Interpreting Temporal Changes in OMC/OC Ratios - An Alternative to OC-LAC Thermal Evolution Measurements

The thermal evolution procedure used by most monitoring programs in the United States to determine carbonaceous aerosol concentrations is referred to as the thermal optical reflectance (TOR) method, where an aerosol sample that has been collected on a quartz filter is heated, and evolved carbon is characterized as either organic (OC) or light absorbing carbon (LAC). Evolved carbon assigned to OC is multiplied by a factor, Roc, to achieve an estimate of organic mass (OMC). Over the last 10-15 years, Roc, estimated through multiple linear regression analysis of data collected in the Interagency Monitoring of Protected Visual Environments (IMPROVE) program, has increased at about a rate of 2% per year, reaching values above 2.0 in many regions of the United States. The fundamental problem with assigning an OC-LAC split is that there is not an objective definition of the two analytes. Data from long-term monitoring programs such as IMPROVE that use TOR to derive estimates of OC and LAC offer unique opportunities to employ statistical methodologies to discern biases in the OC-LAC split. Analysis of evolved carbon concentration temporal trends suggests that TOR analysis inaccurately bifurcates particulate carbon into the OC and LAC fractions with some LAC being inadvertently and wrongly assigned to the OC fraction. Without an objective definition of the distinction between LAC and OC, there is no way this distinction can be traced to fundamental principles. Perhaps total carbon and filter absorption measurements are a better alternative to identifying and tracking refractory and absorbing carbon than TOR derived estimations. In theory, both measurements can be calibrated to traceable standards resulting in stable long-term trends that are directly relatable to the relevant aerosols and emissions causing the measured values.