CRC Report No. A-34 - Coordinating Research Council

April 2005
emissions contributions, as discussed in more detail below where relationships between
emissions and air concentrations are considered.
Two CMB categories were identified by name (CNG/aged and LPG) that were not used in
preparing the emissions for experiment 1 (see Table 2-9). This emphasizes that CMB category
names describe the chemical characteristics of source profiles (fingerprints) rather than specific
activities tracked in emission inventories.
The CNG/aged profile accounted for ambient ethane whereas the LPG profile accounted for
ambient propane. The main sources of ethane and propane in the emissions for experiment 1
were oil/gas production, gas-fueled engines, refineries and hypothetical industrial emissions (see
Section 2). The amount of ethane in experiment 1 (and experiments 2-8) was higher than
intended because of an artifact. The emissions fraction for each A-34 source category (Table 2-
9) was defined as a ROG fraction. The PAMS species are a subset of ROG species with the
exception of ethane. Therefore, ethane rich source profiles (e.g., gas-fueled engines) can have
PAMS/ROG ratios greater than 1 (Table 2-11). The consequence was that contributions of
ethane rich source categories (PAMS/ROG > 1 in Table 2-11) to the sum of PAMS species were
artificially high. DRI accounted for the high ethane and propane backgrounds using the CMB
categories CNG/aged and LPG. This outcome did not perturb the study because urban samples
often contain elevated ethane and propane that are explained in CMB using the same CNG/aged
and LPG categories.
CMB accounted for the remaining PAMS species in the “ambient” samples using background
profiles for the Los Angeles area. The real background introduced into the “ambient” samples
was low (~10 ppbC) and so the CMB category “background” really corresponded to unidentified
emissions. Once again, the label “background” attached to this CMB category reflects the origin
of the profile rather than the identity of the emissions source. The CMB apportionments for
background were biased low because the apportionments for CNG/aged and LPG (and to a lesser
extent gasoline) were biased high. The background, CNG/aged and LPG categories in CMB
together represent a combination of PAMS species including excess ethane and propane that
cannot be explained by other source categories and CMB performance for the sum of these three
categories was better than for the individual categories.
Experiment 1 in Round 2
The difference between Round 2 and Round 1 was that DRI had additional information to help
identify source profiles. In particular, DRI had a tunnel study and gasoline samples to derive
mobile source profiles for Round 2 which may be expected to help the apportionment of the
“gasoline” CMB category. The results of the Round 2 analysis for experiment 1 are shown in
Figure 4-1b. Overall, the apportionment of gasoline was similar in Round 2 to Round 1 with
CMB showing skill in sorting out low/high contribution sites, but tending to over-estimate the
contribution of gasoline. As mentioned in Section 3, the gasoline profile derived from the virtual
tunnel study is very similar to the gasoline profile that DRI used in Round 1. CMB performance
for solvents was better in Round 2 than Round 1, but CMB performance for diesel was poorer in
Round 2. Performance for biogenics was the same in Round 2 as Round 1. The remaining
emissions were classified as CNG/aged, LPG and background in Round 2 as in Round 1.
H:\crca34-receptor\report\Final\sec4.doc 4-5