Article pubs.acs.org/est
Secondary Organic Aerosol Formation from Photo-Oxidation of Unburned Fuel: Experimental Results and Implications for Aerosol Formation from Combustion Emissions Shantanu H. Jathar,† Marissa A. Miracolo, Daniel S. Tkacik, Neil M. Donahue, Peter J. Adams, and Allen L. Robinson* Center for Atmospheric Particle Studies, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States S Supporting Information *
ABSTRACT: 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.
1. INTRODUCTION Secondary organic aerosol (SOA) is formed from the atmospheric oxidation of gas-phase organic emissions. SOA accounts for a significant fraction of the submicrometer atmospheric aerosol mass or fine particulate matter (PM) 1 and therefore has important impacts on the Earth’s energy budget 2 and human health.3 Yet, there are large uncertainties surrounding the sources, atmospheric evolution, and properties of SOA. Combustion sources, such as cars, trucks, and aircraft, emit a complex mixture of unburned and partially burned hydrocarbons some of which are known SOA precursors. However, these emissions cannot be completely speciated using conventional one-dimensional gas chromatography,4−8 preventing us from quantifying their contribution to ambient SOA. Nevertheless, unburned fuel is an important component of tailpipe emissions from combustion systems. For example, Presto et al.9 and Miracolo et al.10 reported that aircraft engine emissions at low engine loads resembled a mixture of unburned Jet Propellent-8 fuel and oil. Gentner et al.11 recently argued that the SOA-relevant emissions from gasoline and diesel engines have the same chemical signature as unburned fuel. In addition, direct evaporation of fuel (gasoline in particular) can be an important source of organic emissions.12 Therefore, the ability of unburned fuel to form SOA should provide insight into the SOA formation potential of combustion emissions. © 2013 American Chemical Society
Fuel reformulation has been used to reduce anthropogenic pollution from combustion sources. For example, the 1990 amendments to the Clean Air Act required the use of reformulated gasoline to abate ozone production. Over a decade ago, Odum and co-workers13−15 investigated SOA formation from unburned gasoline and demonstrated that SOA was primarily formed from the oxidation of single-ring aromatics in the fuel. They hypothesized that, by removing aromatics, one could possibly reduce SOA formation from gasoline engine emissions. The hypothesis remains untested. In fact, few studies have explored the effect of varying fuel composition on SOA formation from engine emissions. Miracolo et al.10 reported significantly less SOA formation from diluted exhaust of a gas-turbine engine operating on Fischer−Tropsch fuel versus Jet Propellent-8. The Fischer− Tropsch fuel was derived from coal; it contained no aromatics (versus about 20% for Jet Propellent-8) and was slightly more volatile than the Jet Propellent-8. Gordon et al.16 found no measurable change in SOA formation when light-duty diesel trucks were operated on biodiesel. Hence, more data are needed to connect fuel composition to SOA formation. Received: Revised: Accepted: Published: 12886
August 5, 2013 October 16, 2013 October 21, 2013 October 21, 2013 dx.doi.org/10.1021/es403445q | Environ. Sci. Technol. 2013, 47, 12886−12893
Environmental Science & Technology
Article
Figure 1. Carbon number and molecular structure distributions for (a) gasoline, (b) Fischer−Tropsch from coal, (c) Fischer−Tropsch from natural gas, (d) Jet Propellent-8, and (e) Diesel.9 For each plot, the bars sum up to one. The inset pie shows the relative fractions of n-alkanes, branched/ cyclic alkanes, and aromatics in each fuel. The magenta arrow indicates the mass-weighted carbon number for each fuel.
from Corporan et al.,28 who characterized the fuels using ASTM D6379 and ASTM D2425. These methods provided speciated data for n-alkanes ranging from n-heptane to nnonadecane but lumped data for branched alkanes, cyclic alkanes, and aromatics. For these fuels, we assumed the unspeciated hydrocarbons had the same carbon-number distribution as the n-alkanes. The Fischer−Tropsch from coal was mostly composed of branched alkanes that were difficult to speciate. We assumed that the distribution of these species was similar to the distribution of n-alkanes in Fischer−Tropsch from natural gas since the vapor pressure distribution of those two fuels was very similar.28 Diesel composition data are from Alnajjar et al.20 where compounds in the fuel that boil below 200 °C were characterized using ASTM D5443 and ASTM D6839 and those above 200 °C were characterized using gas chromatography-field ionization mass spectrometry as described in Briker et al.4,5 To illustrate the key differences in fuel composition, in Figure 1, we plot these data by carbon number, color-coded by molecular structure. The mass-weighted carbon number for each fuel is indicated by a magenta arrow and the aggregated molecular structure by the inset pies. From a carbon-number perspective, we expected gasoline to form the least SOA since it was the most volatile fuel (mean carbon number of 7.3) and diesels to form the most since they were the least volatile fuel (mean carbon number of 12.5). From a molecular structure perspective, we expected the Fischer−Tropsch fuels to form the least SOA since they have essentially no aromatics and diesels to form the most SOA since they have the most aromatics. The seven different diesels vary modestly in carbon number (their mass-weighted carbon number ranged between 11 and 14) but significantly in aromatic fraction (between 25% and 60% by mass). 2.2. Experimental Methods. We performed SOA formation experiments in Carnegie Mellon University’s smog chamber, a 10 m3 Teflon bag suspended in a temperaturecontrolled room. Prior to each experiment, the chamber was cleaned by heating, flushing with HEPA and activated carbon filtered air, and irradiating with UV lights for a minimum of 12 h. The basic experimental procedure was similar to previous chamber studies at Carnegie Mellon.25,29 First, ammonium
In this work, we conducted smog chamber experiments to investigate SOA formation from different unburned fuels that spanned a wide range in carbon number and molecular structure. To evaluate the suitability of using unburned fuel as a surrogate for actual emissions, we compared SOA data from unburned fuels to SOA data from diluted emissions of engines operated on those fuels.10,16−18 Finally, we combined fuel- and emissions-based SOA data to estimate the contribution of evaporative and exhaust emissions from gasoline usage to ambient SOA. In a companion paper, we have used the data to develop and evaluate SOA models that account for the precursor's carbon number and molecular structure.19
2. MATERIALS AND METHODS Smog-chamber experiments were conducted to investigate the SOA formation from unburned fuels. Unburned fuels are both directly (from tailpipe) and indirectly (as evaporative) emitted by combustion sources. In addition, unburned fuel provided a complex but relatively well-characterized model system to test our understanding of the effects of fuel composition, (carbon number and molecular structure), on SOA formation. 2.1. Fuels. A total of 23 experiments was performed with 11 fuels that spanned a wide range in carbon number and molecular structure: commercial California summer gasoline, Fischer−Tropsch made from coal, Fischer−Tropsch made from natural gas, Jet Propellent-8, and seven different diesels. The Fischer−Tropsch fuels were synthetic substitutes for Jet Propellent-8 fuel. The diesels were part of a test fuel matrix (Fuels for Advanced Combustion Engines) designed to span a wide range of three important combustion properties: ignition quality, fuel chemistry, and volatility.20 The experiments are listed in Table S.1, Supporting Information. Fuel composition data were available as a function of molecular structure and carbon number, both of which influence SOA formation potential.21−26 These data are presented in Tables S.2−S.6 in the Supporting Information. The gasoline was characterized by the California Air Resources Board using standard (ASTM D5580, D6839, D6550) and modified ASTM methods.27 These methods provided alkane and aromatic data by carbon number but no information about the specific species and no resolution in the alkene data. The Fischer−Tropsch and Jet Propellent-8 composition data are 12887
dx.doi.org/10.1021/es403445q | Environ. Sci. Technol. 2013, 47, 12886−12893
Environmental Science & Technology
Article
sulfate seed (7−25 μg m−3) was added to the chamber to minimize losses of vapors to the walls and reduce nucleation of SOA products. Second, nitrous acid (HONO) was bubbled into the chamber to serve as a hydroxyl radical (OH) source. Third, the precursor (fuel) and, in some experiments, gas-phase tracers (single-ring aromatics, 2-butanol (d9), and/or acetonitrile) were introduced into the chamber by injecting the precursor/tracer into heated HEPA and activated-carbon filtered air. The injection was performed slowly over the course of a minute and hot air flushed through the injection system for an additional few minutes to ensure that the precursor/tracer was efficiently transferred to the chamber. Photo-oxidation was initiated by turning on the chamber UV black lights (General Electric model 10526), which created an NO2 photolysis rate of 0.2 min−1. The UV light photolyzed HONO to produce OH. NO and NO2 formed as byproducts of HONO irradiation create a low VOC/NOx ratio; the median VOC/NOx was 0.7, comparable to ratios found in polluted regions. The experiments were performed at low relative humidity (
