well. In the past decade there have been major advances in PM measurement techniques, but further advances will be needed to better characterize long-range transport.

Isotopic Signature Studies High-precision, stable isotope ratio measurements are increasingly being applied to both gaseous species and aerosol PM elemental source identification. With this technique the primary identification factor resides at the submolecular level. This technique can be highly sensitive because isotopic species are reduced in abundance and therefore small variations can be detected. Using traditional analytical methods, one often cannot precisely measure the primary species of interest when its background concentration is large (e.g., carbon dioxide). Isotopic measurements do not face such limitations, and are increasingly used to study atmospheric sources and sinks of compounds such as methane, nitrous oxide, carbon dioxide, carbon monoxide, ozone, and aerosol sulfate and nitrate.Recent simultaneous development of methods to measure stable isotopes at a high precision in micro- to nanogram-size samples, and the development of new quantum mechanical theories for the mechanisms that alter stable isotope ratios, have greatly advanced isotopic techniques. A review of the application of stable isotopes for atmospheric composition studies is available (Thiemens, 2006). Precise characterizations of isotopic compositions provide a new fingerprinting technique. This is a particularly powerful application for long-range transport studies because it is one of few measurements that provides information about the chemical processes that have occurred during transit as well as their bulk properties at arrival.

For example, applications of isotopic techniques provide a means by which ship emissions can be recognized in regions with multiple pollution sources that cannot be separated by traditional concentration measurements (Dominguez et al., 2008). Long-range transport of aerosols from Asia to the United States may also be recognized from their characteristic isotopic signature, and their secondary transformations during transit may be resolved (Patris et al., 2007). Isotope ratio measurements can also play a significant role in advancing our understanding of the biogeochemistry of mercury. As discussed in Chapter 4, many important aspects of mercury chemistry are presently unresolved, such as source identification, transformation, and storage in various environmental reservoirs. Isotope measurements can be an effective way to gain new insights on these issues, if used in combination with traditional concentration and chemical speciation measurements. The measurement of mercury isotopes is challenging, and further analytical developments are needed to fully exploit this technique. In the case of POPs there have thus far been few studies at the isotope level. Future developments of carbon and deuterium isotope ratio measurements

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