient PM2.5 levels can be composed

of sulfate, and another 10 to 15% of nitrate Soil-related particles make up only 5% of ambient PM2.5 levels, with combustion-related particles making up the remaining 35% This is

markedly different from the composition of fine particulate matter (PM2.5) in other areas of the continental

US, where most PM2.5 comes from combustion-related sources The composition of PM10 (coarse particulate matter) in eastern urban areas differs from the composition of PM2.5

especially in the proportion of soil-related particles (31% of ambient measurements), with correspondingly lower proportions of PM10 from combustion-related activities (26%) and sulfate (34%)

Like most environmental problems, the effects of particulate matter are complex In addition to

the effects of the principle components (the already-mentioned directly emitted and secondary compounds) themselves, some compounds react with other particles to form reaction products with important effects One of these important reaction products is

of this section to its effects in natural ecosystems, focussing on New Jersey’s most sensitive and unique ecosystems, freshwater, estuarine and coastal waterways, and the Pine Barrens

5.2 Acid Deposition

The fine particulate matter and acid deposition issues are closely linked in the eastern UnitedStates Both share the same dominance of large sulfate fraction in the chemical composition of collected Samples Sulfates are a significant component of fine particles in the East and have been directly linked to ecosystem damage Both direct emissions of particulate matter and secon- dary particle formation caused by oxidation of sulfur dioxide, nitrogen dioxide and aerosol or- ganic carbon species contribute to overall levels of airborne particles In urban environments this greatly affects many construction materials and paints, while its effects in agricultural and natu- ral ecosystems range from reduced productivity to disruption of nutrient cycles to the massive summer fish kills of Chesapeake Bay and Long Island Sound observed through the last two dec- ades

5.2.1 Urban environment: materials

The deposition of airborne particles on the surface of building materials and culturally

important

articles can cause damage and soiling, thus reducing the life usefulness and aesthetic appeal of

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such structures (National Research Council, 1979) Furthermore, the presence of particles on sur-

faces may also exacerbate the physical and chemical degradation of materials that normally oc- cur when these materials are exposed to factors such as sun, wind, temperature fluctuations and moisture Beyond these effects, particles, whether suspended in the atmosphere, or already de- posited on a surface, also adsorb or absorb acidic gases from other pollutants like sulfur dioxide (SO2) and nitrogen dioxide (NO2), thus serving as nucleation sites for these gases The

deposition of “acidified” particles on a susceptible material surface is capable of accelerating chemical deg- radation of the material Therefore, concerns about effects of particles on

materials are related both to impacts on aesthetic appeal and physical damage to material

surfaces, both of which may have serious economic consequences Insufficient data are

available regarding perceptions thresholds with respect to pollutant concentration, particle size, and chemical composition to de- termine the relative roles these factors play in contributing to materials damage

This section briefly discusses the effects of particle exposure on the aesthetic appeal and physical

damage to different types of building materials: metals, paints, stone and cement, and then sum- marizes these effects and their possible economic relevance For more detailed discussion

of the effects of acid gases on materials, see the 1991 National Acid Precipitation Assessment

Program report (Baedecker et al., 1991).

Available information supports the fact that exposure to acid-forming aerosols promotes the cor-

rosion of metals beyond the corrosion rates expected from exposure to natural environmental elements (see Box H) Metals undergo corrosion in the absence of pollutant exposure through

a series of physical chemical and biological interactions involving moisture, temperature, oxygen and various types of biological agents In addition to these environmental factors, atmospheric pollutant exposure may accelerate the corrosion process The rate of corrosion is dependent on deposition rate and the nature of the pollutant, the variability in electrochemical reactions, the amount of moisture present, the presence and concentration of other surface electrolytes and the orientation of the metal surface

Acid-forming aerosols have been found to limit the life expectancy of paints by causing

pollution exposure indicates that particles can result in increased cleaning frequency of the exposed sur- face, and may reduce the life usefulness of the material soiled Data on the effects

of particulate matter on other surfaces are not as well understood

Several types of economic losses result from materials damage and soiling Financial or of-

out-pocket losses include the reduction in service life of a material, decreased utility, substitution

of a more expensive material, losses due to an inferior substitute, protection of susceptible materi- als, and additional required maintenance, including cleaning Economic losses from pollutant

exposure can be estimated using a damage function approach or using direct economic

methods

It is, however, difficult to estimate fully the financial losses because reliable information is not available on many economically important materials Another major problem is the inability to separate pollutant effects from natural weathering processes Attempts have been made to quan- tify the pollutants exposure levels at which materials damage and soiling have been perceived However, to date, insufficient data are available to advance our knowledge

regarding perception thresholds with respect to pollutant concentrations, particle size and chemical composition

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B OX H: T HE EFFECT OF ACID DEPOSITION ON MATERIALS

Iron, steel and steel alloys The rate of corrosion is related to the amount of SO2 in the atmosphere The rate of

corrosion

was also found to depend on the deposition rate of SO2 A separate study (Butlin et al., 1992a) showed that corrosion of steel samples under natural meteorological conditions was highly correlated with long-term SO2 concentrations and only minimally related to nitrogen oxides Stainless steels, incorporating chromium, molybdenum and nickel, are highly corro- sion resistant because of the protective properties of the oxide corrosive film.

Aluminum Aluminum is generally considered corrosion-resistant.

Copper and copper patinas A study in the New York area on the chemical composition of patinas concluded that

long-term corrosion of copper was not controlled by deposition of pollutants, but rather, it was likely controlled by

availability of copper to react with deposited pollutants (Graedel et al., 1987) The patina, that is mostly basic sulfate, is

not readily dissolved by acids and thus provides significant protection for the substrate metal However, this patina can take as long

as 5 years to form and varies with meteorological conditions In bronze, as with many metals, dry deposition between rain events was concluded to dominate soluble corrosion For copper and copper alloys, though, if the patina color has aes- thetic value, and SO2 accelerates the formation, then the presence of SO2 may be beneficial.

The effects of dust alone Only limited information is available on the effects of particles alone on metals Barton

(1958)

found that dust contributed to the early stages of metal corrosion The effect of dust was lessened as the rust layer was formed Other studies report that particles, by forming nuclei for the concentration of active ionic species, increase the corrosion rate of SO2 A laboratory study of the synergistic effects of different types of particles and SOx on the corrosion of aluminum, iron and zinc showed that four most aggressive species of particles were salt and salt/sand from marine and de-iced locations, ash from iron smelters, ash from municipal incinerators and coal mine dusts.

Paints Paints, opaque film coatings, are by far the dominant class of manmade materials exposed to air pollutants in

both

indoor and outdoor environments Paints are used as decorative coverings and protective coatings against environmental elements on a variety of finished including woods, metals, cement and asphalt Paints primarily consist of two compo- nents: the film forming component and the pigments Paints undergo natural weathering processes from exposure to envi- ronmental factors such as sunlight, moisture, fungi and varying temperatures Evidence exists that pollutants affect the durability of paint (National Resources Council, 1979) Paint films permeable to water are also susceptible to

vinyl- acrylic house paint, and vinyl and acrylic coatings for metals (Spence et al., 1975) SO2 and relative humidity markedly affect the erosion of oil-based house paint The presence of NO2 increased the weight of the paint film Blisters formed on acrylic latex house paint at high SO2 levels The vinyl and acrylic coating are resistant to SO2 In addition, the weathering of wood prior to painting decreases paint adhesion.

Automobile finishes Reports indicate that particles can cause damage to automobile finishes The formulation of

the

paint will affect the paint’s durability under exposure to varying environmental factors and pollution However, failure of the paint system results in the need for more frequent repainting and additional cost The New York area, which includes northeastern New Jersey, with California, has the highest costs associated with paint soiling.

Stone and cement In general, stone containing lime is especially susceptible to the effects of acid deposition There

can

be facilitating effects between different constituents of particulate matter For instance, marble in cemeteries in the Los Angeles basin showed that SO2 is more reactive with the calcium in marble under high NO2 conditions.

Soiling An additional, significant and detrimental effect of particle pollution is the soiling of man-made surfaces The

black crust found in the protected areas of buildings is formed from a hard crust of gypsum mixed with dust, aerosols and carbonaceous particles Increased frequency of cleaning, washing or repainting over soiled surfaces becomes an economic burden and can reduce the life usefulness of the material soiled A study of repainting frequency and particulate concen-

tration found that houses in Steubenville, OH, where PM concentrations average 235 µ m per year, needed to be

repainted every year Fairfax, VA, on the other hand, had a mean annual PM concentration of 60 µ m/m3; the time between re- painting was generally 4 years.

5.2.2 Acid deposition and natural ecosystems in New Jersey

New Jersey consists of 19% wetlands and 42% forest area Its five physiological regions

amphibians Because of its latitude, the state has an extraordinary blend of northern and

southern animal and plant species that reach their limit of the ranges here Because New Jersey

is the fourth smallest and

most densely populated state in the nation, many environmental problems surface and need to

be solved here first (Kane et al, 1992).

Today, air pollution is a major global environmental problem In Europe about half the air pollu-

tion crosses borders and kills fish, trees and corrodes buildings and monuments Governments

in Europe have responded to this problem by collaborating within the Long-Range

Transboundary Air Pollution (LRTAP) Convention, as well as by taking measures within the European Commu- nity and at the national level (Levy, 1993) Central to the work plan is the idea to use critical loads as the basis of LRTAP protocols in order to manage transborder pollutants A critical load is defined as “a quantitative estimate of exposure to one or more pollutants below which signifi-

cant harmful effects on specified sensitive elements of the environment do not occur according

to the present knowledge” Critical loads focus negotiators’ attention on scientific issues Usingdose-response data on soils, vegetation and freshwaters, national focal centers are

preparing critical load maps for sulfur nitrogen and total acidity Critical loads vary a great deal across

Europe because ecological sensitivity is highly dependent on geology and weather conditions

Therefore in order to determine the critical load, each country focuses on its most sensitive eco- system We apply this idea to the ecosystems of New Jersey and focus on what we think

are the most sensitive ecosystems in the state: aquatic ecosystems and the Pine Barrens

The effects of deposition on sensitive receptors can range from days to centuries Ecosystems are

complex systems that are simultaneously responding to a variety of inputs, such as climate, land- use patterns and other pollutants besides sulfur and nitrogen oxides These multiple stressors can result in chemical changes within the ecosystem, which can exhibit long lag times before mani- festing a response Therefore, in many effect areas, responses to current

reductions will not be expected for many years Monitoring the changes in the effects areas over time will be necessary to determine whether the expected benefits are realized

5.2.3 Aquatic ecosystems

The following section describes the basic chemical principles behind how acid deposition duces the observed effects in aquatic systems Due to the different chemical properties of fresh and salt water, the major observed effects of acid deposition in freshwater and saltwater

pro-systems are fairly different and are treated under separate subheadings

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Figure 5-1 Physiographic Regions of New Jersey

If there is a change in the concentration of an anion contributed by acid deposition (e.g., sulfate

or nitrate), then other anions and/or cation concentrations must also change, such that the total concentration of anions always equals the total concentration of cations in the surface water

This is called the principle of electroneutrality, whereby positive and negative ions are found in

equal amounts Thus, if acid deposition causes sulfate and nitrate concentrations to increase, then some or all of the following changes will also occur:

• Bicarbonate anion decreases, which causes a reduction in acid neutralizing

capacity

• Base cations (calcium, magnesium, sodium, potassium) increase, which prevent or

minimize acidification of drainage waters, but may deplete soil reserves and

affect forest growth

• Hydrogen cation increases (decrease pH), which can adversely affect aquatic

biota

• Aluminum cation increases which can negatively affect aquatic

biota

If a reduction in acid deposition causes sulfate and nitrate concentrations to decrease, the

changes opposite to those enumerated above will also occur An increase in the concentration

of base cations in drainage waters as a consequence of acid deposition has both positive and nega- tive connotations Removal of base cations from soils to balance sulfate or nitrate from acid deposition minimizes the extent of surface water acidification Over time, however, base cation reserves in the soils can become depleted if they are lost from the soils faster than they are sup- plied from atmospheric inputs and weathering This can delay acidification recovery (NAPAP, 1998)

Although it is too early to detect specific changes in aquatic systems from emission reductionsunder Title IV, significant progress has been made since 1990 in refining understanding of the acidification process and quantifying dose-response relationships This information improves model forecasts of anticipated change in aquatic systems due to reduced emissions Particular areas include naturally occurring organic acidity, the depletion of base cation reserves from soils, nitrogen dynamics in forest and alpine ecosystems, interactions between acid deposition and land use, and the role of aluminum in fisheries response

Rivers, streams and lakes

The concentration of sulfate in surface waters has decreased in many lakes and streams over the

past 10-20 years This decrease has been caused by reductions in emissions and subsequent de- creases in atmospheric deposition of sulfur on a regional basis in the United States during that period In parts of the northeastern United States, approximate reductions of 15% in sulfate con- centrations of lakes and streams have been measured in recent years (NAPAP, 1998) Sulfate concentrations are expected to continue to decline in the Northeast Exactly how that will affect surface water acidity and biological recovery is uncertain and will require continued monitoring On the other hand, lakes in New England do appear to show statistically

significant recovery in acid-neutralizing capability as a result of sulfate reductions However, the majority of Adiron- dack lakes have remained fairly constant, while the sensitive

Adirondack lakes continue to acid- ify (Bulger et al., 1998).

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Concurrent changes in the concentrations of other chemical parameters have been generally lessclear and less consistent than for sulfate and base cations There other parameters are more strongly influenced by factors other than atmospheric deposition Concentrations of key

chemical parameters often vary per season by more than can be accounted for by acid

deposition Seasonal variability is particularly problematic in determining long-term trends Continued monthly monitoring of different types of lakes and streams located in sensitive regions will provide much needed data on seasonal variability

Adverse effects on fish populations and communities of chronically acidified streams comesfrom Shenandoah National Park (Virginia) Fish species richness, population density,

condition factor, age distribution, size and bioassay survival were all reduced in streams with low acid- neutralizing capacity, as compared to those with intermediate and high acid

neutralizing capa- bilities (Bulger et al., 1998).

A study of 13 streams in the Adirondack and Catskill Mountains showed long-term adverse epi-

sodic effects on fish populations Streams with suitable chemistry during low flow, but low pH and high aluminum levels during high flow, had substantially lower numbers and biomass of brook trout than were found in nonacidic streams Streams having acidic episodes showed sig- nificant fish mortality A study of coastal plain streams indicated that larval mortality of river herring due to episodic acidification may be substantial during wet years, which exhibit more frequent and severe episodes Episodic acidification may also be relevant to certain kinds of lakes, depending on the magnitude and duration of the spring snowmelt period Rainbow trout are sensitive to acidification not because of acidity itself, but because of elevated aluminum con- centrations due to low pH levels (lower than 5.0) Aluminum accumulates on gills and disrupts gill ion transport and respiratory function (NAPAP, 1998)

Estuaries and near-coastal waters

It is now obvious that ammonium and nitrate deposition are central concerns to the health ofcoastal ecosystems Although these species are major contributors to acid deposition, their main environmental consequence is eutrophication of coastal waters The problem is not just deposi-

tion to the water bodies themselves, but the transport of airborne nitrogen species through sur- rounding watersheds, streams, ground water into the water bodies that become overenriched with

nutrients Depending on the water body in question, atmospheric deposition is likely to

account for as much as 30-40% of the total nutrient loading received

Nitrogen is the limiting nutrient for the growth of algae in many estuaries and near-coastal tems, rather than phosphorus, which typically limits algal growth in freshwater systems

sys-Chesa- peake Bay is the nation’s largest estuarine system, with a watershed of almost 64,000 square miles, encompassing one-sixth of the Eastern Seaboard The Bay has an important fish and shell- fish industry and serves as a nursery for marine commercial and sport fish There has been con- siderable research and monitoring on the effects of nitrogen and phosphorus loading

to Chesa- peake Bay New Jersey’s southern shore borders Delaware Bay, which has

experienced many problems similar to those of the Chesapeake Changes in atmospheric

nitrogen deposition can have significant impacts on aquatic biology Excess nitrogen entering the Bay produces algal blooms that block sunlight needed for submerged aquatic grasses, and the decomposition of ex- cess algae depletes life-sustaining oxygen needed by invertebrates inhabiting bottom waters The best estimates of atmospheric nitrogen loads to Chesapeake Bay and other estuaries along the

Atlantic and gulf Coasts range from 10% to 45% of the total nitrogen inputs from all

sources

Additional research is needed to quantify the current effects and the expected benefits from re- ducing nitrogen deposition on estuary systems

5.2.4 New Jersey Pine Barrens

The Pine Barrens of New Jersey are unique and of global interest The Pine Barrens region, orPinelands, is a large, mostly forested region that harbors many unique plant and animal

species Ecologically, it consists of some 2000 to 2250 square miles of generally flat, sandy, acidic, and sterile soils which constitute a major part of the Outer Coastal Plain section of the Atlantic Coastal Plain in New Jersey (Boyd, 1991) For a detailed description of this

ecosystem see For- man (1979) In addition to agriculture, some of the principle uses of the Pine Barrens are recrea- tional: camping, canoeing, hiking and hunting Another important use

is valuable scientific re- search

The possible impacts of acid deposition on the vegetation of the Pine Barrens, and upon plantand animal life in its streams are currently under study by the Division of Pinelands Research at Rutgers University The nature and source of acid rain is changing the type of acidity in Pine Barrens streams from organic to mineral This is shown by increased levels of sulfate in the wa- ter and decreased concentrations of dissolved organic matter The acidity characteristic of soils

in the Pine Barrens is created when decaying vegetation produces an organic acid that washes down through and is absorbed by sandy soils These organic acids leach out aluminum found in Pine- land soils, making it harmless in Pine Barren waters However, the sulfuric acids in acid rain do not bond with aluminum, so this mineral is washed, in its pure form, into streams (Morgan, 1984) It is concluded that the pollution caused by these increased sulfates, nitrates, and alumi- num may be toxic to aquatic life, but long-term effects of this pollution are

unknown at this time (Section 5.4.3.1 discusses the toxic effects of aluminum on fish in

freshwater streams) Acid deposition has also been shown to increase levels of mercury and nitrate in pine needles and

other vegetation Animals eating this vegetation will tend to concentrate those toxic compounds

in their body tissues, possibly increasing mortality While the physiological pathways for in- creased plant adsorption of these toxins are well-known, the mechanism by which they

accumu- late in animals and the effects on morbidity and mortality have not been rigorously demonstrated and await further scientific evidence

Significant impacts of acid deposition on forest health have not been detected in the southernpine and pine-hardwood region, to which the New Jersey Pine Barrens are similar However, acid deposition is a major contributor to the depletion of base cations in many poorly buffered soils supporting southern pines and will, most likely, over the long term (decades), impede pro- ductivity Short-term positive effects on growth are expected for some nitrogen-deficient soils, while negative effects are expected to be limited to the most acidic, base-depleted soils

A synthesis of studies that originated as part of a NAPAP to evaluate the sensitivity of

ambient levels

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acid rainfall However, longer-term exposures are expected to have cumulative negative effects

on soil nutrition While these impacts will most likely have negative long-term consequences, the inputs of atmospheric sources of nitrogen to many soils with low nitrogen reserves should have small cumulative positive effects on productivity for as long as decades (NAPAP, 1998)

5.2.5 Trends in acid deposition

Title IV of the 1990 Clean Air Act Amendments (CAAA) requires the reduction of acid rain pre-

cursors—namely, emissions of sulfur dioxide (SO2) and nitrogen oxides (NOx) from electric utilities Emissions of SO2 in the United States have decreased significantly since the

enactment of Title IV of the Clean Air Act, from 26 million tons per year in 1980 to18 million tons in 1995 Most, if not all, of those reductions in SO2 emissions have come from reductions

in the emis- sions of Phase I utilities, especially electric utilities Seasonal trends in

fine-particulate sulfur show increases for the summer months and slight decreases during the winter

in Great Smokey Mountains and Shenandoah National Parks from 1982 to 1994 Total Sulfur Ambient Air Con- centrations from 1989 to 1995 have decreased an average of 30% (NAPAP, 1998) While annual mean pH of precipitation in New Jersey continues to be very low (4.3), the total annual deposi- tion of sulfate ions (kg/ha) is below the threshold considered to be

detrimental to natural ecosys- tems (20 kg/ha)

Analysis of the 1995 wet deposition monitoring data demonstrates that the 1995 reduction inSO2 emissions in the Midwestern and northeastern United States resulted in a substantial reduc- tion of the acidity and sulfate concentration of precipitation in those regions Unlike sulfate and hydrogen ions, nitrate concentrations in 1995 were greater than estimated

concentrations at most of the sites in the eastern and western regions of the country This is not unexpected, since im- plementation of Title IV NOx reductions only began in January 1996

In 1995, dry deposition rates of sulfur at State College, Pennsylvania, decreased to their lowestrates since 1986 Deposition rates of nitrogen have slowly increased Current capabilities for un- derstanding the processes controlling dry deposition are still exploratory Work conducted

in the United States on dry deposition of SO2 indicates that the historic evaluation of dry deposition of gaseous sulfur may underestimate actual rates, perhaps as much as 15-20%, depending on the site

5.3 Visibility

The National Research Council’s Committee on Haze in National Parks and Wilderness Areas

said, “Visibility is the degree to which the atmosphere is transparent to visible light.”

Section 169A of the 1977 Clean Air Act (CAA) Amendments (42 U.S.C 7491) and the 1979 Report to Congress (U.S Environmental Protection Agency, 1979) define visibility

impairment as a re- duction in visual range and atmospheric discoloration Equating visibility

to the visual range is consistent with historical visibility measurements at airports, where human observers recorded the greatest distance at which one of a number of pre-selected targets could be perceived

Visibility may also be defined as the clarity (transparency) and color fidelity of the

atmosphere

Transparency can be quantified by the contrast transmittance of the atmosphere This

definition of visibility is consistent with both (1) the historical records based on human

observation of the perceptibility of targets, which include both the longest duration and most widespread records now available, and (2) the definition of visibility recommended by the National Research Coun- cil

5.3.1 Air pollution and visibility

Air pollution can also alter the colors of the atmosphere and the perceived colors of objectsviewed through the atmosphere A complete quantification of visibility should include a measure of the color changes caused by the atmosphere Such measures have been included in plume visibility models, but there is no consensus on the best parameter to quantify color changes caused by air pollution from many sources

The perception of color depends on illumination and setting For example, when there is a liant sunset, a white picket fence will appear to be white, but will be distinctly yellow in a photo- graph A nitrogen dioxide-containing plume appears to be yellow against a blue sky even when a photograph or spectral measurement shows that the plume is blue, but less blue than the sur- rounding sky The eye correctly perceives that a yellow gas is present in the plume These prop- erties of human vision have been explored elsewhere (EPA, 1997d)

bril-One way to measure the effect of particulate matter on visibility is to measure the light

scattering

efficiency of particles The higher the light scattering efficiency, the less light from any given object reaches an observer’s eyes, decreasing visibility The light-scattering efficiency differs considerably for fine and coarse particles, ranging from 2.4 to 3.1 m2/g for fine particles and 0.2 to 0.4 m2/g for coarse particles (EPA, 1997f) Larger light-scattering efficiencies for fine parti- cles have been observed when significant numbers of the particles are in the 0.5 to 1.0

µm size range The great majority of light absorption by particles is caused by elemental carbon Deter- minations of the mass-specific light-absorption emission of elemental carbon give values in the range of 9 to 10 m2/g Great reductions in visibility occur when water condenses to form fog or clouds Water is also present in all ambient particles, even on

relatively clear days The increase in the amount of water in particle phase that occurs at high relative humidity (RH) has a signifi- cant effect on visibility Light-scattering efficiency of fine particles also increase with high RH, and ammonium sulfate particles are an aerosol component that contributes to the absorption of

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