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

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 after three hours of oxidation inside the chamber at typical atmospheric oxidant levels. 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 photo-oxidizing exhaust from newer (LEV1 and LEV2) vehicles was only modestly lower (38%) than that formed from exhaust emitted by older (pre-LEV) vehicles, despite much larger reductions in non-methane organic gas emissions. These data suggest that a complex and non-linear 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 time scale of these experiments, the mixture of organic vapors emitted by newer vehicles appear to be more efficient (higher yielding) in producing SOA than the emissions from older vehicles. About 30% of the non-methane 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. These results for light-duty gasoline vehicles contrast with the results from a companion study of on-road heavy-duty diesel trucks; in that study late model (2007 and later) diesel trucks equipped with catalyzed diesel particulate filters emitted very little primary PM, and the photo-oxidized emissions produced negligible amounts of SOA

On-road vehicle emissions of glyoxal and methylglyoxal from tunnel tests in urban Guangzhou, China

“…As smog chamber studies indicate that SOA formed from photochemical aging of vehicle exhausts overwhelmingly dominate over the directly emitted primary organic aerosols (POA) (Nordin et al, 2013;Gordon et al, 2014;Liu et al, 2015), it is important to get a full spectrum of SOA precursors, including dicarbonyls, in the vehicle exhausts. From our tests in the Zhujiang Tunnel in urban Guangzhou, we obtained the EFs of glyoxal and methylglyoxal for on-road vehicles in China for the first time.…”

Role of ammonia in forming secondary aerosols from gasoline vehicle exhaust

“…Nordin et al [26] found that SOA formed from the exhaust of idling gasoline vehicles could be 1-2 orders of magnitude higher than POA. Gordon et al [18] and Platt et al [27] also observed an enhancement of SOA/POA from 1 to 15 during the aging of emissions from LDGVs. Significant nitrate and ammonium were observed to be formed synchronously with SOA during the photooxidation of LDGV exhaust [18,26].…”

“…Gordon et al [18] and Platt et al [27] also observed an enhancement of SOA/POA from 1 to 15 during the aging of emissions from LDGVs. Significant nitrate and ammonium were observed to be formed synchronously with SOA during the photooxidation of LDGV exhaust [18,26]. However, the formation of secondary aerosols including SOA, nitrate, and ammonium from vehicle exhaust still remain uncertain when NH 3 is removed.…”

“…By applying this rate to the entire experiment, we used the same method as Pathak et al [44] to correct the wall loss of the particles. In addition, the aerosol mass measured by HR-TOF-AMS was corrected with SMPS data with the same method used by Gordon et al [18].…”

“…In addition, recent smog chamber simulations indicate that NH 3 can enhance the secondary organic aerosol (SOA) formation from ozonolysis of biogenic precursors by reacting with organic acids [15,16], but reduce SOA formation from the styrene-ozone system [17]. However, the potential influence of NH 3 on SOA formation is still poorly understood, especially for complex chemical mixtures such as vehicle exhausts brimming with thousands of gaseous and particle-phase components [18]. SOA, which is ubiquitous in various atmospheric environments, respectively accounts for approximately 66% and 90% of the total OA in urban as well as urban downwind and rural sites in Northern Hemisphere mid-latitudes [19].…”