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Atmospheric Change and the North American Transportation Sector:: Summary of a Trilateral Workshop Atmospheric Changes Resulting from Transportation Activities When fossil fuels are burned for transportation, gases and particles are emitted which lead to changes in atmospheric composition and functioning on local, regional, and global scales. The large contribution that transportation related emissions make to the total air pollution burden is illustrated by the data in Table 1. Table 1. Contribution of the transportation sector to total emissions of major air pollutants for 1996 (comparable data for Mexico were not available) United States a(%) Canada b(%) carbon dioxide (CO2) 35 32 nitrogen oxides (NOx) 49 40-80 volatile organic compounds (VOCs) 42 35-55 carbon monoxide (CO) 79 benzene --- 60-70 particulate matter (PM10) 26 10-40 a U.S. EPA, b Environment Canada Ruth Reck, currently with the National Institute for Global Environmental Change, noted that in Canada and the United States, emissions of many of these pollutants have been dramatically reduced since the early 1970s. Lead has been eliminated as a gasoline additive, and emissions of criteria pollutants such as VOCs, CO, and NOx have been cut by over 90 percent. Although Mexico City has much higher atmospheric concentrations of many criteria pollutants (Figure 3), it too has managed to reduce emissions of CO, VOCs, and lead significantly in recent years. However, in all three countries there has been only limited progress in meeting some air quality goals. Secondary pollutants such as ozone and some particulates have proven very difficult to control. Total emissions of greenhouse gases such as CO2 are increasing. These continuing problems are largely attributable to the following factors: many old, highly polluting cars are still on the road, the total number of vehicle miles traveled keeps increasing, and few or no gains in fleet fuel efficiency have been made since the early 1980's. Mobile source emissions Michael Rodgers, from the Georgia Institute of Technology, explained that when looking at mobile source emissions (MSEs), most of the attention is usually given to passenger cars. However, other categories of surface transportation, including heavy and light duty trucks, off-road vehicles (such as farm machinery and construction vehicles), buses, trains, inland marine, and small engines (such as lawn mowers), also contribute significantly to MSEs. There also are several different ‘mechanisms' of mobile source emission to consider: tailpipe emissions — both hot-stabilized and startup phase (latter is responsible for a large fraction of emissions) evaporative — losses from refueling and fuel storage, losses while the engine is running and immediately after the engine is shut off excess — unintentional losses from broken vehicles; deliberate losses when vehicles use extra power for accelerating (emissions increase tremendously during this phase) Because of this multitude of sources and emission mechanisms, MSE estimates are highly uncertain; emission inventories for some gases are thought to be incorrect by as much as a factor of two. Predicting future emission levels is even more challenging, as one must try to estimate fleet turnover rates, future travel demand, the market penetration of alternative fuel vehicles, and several other factors. Local and regional scale atmospheric changes Some pollutants, such as lead, carbon monoxide, benzene, and other hydrocarbons, have relatively short atmospheric lifetimes and thus their effects are generally confined to the local scale. Other pollutants, however, have lifetimes on the scale of days to weeks, and thus atmospheric transport plays an important role in determining their fate. Ann McMillan, from Environment Canada, described these regional scale atmospheric changes, including acidic deposition, photochemical smog/ozone, and particulates. The worst impacts of these pollutants often occur in different states, provinces, or even different countries than where they are emitted. For example, studies have shown that about half the smog pollution occurring in southern Ontario is due to emissions from the United States. Similar pollution transport issues could be expected for the U.S./ Mexico border as well.
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Atmospheric Change and the North American Transportation Sector:: Summary of a Trilateral Workshop Figure 3. Average measured concentrations of the air pollutants O3, NO2, and PM10 in selected cities during the years 1988-1993. The values in parentheses denote the number of monitoring sites used in each city. (Note that the U.S. NAAQS 03 limit will be changing to an eight-hour average of 80 ppbv.) From the Ontario Ministry of Environment.
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Atmospheric Change and the North American Transportation Sector:: Summary of a Trilateral Workshop Acid rain: SO2 and NOx, which are emitted by both transportation related and other sources, are oxidized in the atmosphere to form sulfuric and nitric acid, leading to the well known phenomenon of acid rain. There is a relatively good scientific understanding of the issue of acid rain, and strong policies are in place to mitigate SO2 emissions in the United States and Canada. With declining sulfur emissions, the role of nitrogen compounds in acidic deposition is now growing more important, and NOx emissions from the transportation sector are still a major problem. Photochemical ozone: Tropospheric ozone pollution continues to plague all three countries. Emissions of ozone's precursors, VOCs and NOx, are not easy to characterize, and as the relevant chemistry is complex, drawing simple relationships between precursor emissions and the occurrence of ozone is very difficult. In addition, surface ozone concentrations are highly dependent on local meteorology. There is now a major trilateral research program in place (the North American Research Strategy for Tropospheric Ozone, NARSTO) which is aimed at improving our understanding of the formation and transport of ozone. Particulates: Particulate matter can affect human health, visibility, and climate and thus is the subject of intense research attention. Transportation related particulate sources include dust from unpaved roads, direct emission from fossil fuel combustion, and atmospheric formation from combustion gases. Measurements of the concentration, size distribution, and chemical composition of particulates are very sparse and need to be improved if we are to better characterize the impacts. McMillan emphasized that these three issues are closely linked, and to whatever extent possible, assessments, research programs, and regulations should try to handle them in an integrated fashion. Because of the transboundary nature of the problems, they are most effectively addressed on a multinational basis. Michelle Broido, from the U.S. Department of Energy, pointed out that the NARSTO program provides a good example of this multi-national, cooperative approach. Global scale atmospheric changes Mary Anne Carroll, a professor from the University of Michigan, explained that the transportation sector also contributes to global scale atmospheric changes including perturbations in tropospheric oxidant levels, increased concentrations of greenhouse gases and aerosols, and stratospheric ozone depletion. Compared to the issues discussed in the previous section, these global scale problems can be much more difficult to address, because the ultimate impacts are usually indirect and difficult to quantify. Tropospheric oxidants: Oxidants cleanse the atmosphere of many pollutants. The hydroxyl radical (OH) is the primary atmospheric oxidant, but is it hard to measure quantitatively due to its small concentrations and very short lifetime; as a consequence, ozone, which is the primary precursor of tropospheric OH, is often measured as a surrogate. Although tropospheric ozone is usually regarded as a regional scale pollutant, it can be affected on much larger scales. Evidence indicates that there has been a two-fold increase in levels of tropospheric ozone in the Northern Hemisphere since pre-industrial times. However, significant uncertainties remain about the distribution and shorter-term trends of ozone and other oxidants, as well as the impacts their changing concentrations may have on global atmospheric chemistry. Greenhouse gases and aerosols: Decades of measurements document the accumulation of CO2 in the atmosphere, and this increase is well correlated with emissions from fossil fuel combustion. The increased radiative forcing due to this build-up of CO2 (and other greenhouse gases) is relatively well quantified. Atmospheric concentrations of particulate matter also have increased enough to cause significant perturbation of the global radiative budget. However, aerosol radiative forcing, which occurs both directly by scattering incoming UV, and indirectly by affecting cloud formation, is very poorly quantified at this time. Stratospheric ozone depletion: At this point, both the gas phase and the heterogeneous mechanisms that lead to stratospheric ozone loss are relatively well understood. In response to the Montreal Protocol, the use of CFCs in vehicle air conditioning systems has been sharply curtailed, and now the atmospheric concentrations of these ozone depleting species are decreasing or at least leveling off. But it will still take decades for stratospheric ozone levels over the Antarctic to recover, and meanwhile, there have been measurable ozone losses over the arctic and temperate latitudes. Carroll noted that, as with the regional scale issues, these global changes are highly interrelated. There are numerous feedbacks between atmospheric composition, climate, UV flux, biological productivity and emissions.
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Atmospheric Change and the North American Transportation Sector:: Summary of a Trilateral Workshop There is much research that needs to be done to gain a comprehensive, quantitative understanding of these relationships. Surface transportation is not the only contributor to atmospheric change — aviation constitutes the fastest growing component of the transportation sector, and it may contribute to the global scale atmospheric changes described above. Howard Wesoky, from the National Aeronautics and Space Administration, talked about the major research and assessment programs (both in the U.S. and Europe) designed to quantify the atmospheric impacts of subsonic and supersonic aircraft. Aircraft emissions of CO2, water vapor, and particulates can cause direct radiative forcing, although this effect is estimated to be quite small. A more recent concern is that contrails and particulate emissions may increase the abundance of cirrus clouds, which could, in turn, have significant climatic impacts. There is also concern that emissions of NOx and other species can perturb the concentrations of ozone in the troposphere and stratosphere.
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