Comment Log Display

In the discussion of alternative transportation fuels, ARB staff
rightly draws attention to the issue of well-to-wheels methane
leakage associated with the use of natural gas fueled vehicles and
engines.  In addition to this methane leakage issue, ARB-sponsored
work with West Virginia University has also recently reported that
two high mileage stoichiometric, natural gas bus engines emitted
ultrafine particulates that were associated with lubricant
consumption in these engines.  The PM composition of these buses
correlated with oil-derived elemental species.  Chemical speciation
showed that lube oil-based additives and wear metals were a major
fraction of the PM from these buses.  The composition of the PM
contained oil-derived (Ca, Mg, Zn, P) and engine-wear (Fe, Cu, Al)
metals.  Detectable levels of K and Mn were also found. The
mass-based, lube oil-derived metal emissions were an order of
magnitude higher than those from DPF-equipped diesel engines. 
Although the two buses operated well within the U.S. EPA 2010
standards for PM and NOx, the study does indicate the possibility
of PM emissions from natural gas engines to increase due to
deterioration and increased lube oil consumption.  This study
suggests that, although the PM mass emissions from natural gas
vehicles are low, the presence of nucleation mode solid metal
particles could pose significant health risks in the alveolar
regions of the respiratory system due to the higher surface area of
these nanoparticles.  The complete reference of the published
report is: A. Thiruvengadam, et al., “Characterization of
Particulate Matter Emissions from a Current Technology Natural Gas
Engine,” Environmental Science & Technology, DOI:
10.1021/es5005973, June 30, 2014,
pubs.acs.org/doi/abs/10.1021/es5005973.  

Lubricant consumption in DME-fueled engines would also be expected
to generate ultrafine particles that would likely contain similar
solid metal particles to those observed on these stoichiometric,
natural gas bus engines.  Exhaust filter technology similar to DPFs
could be applied to stoichiometric, natural gas engines or engines
fueled by DME that would largely eliminate these ultrafine
particulate emissions.  If natural gas or DME-fueled engines are to
become significant actors in the transportation sector, the issue
of their ultrafine particulate emissions must be dealt with through
the application of exhaust filters.  Policies such as controlling
ultrafine particulate emissions through the use of a particulate
number emission limit for mobile source engines would provide ARB
with a mechanism for ensuring that these alternatively fueled
engines do not create a potentially significant health risk for the
state of California.