Determination of Chemical Species in Individual Aerosol Particles Using Ultrathin Window EPMA

The determination of low-Z elements such as carbon, nitrogen, and oxygen in individual atmospheric aerosol particles is of interest in studying environmental pollution. By the application of a newly developed EPMA technique, which employs either windowless or thin-window EDX detector, chemical compositions, including the low-Z components, of individual particles can quantitatively be elucidated. The determination of low-Z elements in individual environmental particles allows to improve the applicability of the single particle analysis; many environmentally important atmospheric particles, e.g. sulfates, nitrates, ammonium, and carbonaceous particles, contain low-Z elements, which cannot be characterized using conventional energy dispersive-EPMA (ED-EPMA). Furthermore, the diversity and the complicated heterogeneity of atmospheric particles in chemical compositions can be investigated in details, using the new EPMA technique. This work demonstrates that the quantitative determination of chemical species in individual particles is possible using ultrathin window EPMA coupled with Monte Carlo based quantification. Using the new EPMA method, molar concentrations of major chemical species in individual environmental particles can be determined. For example, the molecular concentra tions of ammonium sulfate and nitrate in single particle were analyzed for particles internally mixed with ammonium sulfate and nitrate species. When particles are composed of several chemical species so that the number of equations is smaller than the number of chemical species to be determined, the quantitative analysis of each chemical species can be ambiguous; however, many particles are composed of one or two major chemical species, and thus this technique could provide direct observation of atmospheric chemistry for airborne particles in more detail.