Multiscale Impact of Fuel Consumption on Air Quality - Energy & Fuels

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Multiscale Impact of Fuel Consumption on Air Quality G. M. Hidy Envair/Aerochem, Placitas, New Mexico 87043 Received July 17, 2001. Revised Manuscript Received October 1, 2001

Energy production from combustion of fossil fuels tends to dominate the emissions of criteria pollutants. Emissions derive both from large stationary sources with tall stacks such as fossilfueled power plants, and from the ground level use of fuels in transportation. Management of these sources presents a challenge in the light of multi-scale processes that influence ambient concentration and exposure patterns. Directly emitted pollutants and those resulting from atmospheric chemistry, like O3 and sulfate, nitrate and some organic material in fine particles, are affected by phenomena extending over a range of less than a meter to 107 meters in spatial scale, and minutes to many years in temporal scale. Their environmental effects have an analogous wide range of descriptive spatial and temporal scales. Pollution phenomena can be thought of in terms of three major groupings: neighborhood-urban, regional, and continental-global. Currently, decision-makers are developing emission reduction strategies that conceptually integrate considerations over this entire range of scales. In keeping with conceptual integration, recent studies and analyses are bridging different spatial and temporal scales in observations and in mathematical descriptions. Some examples of contemporary issues falling within different scales are described that illustrate approaches to add insight for developing regulatory strategies. A key element in the technical approaches is the application of air quality and exposure modeling using spatially nested descriptions of atmospheric phenomena. The reliability of multi-scale models remains a concern so that analyses for U.S. regulatory applications combine the results of modeling with observations, and knowledge of spatially and temporally differentiated emissions.

Introduction Most of the designated, “criteria pollutants” governed by the United States (U.S.) Clean Air Act (CAA) and its amendments, including CO, SO2, NOx, O3, and finely divided airborne particles (PM2.5, particles with aerodynamic equivalent diameter nominally less than 2.5 µm in diameter), originate from fossil fuel combustion. Combustion emissions are ubiquitous, coming from both stationary and transportation sources. Their effective management to achieve air quality goals depends on knowledge and understanding not only of the nature of the combustion related emissions in space and time, but also the influence of atmospheric processes on emitted material. The latter ultimately results in the formation of products such as O3 and sulfate salts, and dictates the distributions of ambient pollutant concentrations or deposition patterns. The atmospheric processes take place over a very wide range of spatial and temporal scales of importance.1,2 These range over more than 7 orders of magnitude in meters and minutes from a microscale of much less than a meter and seconds to the global scale exceeding a 10 000 km and many years. The scales of concern characterizing atmospheric phenomena also can be identified with analogous “scales” for various effects of air pollution that are dealt with in 1968 within the CAA. In its initial conception, the (1) Hidy, G. M.; Tong, E. Y.; Mueller, P. K. Atmos. Environ. 1978, 12, 735-752. (2) Seaman, N. L. Atmos. Environ. 2000, 34, 2231-2260.

legislation consciously focused on human health at first priority and secondarily human welfare issues, including visual aesthetics and material corrosion. Later, the 1977 CAA amendments added provisions for protection against air quality degradation in pristine areas far from sources, and initiated the concepts for protection of air quality related values (AQRVs) in such areas, including visibility and ecosystem stress. In the 1990 CAA amendments, ecological issues associated with acidification of surface waters and stress on aquatic and terrestrial ecosystems in remote areas were also included in the AQRVs. Separately in the 1980s the role of the criteria pollutants in climate alteration, as well as CO2 and other “noncriteria” greenhouse gases, was debated,3 but remains unresolved in air pollution regulation. Interestingly, the characteristic “scaling” of environmental effects and atmospheric processes of importance range over similar spatial scales as the atmospheric processes. In contrast, the temporal scales that describe processes in the atmosphere are generally distinct from the effects of exposure. Effects are judged to reflect acute responses vs long-term chronic stress in humans or in ecosystems, and thus may differ from atmospheric processes. The ranges of spatial and temporal scaling for air pollution are illustrated in Table 1. (3) Interagency Panel on Climate Change (IPCC). Climate Change 2001: Impacts, Adaptation, and Vulnerability; Contribution of Working Group I to the IPCC Third Assessment Report. Cambridge University Press: Cambridge, 2001.

10.1021/ef0101659 CCC: $22.00 © 2002 American Chemical Society Published on Web 01/08/2002

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Table 1. The Spatial and Temporal Scaling Atmospheric Processes and Associated Environmental Effects spatial scalea

characteristic distance (km)

characteristic time

atmospheric phenomenab localized air stagnation; near source mixing; local chemical processing

environmental effectsc

decision focus

unresolved issues

neighborhood microscale

0.01-4

1-8 h; 24 h; 1 year

urban

4-100+

1-8 h.; 24 h.; air stagnation multiday event; extremes; land seasonal; sea breeze; >1 year. convective Haze hourly mixing daytime and surface layer mixing; photochemical smog processes

regional

100-1000+

24 h.; meso-scale regional scale regional scale regional scale multiday event; stagnation, influence on pollution and chemical 1 year; transport events; urban air quality; cross geo-political transport 6 multiyear. interaction with LRTAP phenomena boundary issues models Haze hourly large storm related to AQRVs5combined with daytime systems acid deposition and air chemistry ecosystem impacts observations provide scaling definitions

continentalglobal

1000+ >10,000

multiday events; 1 year to multiyear; very long-term outlook and trends for nonreactive pollutants

hemispheric LRTAP;f planetary scale vertical mixing

human health; individual and sub-population exposure;d materials corrosion; microscale effects on sensitive ecosystems

conventional jurisdiction for public health protection; protection of sensitive ecosystems

translation of ambient concentrations to exposure metrics

human population exposure; visibility impairment; AQRV5 ecosystems population effects

conventional local jurisdiction for criteria pollutant management

Eulerian grid based chemical transport models used extensively for pollution analysis

global contamination; international global scale global scale geo-political models have phenomena-climate concerns of recently focused alteration; interface between on climate and O3 layer disturbance energy and on stratospheric environmental phenomena policies

a Spatial scales also may be defined in terms of air monitoring site representation, see ref 4. This scaling differs somewhat from this list, especially for the smallest dimensions. b For a detailed description of meteorological features of air pollution, see refs 7 and 8. c For a detailed discussion of environmental effects, see refs 5 and 6. d Exposure is defined in terms of the combined impact of ambient air concentrations outdoors, inside, and personal micro-environments associated with activity patterns. e Air Quality Related Values -AQRVs (Identified with protection of pristine areas, including U.S. National Parks and Wildernessessacid deposition, and consequent effects on human and natural ecosystems; visibility impairment). f Long-range transport of air pollutants (LRTAP).

In the past, pollution phenomena have been isolated from one another by spatial and temporal scaling for purposes of analysis. This was done partly to focus on specific environmental issues, and partly to simplify the analysis of the relevant physical and chemical processes that lead to estimating the atmospheric concentrations of pollutants. Because of the early emphasis on human exposure to, and health consequences of, air pollution, most of the attention through the 1970s was confined to the urban or smaller spatial scale. Temporal scales were dealt with in terms of averaging times believed to be related to acute and chronic effects of exposure. With the advent of issues associated with the effects of pollution deposition (acid rain) and visibility impairment, attention turned to larger scale spatial and temporal phenomena. These focus on the importance of the regional scale, and the apparent long-range transport of air pollution (LRTAP) in the troposphere over distances of 1000 km or more with time periods from days to many years.4-7 Long ago, continental to global scale contamination was recognized from observations (4) Watson, J. G.; Chow, J. C.; DuBois, D. W.; Green, M. C.; Frank, N. H.; Pitchford, M. L. Guidance for Network Design and Optimal Site Exposure for PM2.5 and PM10. EPA-454/R-99-022. U.S. Environmental Protection Agency: Research Triangle Park, NC, 1997.

of the releases of radioactivity in the high atmosphere during nuclear bomb testing in the 1950s, and from the ubiquitous spread of long-lived pollutants represented by certain pesticides in the 1960s. The impact of energyrelated pollution on global processes was highlighted later in the 1980s in conjunction with concerns about climate alteration from radiative forcing from greenhouse gases and suspended particles.3 In the past few years, regulators have realized that future efforts to reduce air pollution should not deal with isolation of phenomena in a relatively narrow range of spatial and temporal scales. Instead, investigators are beginning to integrate the description of air pollution phenomena at all relevant scales of atmospheric processes and environmental effects. Today, methods for analysis have reached the stage where ambient concentrations and pollutant exposure result(5) U.S. Environmental Protection Agency. Air Quality Criteria for Particulate Matter. EPA 600/P-99/002. Office of Research and Development: Washington, DC, 2001; Chapter 5. (In preparation.) (6) U.S. Environmental Protection Agency. Air Quality Criteria for Ozone and Related Photochemical Oxidants. EPA/600/P-93/004. Office of Research and Development: Washington, DC, 1996. (7) Hidy, G. M. Atmospheric Sulfur and Nitrogen Oxides: Eastern North American Source Receptor Relationships; Academic Press: San Diego, CA, 1994.

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ing from emission changes can be computed across essentially all of the spatial and temporal scales, except for the microscale (micro-) environments, corresponding to individual or small sub-population exposure patterns. In this paper, the concept of multi-scale pollution phenomena from fossil fuel combustion is discussed, with particular emphasis of fine particles, and atmospheric oxidants, with O3 as a surrogate. Examples of important phenomena in the atmosphere are summarized. The survey concludes with a brief discussion translating scaling concepts to knowledge about potentially important atmospheric phenomena that will assist in informed decision-making. Combustion Emissions and the Scaling Concept Combustion of fossil fuels involves two broad classes of sourcesstransportation and stationary sources. Transportation sources and some small stationary sources from industry, and commercial or residential activity emit at or near ground level. They influence ambient pollutant concentrations of CO, NOx, volatile organic compounds (VOC), to a lesser extent SO2, and particles (PM) locally in the neighborhood of the sources and in the urban environment. The reasoning behind this relates to the fact that these sources generally are located in cities or populated areas near the sources. Their emissions spread at ground level as a function of surface winds. At the same time, intense vertical mixing in the turbulent air layers near the ground dilute the pollutants rapidly to approach rural levels. In contrast with ground level emitters, some large stationary sources such as electricity generating stations and industrial facilities can release pollutants well above ground level from stacks higher than 50 m. These sources are often located in rural or semi-rural areas to take advantage of flexible land use, local material supplies, and minimization of population exposure. Emissions from these sources rise to heights well above the ground where dispersion of contaminants is affected by winds and mixing aloft, while creating a minimal influence nearby ground level conditions. However, these sources can have an influence on conditions relatively far from the sources, sometimes well in excess of ground level sources. Many of the large power plants in the United States are fueled with coal, and are known to be major contributors to ambient SO2 and NOx levels, as well as PM2.5 pollution over a broad range of spatial scales, including the regional category. Scaling of pollution phenomena implicitly is an important element in the impact assessment of both kinds of sources. Scaling in space and time serves as a descriptor for two aspects of air quality. First, scaling characterizes the nature of the pollution receptor, or the response to an air quality impact, for example, individual human exposure, vs regional haze or chemical deposition. Second, the scaling characterizes the atmospheric phenomena of interest, and provides a“calibration” for the geographical relationship between sources and receptors (Table 1). Atmospheric phenomena that affect pollutant concentration distributions range from “molecular to microscale” processes, including chemical reactions and cloud-particle interactions, to macroscale scale phenomena, including urban and regional events. The

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molecular and microscales are embedded in neighborhood and urban scale phenomena. These respond not only to local influences on the winds and mixing, but also center around the chemistry of SO2, NOx, and VOC. Microscale air motion including turbulence tends to diminish the influence of chemical reactions in favor of transport and mixing, since the reaction rates are slowed by dilution. Regional phenomena involve meteorological processes taking place over large areas after the SO2, NOx, and VOC reactions in polluted air are believed to be largely complete. Processes that are important result in persistent air mass stagnation during which air pollution can accumulate locally and over a large part of a continent for periods of several days. Such pollution events are followed by large scale intrusions of stormy weather, tending to sweep away and dilute the pollution, or facilitate its precipitation removal. The sequence of stagnation and stormy conditions follows a progression of high and low barometric pressure regimes that are well described in the media weather reports. For events on the largest global or planetary scale, pollution phenomena involve slowreacting species such as CO2, and other greenhouse gases that extend their influence insidiously through driving forces on air motion. These influence the heat balance of the lower atmosphere, the troposphere, as well as the upper layers of the atmosphere, including the lower and middle stratosphere. Today, neighborhood and urban scale pollution remain the principal concern for human population exposure, and are generally the center of attention for the measurements of attainment of the National Ambient Air Quality Standards (NAAQS). However, there are the parallel requirements that concern larger scale phenomena, not only in areas near urban centers, but also in relatively pristine areas far distant from sources. Conceptually, scaling of atmospheric processes affecting pollution can follow in two different pathways. As described below, these focus on chemical processes, and on meteorological processes, particularly air mass transport and mixing. The pathways are not independent, but tend to overlap such that emphasis on one or the other depends on spatial and temporal considerations. Emissions and Tropospheric Chemistry. The emissions from fossil fuel combustion contribute critically to chemical processes in the atmosphere. The recent concern for fine particle pollution (PM2.5) has focused both on primary particles emitted directly from combustion, and on secondary material formed in the air through the oxidation of the gases, SO2, NOx, and a fraction of the VOC present. The primary emissions range from metal salts and oxides in ash to carbonaceous material, including organic compounds (OC) and black carbon (BC). The oxidation of SO2 produces sulfate, which is a major fraction of PM2.5 either as ammonium salts or sulfuric acid. SO2 oxidation takes place in the gas phase or in the condensed (frequently aqueous) phase with oxidants including OH radical, H2O2, and O3. The latter is relatively rapid, as high as 10% h-1 in cloud droplets if H2O2 is present; the former typically occurs at a rate of about 0.5-1% h-1 with OH present.7,8 Nitrate primarily as ammonium salt is another important PM2.5 component from atmospheric reactions. NOx is oxidized


relatively rapidly to HNO3 vapor in the air as part of the O3-forming photochemistry.8 In the presence of ammonia, HNO3 comes to equilibrium with the condensed phase as ammonium nitrate. OC in fine particles is generally a fraction as large or larger than the sulfate and nitrate components. OC comes directly from fuel combustion as well as from atmospheric reactions. Much of the total VOC is oxidized rapidly in the troposphere by O3, OH, or NO3. Thus the presence of OC from secondary sources is closely associated with gas-phase oxidation chemistry. Most of the VOC present in the air is low molecular weight material whose oxidation products remain in the vapor phase. VOCs nominally of carbon number above 6 or 7, including aromatic compounds, will involve a fraction of oxidation products with sufficiently low vapor pressure to form particles, or can be adsorbed on existing particles. Generally the yields of condensable material from oxidation of these VOCs are a small fraction of the total products, unless the VOC carbon number is very large. Historical measurements indicate that large quantities of relatively high molecular weight vapors are rarely present in the troposphere, especially as combustion byproducts. Some areas may have significant local emissions of this type, including terpene vapors from vegetation. The chemistry of particle production in polluted air generally takes place most rapidly near sources in regimes of high concentration over a span of hours or less. Although the chemical processes are reactant concentration dependent, they are at work continuously even at low concentrations far from sources. The loading of particles in the air is modulated by losses from dry deposition at the surface, from the condensation and evaporation cycle in clouds, or by wet removal by precipitation. Because the deposition processes are relatively slow compared with atmospheric chemical processes of SO2, NOx, and VOC, they tend to increase in importance in regional and larger scale phenomena. Clouds and precipitation occur intermittently everywhere; they scavenge preferentially the soluble material present at all spatial scales, with removal of insoluble components less efficiently. Meteorological Processes and Transport. Spatial and temporal scaling of meteorological processes is a well-known technique for analyzing such phenomena.2 As a rule of thumb in air pollution analyses, the greater the distance from a source the less influence it will have on receptor air quality. The less well tested rule suggests the strongest influence of air chemistry on criteria pollutant concentrations is nearest sources where concentrations are high, and consequently conversion rates are elevated. However, polluted air mass transport and mixing resulting from air motion is important at all scales. In a conceptually simple way, the distance traversed is related to time by the time of travel and the wind speed, or alternately, the pollutant residence time. The residence time of a contaminant in the troposphere is an average time for a molecule or a particle to remain airborne. This time scale is crudely estimated as a first-order process, with the contaminant’s half(8) Seinfeld, J.; Pandis, S. Atmospheric Chemistry and Physics: From Air Pollution to Climate Change; Wiley-Interscience: New York, 1998.


life for an exponential decay of concentration in the air. The species residence time depends on many interactive factors, including its reactivity in the atmosphere, its solubility in water, and its reactivity with surfaces at the ground, including soils, vegetation, water or ice, and or building materials.7 Most pollutant gases from combustion have a residence time of a few days or less. Slowly reacting species such as CO2, however, have multi-year residence times. Typically O3, for example, has a residence time near the ground of minutes to hours, but is much longer aloft where the O3 is distant from reactive surfaces. Airborne particles effectively have a range of residence times depending on their size and their ability to stick to surface on contact, their solubility, and the presence of clouds and precipitation. In the absence of cloud processes, particles larger than approximately 10 µm diameter, for example, are strongly influenced by gravitational settling and have typical residence times of 1-3 days. Very small particles less than a few tenths of a micrometer diameter are subject to impaction and Brownian diffusion to surfaces and have short residence times of about a day or two. Because the loss or combining processes are relatively slow in the size regime for fine particles from about 0.3 µm to about 2 µm, they have relatively long residence times that are believed to range from 3 to 10 days. This gives the potential for not only a local influence, but also an influence on regional and larger scale phenomena. The fine particle category covers most of the combustion-related particles, including carbonaceous material as well as sulfate and nitrate salts. For regulatory purposes, the USEPA has defined the fine particle range as a key component of the cumulative particle mass less than about 2.5 µm diameter (PM2.5). This focuses attention on the multiscale aspects of PM2.5 characterization. A spatial scale alternative to describing pollution by a scale of residence time is sometimes defined in terms of a zone of influence.1 This “zone” or extent of source influence represents a characteristic distance traveled between a source and a receptor. In analogy to the exponential time decay, the influence distance can be scaled to an intersite correlation decrease from 1 to 0.5 (or less). The influence of a source on surrounding receptors will depend on the statistics of air mass transport by prevailing winds in speed and direction. Roughly speaking, a transport time analogy to the residence time is the ratio of the characteristic distance of zone of influence to the average wind speed. The spatial and temporal scaling reflect such considerations. Typically, intersite correlations for sulfate in the East range from 300 to 600 km.7 In a distinctly different sense, the temporal scaling also concerns the average exposure time imbedded in the NAAQS. The statistics of the frequency of occurrence of high ambient pollutant concentration (extreme events) and other ranges of concentration are an important consideration for exposure, and consequently for constraining temporal scales in the regulatory boundaries. At all spatial scales, meteorological processes, including the transport of polluted air by the winds, and the mixing of contaminated material through the lower and middle layers of the troposphere are critical elements affecting pollutant concentration distributions, and

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source-receptor relationships. Air mass transport pathways are rarely one-dimensional, but are multidimensional in character. The longer contaminants remain in the air, the more difficult it is to establish their pathways downwind making the regional scale sourcereceptor relationships difficult to determine with reasonable certainty.7 Turbulent mixing that takes place generally is most intense in the vertical direction, extending from the regime of frictional forces acting on the air near the ground upward through 3 km or more as a result of thermally driven, convective air motion. Conceptually incorporating relevant tropospheric phenomena, including air chemistry and air mass transport and mixing, has proven to be a formidable task for the analyst. In general, combined air quality and meteorological observations on important spatial and temporal scales have not been available and are expensive to obtain. As an alternative, such analyses are generally facilitated using air quality models, which have become increasingly complex over the years, as they have been expanded to include parametrization of known atmospheric processes affecting air pollutants.2,8 Some Examples for Ozone and Fine Particles The integration of concepts based on scaling is increasingly important for pollution management strategies. However, the regulatory guidelines for technical analysis of most air pollution problems continue to separate phenomena in terms of the most significant spatial and temporal scale of concern. In some cases, however, regulatory analyses have begun to bridge the different scaling. Some examples below illustrate both unresolved technical issues in distinct scaling categories, and cases where the bridging has added significant insight into problems. The Neighborhood and Urban Scale. The neighborhood-urban scale analysis of ambient concentrations is the most common found in the literature dating back to the 1940s or earlier. Myriads of such analyses and a variety of air quality models have been conducted in response to CAA requirements since the 1970s. This range of spatial and temporal scales has long been central to standard setting for human exposure, and for addressing the visual impact of pollution seen as smoke plumes and urban haze and smog. Despite the depth of knowledge about pollution phenomena at this scale, problems remain unresolved as exemplified by the nonattainment of O3 and PM standards in some major areas. Two challenging contemporary problems have emerged at the neighborhood-urban scale which illustrate a significant issue to resolve. The first concerns inferences of individual and sub-population exposure considering indoor conditions, mixed with outdoor air, and human mobility as activities dictate. The second concerns characterizing urban emissions from the large number of disparate small sources present in cities. Of these, transportation sources are by far the most important in magnitude and concern. Emissions from transportation sources are perhaps best characterized using onroad fleet applicable emission factors, in combination with spatial and temporal traffic networking and driving activity patterns. A resurgence of recent research has been reported that helps to clarify the nature of human exposure in

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relationship to neighborhood and community-wide ambient air quality.5 Much of the personal exposure to fine particles related to combustion of fuels on scales that are both difficult to integrate and resolve from models. Studies have found that the bulk of the urban population spends more than 80% of their time indoors. For example, the time spent indoors by active healthy individuals is divided roughly half at home, and more than a third either at work, or in various buildings, and perhaps 5-10% in transportation.5,9 In contrast, people with severe impairment from disease or disability are likely to spend virtually all of their time in their homes, or in health-care facilities. Normal human activities span a variety of neighborhood scales, or even to micro-environments of a few meters or less, and time intervals during the course of a day and longer. Seasonally the fraction of time spent indoors remains about the same, but indoor conditions can vary substantially by geographical location and by season. Research has indicated that outdoor air pollution tends to penetrate the indoor environment significantly and variably, depending on the building ventilation, and other factors.5,9 Studies indicate that a substantial fraction of indoor or activity-based personal exposure to particles is correlated with ambient air quality. This has been found to apply ubiquitously to some chemical constituents such as sulfate salts, but not to others, including metals or metal compounds, and carbonaceous material. Semivolatile species such as ammonium nitrate also appear to be modulated inside and outside as a function of equilibrium conditions, including humidity and temperature.10 A number of indoor sources of contamination are known to exist, especially airborne organic and biological material. Exposure to this material in combination with contaminants from outdoor air tend to confound the interpretation of epidemiological results describing adverse health response based on outdoor air monitoring. Furthermore, investigators have found evidence that, in some cases, measurements of personal exposure during human activities create a so-called “personal cloud” of PM surrounding an individual which is distinct from static conditions in either indoor or outdoor surroundings.11 To date, accounting for the physical and chemical ambiguities in exposure is problematic. However, several investigators have hypothesized that one can separate outdoor exposure and the indoor component of exposure from outdoor air from exposure to indoor sources and from micro-environments associated with human activity.5 The outdoor components of exposure are assumed to be independent of the other two components. Once this is done, the next logical step is to assume that concentrations of pollutants in outdoor air are represented reliably by community monitoring stations that are believed to reflect the urban scale. Then it follows that air quality modeling can be applied to estimate pollutant concentration at all relevant receptors based on consistency with these observing stations. (9) Lachenmyer, C.; Hidy, G. M. Aerosol Sci. Technol. 2000, 32, 3451. (10) Hidy, G. M.; Lachenmyer, C.; Chow, J.; Watson, J. Aerosol Sci. Technol. 2000, 33, 357-375. (11) Wilson, W. E.; Mage, D.; Grant, L. J. Air Waste Manage. Assoc. 2000, 50, 1167-1183.


Researchers have cited evidence that community air monitoring observations are “spatially correlated” in cities for O3 and for PM2.5.5,9 O3 is a product of atmospheric chemistry, and PM2.5 often contains a large fraction of secondary material such as sulfate. Products of atmospheric reactions tend to have relatively weak spatial gradients of concentration in many urban areas. However, other pollutants directly emitted into the air such as CO, NOx, and PM10 tend to have stronger gradients in the urban setting. Spatial correlation is insufficient evidence for assuming that concentrations are spatially uniform. Historical evidence1 and contemporary measurements of PM2.5 and sulfate indicate that ambient concentrations are nonuniform within some large cities such as Los Angeles and New York, sometimes varying by a factor of 2 for annual averages. This ambiguity is important for application to epidemiological studies that assume that spatial uniformity exists, and a few community monitors can represent an entire urban environment. Substantial progress has been made correlating neighborhood scale outdoor air concentrations with indoor air concentrations. This has permitted estimation of the fraction of the latter than derives from outdoor air pollution.5 Quantification of the connection between indoor and outdoor pollution and personal exposure remains to be done, taking into account the experience of sub-population activities that represent both normal, healthy individuals in different socio-economic settings, and people with cardio-pulmonary diseases that are particularly susceptible to air contamination. Activities of a healthy, mobile population are particularly relevant to exposure from fuel combustion sources, since many people spend significant time in vehicles where inhaled or ingested, microscale concentrations are significantly higher than the surroundings. Estimation of source contributions to ambient pollutant concentrations through the use of mathematical models is an important adjunct to air quality management. Contemporary models are based on an Eulerian frame of reference, with rectangular grids identifying locations the horizontal dimensions. Today’s models such as the urban airshed model (UAM V) are applied routinely for neighborhood and urban scale studies using a grid resolution of 5 km × by 5 km or less over a domain of 105 km2 or more. Temporal averaging of an hour or less is used for episode modeling, while averaging seasonally or annually is done for long term ambient concentration estimates. Within these gridded air volumes, air quality components are assumed to be well mixed. The reliability of the modeled ambient concentrations depends on its input data, including meteorological factors, and an emissions processing computation. The processed emissions data rely on the (12) NARSTO. An Assessment of Tropospheric Ozone Pollution: A North American Perspective. Electric Power Research Institute: Palo Alto, CA, 2000. (Also, www.cgenv.com/Narsto.) (13) Pierson, W. R.; Gertler, A.; Robinson, N.; Sagebiel, J.; Zielinski, B.; Bishop, G.; Stedman, D.; Zweidinger, R.; Ray, W. Atmos. Environ. 1996, 30, 2233-2256. (14) Lawson, D.; Smith, R. The Northern Front Range Air Quality Study; Cooperative Institute for Research in the Atmosphere, Colorado State University: Fort Collins, CO, 1998. (http://nfraqs.cira. colostate.edu). (15) NARSTO. Assessment of Airborne Particulate Matter Pollution in North America; Environment Canada: Downsview, Ontario, Canada, 2001. (In preparation.)


conventional emission inventories, combined with temporally resolved activity and operational data.12 The U.S. National Inventory for 1999 is readily available by electronic access (www.epa.gov/ttn/chief/efig.). Some investigators believe that the most significant uncertainty in model calculations comes from the emissions component. Considerable effort has been made to improve emissions inventories based in part on reconciliation of ambient concentration data, and source profile estimates. One of the critical but historically “unreliable” elements of emissions processing for air quality modeling is the transportation sector, particularly those emitted from light duty and heavy duty onroad vehicles.12 Spatially and temporally resolved emission processing for on-road vehicles depends on knowledge of an emission factor in fuel consumption as a function of road network, driving conditions, traffic, and time of day. It is far more complex than simply estimation by mass/ distance traveled emitted, and vehicle distance-miles traveled (VMT). Nevertheless, for vehicle emissions, the historical approach has been to use laboratory dynamometer tests to establish emission factors, which are adjusted for vehicle type, and maintenance. Vehicle use is estimated in cities from prototype calculations that provide “typical”, standardized VMT patterns by major roadway traffic counts, average distance traveled, etc. Temporal patterns take into account “generic” weekday commuting patterns, with morning and evening traffic peaks, and weekday-weekend differences. Spatial patterns focus on networks of major thoroughfares, railroad right-of-ways, and airport locations. Recent experimental campaigns have attempted to reconcile the vehicle emission factor estimates from air quality models with specialized observations near sources, or community-based ambient observations spatially and temporally averaged concentrations. Roadside observations of vehicle emissions have been made using remote sensing for gases, and mobiles sampling for particles using on-road equipment. Laboratory tests have been conducted with in-use vehicles, and measurements have been made in road tunnels to capture “average” emissions.13 These in combination with ambient gas and PM2.5 concentration data have indicated large deviations of a factor of 2 or more from the emissions processing estimates and actual conditions for gases and airborne particles.14 The use of ambient measurements for real world conditions with air quality modeling, including receptor oriented modeling15 to reconcile emissions processing results has added an important tool for improvement of emissions inventories. Such methods are now considered an essential part of the testing and verification of both the emissions component and the output of the air quality models. To determine uncertainties in the urban vehicle use patterns beyond isolated traffic monitoring, special studies have been developed that attempt to map emissions patterns as a function of on-road traffic flow by roadway segment, in-use vehicle fleet characteristics, and global positioning system (GPS) location of congestion.16 These models provide a highly sophisticated simulation of vehicle emissions suitable for episodic air quality model calculations, as well as a semi-indepen-

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dent check on the coarser calculations. However, they are input data intensive for location, traffic, and vehicles, and elaborate methods are required to describe major human activity input as well as computational capacity. So far they have been used for microscale to neighborhood scale transportation analyses in only a few locations, such as the eastern part of the Los Angeles air basin. They have not reached the stage where they will find common use in air quality studies. They have indicated that significant differences exist between conventional emissions processing on a given day and more precise traffic-vehicle simulations. But this work has not yet quantified the errors to be expected in the conventional, aggregated approaches to transportation emissions. The Regional Scale. The examples above illustrate two unresolved problems categorized by in small-scale interactions that concern the interface between ambient air quality and human population exposure, and the ambiguities associated with nonstationary source urban pollutant emissions. If the spatial scale is expanded to regional coverage, a complementary set of unresolved problems can be identified. Analyses of regional scale air quality use models such as the community model for air quality (CMAQ) or the regional modeling system for atmospheric deposition (REMSAD). These generally have a horizontal resolution from 50 km × 50 km to 100 km × 100 km in a domain of 109 km2 or more, with a vertical spacing that is variable with height.15 Focus on the regional scale then, by definition and approximation, deals with neighborhood and urban phenomena as a “sub-grid” process. Historically focus on the regional scale phenomena involving derivatives of acidic criteria pollutants, SO2 as sulfate and NOx as nitrate, centered on acid precipitation and visibility impairment.7 Later, attention to concerns about O3 production and dispersion emerged.12 Observations and air quality modeling have at least qualitatively established the importance of LRTAP, with certain visual and ecological consequences. The existence of LRTAP also creates significant problems for the application of CAA strategies employing local emission controls in the absence of controls on neighboring geopolitical jurisdictions12,15 because the ambient concentrations upwind of locales are frequently elevated, and may approach the ambient standards. Both acid precipitation and visibility impairment (as regional haze) have been identified in North America with the emissions from large, rurally located electricity generating stations, many of which use coal as a fuel. These plants emit large quantities of SO2 and NOx from tall stacks, which oxidize in the atmosphere to form sulfuric and nitric acids. The ecological systems susceptible to acid or chemical deposition are localized in remote areas with vulnerable weakly buffered lakes, and granitic or poorly developed soils with little acid neutralizing capacity. Although there is natural acidity in the air, the added acid from pollution, even low ambient concentration, creates potentially significant stress on certain surface waters and forests. The spatial scale of these effects depends on the region of concern, (16) Barth, M.; An, F.; Norbeck, J.; Ross, M. Transportation Record, 1520, Transportation Research Board, National Research Council: Washington, DC., 1996; pp 81-88.

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and may range from neighborhood to the regional scale. Temporally, the effects are cumulative and may been seen after many years of exposure to acid deposition. Chemical deposition effects have been identified mainly in the eastern United States and southeastern Canada,17 but some evidence of deposition effects has been reported in the western United States.18 Visibility impairment in the form of regional haze has been observed in many areas across the United States,15 and has been seen in the arctic environment.19 In the United States, visibility is affected by plumes from large power plants, as well as from widespread degradation by pollution from distant sources drifting over pristine areas. For example in the Southwest, regional haze has been observed in and around the national parks in the golden triangle, corresponding to the Grand Canyon, Bryce, and Zion National Parks. In the East, it wellknown that relatively dense haze can be seen which can extend from regions in the Southeast to New England at times, especially in summer and early fall.15 Evidence suggests that these persistent haze events are associated with fine particles composed mainly of ammonium salts of sulfate, and carbonaceous material originating from fossil fuel combustion. The extent of the hazes is associated with long-range transport of particles formed from SO2 emissions from large sources, as well as carbon emissions from a variety of sources, including forest fires. The potential for significant production of carbon-containing particles from atmospheric reactions of VOC also may be a factor, particularly in the Southeast, where extensive forests abound, which emit biogenic VOC, including highly reactive, particle-forming terpene compounds. Quantification of the LRTAP phenomenon has been difficult, and analyses have been controversial. In the West, evidence20 has been reported of the transport of haze from southern California over 600 km into the Grand Canyon area, along with local pollution from large power stations, and from urban areas such as Phoenix, AZ and parts of Texas. These results and others have been accepted qualitatively, though the frequency of occurrence of LRTAP in the West compared with local influence remains to be determined. In the East, the presence of LRTAP is more difficult to elucidate unambiguously.7 The reasons for this trace back to the variation in persistence and direction of the prevailing winds in relationship to the spatial distribution of emissions. East of the Mississippi River, there are many urban areas present with separations of 100 km or less. Further, the siting of power plants and heavy manufacturing facilities follow major rivers, such as the Ohio River. The spatial orientation of these sources tends to be north-south or southwest to northeast along the direction of prevailing winds. Thus emissions of air (17) Acid Deposition: State of Science and Technology; Irving, P., Ed.; National Acid Precipitation Assessment Program (NAPAP): Washington, DC, 1990; Vols. I, II, and III. (18) Peterson, D. L.; Sullivan, T.; Eilers, J.; Brace, S.; Horner, D.; Savig, K. Assessment of Air Quality and Air Pollutant Impacts in National Parks of the Rocky Mountains and Northern Great Plains. U.S. Department of Interior, National Park Service, Air Resources Division: Denver, CO, 1998. (19) Barrie, L. Atmos. Environ. 1986, 20, 643-663. (20) Henry, R. C.; Wang, Y. N.; Gebhart, K. A. Atmos. Environ. 1991, 25A, 503-509.


pollution tend to be oriented such that they accumulate eastward, confounding the effects of local exposure as contrasted with LRTAP.7 The blending of local and distant contributions appears to be most pronounced for airborne sulfate particles, and for O3. High concentrations of particulate sulfate are associated with the presence of dense haze across the United States east of the Mississippi River, mainly in summer and early fall. These haze occurrences also frequently contain widespread elevated O3 concentrations at ground level and aloft. O3 and PM2.5 concentration patterns do not always correlate, which complicates their interpretation in the light of knowledge of air chemistry.7,15 Attempts have been made to distinguish between the formation and transport of these pollutants since the mid-1970s using analysis of observations, and air quality modeling. Early work that arose from a major regional scale field campaign, the 1977-1978 Sulfate Regional Experiment (SURE). The SURE was the first major field program designed to investigate regional scale sulfur oxide characteristics in the greater northeastern United States from the Mississippi River to the Atlantic coast, and from southeastern Canada to southern TennesseeNorth Carolina. The study involved operation for approximately 15 months (1977-1978) with 49 rural SO2SO4 monitoring stations located over the region of about 107 km2. The station intersite spacing was between approximately 100 km and 300 km. The network included 9 rural research sites instrumented for particle chemistry, as well as the gases, SO2, NOx, and O3, and meteorological parameters. The research stations were operated to collect continuous or daily data during the experiment, while the remaining stations were operated intermittently with daily sampling for one month in each season. Considerable care was taken in siting location of stations to minimize the influence of local sources, to provide data representative of regional scale processes. Siting was based on design analyses using earlier observations, and preliminary air quality modeling. Complementary observations of conditions aloft were included using enhanced National Weather Service rawinsonde measurements, and intermittent aircraft flights. The SURE observations were analyzed extensively using various methods, including extensive meteorological and phenomenological studies, and air quality modeling. The results from the SURE contributed substantially to the knowledge about conditions for LRTAP, as compared with local or regional air mass stagnation events.1,7 The experiment resulted in a conceptual description for regional sulfate events in the East that were linked with regional scale meteorological features. Historical studies recognized that the eastern United States is subject to frequent summer occurrences of persistent regional scale air mass stagnation lasting 3-7 days, and associated with large areas of high barometric pressure.21 These conditions are characterized by weak winds, intense heating at the ground, and relatively weak vertical mixing. The center of these areas generally falls over the Deep South. These large, (21) Korshover, J. Climatology of Stagnating Anticyclones East of the Rocky Mountains 1936-1975. Technical Memorandum, ERL-ARL55. National Oceanic and Atmospheric Administration: Silver Springs, MD, 1976.


regional zones of stagnation are eroded by the onset of large scale frontal systems from the north, oriented southwest to northeast and extending as far as 1000 km. The winds tend to increase as the flow nearly parallels the frontal weather systems promoting transport parallel to the frontal system and carrying polluted air already present toward the Northeast crossing the Atlantic coast. The eastern LRTAP events were found to have characteristics quite distinct from the regional stagnation conditions,2 wherein pollution accumulates over multi-day periods under a large stagnating air mass. The LRTAP conditions along encroaching frontal systems were observed with increasing, persistent winds along the front, and an increased height of mixing normal to the front. This condition is likened to the development of a channeling of persistent polluted air flow lasting 1-4 days, but is quite distinct from stagnation conditions where pollution accumulates during periods of sustained weak winds, and low mixing heights. Eventually the development of stormy weather along the frontal zone tends to dilute and erode away the polluted air mass, and provide for removal of pollution through precipitation. The description of meteorological conditions associated with local and regional events have been refined substantially since the SURE as a result of a series of field campaigns in the Northeast. The evolution of airflow at the ground and aloft during LRTAP events has been reported from O3 studies associated with NARSTO-NE.2,22 One of the key questions asked by regulators of pollution in the Northeast concerns the development of emission reduction strategies that cross geo-political jurisdictions. When regional scale pollution events take place they cannot be managed effectively within local constraints. Thus emission controls have been suggested and adopted that extend beyond local authorities. With the need to develop local and regional strategies to manage O3 and PM2.5 in the East, authorities need information on the frequency of occurrence of regional pollution events vs local events. Ironically, little information is published on this subject. Although it is relatively old, and based on limited data, a descriptive analysis of rural observations taken during the SURE provides insight about the frequency of occurrence of these major regional events. The results are summarized in Table 2, classifying synoptically events with elevated sulfate concentrations, a surrogate for PM2.5 concentrations in the East. The frequency of occurrence of different classes of events based on the data cited in Table 2 is shown in Table 3. The meteorological conditions analyzed in the SURE period were determined to be reasonably representative of normal meteorological conditions, one could expect that the regional pollution events occur about 30% of the time, mainly in summer and early fall, whereas more localized pollution occurs more frequently at other times. The level of occurrence of regional scale conditions represents a significant fraction of the time, especially if it is concentrated in the summer months. The uncertainty in estimation of the frequency of occurrence of regional scale events remains ill defined, (22) Hidy, G. M. Atmos. Environ. 2000, 34, 2001-2022.

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Table 2. Definition of Event Days by Observed Sulfate Concentrations and Spatial Extent of Elevated Concentrations7 percentage of observation stations with 24 h sulfate concentrationa event groupb enlarged regional event day regional event day sub-regional event day non-regional event day non-event day

>10 µg/m3 >15 µg/m3 >20 µg/m3 40-93 40-70 25-50

Multiscale Impact of Fuel Consumption on Air Quality - Energy & Fuels

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