Andrew A. May

Gas-Particle Partitioning of Primary Organic Aerosol Emissions: (2) Diesel Vehicles

Experiments were performed to investigate the gas-particle partitioning of primary organic aerosol (POA) emissions from two medium-duty (MDDV) and three heavy-duty (HDDV) diesel vehicles. Each test was conducted on a chassis dynamometer with the entire exhaust sampled into a constant volume sampler (CVS). The vehicles were operated over a range of driving cycles (transient, high-speed, creep/idle) on different ultralow sulfur diesel fuels with varying aromatic content. Four independent yet complementary approaches were used to investigate POA gas-particle partitioning: artifact correction of quartz filter samples, dilution from the CVS into a portable environmental chamber, heating in a thermodenuder, and thermal desorption/gas chromatography/mass spectrometry (TD-GC-MS) analysis of quartz filter samples. During tests of vehicles not equipped with diesel particulate filters (DPF), POA concentrations inside the CVS were a factor of 10 greater than ambient levels, which created large and systematic partitioning biases in the emissions data. For low-emitting DPF-equipped vehicles, as much as 90% of the POA collected on a quartz filter from the CVS were adsorbed vapors. Although the POA emission factors varied by more than an order of magnitude across the set of test vehicles, the measured gas-particle partitioning of all emissions can be predicted using a single volatility distribution derived from TD-GC-MS analysis of quartz filters. This distribution is designed to be applied directly to quartz filter data that are the basis for existing emissions inventories and chemical transport models that have implemented the volatility basis set approach.

Intermediate-Volatility Organic Compounds: A Large Source of Secondary Organic Aerosol

Secondary organic aerosol (SOA) is a major component of atmospheric fine particle mass. Intermediate-volatility organic compounds (IVOCs) have been proposed to be an important source of SOA. We present a comprehensive analysis of atmospheric IVOC concentrations and their SOA production using measurements made in Pasadena, California during the California at the Nexus of Air Quality and Climate Change (CalNex) study. The campaign-average concentration of primary IVOCs was 6.3 ± 1.9 μg m(-3) (average ± standard deviation), which is comparable to the concentration of organic aerosol but only 7.4 ± 1.2% of the concentration of speciated volatile organic compounds. Only 8.6 ± 2.2% of the mass of the primary IVOCs was speciated. Almost no weekend/weekday variation in the ambient concentration of both speciated and total primary IVOCs was observed, suggesting that petroleum-related sources other than on-road diesel vehicles contribute substantially to the IVOC emissions. Primary IVOCs are estimated to produce about 30% of newly formed SOA in the afternoon during CalNex, about 5 times that from single-ring aromatics. The importance of IVOCs in SOA formation is expected to be similar in many urban environments.

Volatility of Organic Molecular Markers Used for Source Apportionment Analysis: Measurements and Implications for Atmospheric Lifetime

Molecular markers are organic species used to define fingerprints for source apportionment of ambient fine particulate matter. Traditionally, these markers have been assumed to be stable in the atmosphere. This work investigates the gas-particle partitioning of eight organic species used as molecular markers in receptor models for biomass burning (levoglucosan), motor vehicles (5α-cholestane, n-hexacosane, n-triacontane, 1,2-benz[a]anthracene, coronene), and meat cooking (cholesterol, oleic acid). Experiments were conducted using a thermodenuder to measure the evaporation of single component particles. The data were analyzed using the integrated volume method to determine saturation concentrations and enthalpies of vaporization for each compound. The results indicate that appreciable quantities (>10%) of most of these markers exist in the gas phase under typical atmospheric conditions. Therefore, these species should be considered semivolatile. Predictions from a chemical kinetics model indicate that gas-particle partitioning has important effects on the atmospheric lifetime of these species. The atmospheric decay of semivolatile compounds proceeds much more rapidly than nonvolatile compounds because gas-phase oxidation induces evaporation of particle-phase material. Therefore, both gas-particle partitioning and chemical reactions need to be accounted for when semivolatile molecular markers are used for source apportionment studies.

Aerosol emissions from prescribed fires in the United States: A synthesis of laboratory and aircraft measurements

Aerosol emissions from prescribed fires can affect air quality on regional scales. Accurate representation of these emissions in models requires information regarding the amount and composition of the emitted species. We measured a suite of submicron particulate matter species in young plumes emitted from prescribed fires (chaparral and montane ecosystems in California; coastal plain ecosystem in South Carolina) and from open burning of over 15 individual plant species in the laboratory. We report emission ratios and emission factors for refractory black carbon (rBC) and submicron nonrefractory aerosol and compare field and laboratory measurements to assess the representativeness of our laboratory-measured emissions. Laboratory measurements of organic aerosol (OA) emission factors for some fires were an order of magnitude higher than those derived from any of our aircraft observations; these are likely due to higher-fuel moisture contents, lower modified combustion efficiencies, and less dilution compared to field studies. Nonrefractory inorganic aerosol emissions depended more strongly on fuel type and fuel composition than on combustion conditions. Laboratory and field measurements for rBC were in good agreement when differences in modified combustion efficiency were considered; however, rBC emission factors measured both from aircraft and in the laboratory during the present study using the Single Particle Soot Photometer were generally higher than values previously reported in the literature, which have been based largely on filter measurements. Although natural variability may account for some of these differences, an increase in the BC emission factors incorporated within emission inventories may be required, pending additional field measurements for a wider variety of fires.

Temperature Dependence of Gas–Particle Partitioning of Primary Organic Aerosol Emissions from a Small Diesel Engine

A new experimental technique has been developed to study the gas–particle partitioning behavior of primary organic aerosol (POA) emissions from combustion sources at atmospherically relevant concentrations. The technique involves slowly filling a Teflon chamber with a constant emission source. As aerosol concentrations increase inside the chamber, the gas–particle partitioning of semivolatile organics shifts to the particle phase, thus increasing the fuel-based POA emission factor. The technique allows characterization of partitioning under isothermal conditions and atmospherically relevant concentrations. The technique was evaluated using emissions from a small diesel engine; the measured changes in gas–particle partitioning agreed well with previously published data for this engine measured with a dilution sampler. The temperature dependence of the gas–particle partitioning was investigated by conducting experiments at three different temperatures (15°C, 26°C, and 33°C). Increasing organic aerosol concentration and decreasing temperature increased the fuel-based POA emission factor. The gas–particle partitioning data were fit using absorptive partitioning theory to determine the volatility distribution and enthalpy of vaporization (ΔH v) of the emissions. We have derived two fits; one using the volatility basis set approach and a second using a two-product model. Both fits are suitable for use in chemical transport models. These fits were tested using previously published thermodenuder data. Partitioning calculations predict that the gas–particle partitioning from POA emissions from this engine vary by about a factor of 4 across the atmospherically relevant range of temperature and organic aerosol concentrations. This underscores the semivolatile nature of POA emissions. Copyright 2012 American Association for Aerosol Research

Intermediate Volatility Organic Compound Emissions from On-Road Diesel Vehicles: Chemical Composition, Emission Factors, and Estimated Secondary Organic Aerosol Production.

Emissions of intermediate-volatility organic compounds (IVOCs) from five on-road diesel vehicles and one off-road diesel engine were characterized during dynamometer testing. The testing evaluated the effects of driving cycles, fuel composition and exhaust aftertreatment devices. On average, more than 90% of the IVOC emissions were not identified on a molecular basis, instead appearing as an unresolved complex mixture (UCM) during gas-chromatography mass-spectrometry analysis. Fuel-based emissions factors (EFs) of total IVOCs (speciated + unspeciated) depend strongly on aftertreatment technology and driving cycle. Total-IVOC emissions from vehicles equipped with catalyzed diesel particulate filters (DPF) are substantially lower (factor of 7 to 28, depending on driving cycle) than from vehicles without any exhaust aftertreatment. Total-IVOC emissions from creep and idle operations are substantially higher than emissions from high-speed operations. Although the magnitude of the total-IVOC emissions can vary widely, there is little variation in the IVOC composition across the set of tests. The new emissions data are combined with published yield data to investigate secondary organic aerosol (SOA) formation. SOA production from unspeciated IVOCs is estimated using surrogate compounds, which are assigned based on gas-chromatograph retention time and mass spectral signature of the IVOC UCM. IVOCs contribute the vast majority of the SOA formed from exhaust from on-road diesel vehicles. The estimated SOA production is greater than predictions by previous studies and substantially higher than primary organic aerosol. Catalyzed DPFs substantially reduce SOA formation potential of diesel exhaust, except at low speed operations.

Intermediate Volatility Organic Compound Emissions from On-Road Gasoline Vehicles and Small Off-Road Gasoline Engines.

Dynamometer experiments were conducted to characterize the intermediate volatility organic compound (IVOC) emissions from a fleet of on-road gasoline vehicles and small off-road gasoline engines. IVOCs were quantified through gas chromatography/mass spectrometry analysis of adsorbent samples collected from a constant volume sampler. The dominant fraction (>80%, on average) of IVOCs could not be resolved on a molecular level. These unspeciated IVOCs were quantified as two chemical classes (unspeciated branched alkanes and cyclic compounds) in 11 retention-time-based bins. IVOC emission factors (mg kg-fuel(-1)) from on-road vehicles varied widely from vehicle to vehicle, but showed a general trend of lower emissions for newer vehicles that met more stringent emission standards. IVOC emission factors for 2-stroke off-road engines were substantially higher than 4-stroke off-road engines and on-road vehicles. Despite large variations in the magnitude of emissions, the IVOC volatility distribution and chemical characteristics were consistent across all tests and IVOC emissions were strongly correlated with nonmethane hydrocarbons (NMHCs), primary organic aerosol and speciated IVOCs. Although IVOC emissions only correspond to approximately 4% of NMHC emissions from on-road vehicles over the cold-start unified cycle, they are estimated to produce as much or more SOA than single-ring aromatics. Our results clearly demonstrate that IVOCs from gasoline engines are an important class of SOA precursors and provide observational constraints on IVOC emission factors and chemical composition to facilitate their inclusion into atmospheric chemistry models.

Comparison of Gasoline Direct-Injection (GDI) and Port Fuel Injection (PFI) Vehicle Emissions: Emission Certification Standards, Cold-Start, Secondary Organic Aerosol Formation Potential, and Potential Climate Impacts

Recent increases in the Corporate Average Fuel Economy standards have led to widespread adoption of vehicles equipped with gasoline direct-injection (GDI) engines. Changes in engine technologies can alter emissions. To quantify these effects, we measured gas- and particle-phase emissions from 82 light-duty gasoline vehicles recruited from the California in-use fleet tested on a chassis dynamometer using the cold-start unified cycle. The fleet included 15 GDI vehicles, including 8 GDIs certified to the most-stringent emissions standard, superultra-low-emission vehicles (SULEV). We quantified the effects of engine technology, emission certification standards, and cold-start on emissions. For vehicles certified to the same emissions standard, there is no statistical difference of regulated gas-phase pollutant emissions between PFIs and GDIs. However, GDIs had, on average, a factor of 2 higher particulate matter (PM) mass emissions than PFIs due to higher elemental carbon (EC) emissions. SULEV certified GDIs have a factor of 2 lower PM mass emissions than GDIs certified as ultralow-emission vehicles (3.0 ± 1.1 versus 6.3 ± 1.1 mg/mi), suggesting improvements in engine design and calibration. Comprehensive organic speciation revealed no statistically significant differences in the composition of the volatile organic compounds emissions between PFI and GDIs, including benzene, toluene, ethylbenzene, and xylenes (BTEX). Therefore, the secondary organic aerosol and ozone formation potential of the exhaust does not depend on engine technology. Cold-start contributes a larger fraction of the total unified cycle emissions for vehicles meeting more-stringent emission standards. Organic gas emissions were the most sensitive to cold-start compared to the other pollutants tested here. There were no statistically significant differences in the effects of cold-start on GDIs and PFIs. For our test fleet, the measured 14.5% decrease in CO2 emissions from GDIs was much greater than the potential climate forcing associated with higher black carbon emissions. Thus, switching from PFI to GDI vehicles will likely lead to a reduction in net global warming.