Research Program Area: Emissions Monitoring & Control
The California Air Resources Board (CARB) is interested in improving its area source methodology for estimating emissions of total organic gases (TOG) and reactive organic gases (ROG) from the solvent cleaning and degreasing source category. In particular, CARB is interested in an inventory method that provides greater detail in both the types of solvents used and the type of equipment involved (e.g., vapor degreaser, cold cleaner). The current area source methods for this source category do not incorporate this level of detail, which limits regulatory analysis. CARB is also in need of a method to update the base year emission inventory developed during this project.
A comprehensive review of sources of data that could be used to develop base year emission estimates or inventory updates was conducted. These sources included government agencies, trade associations, and other industry sources. The results of this review showed that the only way to develop a base year inventory, with the level of detail desired by CARB, was to perform solvent surveys of the significant end-users. End-user surveys do not result in complete coverage of the source category, since only a predetermined end-user universe is sampled. Ideally, a comprehensive inventory would be developed through both end-user surveys and reliable solvent sales or production figures. Unfortunately, no sources of sales / production data were found for many of the solvents covered in this inventory. Further, for those solvents where sales data were available, reliable data for use in determining the fraction of solvent used for solvent cleaning purposes were often lacking.
To develop the base year inventory, an approach was developed that included two surveys: a comprehensive mail out survey for facilities likely to use solvent during the cleaning of parts that are incorporated into products (manufacturing users); and a simpler telephone survey to gather information from facilities likely to use solvents during maintenance activities (maintenance users - e.g., auto repair facilities, maintenance users at manufacturing facilities).
Methods used to reduce and statistically analyze the data in order to develop an emissions model are discussed. There were 32 unique combinations of equipment and solvent identified during the end-user surveys that make up the model. These 32 combinations are characterized by three equipment or operation types: cold cleaners (e.g., batch-loaded cold cleaners, conveyorized cold cleaners, spray gun cleaners), vapor degreasers (e.g., batch and conveyorized vapor degreasers), and hand wiping activities. The handwiping emissions presented in this study are the first to be reported for this source category by CARB. There are also 15 solvent categories that make up the 32 combinations of equipment and solvents, some of which are single solvents (e.g., 1,1,1-trichloroethane) and others being combinations (e. g., petroleum distillates).
The use of the model to develop county-, air district-, and state-level inventories is described, as well as methods that can be used to assess the uncertainty of the emission estimates. State-level emission estimates for the 1993 base year were 78,579 tons of TOG. CARB's 1993 TOG estimate for this category was 58,400 tons. One significant difference in the two estimates is that, at the state level, the fraction of ROG in the revised estimates is over 70% compared to the value of 40% in CARB's 1993 inventory (CARB, 1995). The emissions model developed during this project will provide CARB with the ability to develop emission estimates and allocate emissions to counties and districts in a much more detailed and realistic fashion. In part, this is due to the allocation of emissions being based on employment population and not the general population.
Almost 80% of these emissions were estimated to occur within the South Coast and Bay Area districts. The previously unquantified hand wiping operations contributed nearly 27% of the base year TOG emissions. From a solvents perspective, petroleum distillates were found to account for about half of the total base year TOG emissions.
A limited uncertainty assessment was performed for several equipment and solvent combinations. This assessment showed varying levels of uncertainty depending on the specific combination. For example, estimated annual 1993 TOG emission in Los Angeles (L.A) county for the trichloroethylene batch loaded vapor degreasing combination were shown to range from 60 tons (2.5th percentile) to 140 tons [97.5th percentile (mean = 99 tons)]. For cold cleaning using miscellaneous solvent blends, estimated annual L.A County emissions ranged from 45 tons (2.5th percentile) to 2,928 [97th percentile (mean = 1,055)].
Since variability in activity and emission factors drives the uncertainty estimates, additional resolution of certain solvent-equipment pairings is needed to decrease the uncertainty. For example, additional data (e.g., surveys) for the miscellaneous blends-cold cleaning category is needed in order to further resolve the solvent usage patterns in this grouping. For example, the additional data may lead to disaggregation of the activity or emission factors for certain industry groups. Assuming that usage patterns are similar within industry groups, the newly formed industry specific emission / activity factors will likely have lower variability. A similar argument could be made for disaggregation (increased resolution) for specific geographic regions.
Finally, while not representing the ideal source of inventory update data, information on industry employment was found to be the best data source for preparing inventory updates. Recommended methods for updating the base year estimates using the emissions model and employment data from U.S. Bureau of Census are presented, along with recommendations for improving the emission estimates in future years.
For questions regarding this research project, including available data and progress status, contact: Research Division staff at (916) 445-0753
Stay involved, sign up with CARB's Research Email Distribution List