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" From Calvert et al. (1993). h Evaporative emissions.

' Crankcase emissions of 4.1 g/mi not included; fully controlled. '' Emissions of NO,, (no standard) increased with control of HCs and CO. ' Change in test procedure.

' Non-methane HC standard (or 0.41 g/mi for total HCs).

s Optional HC standard, 0.3 g/mi, requires 7-year or 75,000-mi limited recall authority.

h A 0.7 NOa optional standard for 1983 and later, but requires limited recall authority for 7 years or 70,000 mi. ' Diesel passenger cars only.

" From Calvert et al. (1993). h Evaporative emissions.

' Crankcase emissions of 4.1 g/mi not included; fully controlled. '' Emissions of NO,, (no standard) increased with control of HCs and CO. ' Change in test procedure.

' Non-methane HC standard (or 0.41 g/mi for total HCs).

s Optional HC standard, 0.3 g/mi, requires 7-year or 75,000-mi limited recall authority.

h A 0.7 NOa optional standard for 1983 and later, but requires limited recall authority for 7 years or 70,000 mi. ' Diesel passenger cars only.

Fraction of cars (ranked by emissions)

FIGURE 16.33 Distribution of exhaust CO and hydrocarbon (HC) vehicle exhaust emissions as a function of model year in the United States (adapted from Stephens, 1994).

Fraction of cars (ranked by emissions)

FIGURE 16.33 Distribution of exhaust CO and hydrocarbon (HC) vehicle exhaust emissions as a function of model year in the United States (adapted from Stephens, 1994).

decline in CO has been observed in tunnel study measurements (Pierson, 1995).

A similar observation has been made in other countries where exhaust controls have been instituted. For example, vehicle emissions measurements made by remote sensing in Monterrey, Nuevo Léon, Mexico, showed that 1995 model years emitted 75% less CO, 70% less hydrocarbons, and 65% less NO compared to the pre-1991 vehicles without emission controls (Bishop et al., 1997). In Australia, emissions for pre-1986 vehicles were substantially larger than those from newer, catalyst-equipped cars, a factor of ~4 for hydrocarbons, ~2.5 for CO, and ~2 for NOx; in addition, the reactivity (see Section 16.B) of the exhaust emissions was also lower for the catalyst-equipped vehicles, 2.1 g of 03 per km driven compared to 8.2 g of 03 per km, due to a reduction in the relative amounts of alkenes and substituted aromatics (Duffy et al., f 999).

In Sweden, three-way catalysts have been required on all cars since 1989, and tax incentives were offered to purchase such vehicles in the 1987 and 1988 model years. Figure 16.34 shows the CO and hydrocarbon exhaust emissions as a function of model year of gasoline-powered cars, measured using a remote-sensing technique (Sjodin, 1994). There is a large decrease in the emissions from f987 to 1988 and f989, supporting the effectiveness of these motor vehicle exhaust controls.

For a description of some of the issues involved in motor vehicle emissions control, see Calvert et al. (1993), for those involving diesel emissions, see Walsh (1995), and for motorcycle emisions compared to passenger cars, see Chan et al. (1995).

While there has been a great deal of work on emissions from motor vehicles, with emphasis on why the VOC and CO emissions have been historically underestimated, a similar problem appears to exist with respect to stationary source emissions, at least in some areas. For example, Henry et al. (1997) measured organic gases in an industrial area in Houston, Texas, and compared them to reported emissions inventories. Application of a multivariate receptor model revealed large inconsistencies between the measurements and expected concentrations.

Another group of sources that have been increasingly recognized as being potentially important are biogenic in nature (see Section A.2 in Chapter 2 and Sections J.l and J.2 in Chapter 6). Organics such as isoprene are emitted by deciduous (hardwood) trees and to a smaller extent from other sources such as phytoplankton in the ocean (Graedel et al., 1986; Bonsang et al., 1992; Moore et al., 1994; Milne et al., 1995; McKay et al., 1996), and conifers (softwoods) are sources of organics such as a-pinene. It should be noted that recent studies indicate that some of the organics that are normally thought of as solely biogenic in origin, such as isoprene, may also be produced in automobile exhaust (e.g., see McLaren et al., 1996a). Methyl vinyl ketone and methacrolein, the major atmospheric oxidation products of isoprene, also appear to be generated in automobile exhaust (Biesenthal and Shepson, 1997). <i/-Limonene has been reported to be formed in significant yields in the pyrolysis of used automobile tires (Pakdel et al., 1991).

Such organics are highly reactive with essentially all oxidants of tropospheric interest, including OH, 03, N03, and chlorine atoms. For example, the lifetimes of

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