Development of a Photochemical Chamber System for Determining Measures of Reactivity from Exhaust of Alternative-Fuel Vehicles 941906

A laboratory system has been developed to evaluate the photochemical characteristics of exhaust generated from alternative-fuel and new technology vehicles. This system is built around an 8-m³ photochemical chamber constructed of Teflon film. The chamber has been designed to be portable and allow both indoor and outdoor irradiations. The indoor irradiation system uses a combination of ultraviolet fluorescent bulbs to match, to the extent possible, solar radiation. Exhaust from the test vehicles is generated in an adjoining dynamometer facility. Exhaust can be added directly to the chamber from the dilution tunnel or collected in intermediate Teflon bags and then added to the chamber. For vehicles having relatively low emissions the second approach is required. Instrumentation associated with the chamber system includes continuous monitors for measuring NO_x and ozone, hydrocarbons from both reactant and product mixtures, peroxyacyl nitrates, and carbonyl compounds.

Irradiations were conducted on 10 vehicle-fuel combinations. The fuels include reformulated and other alternative fuels. Emissions were collected during Bag 1 of the Federal Test Procedure to ensure sufficient exhaust hydrocarbon during the irradiation. Similar initial conditions were set for each vehicle-fuel combination to allow evaluation (to the extent possible) of differences in reactivity of the exhaust mixture. Reactivity parameters, such as the rate of NO oxidation and the rate of ozone formation, were examined for irradiations of the exhaust mixtures. Formation of toxic species (peroxyacyl nitrates and carbonyls) was also determined during the study. This paper describes the motivation for the experiments, details on the experimental design, and the data from irradiations of the exhaust of three fuels using a single vehicle.

The reduction in air pollution resulting from the advances in motor vehicle emission control technology of the 1980s and 1990s is being offset by increased numbers of vehicles on the nation's roadways (growth in vehicle miles traveled), and by a persistence of malfunctioning vehicles. As a consequence, the U.S. Congress, in the 1990 Clean Air Act Amendments [1], has required further reduction of emissions from new motor vehicles, introduction of environmentally favorable reformulations of conventional diesel and gasoline fuels, and introduction of alternative fuels such as methanol, ethanol, and natural gas with associated vehicle technologies. Gasoline reformulation must result in reduction of both ozone-forming volatile organic compound (VOC) and toxic compound (formaldehyde, acetaldehyde, polycyclic organic matter, benzene, and 1,3-butadiene) emissions. The Alternative Motor Fuels Act of 1988 (AMFA) also mandated the introduction of alternatives to conventional vehicle and fuel technologies.

This legislation and the considerable difficulty in meeting the National Ambient Air Quality Standard for ozone and CO in metropolitan areas have resulted in initiatives to develop reformulated and alternative fuels. With historical phase-out of leaded gasoline additives, aromatic hydrocarbon levels have increased to maintain necessary fuel octane. However, aromatic compounds can be important ozone precursors and contribute to elevated benzene emissions. Part of the developing strategy to maintain octane while removing aromatic compounds has been the introduction of alcohols and aliphatic ethers into gasoline blends. For example, methyl tertiary-butyl ether (MTBE) and ethanol are being used in many areas in the country as additives in winter grade gasolines for CO emissions reduction. Oxygenates will also be used in summer gasolines in areas requiring reformulated gasoline for ozone abatement.

In addition to changes and improvements in fuels, considerable efforts are being made by automotive companies to produce vehicles that generate lower levels of exhaust and evaporative emissions. New technology vehicles are also being developed to use alternatives to conventional fuels. The federal government, through AMFA, is leading in the purchase of vehicles that operate on methanol, ethanol, and compressed natural gas (CNG). This effort is expected to demonstrate feasibility for introduction of large numbers of these vehicles into the general population over the next dozen years. As new fuels and vehicle technologies are introduced and ultimately become predominant, the distribution of compounds introduced into the atmosphere, particular the urban atmosphere, will change considerably.

Thus, the potential air quality effects of vehicle exhaust and evaporative emissions resulting from use of these fuels and their photodegradation products must be assessed. This information is vital for determining tropospheric lifetimes, effects on ozone formation, and possible generation of toxic and genotoxic products in the atmosphere. In response to these needs, a multitude of changes in vehicle design, fuel composition, and emission control technology are being investigated by the U.S. Department of Energy, the National Renewable Energy Laboratory, the U.S. Environmental Protection Agency, automotive manufacturers, and petroleum producers.

The formation of ozone and other toxic compounds from photochemical transformations of automotive exhaust are frequently studied in laboratory smog chambers. Frequently, surrogate mixtures are used to represent automotive exhaust and/or evaporative emissions. This approach allows reproducible mixtures to be produced in the chamber to examine the effect of individual compounds or other physical variables. However, this approach makes it difficult, if not impossible, to experimentally compare the reactivity and toxics formation of the whole exhaust and evaporative emissions mixtures associated with the use various fuels.

The experimental program described in this paper has been designed to test reactivity parameters by experimental means. In the first phase of this program, the reactivity of emissions from a series of vehicles utilizing different types of alternative fuels is examined. The use of this approach has required substantial methods development in both the design of the experimental system and the procedures for carrying out the experiments.

A dynamometer/photochemistry chamber system has been developed to examine the reactivity of exhaust from vehicles operating on alternative fuels. Exhaust from test vehicles is generated for injection into a smog chamber by using dynamometer simulations of roadway driving conditions. The exhaust is irradiated, most of the major organic and inorganic constituents are measured, and several reactivity parameters are determined. Preliminary data from a subset of the runs conducted are presented to demonstrate the capabilities of the experimental system.