Chemical Mass Balance Model

The chemical mass balance model, CMB7 (Watson 1997; Watson and others 1990), infers source contributions based on speciated aerosol samples collected at a monitoring site. Chemical elements and compounds in ambient aerosol are "matched" to speciated source emission profiles "fingerprints" by using least-squares, linear regression techniques to apportion the aerosol mass. CMB7 has been widely used within the regulatory community to identify and quantify the sources of particles emitted directly to the atmosphere. The model is based on the relationship between characteristics of the airborne particle (c_i), the summation of the product of the ambient mass concentration contributed by all sources (S_j), and the fraction of the characteristic component in the sources fingerprint (f_ij).

c_i= Σ_jS_jf_ij                              (2)

Given detailed information about the chemical speciation of the ambient aerosol and similar information about all of the emission sources impacting the receptor, the CMB7 model can apportion the aerosol mass among the sources if certain assumptions are met. To minimize error, there must be more aerosol components than sources to be included in the least squares linear regression fit. If there are more components measured than sources, then the comparison of model-estimated concentrations of these additional components provides a valuable internal check on model consistency.

The chemical components in the source "finger-print" must be conserved and not altered during atmospheric transport -- a rather large limitation.

Model resolution is typically limited to five or six source types, and separation of two sources with similar emission profiles (for example, prescribed burning and residential woodstove smoke) is difficult if both sources are active at the same time.

Systematic error analysis procedures have been developed for the CMB7 model, and the results have been published in model validation studies (NRC 1993). However, the model cannot apportion secondary aerosols (sulfate and nitrate); it is limited in its ability to apportion all of the mass to specific sources.

The ability of the model to apportion smoke from fire depends on several factors:

• The presence or absence of smoke from other forms of vegetative burning (woodstoves, agricultural burning, open burning, and others).
• The magnitude of the smoke impact at the receptor (for example, well-dispersed smoke that contributes small amounts of aerosol mass is more difficult to distinguish).
• The uncertainty in both the ambient aerosol and the source "fingerprint" components that the model most heavily weights in the regression analysis, typically organic carbon, potassium, and elemental carbon. The greater the uncertainty of these measurements, the less "fitting pressure" they have in influencing the regression solution.
• Inclusion of multiple aerosol components that are as nearly unique to smoke from fires (endemic signatures) as possible. These include organic compounds such as retene and levoglucosan, as well as gaseous signature such as carbon monoxide and methyl chloride. The more the source profile distinguishes prescribed or wildland fire smoke from other sources, the more accurate the source apportionment is likely to be.