Catalyzed Gasoline Particulate Filters Reduce Secondary Organic

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Environmental Processes

Catalyzed Gasoline Particulate Filters Reduce Secondary Organic Aerosol Production from Gasoline Direct Injection Vehicles Patrick Roth, Jiacheng Yang, Emmanuel Fofie, David R. Cocker, Thomas D. Durbin, Rasto Brezny, Michael Geller, Akua Asa-Awuku, and Georgios Karavalakis Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b06418 • Publication Date (Web): 22 Feb 2019 Downloaded from http://pubs.acs.org on February 23, 2019

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Environmental Science & Technology

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Catalyzed Gasoline Particulate Filters Reduce

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Secondary Organic Aerosol Production from

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Gasoline Direct Injection Vehicles

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Patrick Roth †,‡, Jiacheng Yang †,‡, Emmanuel Fofie †,‡, David R. Cocker III †,‡, Thomas

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D. Durbin †,‡, Rasto Brezny §, Michael Geller §, Akua Asa-Awuku *,†,‡,¥, Georgios

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Karavalakis *,†,‡

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†University

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Research and Technology (CE-CERT), 1084 Columbia Avenue, Riverside, CA 92507,

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USA

of California, Bourns College of Engineering, Center for Environmental

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‡Department

of Chemical and Environmental Engineering, Bourns College of

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Engineering, University of California, Riverside, CA 92521, USA

ACS Paragon Plus Environment

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§Manufacturers

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Arlington, Virginia 22201, USA

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¥Department

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Engineering, University of Maryland, College Park, MD 20742, USA

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Corresponding Authors

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* Georgios Karavalakis. Phone: (951)-781-5799; Fax: (951)-781-5790; Email:

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[email protected].

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* Akua Asa-Awuku. Phone: (301)-405-8527; E-mail: [email protected]

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Abstract

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of Emission Controls Association, 2200 Wilson Boulevard, Suite 310,

of Chemical and Biomolecular Engineering, A. James Clark School of

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The effects of photochemical aging on exhaust emissions from two light-duty vehicles

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with gasoline direct injection (GDI) engines equipped with and without catalyzed gasoline

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particle filters (GPFs) were investigated using a mobile environmental chamber. Both

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vehicles with and without the GPFs were exercised over the LA92 drive cycle using a

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chassis dynamometer. Diluted exhaust emissions from the entire LA92 cycle were

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introduced to the mobile chamber and subsequently photochemically reacted. It was


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found that the addition of catalyzed GPFs will significantly reduce tailpipe particulate

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emissions and also provide benefits in gaseous emissions, including non-methane

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hydrocarbons (NMHC). Tailpipe emissions composition showed important changes with

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the use of GPFs by practically eliminating black carbon and increasing the fractional

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contribution of organic mass. Production of secondary organic aerosol (SOA) was

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reduced with GPF addition, but was also dependent on engine design which determined

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the amount of SOA precursors at the tailpipe. Our findings indicate that SOA production

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from GDI vehicles will be reduced with the application of catalyzed GPFs through the

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mitigation of reactive hydrocarbon precursors.

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Introduction

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Mobile sources are major contributors of emissions, especially in urban areas (1). On-

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road vehicles equipped with internal combustion engines are known to emit nitrogen

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oxides (NOx), carbon monoxide (CO), particulate matter (PM), and volatile organic

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compounds (VOCs) (2). While vehicular NOx, CO, and VOC emissions have been

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reducing over the past years as a result of tightening regulations imposed by

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environmental and governmental agencies (3-4), several urban and rural areas across


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the United States (US) and especially in California are characterized as nonattainment

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for air pollutant emissions (5). Directly emitted PM from the vehicle’s exhaust is comprised

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by a complex mixture of constituents, including sulfate, metals, and black carbon, as well

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as primary organic aerosol (POA) (6). Studies have shown that PM emissions can be

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deposited deep into the lungs, inducing oxidative stress and respiratory diseases (7-8).

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Other studies have shown that PM emissions influence the cardiovascular system (9).

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Emissions of NOx, VOCs, and other semi-volatile compounds are responsible for the

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formation of secondary organic aerosol (SOA) through the photo-oxidation of these

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species in the atmosphere (10-11). Emissions of NOx and VOCs can undergo

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atmospheric reactions resulting in the formation of secondary lower volatility organic

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gases. As the gases continue to react, their volatility decrease until these gases nucleate

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(to form new particles) or condense onto existing particles and ultimately increase the

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atmospheric organic aerosol (OA) mass (the organic fraction of particles). The

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mechanism for the formation of SOA from anthropogenic or biogenic sources is not

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entirely clear as it involves complex processes, with the number of organic compounds

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participating in SOA formation being unknown (12-13). Studies have reported that SOA


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accounts for the largest fraction of atmospheric OA, accounting for approximately 30-90%

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of total OA in megacities (12, 14-15). Recent studies have shown that gasoline vehicles,

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a major source of VOC emissions, dominate the production of SOA in urban areas

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compared to diesel vehicles, at least in the United States (US) (16). This phenomenon

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can be confirmed by the fact that gasoline vehicle sales far exceed those of diesel

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vehicles in the US compared to the European Union, and also by the fact that gasoline

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fuel consists of lighter and more volatile hydrocarbons in the range of C4-C10, which are

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major SOA precursors (11, 17).

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The significant contribution of gasoline vehicles to the SOA budget has been shown in

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several studies (18-20). Gordon et al. (21) tested pre-LEV and newer gasoline vehicles

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and they concluded that SOA formation exceeds primary PM emissions. They also found

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lower SOA production from the newer vehicles compared to the pre-LEV gasoline

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vehicles and less SOA production during the hot-start testing compared to cold-start

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cycles. In another study, the authors confirmed the production of 15 times higher SOA

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compared to POA when they tested a gasoline vehicle over the New European Driving

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Cycle (NEDC) (22). Nordin et al. (23) showed that C6-C9 light aromatic hydrocarbons


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contributed up to 60% of the formed SOA when they tested Euro 1 and Euro 4 compliant

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passenger cars under idling conditions. Liu and co-workers (24) tested Euro 1 and Euro

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4 gasoline vehicles under idling conditions and found that single-ring aromatics and

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naphthalene were responsible for 51%-90% of the formed SOA.

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Over the past decade the transportation sector has changed significantly with the

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introduction of gasoline direct injection (GDI) engines, driven by stringent legislative

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measures to lower vehicle fuel efficiency and greenhouse gas emissions. However, GDI

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engines are known to emit more soot emissions than traditional port fuel injection (PFI)

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engines (25-26). In GDI engines, fuel is sprayed directly into the combustion chamber,

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which leads to incomplete fuel evaporation due to the imperfect mixing of fuel and air,

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resulting in pockets with high temperatures but insufficient oxygen, leading to pyrolysis

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reactions and soot formation. Additionally, as the fuel comes directly into contact with the

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cold cylinder walls and piston, a small amount of fuel may impinge on the piston, which

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during evaporation may lead to diffusion combustion and PM formation (27-29). One

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strategy to reduce PM emissions from GDI vehicles is through the use of a gasoline

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particulate filter (GPF) (30-31). The use of GPFs in GDI vehicles have been shown to


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dramatically reduce PM mass, black carbon, and particle number emissions, as well as

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toxic pollutants such as polycyclic aromatic hydrocarbons (PAHs) and nitrated PAHs (30,

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32). Despite the increased popularity of GDI engines in the light-duty vehicle sector

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across the US and Europe, there is limited information on the SOA production from

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current technology GDI vehicles. In a recent study, Du et al. (33) tested both a PFI and

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GDI vehicle over the NEDC and reported much higher SOA production for the GDI vehicle

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compared to the PFI vehicle. In a different study, on the other hand, Zhao et al. (34)

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showed no differences in SOA production between PFI and GDI vehicles. Finally,

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Karjalainen et al. (35) tested a GDI vehicle over the NEDC and showed reduced SOA

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formation when the engine was warm and higher SOA formation during the cold-start

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phase when the three-was catalyst (TWC) was below its light-off temperature.

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In this study, we investigated the SOA production from two current GDI vehicles with

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and without a catalyzed GPF. To the best of our knowledge, only Pieber et al. (36)

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evaluated SOA production from GDI vehicles equipped with prototype GPFs using a

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batch and a flow reactor. While the authors did not find differences in SOA formation with

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the tested non-catalyzed and catalytically coated GPFs, they suggested that the


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catalyzed GPFs could be installed at the same position as the underfloor TWC, and that

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this should be investigated in future work. Here, for the first time, we report results on

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SOA formation with catalyzed GPFs or 4-way catalysts when exercising GDI vehicles

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over the LA92 test cycle. This is a companion study to Yang et al. (30), in which detailed

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primary gaseous and particulate emissions, as well as toxic pollutants, are reported.

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Experimental

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Test Vehicles and Driving Cycles. This study utilized two 2016 model year passenger

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cars. GDI_1 was equipped with a 2.0 liter (L) wall-guided direct injection SI Atkinson cycle

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engine and GDI_2 was equipped with a 1.5 L downsized turbocharged centrally-mounted

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direct injection engine. Testing on both vehicles was performed over duplicate cold-start

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LA92s cycles with commercial California E10 fuel. Additional information on the test

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vehicles and driving cycle can be found elsewhere (30).

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Both vehicles were tested in their original configuration and also retrofitted with a

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catalyzed GPF, which was installed in place of the underfloor TWC. The original close-

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coupled catalysts were retained in their stock location. The catalyzed GPFs were provided


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by the Manufacturers of Emissions Controls Association (MECA) and more information

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on their technical characteristics can be found in Yang et al. (30). Briefly, both GPFs were

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sized based on the engine displacement of each vehicle and they were catalyzed with

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precious metal loadings typical of underfloor catalysts matching the certification levels of

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the two vehicles. The GPFs had a TWC washcoat with approximately 1.0 g/liter loading

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of palladium (Pd) and rhodium (Rh) (Pd:Rh tatio of 4:1).

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Emissions and Chamber Testing. All tests were conducted in CE-CERT’s Vehicle

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Emissions Research Laboratory (VERL), on a Burke E. Porter 48-inch single-roll electric

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dynamometer. A Pierburg Positive Displacement Pump-Constant Volume Sampling

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(PDP-CVS) system was used to obtain standard bag measurements for total

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hydrocarbons (THC), CO, NOx, non-methane hydrocarbons (NMHC), and carbon dioxide

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(CO2). Bag measurements were made with a Pierburg AMA-4000 bench. More details for

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the measurement methods and analysis of the primary emissions are provided elsewhere

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(30).


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The photochemical aging experiments were carried out in UCR’s (University of

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California, Riverside) Mobile Atmospheric Chamber (MACh). The MACh consists of a 30

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m3 2 mil fluorinated ethylene propylene Teflon film reactor. Briefly, the reactor is enclosed

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in a segmented, lightweight aluminum frame fitted with wheels and a static resistant/UV

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blackout tarp. It is surrounded by 600 15 W, 18”, T8 black light fluorescent bulbs that

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serve as a photochemical light source. More detailed information on the construction and

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characterization of MACh can be found in Vu et al. (37).

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Prior to each irradiation experiment, the chamber was cleaned by injecting O3, H2O2,

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and purified air (AADCO 737 air purifier) and was irradiated with UV light. The AADCO

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air consists of no detectable reactive compounds (i.e., H2O, NOx, CO, O3, hydrocarbons)

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to minimize background reactions in experiments. The chamber was then subsequently

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emptied and filled repeatedly until all gases and particles were measured to be below

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detection limit (H2O < -50°C dew point, NOx, CO, HC, and O3 at ~0 ppb, and PM=0 µg

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m-3) then flushed with purified air overnight. Prior to the injection of vehicle exhaust, the

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chamber was half-filled with the AADCO air. CVS blank tests were conducted during the


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test campaign, with the chamber filled with the CVS air for the same duration as the LA92

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cycle (1735 seconds) and then filled the remaining volume with the AADCO clean air. The

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final mass of aerosol formation (wall loss corrected) was found to average at 0.67 μg/m3,

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which was then subtracted from the actual vehicle exhaust experiments.

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The dilute exhaust was injected from the PDP-CVS into MACh during an LA92 cycle

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(excluding the hot soak). The exhaust was injected utilizing two Ejector Dilutors (Air-Vac

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TD11OH) in parallel, connected to a home built clean air system with filters and

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desiccants to remove the PM (HEPA filters), water (silica gel columns), NOx (Purafil), CO

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(Carulite canister), and hydrocarbons (activated charcoal) from the dilution air. The

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injection lines consisted of 8.5 m of 0.5” stainless steel tubing. The dilution setup was

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designed to allow a variation of dilution from 50:1 up to 200:1 by varying the dilution air

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and CVS flow. MACh utilized gravitational forces to ensure a positive pressure inside the

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reactor, such that potential holes in the Teflon surface of the chamber will force air to flow

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out of the reactor. This minimized the contamination and dilution of the exhaust

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throughout the experiment.


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For the experiments without the GPFs, after the exhaust was collected, the chamber

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was filled to maximum volume with purified air. A concentration of 1.0 ppm of H2O2 was

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also injected to act as an OH radical source. Prior to irradiation, the primary exhaust was

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evaluated for about 30 min to help characterize the diluted tailpipe composition. The

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emissions were then photo-oxidized continuously for 7-10 hours. The exhaust was

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monitored during photo-oxidation utilizing a host of gaseous and particulate instruments

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allowing for real time evaluation of the aged exhaust. All chamber experiments were

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concluded when the wall-loss corrected aerosol mass formation, and the ozone

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concentration plateaued.

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For the experiments with the GPF-equipped vehicles, the partially diluted exhaust

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collected in MACh was characterized for approximately 20-30 min. The total tailpipe PM

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were significantly lower and inert seeds were deemed necessary to mimic a similar

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condensable particulate surface area for low volatility gases to partition on as they age.

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Therefore, an ammonium sulfate (AS) seed was injected into the chamber utilizing an

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atomizer with a 2.40 M, aqueous AS solution (Acros, 99.5 % in Millipore © DI water


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(18mΩ,

Catalyzed Gasoline Particulate Filters Reduce Secondary Organic

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