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LASER
TECHNOLOGY HELPS MEASURE POLLUTION FROM NYC BUSES
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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
, and the results were surprising. The findings may
help other cities determine what kinds of buses to purchase
for their transit systems.
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.
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Hybrid
bus
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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.
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Orion
bus
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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.
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Orion
CNG bus
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"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.
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RTS
bus
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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.
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