Enantiomeric Ratios and Stable Carbon Isotope Analysis of Limonene from Anthropogenic and Biogenic Emission Sources
SHAN GU, Nana Khundadze, Wentai Luo, Christoph Küppers, Avisa Charmchi, Kevin McWhirter, Todd Rosenstiel, James Pankow, Iulia Gensch, Celia Faiola, University of California, Irvine
Abstract Number: 253
Working Group: Source Apportionment
Abstract
Volatile chemical product (VCP) emissions have been implicated as a major source of urban air quality degradation. Limonene, in particular, has been identified as a tracer compound for fragranced VCPs in densely populated areas. However, limonene has also been long recognized as a significant emission from terrestrial vegetation. This presents challenges for teasing apart the relative contribution of limonene from anthropogenic VCPs versus biogenic emissions from urban forests in urban areas. Quantifying a limonene "fingerprint” that can distinguish between these different sources would be useful for improving emission inventories and predicting their impact on urban air quality. In this study, (−)/(+)-enantiomers ratios and stable carbon isotope ratios were quantified from limonene emissions of various VCPs and plant sources. Samples were collected on adsorbent cartridges and analyzed offline using thermal desorption gas chromatography mass spectrometry. Chiral GC columns were integral to both methodologies, allowing the baseline separation of (−)/(+)-limonene. A two-dimensional GC and isotope ratio mass spectrometer was used to quantify stable carbon isotope ratios. The results illustrated distinct differences in the enantiomeric ratios and isotopic signature (δ13C) of limonene sourced from VCPs compared to those from tree emissions. (+)-limonene contributed over 97% to total limonene emissions from VCPs with a mean δ13C value of −27.3‰. Conversely, (-)-limonene tends to be more dominant in tree emission sources (with some exceptions) with a mean δ13C of −31.8‰. The difference in δ13C between VCP and real plant limonene sources was statistically significant (p-value<0.01). This indicates that VCP-sourced limonene was enriched in the heavier isotope, possibly due to fractionation during the citrus peel extraction process leading to preferential loss of the lighter isotope. A comparison of outdoor and indoor urban air samples indicates that VCP-sourced limonene is the major source indoors, but outdoor limonene likely originates from a combination of VCP and plant emission sources. Overall, this study demonstrates the potential usage of enantiomeric analysis and stable carbon isotope analysis for improving VCP emission estimates in urban areas.