Rethinking the Ozone Problem in Urban and Regional Air Pollution (1991) / Chapter Skim
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9 Emmission Inventories
9 Emmission Inventories
Pages 251-302
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From page 251... ...
With CO, which is emitted mostly by mobile sources-cars and trucks-data on the actual emission rates by source (real-time) are not available, and the source contribution is much more ubiquitous, but real-time ambient measurement is possible.
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From page 252... ...
This chapter provides an overview of the anthropogenic emissions inventory: how it is compiled, what the major contributing sources are, and where uncertainties lie. There is a similar overview of the inventory of biogenic emissions, and finally a review of efforts to evaluate the accuracy of emissions inventories.
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From page 253... ...
The formulation of emission factors for mobile sources, the major sources of VOCs and NOx, is based on rather complex emission estimation models used in conjunction with data from laboratory testing of representative groups of motor vehicles. Vehicle testing is performed with a chassis dynamometer, which determines the exhaust emission of a vehicle as a function of a specified ambient temperature and humidity, speed, and load cycle.
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From page 254... ...
. Based on results from this set of vehicle emissions data, a computer model has been developed to simulate for specified speeds, temperatures, and trip profiles, for example, the emission factors to be applied for the national fleet average for all vehicles or any specified distribution of vehicle age and type.
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From page 255... ...
Anthropogenic VOCs Forty percent of anthropogenic VOC emissions result from transportation, according to the 1985 NEDS and NAPAP emissions inventories (see Figure 9-2~; light-duty cars and trucks make up the largest contributing fraction. Solvent emissions, which are distributed across a broad group of sources,
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From page 256... ...
contribute 32% of total VOC emissions; the remaining 28% result from other sources such as industrial manufacturing activities and fuel combustion. An independent analysis by the Congressional Of lice of Technology Assess
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From page 257... ...
An OTA study (1989) reported that, using preliminary emission factors provided by EPA for fleet average running losses, for ambient temperatures of 79°F (26 °C)
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From page 258... ...
As With stationary-source solvent evaporation, this source category is not well quantified, and the combined contributions of the two categories are expected to have significant uncertainties. VOC Speciation by Source Category The evolving knowledge of the chemical reaction mechanisms that provide quantitative information on the VOC-NOx relationship to ozone production has highlighted the need for compound-specific information on the chemical composition of the compounds in the VOC emissions inventory.
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From page 259... ...
Source: EPA, 1989c. 259 used for emission factors.
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From page 260... ...
. _ ~_ _ =~= _ _ ss9 __ == 1 1 ~1 1 1 o 500 1000 1500 2000 2500 Thousand Tons/Year FIGURE 9-3 NAPAP 1985 national emissions inventory for NOX and VOCs by state.
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From page 261... ...
TABLE 9-3 Estimated Annual U.S. NOX Emissions from Anthropogenic Sources Obtained from Recent Inventories Source category Emissions (teragrams of nitrogen/year)
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From page 262... ...
(1990) published a statistical analysis of the measured differences in the emission factors used to construct the annual NOX emissions inventory for power plants in the United States.
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From page 263... ...
Since that study, numerous investigators have focused on biogenic VOCs. Researchers have investigated the speciation of natural VOCs (Rasmussen and Went, 1965; Rasmussen, 1970, 1972)
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From page 264... ...
264 RETHINKING THE OZONE PROBLEM isoprene-emitting deciduous trees and in a factor of 1.5 increase from terpeneemitting conifers (Figure 9-43. This analysis aptly illustrates the fact that, all other factors being equal, natural VOC emissions are generally highest on hot summer days, when the weather also favors ozone generation.
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From page 265... ...
Also similar to the case of anthropogenic emissions inventories, natural VOC inventories typically are developed first by determining an emission factor (actually an emissions rate per unit area) for specific kinds of sources (in the case of biogenic emissions, the source types are biomes such as deciduous forests, coniferous forests, mixed forests, grasslands, and agricultural lands)
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From page 266... ...
and thus cause anomalous emission rates. There are also uncertainties associated with the large extrapolation that must be made to derive an emission rate for an entire forest from the rates measured for a fixed number of branches and trees.
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From page 267... ...
. Despite the inadequacies of each method and the spatial and temporal inhomogeneity inherent in biogenic emissions, the emission factors yielded by the different approaches have all been reasonably consistent; emissions rates obtained by these approaches agree within a factor of two or three (Lamb et al., 1987~.
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From page 268... ...
Furthermore, although the inventories for the other regions of the nation are dominated by a-pinene and other nonmethane VOCs, with relatively small emissions of isoprene, isoprene emissions comprise almost 50% of the total biogenic inventory for the South. TABLE 9-5 Emission Factors, ~g/m2-hr Day Night Compound ER s2 ER s2 TNMHCa 5969.490 2532520.0 1745.940 439545.0
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From page 269... ...
EMISSIONS INVENTORY 269 Day Night Paraffins Olefins Aromatics Methane c'-Pinene ,B-Pinene 573.827 4953.520 423.550 830.981 462.466 428.301 21846.0 2139170.0 11713.9 75674.0 134816.0 168643.0 411.475 1024.990 298.629 798.813 348.456 311.320 10462.6 376886.0 5638.1 71427.9 69977.9 82895.6 d-L~monene 53.234 2243.0 36.998 1085.5 Isoprene 3539.270 1372810.0 0.000 0.0 Myrcene 7.806 1030.3 5.426 498.1 Unknown -4.658 614.625 -2.832 285.2 Terpenes 2LA 2.196 0.0 1.531 0.0 18 0.170 0.668 0.115 0.313 20 0.081 0.153 0.055 0.072 21 90.549 62997.8 61.404 31599.1 22 4.063 170.0 2.749 79.9 23 1.261 6.479 0.880 3.156 24 2.179 35.7 2.179 35.7 27 0.756 1.593 0.525 0.767 28 1.427 1.879 0.991 0.907 29 6.719 132.4 3.997 55.8 ^3-Carene 102.137 71.374 26A 10.842 2253.4 7.522 1083.
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From page 270... ...
Because of their extensive land area and high biomass density, forests are by far the major contributor to the biogenic VOC inventory; agricultural crops contribute only a few percent. In addition to being a significant source of VOCs in the continental United States as a whole, biogenic emissions can also represent a significant source of VOCs in urban airsheds.
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From page 271... ...
However, biogenic emissions inventories for yearly or seasonal periods are not yet well-quantiE~ed and could be in error by as much as a factor of two or three (Places et al., 1990~. Moreover, because of the extreme variability in emissions that can occur over the growing season, much larger errors can be incurred when annual or even seasonal inventories are applied to a given single- or multiple-day episode of high concentrations of ozone.
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From page 272... ...
The remaining variability, which is caused by some combination of environmental as well as biologic factors, is not accounted for in current biogenic emissions models and must therefore be removed by averaging to obtain a "representative" emission factor. However, because the variability is so great, the averaging method can affect the resulting emission factors; it has been found, for instance, that emission factors formed by using a geometric mean of the data-which tends to minimize the influence of members of the sample population found to be gross or large emitters is typically a factor of two or three lower than are emission factors formed from an arithmetic mean of the data (Places et al., 1990~.
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From page 273... ...
The remaining two-thirds of lightning-produced NOX is generated by CG discharges. These NOX emissions, which are generated at lower altitudes, and thus are similar to those from anthropogenic sources, are more readily removed from the atmosphere and therefore less available for long-distance transport.
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From page 274... ...
Less than 10% occurs over North America, and of that, about 75% occurs from May to September (Logan, 1983~. Quantitative estimates of NOX from lightning usually are derived from a combination of three factors: the frequency of lightning flashes (a flash consists of one or more strokes closely spaced in time)
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From page 275... ...
EMISSIONS IVORY 275 TABLE 9-6 Production of NOX by Lightning over the United States as a Function of Season, Tg-N Latitude Longitude Flux (0.001 Tg-N) Winter (January, February, and March)
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From page 276... ...
276 RETHINKING THE OZONE PROBLEM Latitude Longitude Flux (0.001 Tg-N) Summer (July, August, and September)
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From page 277... ...
NOX is produced in soils by the microbial processes of Vitrification and denitrif~cation and by several chemical reactions that involve nitrite (Galbally and Roy, 1978; Galbally, 1989~. The local variables that frequently influence NOX emissions are soil temperature, moisture content, soil nutrient level, and vegetation cover (Johansson, 1984; Johansson and Granat, 1984; Slemr and Seller, 1984; Galbally et al., 1985; Anderson and Levine, 1987; Galbally et al., 1987; Williams et al, 1987; Johansson et al., 1988; Kaplan et al., 1988; Williams et al., 1988~.
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From page 278... ...
However, given the increasing number of measurements of NOX emissions from soil by type, a somewhat more detailed estimate of NOX soil emissions for the continental United States is now feasible. Despite variability and uncertainties, various distinct patterns emerge.
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From page 279... ...
Outside the active growing season, the NOX emissions from these agricultural lands are assumed to be the same as for the grasslands. Excluded from the inventory is the remaining 30.65to of the total land area of the United States.
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From page 280... ...
ACCURACY OF EMISSIONS INVENTORIES Knowledge of the precision and accuracy of emissions estimates for VOCs and NOx is essential to quantify the relationship of these precursor species to ozone air quality. The desired accuracy and precision of the emissions inventory is governed by its intended application.
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From page 281... ...
TABLE 9-8 Sources of Emission Variability Emission Factor Variability Temporal variability as a result of operating conditions Inherent variations within source category due to equipment configuration Location specific factors: ambient temperature, wind speed, or fuel characteristics Variability in Btu and sulfur content of fuel Annual Activity Level Variability Temporal variability in fuel consumption or production levels Variability in activity values disaggregated using surrogate activity levels Variability of Allocation Factors Temporal variability due to changes in demand or production distribution, maintenance, and random outages Variability in chemical speciation due to process changes and product variability Variability of Emission Control Efficiency Variability in removal efficiency due to changes in operating conditions, equipment failures, and fuel characteristics. Adapted from Placet et al., 1990 Estimates of uncertainty in emissions data have been derived mainly from subjective judgments of the variability of the various components identified in
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From page 282... ...
Variation 90% RCI Lower variability cased Electric Utilities 6.09 0.07 0.11 Industrial Combustion 2.25 0.15 0.25 Industrial Process 0.84 0.25 0.41 Residential/ 0.62 0.30 0.49 Commercial 8.03 0.02 0.03 Transportation All Other 0.81 0.40 0.66 An illustrative example showing the application of Goodman's formula to two power plants with varying CVs (coefficient of variation; CV = standard deviation/mean) with respect to their activity levels and emission factors can be found in Placet et al.
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From page 283... ...
MOTOR VEHICLE EMISSIONS Accurately estimating emissions from the population of motor vehicles in an area is one of the most difficult problems in the entire emissions inventory process. The problem is compounded because motor vehicle emissions typi
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From page 284... ...
Although this section addresses only vehicle emissions and not driving cycles, it is important to note that determination of a representative cycle for an area is an essential aspect of the overall motor vehicle emissions inventory and is generally overlooked, due to expense and a lack of proven techniques. Dynamometer testing of new and in-use vehicles has been performed over the years on limited numbers of vehicles to assess pro- and postcontrol tailpipe emissions, emissions control deterioration factors, and inspection and maintenance programs (Bonamassa and Wong-Woo, 1966; Black and High, 1977,1980; Sigsby et al., 1987)
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From page 285... ...
. According to the SCAQS tunnel study, measured CO and VOC emissions rates in the tunnel were a factor of 2.7 + 0.7 and 3.8 + 1.5 higher, respectively, than predicted by CARB's EMFAC7C model for estimating emissions from on-road vehicles; NOX emissions rates agreed reasonably well with model predictions.
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From page 286... ...
Roadside Measurements Three approaches to monitoring roadway vehicle emissions for the purpose of evaluating emissions factors and emissions models have been considered. Two methods use ambient measurements near roadways to estimate vehicle emissions indirectly; a third uses remote sensing to monitor vehicle emissions directly on the road.
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From page 287... ...
The study reported that half of the CO emissions produced by the sampled vehicle population were from 7.0-10.2% of the vehicles, depending on location and time of day. More than 70% of the vehicles were measured to be emitting less than 1% CO.
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From page 288... ...
ATMOSPHERIC MEASUREMENTS VERSUS EMISSIONS INVENTORIES The distribution of chemical compounds in the atmosphere is governed by source characteristics and locations, air mass transport patterns, and the nature of the processes through which individual species are removed. All of these processes acting in concert must be understood in detail if the atmospheric measurements of the compounds are to be used to extract information concerriing any one of them.
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From page 289... ...
The natural sources, by contrast, tend to be widely dispersed, highly variable, and seasonal, with peak emissions usually occurring in the summer. In the case of the VOCs the principal compounds emitted from natural sources, the terpenoid compounds, are not emitted by anthropogenic sources.
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From page 290... ...
In all cases, the percentage of VOCs from mobile sources is much greater than the percentage of the mobile-source emissions used in each State Implementation Plan. Again, this difference is attributed to higher emissions of VOCs from operating vehicles than are estimated from emission factors deduced from measurements made on test vehicles.
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From page 291... ...
of isoprene and a-pinene for 10 cities that were studied during the summers of 1984 and 1985 are listed in Figure 9-10. For comparison with the anthropogenic VOCs, the percentage concentrations of the natural VOCs to the total are plotted in Figure 9-11 for these 10 cities.
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From page 292... ...
First, as discussed in Chapter 5, the natural VOCs are, in general, more reactive than are the anthropogenic VOCs. As a consequence, any chemical processing of the VOCs that occurs before early-morning samples are taken Hill tend to increase the concentrations of the anthropogenic VOCs relative to those of the more reactive natural VOCs.
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From page 293... ...
Third, because of the temperature and light dependence, the emissions of the natural VOCs will tend to increase during the daytime and peak during the periods of greatest photochemical activity. Finally, the concentration of samplers near street-level in downtown areas tends to discriminate against natural VOCs, whose sources tend to be concentrated in areas outside the core city.
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From page 294... ...
Rural Measurements As air masses leave the urban areas, they mix with air masses from other urban centers that can contain a somewhat different mix of ozone precursors. To this mixture are added the emissions from more isolated industrial sources or power plants, with their characteristic emission patterns.
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From page 295... ...
deposition, the difficulties associated with measurement increase the chance of sizable errors in measurement. This makes the extraction of useful information for emissions inventories extremely difficult.
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From page 296... ...
The measurements were made at the surface in forest clearings. Hence, they tend to amplify the concentration of the natural VOCs relative to anthropogenic VOCs.
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From page 297... ...
comm., 1990~. ATMOSPHERIC MEASUREMENTS VERSUS EMISSIONS INVENTORIES Fujita et al.
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From page 298... ...
Similar ambient CO/NOX ratios at each of the monitoring locations measured at the same times suggested a common emissions source. Both the ambient CO/NOX and VOC/NOX ratios were about 60-80~o higher than corresponding ratios derived from emissions inventories (Figures 9-13 and 9143.
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From page 299... ...
EI refers to CO/NOx ratios derived from emissions inventory data Gil. Ambient ratio refers to ratios obtained from ambient measurements taken between 7:00 a.m.
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From page 300... ...
The methods used to estimate emissions have not been adequately checked by intercomparison or field measurements. Ambient monitoring data from many urban and rural areas of the United States, along with data from roadside motor vehicle emissions tests, tunnel studies, and remote sensing studies of on-road vehicle exhaust, show that current inventories underestimate anthropogenic VOC and carbon monoxide (CO)
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From page 301... ...
Worth Los Angeles 301 1 0 2 4 ~ 8 10 12 VOC/N Ox [~1 Emissions inventory ~ Ambient measurements FIGURE 9-15 Comparison of VOC/NOx ratios derived from ambient measurements and emissions inventories for seven cities. Source: Morris et al., 1990.
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From page 302... ...
Biogenic VOC emissions appear to be of comparable magnitude to anthropogenic VOC emissions in the United States as a whole. Biogenic emissions can also be a significant source of VOCs in urban airsheds.
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