Molecular signatures of organic particulates as tracers of emission sources

Abstract As tool for identifying the pollution sources and their impact on human health, chemical signature of airborne particulates and deposition dusts is a subject of study since decades. Usually, three complementary ways are adopted for this purpose; they are: specific source markers, concentration ratios of pairs of congeners, and percent distributions of homologues within groups. After presenting the state-of-the-art about non-polar aliphatic (i.e., alkanes and alkenes), aromatic (PAHs, Nitro-PAHs) and polar (fatty acids, organic halides, polysaccharides) compounds associated with emissions, some unusual molecular signatures of non-polar organics are discussed, as possible tracers of emission sources. Non-polar organic fraction of particulates comprises short/medium-chain alkenes and alkanes (with carbon numbers ranging from 12 to 23), which display distinct relative abundances in petrol-derived spills and exhausts, release by microorganisms and high vegetation leaf debris. Long-chain alkanes associated with tobacco smoke show a peculiar iso/anteiso/normal homologue fingerprint and n-hentriacontane percentages higher than other emissions. Several concentration ratios of PAHs were analyzed in scientific literature as diagnostic of nature of emissions; meanwhile, the possible sources of uncertainty were elucidated. Due to its intrinsic limits (e.g., diagnostic ratios variability caused by decomposition and gas/particle exchange), the identification of sources exploiting molecular signature is overall restricted to qualitative purposes. Nevertheless, parameterizing molecular signatures through numerical indexes can aid in apportioning the pollution sources providing preliminary information for modeling. Future investigations would allow understanding the sources of uncommon molecular fingerprints (e.g., alkane/alkene distributions) and elucidating the role of humans as actual emitters of chemicals affecting internal environments.