Receptor models A large body of literature describes the theory and use of receptor models to
identify and/or apportion sources which may be contributing to monitored air quality. Seigneur,
(1997), summarized in Seigneur, (1999), provides a review of receptor models, contains a more
complete description of the major approaches, summarizes findings obtained in a number of
applications, and provides an extensive list of references. Receptor models are most useful for
identifying contributions of various source categories to observedprimary components of
particulate matter, although, to a lesser degree, they can also be used to infer contributions from
ozone and secondary PM precursors.
There are two major types of receptor models. The first type is the chemical mass
balance model (CMB). A description and user’s guide is available for the CMB model (U.S.
EPA, 2004d). This model assumes that the user already has a good idea of what source
categories potentially contribute to observations at a monitoring site. Speciated emissions
Indicator Species Indicator species approaches are based on the predicted sensitivity of a
secondary pollutant’s concentration to changes in different precursors for that pollutant. It is
possible to identify ratios of certain species which are good indicators of whether a secondary
pollutant is sensitive to reductions in precursor A or precursor B. Measurement of “indicator
species” is a potentially useful means for assessing which precursor category (e.g., VOC or
NOx) limits further production of ozone or secondary PM2.5 at a monitor’s location at various
times of day and under various sets of meteorological conditions. Several indicator ratios have
been developed to examine the sensitivity of ozone to changes in NOx and VOC, and the
sensitivity of particulate nitrate to changes in NOX, VOC, and ammonia.
Sillman (1998, 2002) and Blanchard, (1997, 1999, 2000, 2001) identify several sets of
indicator species which can be compared to suggest whether ozone is limited by availability of
VOC or NOx. Comparisons are done by looking at ratios of these species. States/Tribes should
consult the Sillman (1998, 2002) and Blanchard, (1997, 1999, 2000, 2001) references for further
details on measurement requirements and interpretation of observed indicator ratios (also see
Section 18.5.1 for more details).
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