Applications of optical spectroscopy and stable isotope analyses to organic aerosol source discrimination in an urban area

Understanding the chemical character of organic aerosols is extremely important for evaluating their role in climate forcing and human respiratory health. Aerosol columnar properties retrieved by sun photometry represent a large dataset of information about the physical and light absorbing and scattering properties of the total aerosol, but lack more detailed chemical information about the organic fraction of atmospheric particulate matter. To obtain additional information about relationships between organic aerosol sources and columnar properties, we simultaneously examined stable isotope properties of PM10 aerosols from urban (Granada, Spain) and remote (Sierra Nevada, Spain) sites and diesel exhaust, spectroscopic properties of water soluble organic carbon (WSOC) of PM10 aerosols, and sun photometry measurements. We demonstrated that C and N stable isotopes and parameters from UV–vis and fluorescence spectroscopy are able to discriminate between aerosols receiving substantial fossil fuel pollution and those influenced by Saharan dust in an urban area. More depleted δ13C was associated with low asymmetry parameter, gλ, and high values of the spectral slope ratio, SR, were associated with high effective radius, typical of pollution situations. The humification index (HIX), used predominantly to evaluate the degree of organic matter humification, was significantly related to gλ and the radius of fine mode particles, rf, and may reflect aging of the Saharan dust-influenced aerosols. Parallel factor analysis (PARAFAC) modeling identified a fluorescent component (C3) with a spectrum similar to that of naphthalene, which was significantly related to gλ and rf. The diesel exhaust sample represented a pollution end-member, with the lightest δ13C value (−26.4‰), lowest SR (0.95), lowest HIX (2.77) and highest %C3 (20%) of all samples.