Researchers from Aerodyne Research Inc. of Billerica, Mass., the New York state Department of Environmental Conservation in Albany and State University of New York at Albany reported their findings on the emissions of passenger buses at the fall meeting of the American Geophysical Union in San Francisco in December. The work, which employed tunable infrared laser direct absorption spectroscopy and aerosol mass spectrometry from a mobile step van, monitored the exhaust of diesel, natural gas and hybrid diesel/electric buses in use in New York City traffic.Under the auspices of the Environmental Protection Agency's Supersite program, the university's technology assessment and characterization study determined the emission ratios of NO, NO2, SO2, N2O, CO, CH4 and CH2O relative to CO2 and sampled particulate pollution. The team found that retrofitting diesel buses with continuous regenerative technology systems reduced the emission of particulates but increased the ratio of NO2 to total nitrous oxides. Natural gas buses produced high quantities of CH4 and CH2O. Hybrid and diesel vehicles using a new ultralow-sulfur fuel produced much less SO2.