Secondary organic aerosol formation from photo-oxidation of unburned fuel: experimental results and implications for aerosol formation from combustion emissions.

We conducted photo-oxidation experiments in a smog chamber to investigate secondary organic aerosol (SOA) formation from eleven different unburned fuels: commercial gasoline, three types of jet fuel, and seven different diesel fuels. The goals were to investigate the influence of fuel composition on SOA formation and to compare SOA production from unburned fuel to that from diluted exhaust. The trends in SOA production were largely consistent with differences in carbon number and molecular structure of the fuel, i.e., fuels with higher carbon numbers and/or more aromatics formed more SOA than fuels with lower carbon numbers and/or substituted alkanes. However, SOA production from different diesel fuels did not depend strongly on aromatic content, highlighting the important contribution of large alkanes to SOA formation from mixtures of high carbon number (lower volatility) precursors. In comparison to diesels, SOA production from higher volatility fuels such as gasoline appeared to be more sensitive to aromatic content. On the basis of a comparison of SOA mass yields (SOA mass formed per mass of fuel reacted) and SOA composition (as measured by an aerosol mass spectrometer) from unburned fuels and diluted exhaust, unburned fuels may be reasonable surrogates for emissions from uncontrolled engines but not for emissions from engines with after treatment devices such as catalytic converters.

Fuel composition and secondary organic aerosol formation: gas-turbine exhaust and alternative aviation fuels.

A series of smog chamber experiments were performed to investigate the effects of fuel composition on secondary particulate matter (PM) formation from dilute exhaust from a T63 gas-turbine engine. Tests were performed at idle and cruise loads with the engine fueled on conventional military jet fuel (JP-8), Fischer-Tropsch synthetic jet fuel (FT), and a 50/50 blend of the two fuels. Emissions were sampled into a portable smog chamber and exposed to sunlight or artificial UV light to initiate photo-oxidation. Similar to previous studies, neat FT fuel and a 50/50 FT/JP-8 blend reduced the primary particulate matter emissions compared to neat JP-8. After only one hour of photo-oxidation at typical atmospheric OH levels, the secondary PM production in dilute exhaust exceeded primary PM emissions, except when operating the engine at high load on FT fuel. Therefore, accounting for secondary PM production should be considered when assessing the contribution of gas-turbine engine emissions to ambient PM levels. FT fuel substantially reduced secondary PM formation in dilute exhaust compared to neat JP-8 at both idle and cruise loads. At idle load, the secondary PM formation was reduced by a factor of 20 with the use of neat FT fuel, and a factor of 2 with the use of the blend fuel. At cruise load, the use of FT fuel resulted in no measured formation of secondary PM. In every experiment, the secondary PM was dominated by organics with minor contributions from sulfate when the engine was operated on JP-8 fuel. At both loads, FT fuel produces less secondary organic aerosol than JP-8 because of differences in the composition of the fuels and the resultant emissions. This work indicates that fuel reformulation may be a viable strategy to reduce the contribution of emissions from combustion systems to secondary organic aerosol production and ultimately ambient PM levels.

Photo-oxidation of low-volatility organics found in motor vehicle emissions: production and chemical evolution of organic aerosol mass.

Recent research has proposed that low-volatility organic vapors are an important class of secondary organic aerosol (SOA) precursors. Mixtures of low-volatility organics were photo-oxidized in a smog chamber under low- and high-NO(x) conditions. Separate experiments addressed emission surrogates (diesel fuel and motor oil) and single components (n-pentacosane). Both diesel fuel and motor oil are major components of exhaust from diesel engines. Diesel fuel is a complex mixture of intermediate volatility organic compounds (IVOCs), whereas motor oil is a complex mixture of semivolatile organic compounds (SVOCs). IVOCs exist exclusively in the vapor phase, while SVOCs exist in both the aerosol and vapor phase. Oxidation of SVOC vapors (motor oil and n-pentacosane) creates substantial SOA, but this SOA is largely offset by evaporation of primary organic aerosol (POA). The net effect is a cycling or pumping of SVOCs between the gas and particle phases, which creates more oxygenated organic aerosol (OA) but little new OA mass. Since gas-phase reactions are much faster than heterogeneous ones, the processing of SVOC vapors likely contributes to the production of highly oxidized OA. The interplay between gas-particle partitioning and chemistry also blurs traditional definitions of POA and SOA. Photo-oxidation of diesel fuel (IVOCs) rapidly creates substantial new OA mass, similar to published aging experiments with dilute diesel exhaust. However, aerosol mass spectrometer (AMS) data indicated that the SOA formed from emission surrogates is less oxidized than either the oxygenated organic aerosol (OOA) measured in the atmosphere or SOA formed from the photo-oxidation of dilute diesel exhaust. Therefore, photo-oxidation of IVOCs helps explain the substantial SOA mass produced from aging diesel exhaust, but some component is missing from these emission surrogate experiments that leads to the rapid production of highly oxygenated SOA.

Secondary organic aerosol formation from high-NO(x) photo-oxidation of low volatility precursors: n-alkanes.

Smog chamber experiments were conducted to investigate secondary organic aerosol (SOA) formation from photo-oxidation of low-volatility precursors; n-alkanes were chosen as a model system. The experiments feature atmospherically relevant organic aerosol concentrations (C(OA)). Under high-NO(x) conditions SOA yields increased with increasing carbon number (lower volatility) for n-decane, n-dodecane, n-pentadecane, and n-heptadecane, reaching a yield of 0.51 for heptadecane at a C(OA) of 15.4 microg m(-3). As with other photo-oxidation systems, aerosol yield increased with UV intensity. Due to the log-linear relationship between n-alkane carbon number and vapor pressure as well as a relatively consistent product distribution it was possible to develop an empirical parametrization for SOA yields for n-alkanes between C(12) and C(17). This parametrization was implemented using the volatility basis set framework and is designed for use in chemical transport models. For C(OA) < 2 microg m(-3), the SOA mass spectrum, as measured with an aerosol mass spectrometer, had a large contribution from m/z 44, indicative of highly oxygenated products. At higher C(OA), the mass spectrum was dominated by m/z 30, indicative of organic nitrates. The data support the conclusion that lower volatility organic vapors are important SOA precursors.

Effective rate constants and uptake coefficients for the reactions of organic molecular markers (n-alkanes, hopanes, and steranes) in motor oil and diesel primary organic aerosols with hydroxyl radicals.

Hydroxyl radical (OH) uptake by organic aerosols, followed by heterogeneous oxidation, happens nearly at the collision frequency. Oxidation complicates the use of organic molecular markers such as hopanes for source apportionment, since receptor models assume markers are stable during transport. We report the oxidation kinetics of organic molecular markers (C(25)-C(32) n-alkanes, hopanes and steranes) in motor oil and primary organic aerosol emitted from a diesel engine at atmospherically relevant conditions inside a smog chamber. A thermal desorption aerosol gas chromatograph/mass spectrometer (TAG) and Aerodyne high resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) were used to measure the changes in molecular comosition and bulk primary organic aerosol. From the measured changes in molecular composition, we calculated effective OH rate constants, effective relative rate constants, and effective uptake coefficients for molecular markers. Oxidation rates varied with marker volatility, with more volatile markers being oxidized at rates much faster than could be explained from heterogeneous oxidation. This rapid oxidation can be explained by significant gas-phase OH oxidation that dominates heterogeneous oxidation, resulting in overall oxidation lifetimes of 1 day or less. Based on our results, neglecting oxidation of molecular markers used for source apportionment could introduce significant error, since many common markers such as norhopane appear to be semivolatile under atmospheric conditions.

Intermediate-volatility organic compounds: a potential source of ambient oxidized organic aerosol.

Smog chamber experiments were conducted to investigate secondary organic aerosol (SOA) formation from intermediate volatility and semivolatile organic compounds (IVOCs and SVOCs). We present evidence for the formation of highly oxygenated SOA from the photooxidation of n-heptadecane, which is used as a proxy for IVOC emissions. The SOA is consistent with multiple generations of oxidation chemistry resulting from OH radical exposure equivalent to approximately 0.5 days of atmospheric processing under high-NO(x) and low-CoA conditions. The SOA has a calculated O/C ratio of 0.59, which is higher than typical for chamber-generated SOA. The mass spectrum of the SOA, as measured with a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS), is similar to the OOA-2 factor determined for Mexico City. SOA formed from the low-NO(x), low-C(OA), oxidation of n-heptadecane is less oxidized because of differences in the chemical mechanism and lower integrated OH exposure. SOA formed from both the oxidation of n-heptadecane under high-NO(x), high-C(OA) conditions and the oxidation of n-pentacosane, a proxy for semivolatile organic emissions, does not produce highly oxygenated SOA, largely because of the condensation of early generation oxidation products.

Constraining the volatility distribution and gas-particle partitioning of combustion aerosols using isothermal dilution and thermodenuder measurements.

The gas-particle partitioning of primary organic aerosol (POA) emissions from a diesel engine and the combustion of hard- and soft-woods in a stove was investigated by isothermally diluting them in a smog chamber or by passing them through a thermodenuder and measuring the extent of evaporation. The experiments were conducted at atmospherically relevant conditions: low concentrations and small temperature perturbations. The partitioning of the POA emissions from both sources varied continuously with changing concentration and temperature. Although the POA emissions are semivolatile, they do not completely evaporate at typical atmospheric conditions. The overall partitioning characteristics of diesel and wood smoke POA are similar, with wood smoke being somewhat less volatile than the diesel exhaust. The gas-particle partitioning of aerosols formed from flash-vaporized engine lubricating oil was also studied; diesel POA is somewhat more volatile than the oil aerosol. The experimental data from the dilution- and thermodenuder-based techniques were fit using absorptive partitioning theory to derive a volatility distribution of the POA emissions from each source. These distributions are suitable for use in chemical transport models that simulate POA concentrations.