CRC Report No. A-34 - Coordinating Research Council

April 2005
CMB
Category
Description/Interpretation
predominantly propane.
Biogenic Isoprene. 18
Background Composite ambient background for Southern
California in 1997 with isoprene removed.
Morning and afternoon background profiles
were used.
Notes:
# 9.88 ppbC is the background added to each “ambient” sample
Matched to A-34
Categories
12, 13, 19, 20, 21,
22 + 9.88 ppbC #
Experiment 1 in Round 1
In Round 1, DRI applied CMB with minimal information to support the analysis. The results of
the Round 1 analysis for experiment 1 are shown in Figure 4-1a. The main source category
positively identified by CMB in experiment 1 was gasoline. CMB showed skill in correctly
apportioning the amount of gasoline emissions and correctly rank ordering receptor locations
from low to high gasoline contribution (Long Beach to Van Nuys). CMB systematically overestimated
the contribution of gasoline emissions. The bias was greatest when the actual gasoline
contribution was low (bias > 50% at Long Beach) and least when the actual gasoline contribution
was high (bias < 5% at Van Nuys). Over-estimations at Long Beach, Hawthorne and LAX were
most likely due to omission of a source profile that resembles the hypothetical industrial source
emissions. There is also a general tendency at downwind locations such as Lake Perris and
Crestline, and with aged afternoon samples at mid-basin sites, for total predicted VOC
concentrations to exceed measured levels because relatively nonreactive species are used in the
CMB calculations. The average ratios of predicted and measured VOCs were 1.10 for samples
having xylene/benzene ratios between 1.2 and 1.6 and 1.15 for ratios less than 1.2.
CMB correctly identified three other source categories in Round 1: biogenics, diesel and
solvents. Biogenics were quantified accurately because the profile (in terms of PAMS species)
was dominated by a single compound, isoprene. Solvents were generally under-estimated by
CMB with a bias of about a factor of two. CMB accurately determined that the diesel
contribution (to the sum of PAMS species) was small when the Type 2 am set of fitting species
was used in the fit. This set includes decane and undecane, which CMB needs to apportion
surface coatings and to distinguish between gasoline and diesel exhaust. CMB detected diesel
exhaust in only 23% of the urban-fresh samples and most source contributions were significantly
lower than the propagated 1-sigma uncertainties. In contrast, diesel exhaust was detected in 79%
of the urban-aged samples and is often co-linear with gasoline exhaust. The relative contributions
of diesel exhaust are larger for the urban-aged samples but so are the corresponding propagated
uncertainties. The composite surface coating profile is detected in 87% of the urban-fresh
samples when nonane, decane and undecane are used in the fit, but is detected in only 28% of the
urban-aged samples when the heavy hydrocarbon is left out of the calculation. Biogenic
emissions are detected in most samples at negligible levels in urban samples. Their contributions
are substantially higher relative to total VOC in many of the low-aged samples, which are from
Crestline. CMB accurately apportioned the biogenic contribution to air concentrations (in terms
of PAMS species) because the profile is dominated by a single compound, isoprene. Because
isoprene is reactive, the biogenic contributions reported by CMB represent lower limits of the
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