Secondary organic aerosol formation exceeds primary particulate matter emissions for light-duty gasoline vehicles

T D Gordon,P Rieger,C Maddox,H Maldonado,A A Presto,M Zhang,A L Robinson,A A May,A Gutierrez,N M Donahue,E M Lipsky,S Chattopadhyay,M M Maricq,N T Nguyen

doi:10.5194/acp-14-4661-2014

Abstract

Abstract. The effects of photochemical aging on emissions from 15 light-duty gasoline vehicles were investigated using a smog chamber to probe the critical link between the tailpipe and ambient atmosphere. The vehicles were recruited from the California in-use fleet; they represent a wide range of model years (1987 to 2011), vehicle types and emission control technologies. Each vehicle was tested on a chassis dynamometer using the unified cycle. Dilute emissions were sampled into a portable smog chamber and then photochemically aged under urban-like conditions. For every vehicle, substantial secondary organic aerosol (SOA) formation occurred during cold-start tests, with the emissions from some vehicles generating as much as 6 times the amount of SOA as primary particulate matter (PM) after 3 h of oxidation inside the chamber at typical atmospheric oxidant levels (and 5 times the amount of SOA as primary PM after 5 × 106 molecules cm−3 h of OH exposure). Therefore, the contribution of light-duty gasoline vehicle exhaust to ambient PM levels is likely dominated by secondary PM production (SOA and nitrate). Emissions from hot-start tests formed about a factor of 3–7 less SOA than cold-start tests. Therefore, catalyst warm-up appears to be an important factor in controlling SOA precursor emissions. The mass of SOA generated by photooxidizing exhaust from newer (LEV2) vehicles was a factor of 3 lower than that formed from exhaust emitted by older (pre-LEV) vehicles, despite much larger reductions (a factor of 11–15) in nonmethane organic gas emissions. These data suggest that a complex and nonlinear relationship exists between organic gas emissions and SOA formation, which is not surprising since SOA precursors are only one component of the exhaust. Except for the oldest (pre-LEV) vehicles, the SOA production could not be fully explained by the measured oxidation of speciated (traditional) SOA precursors. Over the timescale of these experiments, the mixture of organic vapors emitted by newer vehicles appears to be more efficient (higher yielding) in producing SOA than the emissions from older vehicles. About 30% of the nonmethane organic gas emissions from the newer (LEV1 and LEV2) vehicles could not be speciated, and the majority of the SOA formed from these vehicles appears to be associated with these unspeciated organics. By comparing this study with a companion study of diesel trucks, we conclude that both primary PM emissions and SOA production for light-duty gasoline vehicles are much greater than for late-model (2007 and later) on-road heavy-duty diesel trucks.

Highlights

Ambient fine particulate matter (PM) is comprised of a complex mixture of constituents, including sulfates, nitrate, ammonium, organic material, elemental carbon (EC), crustal materials, trace elements, and water
LEV1 gasoline vehicles already met the LEV2 PM emissions standard; changes to engine control/aftertreatment from LEV1 to LEV2 were not aimed at reducing nonvolatile EC particles or primary PM more broadly, and this fact is reflected in the relatively constant EC value shown in Fig. 2 across the LEV classes
For the pre-LEV vehicles, the cold-start end-of-experiment secondary organic aerosol (SOA) levels were similar to the primary PM and Primary organic aerosol (POA) emissions measured in the constant volume sampling (CVS)

Summary

Introduction

Ambient fine particulate matter (PM) is comprised of a complex mixture of constituents, including sulfates, nitrate, ammonium, organic material (organic aerosols), elemental carbon (EC), crustal materials, trace elements, and water. Organic aerosols often contribute a third or more of fine PM mass, but their sources are poorly understood (Kanakidou et al, 2005; Turpin et al, 2000). Primary organic aerosol (POA) is emitted directly “from the tailpipe”; secondary organic aerosol (SOA) is formed in the atmosphere from the oxidation of gaseous precursors. Numerous reports have shown that the secondary fraction of fine organic PM (SOA) dominates POA, even in urban areas with substantial fresh POA emissions (Jimenez et al, 2009; Subramanian et al, 2007; Stone et al, 2009). Motor vehicle emissions contribute to both POA and SOA concentrations

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