Sources and their contribution to two water-soluble organic carbon fractions at a roadway site

Abstract

24-h PM2.5 samples were collected at a roadway site every 6th day for one year (September 2010 through August 2011) and analyzed for organic carbon (OC), elemental carbon (EC), water-soluble organic carbon (WSOC), hydrophilic and hydrophobic fractions of WSOC (WSOCHPI and WSOCHPO), and ionic species, to provide important seasonal quantitative information on the primary and secondary sources of two WSOC fractions. Five minute black carbon (BC) concentrations were also measured using a seven-channel wavelength aethalometer to investigate the relationship of biomass burning (BB)-derived BC data from a BC@880 nm tracer method and WSOC. There has been increased interest in the light adsorption of WSOC and water-insoluble OC but most of the tools that have been used to understand these relationships have limited to extracts of filter-based samples. The impact of BB emissions on WSOC fractions was examined using the relationship between ΔBC (=BC@370 nm − BC@880 nm) and WSOC (or K+), and between BB tracers (WSOC and K+) and BB-derived BC (BCBB) estimated. The moderate correlation (R2 = 0.41) of WSOC and ΔBC during the cold months of November through April may support the contribution of BB emissions to the observed WSOC. Predicted BCBB correlated well with K+, WSOCHPI, and WSOCHPO concentrations (R2 of 0.65, 0.43, and 0.61, respectively), suggesting BB emissions may have an influence on the WSOC fractions observed.Contributions of non-BB, BB, and secondary OC (SOC) to both WSOCHPI and WSOCHPO were estimated using a multiple linear regression analysis. The monthly average contribution of non-BB emissions ranged from 12.6% to 29.4% of the WSOCHPI and from 21.5% to 44.1% of the WSOCHPO, with high contributions occurring during the cold months and low contributions occurred during the warm months. BB emissions contributed more to WSOCHPI (2.7%–13.1%) than WSOCHPO (0.2%–1.1%), and the SOC contribution to both WSOC fractions was significant. SOC accounted for 57.2%–79.7% of the WSOCHPI, and for 55.3%–78.0% of the WSOCHPO, with the higher fractions occurring in summer. During the summer the high SOC contributions were consistent with high OC/EC and WSOC/OC ratios. Results of this study suggest that non-BB (“fossil”) emissions are likely an important source to WSOCHPI and WSOCHPO observed at this roadway site.