AAAR 37th Annual Conference October 14 - October 18, 2019 Oregon Convention Center Portland, Oregon, USA
Abstract View
Interpreting Temporal Changes in OMC/OC Ratios - An Alternative to OC-LAC Thermal Evolution Measurements
William Malm, BRET SCHICHTEL, Jenny Hand, Anthony Prenni, Colorado State University
Abstract Number: 151 Working Group: Carbonaceous Aerosol
Abstract 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 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. Analysis of evolved carbon concentration temporal trends suggest 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. The fundamental problem with assigning a LAC-OC split is that there is not an objective definition of the two analytes we claim to be distinguishing between. Without an objective definition of the distinction between LAC and OC, there is no way this distinction can be traced to fundamental principles. There is only the ostensive definition that the desired distinction is just what this [or that] method delivers. 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.