Insights into the Origin of Water Soluble Organic Carbon in Atmospheric Fine Particulate Matter

Concentrations of fine carbonaceous aerosols (PM 2.5 ), including elemental carbon (EC), organic carbon (OC), and water-soluble organic carbon (WSOC) were measured from filters collected every 6th day for one year at three urban/industrial sites in the Midwestern United States: Cincinnati, Cleveland, and Mingo Junction, Ohio. The water-soluble fraction of fine particulate OC varied considerably from site to site and monthly averages were between 39-85% at Cincinnati, 35-68% at Cleveland, and 32-65% at Mingo Junction. Average monthly concentrations of WSOC were compared with measurements of organic source tracers, including levoglucosan, to better understand the spatial and temporal distribution and sources of WSOC. Two methods of predicting the non-biomass burning portion of WSOC (WSOC NB ) were compared including estimation of secondary organic carbon (SOC) by the EC-tracer Method and comparison of the unapportioned OC from a chemical mass balance (CMB) source apportionment analysis. Poor correlations between SOC estimated by the EC-tracer method and WSOC NB suggested that the use of the EC-tracer method to estimate SOC may be significantly flawed with respect to low time-resolved measurements, such as one-in-six day measurements. Good correlations between CMB unapportioned OC and WSOC NB at all three sites (R 2 = 0.68 to 0.91) suggested that direct measurements of levoglucosan and WSOC could provide a reasonable estimate of secondary organic carbon concentrations in some locations. However, application of this method to daily measurements made in Detroit, MI during July of 2007 and January/February of 2008 demonstrated that, on some individual days near large point sources, non-biomass burning sources of WSOC were important contributors to WSOC concentrations.