A review of source apportionment techniques and marker substances available for identification of personal exposure, indoor and outdoor sources of chemicals

Introduction. The need of knowledge on occurrence, source strengths, distribution and fate of chemical substances is increasing. This knowledge is needed to effectively reduce levels of chemical substances in microenvironments that are harmful for human health and the environment. The only mechanism that ultimately will lead to cost effective reduction in population exposures is by the identification of sources that release these substances. Knowing the sources allows prioritization of substances/sources that most significantly contribute to target (sub)population exposures. To apportion personal exposures to sources of substances present in indoor and outdoor air through daily time-activity patterns is a highly complex task. This requires reliable exposure data and the application of source apportionment techniques to all kinds of microenvironmental samples (indoor, outdoor and commuting). The present report provides an overview of the existing source apportionment techniques and source tracers and their respective data requirements and serves as a background and guidance document for the development of harmonized exposure assessment practises. Methods. A literature survey was undertaken to identify source apportionment studies for two air pollutant groups, i.e. airborne particulate matter and volatile organic compounds. The results of this survey were used to compose an overview of the most commonly existing source apportionment techniques - also called receptor modelling - used to identify source contributions, including a description of the strengths and weaknesses. Results. The fundamental principle of receptor modelling is based on the assumption that mass is conserved and that on this basis a mass balance analysis can be used to identify and apportion sources in the atmosphere. All techniques that were considered are based on assumptions regarding the source, chemical species and measurement methodology. They require a certain degree of knowledge about the source regarding the number of sources, source profile (which substances are emitted by which source) or source strength regardless the origin of the source (outdoors or indoors). Of all considered techniques, conventional factorization and chemical mass balances represent the two extremes. Conventional factorization requires little knowledge, while chemical mass balance strategies require exact knowledge about the source(s). Other techniques, such as Positive Matrix Factorization or UNMIX, can be considered as intermediate strategies and are based on partly overlapping or slightly different assumptions and source knowledge requirements. Discussion and Conclusions. An ideal source apportionment study starts with selecting the trace elements or compounds representing the obvious sources that might contribute to the location where the measurements will be carried out. Consequently, the collection methods and chemical analyses should be selected in a way that the important marker species can be analyzed accurately. The number of samples is another major requirement for a successful source apportionment analysis. Typically at least tens of samples are needed for a reliable source apportionment analysis. From the methods considered in this report, chemical mass balance requires fewest samples, however, it requires the most detailed quantitative description of the number of sources and the respective chemical emission profiles. When identifying and interpreting emission sources of personal exposures or those of indoor air samples sufficient number of samples is required, i.e. preferably more than 50 samples for each receptor site.