species approaches.

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 observed primary 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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