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How do Vehicle Emissions Affect Climate?
Earth Science Division - NASA Ames Research Center • 2006
Heavy and light duty vehicles exiting theCaldecott Tunnel near San Francisco, CA
The objective of this study is to measure the radiativeproperties of vehicle emissions in a real world situation andassess the effects of aerosol on regional climate.
It is well known that aerosol have a profound effect onregional and global climate, however, there still exist largeuncertainties in the measurement of key aerosol radiativeproperties, such as absorption and single scattering albedoand their distribution in the atmosphere. These effects arefelt directly, through direct scattering and absorption of solarradiation and indirectly by modifying cloud properties andprecipitation. Single scattering albedo, the ratio of scatteringto extinction, is an important metric for aerosol radiativeeffects and an important input into global climate models.
Measurements within the Caldecott TunnelMeasurements within the Caldecott Tunnel
The Caldecott Tunnel is a heavily-used commuter tunneleast of San Francisco. One unique feature is that heavyvehicles are restricted from bore 2 affording an opportunityto study light duty and mixed duty vehicles under realdriving conditions.Gas phase pollutants (CO2, CO, NOx) are measured at theinlet (I) and outlet (O) of tunnel. Aerosol size distribution(TSI SMPS) and optical properties (Cadenza,nephelometers) measured at outlet, just above traffic inexhaust duct. Black carbon (BC) mass is measured onfilters and aethalometers.Two tunnel bores: 1) light-duty (gasoline) vehicles and 2)mixed light-duty and heavy-duty (diesel) vehicles.
Schematic layout of the Caldecott Tunnel. Our instrumentationwas set up in an exhaust duct which is separated from the vehicletraffic.
Key Findings
1. Black Carbon (BC) dominates theradiatively active aerosol, especiallyduring peak traffic periods. The singlescattering albedo during these periodsapproaches that of pure BC emitted froma sooting diffusion flame.
2. The mass absorption efficiency of thetunnel aerosol is about 6 gm-2, which isat the low end of the range observed inthe atmosphere. This suggests that theefficiency of BC emitted from trucksincreases as it ages in the atmosphere.
3. The number size distribution of aerosolis dominated by freshly nucleatedparticles, but there is a radiatively activesoot accumulation mode (D~150 nm)and a larger mode (D~600 nm) ofunknown composition.
How do Vehicle Emissions Affect Climate?
Points of Contact:Dr. A.W. StrawaProject Principal Investigator650-604-3437, [email protected]://geo.arc.nasa.gov/sgp/aerocloud_web/homepage.html
ObservationsThe measurements of optical properties show a distinctdiurnal variation with peaks in the coefficients occurringduring the midday commute.
The single scattering albedo during the commute periods isvery close to the values for pure black carbon. Values ofoptical coefficients are much lower on weekends and singlescattering albedo is higher as expected.
While particle concentrations and optical coefficients are veryhigh, they are not unlike those from other tunnel studies.
The magnitude of extinction and absorption coefficients washigher in the dual use bore than in the in the light duty bore,however, the single scattering albedo appears to be lower inthe light duty bore. This is a surprising result and will beinvestigated further.
Schematic of Cadenza.
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Features of CadenzaCadenza [Strawa et al., 2003; Strawa et al., 2006] is a newinstrument that uses continuous wave cavity ring-down (CW-CRD) to measure the aerosol extinction coefficient for 675 nmand 1550 nm light, and simultaneously measure thescattering coefficient at 675 nm using a recipricalnephelometer [Mulholland and Bryner, 1994] concept. Bothmeasurements are made in the same optical and flow cell atexactly the same conditions with a temporal resolution ofabout 1 sec. Absorption coefficient is obtained from thedifference of measured extinction and scattering within theinstrument and single scattering albedo from their ratio.
At left: Instrumentation in the ventilation area above the tunneltraffic bore.
Below: Measurements of aerosol size distribution (top), extinctioncoefficient (middle), and single scattering albedo (bottom) duringthe day.
