Mobile Lasers Help Test Bus Emissions
Atmospheric scientists used laser technology while riding in traffic behind New York City transit buses to find out exactly how much and what type of pollution different types of buses emit in their exhausts.
The findings may help other cities determine what kinds of buses to purchase for their transit systems.
The study found that while conventional diesel buses are more fuel efficient than other types of buses, they 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.
The mobile laboratory used to test emissions from buses driving their regular routes. (Photo courtesy Aerodyne Research, Inc.)
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.
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 buses went along their normal routes.
“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 types of buses the researchers’ 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. The team determined that each type of bus poses different pollution problems.
Herndon and Kolb analyzed the levels of nitrogen oxides (NO2), formaldehyde, methane, and sulfur dioxide (SO2) emissions read by the lasers.
The measurements showed that diesel buses fitted with traps produced less fine particle emissions, but increased the fraction of nitrous oxides emitted as NO2, rather than the less toxic NO, from five to 40 percent. 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 less SO2 than other New York City diesel buses and trucks, because the agency now supplies diesel fuel with a lower sulfur content for all of its diesel powered buses.
Provided by theEnvironmental News Service.
