Secondary organic aerosol production from diesel vehicle exhaust: impact of aftertreatment, fuel chemistry and driving cycle

摘要

Environmental chamber ("smog chamber") experiments were conducted toinvestigate secondary organic aerosol (SOA) production from dilute emissionsfrom two medium-duty diesel vehicles (MDDVs) and three heavy-duty dieselvehicles (HDDVs) under urban-like conditions. Some of the vehicles wereequipped with emission control aftertreatment devices, including dieselparticulate filters (DPFs), selective catalytic reduction (SCR) and dieseloxidation catalysts (DOCs). Experiments were also performed with differentfuels (100% biodiesel and low-, medium- or high-aromatic ultralow sulfurdiesel) and driving cycles (Unified Cycle,~Urban Dynamometer DrivingSchedule, and creep + idle). During normal operation, vehicles with acatalyzed DPF emitted very little primary particulate matter (PM).Furthermore, photooxidation of dilute emissions from these vehicles producedessentially no SOA (below detection limit). However, significant primary PMemissions and SOA production were measured during active DPF regenerationexperiments. Nevertheless, under reasonable assumptions about DPFregeneration frequency, the contribution of regeneration emissions to thetotal vehicle emissions is negligible, reducing PM trapping efficiency byless than 2%. Therefore, catalyzed DPFs appear to be very effective inreducing both primary PM emissions and SOA production from diesel vehicles.For both MDDVs and HDDVs without aftertreatment substantial SOA formed in thesmog chamber – with the emissions from some vehicles generating twice asmuch SOA as primary organic aerosol after 3 h of oxidation at typicalurban VOC / NO_x ratios (3 : 1). Comprehensive organic gasspeciation was performed on these emissions, but less than half of themeasured SOA could be explained by traditional (speciated) SOA precursors.The remainder presumably originates from the large fraction (~30%) ofthe nonmethane organic gas emissions that could not be speciated usingtraditional one-dimensional gas chromatography. The unspeciated organics –likely comprising less volatile species such as intermediate volatilityorganic compounds – appear to be important SOA precursors; we estimate thatthe effective SOA yield (defined as the ratio of SOA mass to reactedprecursor mass) was 9 ± 6% if both speciated SOA precursors andunspeciated organics are included in the analysis. SOA production from creep+ idle operation was 3–4 times larger than SOA production from the samevehicle operated over the Urban Dynamometer Driving Schedule (UDDS). Fuelproperties had little or no effect on primary PM emissions or SOA formation.