Motor Vehicle Emissions Trends

Motor vehicles emit large quantities of carbon monoxide (CO), hydrocarbons (HC), nitrogen oxides (NOx), particulate matter (PM), sulfur oxides (SOx) and such toxic substances as benzene, formaldehyde, acetaldehyde, 1,3-butadiene and (where lead is still added to gasoline) lead. Each of these, along with secondary by-products such as ozone and small particles (e.g. nitrates and sulfates), can cause serious adverse effects on health and the environment. Because of growing vehicle populations and resulting emissions, the fraction of health damaging pollution due to motor vehicles remains significant throughout the developed world and is rising in many cities in the developing world.

The greenhouse gases (GHGs) most closely identified with the transportation sector are the main Kyoto gases, CO2, nitrous oxide and methane as well as the HFC's in vehicle air conditioning systems. The global warming potentials (GWPs)1 of nitrous oxide and methane, relative to CO2, are identified in Table 6.1. However, it is important to note that other vehicle-related pollutants also contribute to global warming, although their quantification has been more difficult; these include CO, non-methane hydrocarbons (NMHC) and nitrogen dioxide. It is generally agreed for example that CO emitted from vehicles is eventually converted to carbon dioxide in the atmosphere and in the process consumes hydroxyl radicals which might otherwise reduce methane concentrations. Similarly NMHCs and NOx contribute to global background tropospheric ozone, a very potent greenhouse gas. The GWPs listed in Table 6.1, including those attributed to CO, NMHCs and nitrogen dioxide, are from the original (1990) Intergovernmental Panel on Climate Change (IPCC) report and based on a 100 year timeframe.2

The IPCC has changed its GWP estimates with each new assessment report as newer data and information have become available, most recently in 2007 in the fourth assessment report. Even since that report, there has been a growing concern that black carbon (BC) or soot emitted from diesel vehicles and other sources is a

Table 6.1 Global warming potentials of transportation pollutants (Source: Intergovernmental Panel on Climate Change, 1990 and 1996) [4, 5]_

Carbon

Nitrous

Carbon

Nonmethane

Nitrogen

dioxide

Methane

oxide

monoxide

hydrocarbons

dioxide

GWP

(CO2)

(CH4)

(N2O)

(CO)

(NMHC)

(NO2)

100-Year time

1

21

310

3

11

7

horizon

1 Global warming potential is a measure of how much a given mass of a pollutant will contribute to global warming relative to the same mass of carbon dioxide which by definition is given a value of 1.

2 Because of difficulty reaching agreement on the appropriate quantification, specific GWPs for these gases were not contained in the most recent Intergovernmental Panel on Climate Change (IPCC) report.

1 Global warming potential is a measure of how much a given mass of a pollutant will contribute to global warming relative to the same mass of carbon dioxide which by definition is given a value of 1.

2 Because of difficulty reaching agreement on the appropriate quantification, specific GWPs for these gases were not contained in the most recent Intergovernmental Panel on Climate Change (IPCC) report.

potent greenhouse gas in part due to the snow albedo (reflectivity) effect for solar radiation.3 Hansen et al. for example estimate the GWP for soot is ~2,000 for 20 year, ~500 for 100 years, and ~200 for 500 years [6]. Similarly Jacobsen estimates a BC GWP range from about 800-1,200 [7]. Using a different metric, CO2 Equivalency Factor, Mark Delucchi of University of California, Davis ascribes a value of 4,684 to black carbon.4 As recently noted by Ramanathan and Carmichael, "Because of the combination of high absorption, a regional distribution roughly aligned with solar irradiance, and the capacity to form widespread atmospheric brown clouds in a mixture with other aerosols, emissions of black carbon are the second strongest contribution to current global warming, after carbon dioxide emissions. In the Himalayan region, solar heating from black carbon at high elevations may be just as important as carbon dioxide in the melting of snowpack's and glaciers" [8]. Of course, there is not yet a universal consensus on this high ranking of black carbon.

Great progress in reducing emissions of the urban air pollutants and their precursors from gasoline-fueled cars has occurred in the major industrialized countries and stringent requirements for diesel vehicles are starting to be phased in. However, the vehicle population and vehicle kilometers traveled are expected to continue to grow rapidly in the future especially in developing countries which will offset many of the gains to date [3].

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