CRC Report No. A-34 - Coordinating Research Council

April 2005
Selection of Fitting Species
A prerequisite for using receptor models is that the relative proportions of chemical species
change little between source and receptor. Most ambient NMHCs are oxidized in the lowest 2
km of the troposphere with tropospheric lifetimes ranging from hours to several months.
Nominal afternoon summertime residence times for a reactive environment (e.g., Los Angeles)
are estimated in Table 3-1. These estimates provide indications of which components are likely
to remain relatively stable between source and receptor, thereby qualifying as fitting species for
CMB source apportionment. An exception is isoprene, which is included as a fitting species
despite its high reactivity because it serves as a marker for biogenic emissions. The source
contribution estimates under-estimated the actual source contributions of biogenic emissions, i.e.,
they provide a lower limit to biogenic contributions.
Table 3-1 lists three sets of default fitting species, by site location and time of day, which we
have used in past CMB analysis of PAMS hydrocarbon data. Compounds with potential
analytical problems such as coelution of peaks during gas chromatographic analysis are also
excluded as fitting species. Type 2 PAMS sites are located immediately downwind of the area of
maximum precursor emissions and are typically placed near the downwind boundary of the
central business district. An expanded list of hydrocarbons (36 species) are used as fitting
species at Type 2 sites for samples collected in the morning hours prior to 9:00 am since the
emissions are largely unreacted. A shorter list of more stable species (20 species) is used for
samples collected between 9:00 am and 6:00 pm. Type 1 sites characterize upwind background
and Type 3 sites monitor maximum ozone concentrations downwind from the fringe of the urban
area. Type 4 sites characterize the extreme downwind transported ozone and its precursor
concentrations exiting the area and are located near the downwind edge of the photochemical
grid model domain. A shorter list of 11 fitting species is used for Type 1, 3 and 4 PAMS sites.
Reactive species are retained in the CMB modeling as “floating species”, and provide useful
diagnostic information. Because the CMB model calculations are based upon non-reactive
fitting species, the predicted concentrations exceed the measured values by margins that increase
with increasing reactivity of the species.
3.2 APPLICATION OF CMB BY ROUND
The validity of source contribution estimates obtained from CMB depends greatly upon the
selection and application of appropriate sets of source composition profiles and “fitting” species.
In an actual CMB analysis, the location and time of sample collection are known. Available
emission inventory data are used to determine the major emission sources that are likely to
impact the receptor site. Visual surveys of the sampling location identify local sources near the
sampling site that might disproportionately influence the receptor measurements. This
information is then used to select an appropriate set of source composition profiles among
available alternatives and identify sources for which new or updated composition profiles are
needed. The location of the sampling site in relation to source and receptor areas and time of
sample collection provide a basis for selecting “fitting” species that are sufficiently nonreactive
so that relative proportions of chemical species change little between source and receptor. The
expectation in this present experiment is that the accuracy of the source attributions from the
CMB model analysis will improve as more appropriate source profiles and fitting species are
applied to the simulated ambient samples. The receptor model performance was evaluated in
four rounds as described in Section 1.4. This section describes DRI’s approach for performing
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