Source Apportionment of Sulfur and Light Extinction Using Receptor Modeling Techniques

Most visibility impairment is associated with sulfates, carbonaceous material, and soil-related material.1 Therefore, any visibility source apportionment scheme must address both secondary and primary aerosols. The chemical mass balance (CMB) formalism is usually used to apportion primary particles. It relies on known physical and chemical characteristic aerosols, such as ratios of tracer species, natural or man-made, at the receptors and sources to attribute aerosols to single sources or source types. CMB modeling apportions aerosol species on a sampling-period- by-sampling-period basis. However, if the data set contains an adequate number of samples, regressional techniques, along with less restrictive assumptions, can be used to estimate apportionment of secondary as well as primary species. In a regressional approach, the secondary species is the dependent variable, while the independent variables are tracers that are unique to a single source or group of sources. A key assumption associated with this approach is that the chemical species used as tracers must be uniquely emitted by non-overlapping groups of sources. These techniques were successfully used to develop a semiquantitative apportionment of particulate sulfur, total sulfur (particle plus gaseous sulfur), absorption, and extinction to source categories at receptor sites near the Grand Canyon using data gathered in a special study called Project MOHAVE (Measurement of Haze and Visual Effects). Regression models were used to develop links between trace elements and visibility variables and then to link the trace elements to source categories using CMB analysis. As part of the CMB analysis, a new technique was developed for verifying and/or extracting source profiles from the ambient data set. About 50% of the measured particle sulfur is attributable to coal-fired power plants during summer and winter months, while in the winter months, about 50% of the particle sulfur may be associated with primary sulfur emissions from burning activity and urban emissions during the summer. A variable that is responsible for over 30% of the extinction, babs, is predominately associated with burning activity during the winter and to burning, transportation, and suspended soil during the summer months.