A Review Of Polycyclic Aromatic Hydrocarbon And Polycyclic .

White PaperA Review of Polycyclic Aromatic Hydrocarbon and Polycyclic AromaticHydrocarbon Derivative Emissions from Off-Road, Light-Duty, Heavy-Duty,and Stationary SourcesContract #18RD011Prepared for:California Air Resources BoardJune 2020Submitted by:Dr. Georgios KaravalakisMr. Tianyi (Jerry) MaMr. Miguel RobledoUniversity of CaliforniaCE-CERTRiverside, CA 92521951-781-5799951-781-5790 (fax)

DisclaimerThe statements and conclusions in this report are those of the contractor and not necessarily thoseof the California Air Resources Board or other participating organizations and their employees.The mention of commercial products, their source, or their use in connection with material reportedherein is not to be construed as actual or implied endorsement of such products.AcknowledgmentsThis report was prepared at the University of California, Riverside, and Bourns College ofEngineering-Center for Environmental Research and Technology (CE-CERT). The authors thankthe following organizations and individuals for their valuable contributions to this project.We acknowledge funding from California Air Resources Board under contract 18RD011. Wethank CARB staff, including Jenny Melgo, Wenli Yang, Mary Beth Schwehr, Gabe Ruiz, andDavid Edwards from the Air Quality Planning and Science Division, for their helpful reviews ofthis document. Finally, we thank Chris Ruehl in CARB’s Research Division for his valuable helpin initiating this important study.2

Table of ContentsDisclaimer . 2Acknowledgments. 2Executive Summary . 41.Introduction . 52.Off-Road Sources . 142.1Diesel Off-Road Engines . 142.2Diesel Generators (Stationary Sources) . 293.Heavy-Duty Diesel Vehicles and Engines . 524.Light-Duty Gasoline and Diesel Vehicles and Engines . 1645.Critical Review on the Sampling and Analysis of PAH . 3373

Executive SummaryPolycyclic aromatic hydrocarbons (PAHs) are an important class of organic pollutants andtheir primary source in the environment is from the incomplete combustion of carbonaceousmaterials. PAHs are of great concern because of their widespread occurrence and toxic effects onecosystem and human health. Although there is no definitive legislation concerning PAHabatement, the Environmental Protection Agency (EPA) has designated 16 PAH as prioritypollutants, the latest being effective from 1997.In this white paper, an extensive literature review on past and current studies on PAH emissionsfrom different combustion sources was performed. We summarized the results of 14 peer-reviewedstudies conducted on off-road combustion sources, including diesel generators and off-roadengines used for construction or agricultural activities. We also summarized the findings of 37peer-reviewed studies conducted on on-road heavy-duty diesel engines or heavy-duty dieselvehicles. Finally, we included the results from 50 peer-reviewed studies on light-duty diesel andgasoline engines and vehicles.PAH emission concentrations are generally high with older technology diesel generatorswithout emissions control aftertreatment. Diesel generators and off-road diesel engines equippedwith diesel particulate filters (DPFs) show significant reductions in PAH emissions, especially inthe particle-phase. The same observation holds for heavy-duty diesel vehicles and engines. Forgasoline vehicles and engines, PAH emissions are generally higher than diesel vehicles. Ourliterature review showed elevated PAH emissions for gasoline vehicles equipped with gasolinedirect injection (GDI) engines compared to diesel vehicles equipped with DPFs, indicating that thelarger source of airborne PAHs is from the light-duty GDI fleet.4

1. IntroductionPolycyclic aromatic hydrocarbons (PAHs) are a large group of organic chemicals with two toseven fused aromatic rings. Chemically the PAHs are comprised of two or more benzene ringsbonded in linear, cluster, or angular arrangements. PAHs may also contain additional fused ringsthat are not six-sided. Some PAHs are well known as carcinogens, mutagens, and teratogens andtherefore pose a serious threat to the health of humans. The International Agency for Research onCancer (IARC) has classified certain individual PAHs as carcinogenic to animals and probablycarcinogenic to humans (IARC, 2010). PAHs known for their carcinogenic and ene, benzo[k]fluoranthene, and benzo[a]pyrene (C20H12); indeno[1,2,3cd]pyrene (C22H12); and dibenzo[a,h]anthracene (C20H14). Benzo[a]pyrene is being classified asGroup 1 by IARC, while dibenzo[a,l]pyrene and dibenzo[a,h]pyrene are considered as probablycarcinogenic to humans (Group 2A) and possible carcinogenic to humans (Group 2B), respectively.Benzo[a]pyrene, a well-studied carcinogen with high potency, has been considered as an indexcompound and a ‘gold standard’ for its carcinogenic activity defines as 1.0 according to the USEPA (US EPA, 1993).PAHs are formed primarily during the incomplete combustion of fossil and biomass fuels, andother organic material such as coal, and wood (Stogiannidis and Laane, 2015; Samburova et al.,2017; Zhang and Tao, 2009; Ravindra et al., 2008). The most abundant and potent PAHcompounds found as products of incomplete combustion are shown in Figure 1. PAHs formationhas three different pathways: 1. PAH fragments in the fuel can survive the combustion processretaining the original carbon skeleton. 2. Pyrosynthesis during combustion of lower molecularweight aromatic compounds. PAHs isolated from exhaust gases could be produced by the5

recombination of fragments of previous partially destroyed compounds to form new PAHs. 3.Pyrolysis of lubricant oils and unburnt fuel.Figure 1: Most abundant PAHs found in the exhaust. Numbers represent carcinogenic potencyfactors relative to benzo[a]pyrene.When the temperature exceeds 500 C, carbon-hydrogen and carbon-carbon bonds are brokento form free radicals. These radicals combine to form ethylene, acetylene, and 1,3-butadiene whichfurther condense with aromatic ring structures, which are resistant to thermal degradation.Although radical PAH formation mechanisms are favored due to the faster combustion processesin the internal combustion engine, other possible PAH formation pathways exist, including the6

Diels-Alder reactions, rapid radical reactions, and ionic reaction mechanisms (Kislov et al., 2005).It should be noted, however, that the hydrogen abstraction-acetylene addition (HACA) mechanismis the most commonly accepted as the major reaction route leading to the formation of PAHs incombustion engines (Kislov et al., 2013; Richter and Howard, 2000; Frenklach and Wang, 1991).Fuel structure could play an important role in PAH formation (Pena et al., 2018; Yinhui et al.,2016). Aromatic hydrocarbons present in diesel and gasoline fuels, especially mono-aromaticspecies (benzene, toluene, and xylene isomers), may provide a base for PAH and soot growth(Anderson et al., 2000). For instance, benzene combustion generates phenyl radicals, which arehighly reactive and can undergo oxidation and ring fragmentation reactions (Li et al., 2010). Theycan also form phenoxy radicals that oxidizes to cyclopentadiene, a species known to producenaphthalene through cyclopentadienyl recombination (Saggese et al., 2013). In contrast, toluenecombustion generates benzyl radicals that are resonantly stabilized and can survive for a long timein the combustion environment (Talibi et al., 2018). They provide a base for the formation ofnaphthalene and larger PAHs and soot (Anderson et al., 2000).Beside parent PAHs, some derivatives, like nitrated PAHs (nitro-PAHs), oxygenated PAHs(oxy-PAHs), and hydroxyl PAHs (HO-PAHs) are also of growing concern since it is generallyconsidered that these derivatives are more toxic than the parent PAHs (Albient et al., 2008;Andreou and Rapsomanikis, 2009; Durant et al., 1996; Walgraeve et al., 2010; Ravindra et al.,2008). Nitrated and oxygenated PAHs (mostly quinones) can be directly emitted from combustionsources, formed secondary inside the catalyst system from nitration reactions of the parent PAHs,or formed secondary from the radical reactions with the parent PAHs and/or other precursors(Albient et al., 2007; Walgraeve et al., 2010; Perrini et al., 2005; Yaffe et al., 2001; Jakober et al.,2007). Like their parent PAHs, nitro-PAHs and oxy-PAHs are semi-volatile organic compounds,7

partitioning between the particle and vapor phase. Although concentrations are far lower thanlevels of parent PAHs, nitro-PAHs can have stronger carcinogenic and mutagenic activity (Schantzet al., 2001; Khalek et al., 2011). Several nitro-PAHs are direct-acting mutagens, e.g.,dinitropyrene and 1-nitropyrene in diesel particulate matter (Perrini et al., 2005). OxygenatedPAHs, containing one or more oxygen(s) attached to the aromatic structure, are also known fortheir toxicity and mutagenicity, especially several ketones and quinones like benzanthrone andanthraquinone (Durant et al., 1996; Lundstedt et al. 2007). Oxygenated PAHs may affect humanhealth via the formation of proteins and DNA adducts, the depletion of glutathione, and generationof reactive oxygen species (ROS) that can enhance the oxidative stress (Li et al., 2003; Verma etal., 2015). Gas-phase reactions of PAHs to form nitro-PAHs or oxy-PAHs are initiated by eitherOH or NO3 radical attack at the position of highest electron density on the aromatic ring, followedby NO2 addition with a subsequent loss of H2O or HNO3, respectively (Jariyasopit et al., 2014;Atkinson et al., 1990; Keyte et al., 2013). In addition, heterogeneous reactions may also occur,including nitration reactions of pyrene and fluoranthene with NO3/N2O5 to yield differentnitropyrene and nitrofluoranthene isomers (Zimmermann et al., 2013; Atkinson et al. 1990).The literature review was selective and critical. Journals obtained from scientific indices werethe preferred choice, although other non-indexed publications, such as Society of AutomotiveEngineers (SAE) technical papers and some internal and published reports from organizations suchas the California Air Resources Board have also been cited. This literature review emphasizes onthe emissions of PAHs and their nitrated and oxygenated derivatives from on-road and off-roadengines. Figure 2 summarizes the compound-specific emission factors included in this review. Thefollowing sections show PAH emission factors categorized by source, including off-road dieselengines and generators, light-duty gasoline vehicles, light-duty diesel vehicles, and heavy-duty8

diesel vehicles. For the light-duty gasoline vehicles category, our intent is to provide two subcategories that represent PAH emission factors from port fuel injection (PFI) engines and gasolinedirect injection (GDI) engines.Figure 2. Distribution of compound specific emissions factors (EFs) from (a) light-dutyvehicles, (b) heavy-duty vehicles, (c) heavy-duty engines, and (d) off-road engines. EFs aregrouped by compound class: nitrogenated (blue), oxygenated (purple), and hydrocarbon PAHs(yellow). Total number of EFs are given in italics. Superimposed are box plots in blackindicating median and interquartile range.9