The study found that conventional diesel buses are comparatively fuel efficient, but produce nitrogen oxide pollutants that can contribute to photochemical smog as well as large amounts of fine soot and sulfate particles, which are suspected to contribute to heart disease and lung cancer.
Photochemical smog develops when primary pollutants (nitrogen oxides and volatile organic compounds created from fossil fuel combustion) interact with sunlight and produce a mixture of hundreds of different and hazardous chemicals known as secondary pollutants.
Types of buses the researcher's tested included diesel buses with pollution controls called soot particle oxidation traps and without controls, new compressed natural gas fueled buses; and hybrid diesel/electric buses. Buses were tested while on their regular routes. They then determined that each type of bus poses different pollution problems.
Scott Herndon and Charles Kolb, of Aerodyne Research used a mobile step van laboratory with fast response laser sensors that provided emission results every second. The laser sensors generated a low-power light beam that measured pollutant levels in samples of the target vehicle's exhaust plumes drawn into the van as the buses went along their normal routes.
Herndon and Kolb analyzed the levels of nitrogen oxides (NO2), formaldehyde, methane, and sulfur dioxide (SO2) emissions read by the lasers.
Nitric oxide forms inside automobile engines and slowly reacts with oxygen to create nitrogen dioxide. NO2 itself is responsible for the brownish haze that hangs over many cities during the afternoon of sunny days. Formaldehyde also contributes to urban smog; methane is a powerful greenhouse gas; SO2 contributes to additional fine particle formation and acid rain; and both formaldehyde and NO2 are toxic and contribute to allergic reactions in some people.
"Normally, emissions from large vehicles, like buses, are measured at specialized facilities. The ability to measure in-use emissions from a large number of buses and heavy duty trucks during their routine use is new and gives us a much more accurate picture of their impact on air quality issues," Kolb said.
The measurements showed that the CRT technology for diesel buses did reduce fine particle emissions as expected, but increased the fraction of nitrous oxides emitted as NO2, rather than the less toxic NO, from 5 to 40%. The Compressed Natural Gas (CNG) powered buses also emitted much less particulate matter than diesel buses, but emitted troubling quantities of methane and formaldehyde. New York Metropolitan Transit Authority (MTA) diesel buses and diesel-electric hybrids released significantly less SO2 than other New York City diesel buses and trucks, primarily because the agency now supplies diesel fuel with a lower sulfur content for all of its diesel powered buses.
In addition to the lasers, the researchers used a condensation particle counter that counts fine pollution particles and determines their sizes. They also used an aerosol mass spectrometer that separates particles by size and vaporizes their chemical constituents for analysis.
The research was performed under an award from the Atmospheric Sciences Research Center of the State University of New York at Albany as part of its PM2.5 Technology and Assessment and Characterization Study in New York, one of several U.S. EPA Supersite programs operating around the country, and in cooperation with the New York Metropolitan Transit Authority (MTA) that owns and operates many of the buses in NYC.
The laser technology had previously been demonstrated for urban emission measurements in Boston, Mass. and Manchester, N.H. in collaboration with groups from the Massachusetts Institute for Technology, the University of New Hampshire and Washington State University.
The authors will present their poster titled "Gas Phase Emission Ratios From In-Use Diesel and CNG Curbside Passenger Buses in New York City" at the Fall AGU meeting on Friday, December 6th.
This research was funded by U.S. EPA using technology and methods developed under a NASA Earth Observing System/International Working Group grant on Urban Trace Gas Respiration and Metabolism.