Gas-phase composition and secondary organic aerosol formation from standard and particle filter-retrofitted gasoline direct injection vehicles investigated in a batch and flow reactor

Simone M Pieber,Imad El Haddad,Deepika Bhattu,Doǧuşhan Kılıç,Josef Dommen,Norbert Heeb,Jay G Slowik,Alejandro Keller,Urs Baltensperger,Jan Czerwinski,Felix Klein,André S H Prévôt,Pierre Comte,Nivedita K Kumar,Emily A Bruns

doi:10.5194/acp-18-9929-2018

Abstract

Abstract. Gasoline direct injection (GDI) vehicles have recently been identified as a significant source of carbonaceous aerosol, of both primary and secondary origin. Here we investigated primary emissions and secondary organic aerosol (SOA) from four GDI vehicles, two of which were also retrofitted with a prototype gasoline particulate filter (GPF). We studied two driving test cycles under cold- and hot-engine conditions. Emissions were characterized by proton transfer reaction time-of-flight mass spectrometry (gaseous non-methane organic compounds, NMOCs), aerosol mass spectrometry (sub-micron non-refractory particles) and light attenuation measurements (equivalent black carbon (eBC) determination using Aethalometers) together with supporting instrumentation. Atmospheric processing was simulated using the PSI mobile smog chamber (SC) and the potential aerosol mass oxidation flow reactor (OFR). Overall, primary and secondary particulate matter (PM) and NMOC emissions were dominated by the engine cold start, i.e., before thermal activation of the catalytic after-treatment system. Trends in the SOA oxygen to carbon ratio (O : C) for OFR and SC were related to different OH exposures, but divergences in the H : C remained unexplained. SOA yields agreed within experimental variability between the two systems, with a tendency for higher values in the OFR than in the SC (or, vice versa, lower values in the SC). A few aromatic compounds dominated the NMOC emissions, primarily benzene, toluene, xylene isomers/ethylbenzene and C3-benzene. A significant fraction of the SOA was explained by those compounds, based on comparison of effective SOA yield curves with those of toluene, o-xylene and 1,2,4-trimethylbenzene determined in our OFR, as well as others from literature. Remaining discrepancies, which were smaller in the SC and larger in the OFR, were up to a factor of 2 and may have resulted from diverse reasons including unaccounted precursors and matrix effects. GPF retrofitting significantly reduced primary PM through removal of refractory eBC and partially removed the minor POA fraction. At cold-started conditions it did not affect hydrocarbon emission factors, relative chemical composition of NMOCs or SOA formation, and likewise SOA yields and bulk composition remained unaffected. GPF-induced effects at hot-engine conditions deserve attention in further studies.

Highlights

Vehicular emissions are a significant source of air pollution in many urban areas (Platt et al, 2014; Zotter et al, 2014; Bahreini et al, 2012; Borbon et al, 2013; May et al, 2014; Worton et al, 2014; Gentner et al, 2017)
For GDI4 we found lower total emissions during cold-started cycles compared to other vehicles (∼ factor of 3 lower than GDI1-3, median non-methane hydrocarbons (NMHCs) emission factors (EFs): 434 mg kg−fu1el) and a smaller difference between coldand hot-started cycles
EFs of primary NMHC and THC were reduced by up to a factor of 90 under hot-started conditions compared to cold starts, and total emissions were dominated by the pollution during the first few minutes of the driving cycle

Summary

Introduction

Vehicular emissions are a significant source of air pollution in many urban areas (Platt et al, 2014; Zotter et al, 2014; Bahreini et al, 2012; Borbon et al, 2013; May et al, 2014; Worton et al, 2014; Gentner et al, 2017). Depending on vehicle fleet technology, emissions may include fine particulate matter (PM), consisting mainly of sub-micron primary organic aerosol (POA) and black carbon (BC), and reactive gases such as nitrogen oxides (NOx) and organic compounds. Note that we refer to organic gas-phase compounds as nonmethane organic compounds (NMOCs). Numerous strategies have been developed to decrease PM and NOx emissions from on-road vehicles, including optimization of engine settings and implementation of after-treatment systems. Examples of such systems are oxidation catalysts that oxidize gas-phase pollutants (CO, NMOC), three-way catalysts (TWC) for gasoline on-road vehicles and selective catalytic reduction (SCR) systems for heavy-duty diesel engines and large diesel passenger cars, which convert NOx emissions to N2 and O2

Methods

Results

Conclusion