Abstract
Secondary organic aerosol (SOA) formation from in-use vehicle emissions was investigated using a potential aerosol mass (PAM) flow reactor deployed in a highway tunnel in Pittsburgh, Pennsylvania. Experiments consisted of passing exhaust-dominated tunnel air through a PAM reactor over integrated hydroxyl radical (OH) exposures ranging from ∼ 0.3 to 9.3 days of equivalent atmospheric oxidation. Experiments were performed during heavy traffic periods when the fleet was at least 80% light-duty gasoline vehicles on a fuel-consumption basis. The peak SOA production occurred after 2-3 days of equivalent atmospheric oxidation. Additional OH exposure decreased the SOA production presumably due to a shift from functionalization to fragmentation dominated reaction mechanisms. Photo-oxidation also produced substantial ammonium nitrate, often exceeding the mass of SOA. Analysis with an SOA model highlight that unspeciated organics (i.e., unresolved complex mixture) are a very important class of precursors and that multigenerational processing of both gases and particles is important at longer time scales. The chemical evolution of the organic aerosol inside the PAM reactor appears to be similar to that observed in the atmosphere. The mass spectrum of the unoxidized primary organic aerosol closely resembles ambient hydrocarbon-like organic aerosol (HOA). After aging the exhaust equivalent to a few hours of atmospheric oxidation, the organic aerosol most closely resembles semivolatile oxygenated organic aerosol (SV-OOA) and then low-volatility organic aerosol (LV-OOA) at higher OH exposures. Scaling the data suggests that mobile sources contribute ∼ 2.9 ± 1.6 Tg SOA yr(-1) in the United States, which is a factor of 6 greater than all mobile source particulate matter emissions reported by the National Emissions Inventory. This highlights the important contribution of SOA formation from vehicle exhaust to ambient particulate matter concentrations in urban areas.
Highlights
Motor vehicle emissions are an important source of particulate matter (PM) in urban areas,[1,2] adversely affecting public health and influencing climate.[3,4] Vehicles directly emit PM, which is mainly comprised of black carbon (BC) and primary organic aerosols (POA)
Vehicle exhaust forms “secondary” PM through the oxidation of gas-phase organic emissions to form secondary organic aerosol (SOA) and the oxidation of oxides of nitrogen to form nitrate aerosol.[5−8] Substantial SOA production occurs downwind of urban areas, but models based on laboratory-measured SOA yields fail to explain these observations.[9−12] A key question is: what role do vehicle emissions play in ambient SOA production? Addressing this question requires investigating SOA formation from vehicles that are representative of actual traffic fleets and driving conditions
volatile organic compounds (VOC), BC, NOx, and POA emissions measured in the tunnel fall within the range of gasoline and diesel emissions reported in previous studies.[33,34]
Summary
Motor vehicle emissions are an important source of particulate matter (PM) in urban areas,[1,2] adversely affecting public health and influencing climate.[3,4] Vehicles directly emit PM, which is mainly comprised of black carbon (BC) and primary organic aerosols (POA). Recent studies have investigated SOA formation from dilute vehicle emissions using smog chambers.[6,8,13−15] In these studies, emissions from individual vehicles were photo-oxidized via UV-irradiation in a smog chamber under various conditions. Since vehicle exhaust can reside in the atmosphere for up to a week or more, chamber studies may not characterize the full SOA production potential associated with multiple generations of oxidation.[16] In addition, smog chamber experiments have only been performed with exhaust from a small number of vehicles that may or may not be representative of actual in-use vehicle fleets. Many tunnel studies have characterized primary emissions from large fleets of motor vehicles,[17−19] but to date, no tunnel studies have quantified SOA formation from motor vehicle emissions. The data are compared to ambient measurements and scaled to provide an estimate of the contribution of in-use vehicles to SOA formation on the national level
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