Differences in the OC/EC Ratios that Characterize Ambient and Source Aerosols due to Thermal-Optical Analysis

摘要

Different thermal-optical methods used to measure OC/EC and EC/TC ratios in atmospheric aerosols often produce significantly different results due to variations within the temperature programming and optical techniques of each method. To quantify the thermal and optical effects on these ratios, various source (residential cookstoves and diesel exhaust) and atmospheric (rural and urban) aerosols were analyzed using 3 thermal protocols: (1) two modified versions of the Birch and Cary (1996, Elemental Carbon-Based Method for Monitoring Occupational Exposures to Particulate Diesel Exhaust. Aerosol Sci. Technol., 25:221-241) National Institute of Occupational Safety and Health (NIOSH 5040) protocol—designated in this paper as NIOSH and NIST-EPA protocols, and (2) the IMPROVE (the Interagency Monitoring of Protected Visual Environments) protocol outlined by Chow et al. 1993 (The DRI Thermal/Optical Reflectance Carbon Analysis System: Description, Evaluation, and Applications in U.S. Air Quality Studies. Atmos. Environ., 27:1185-1201)—designated in this paper as IMPROVE protocol. The use of a dual-optical instrument permitted simultaneous monitoring of the transmission (TOT [thermal-optical transmission]) and reflectance (TOR [thermal-optical reflectance]) for each protocol. Results show that the aerosols containing components susceptible to charring (such as water-soluble organic compounds typical of cookstove and rural aerosols) had higher OC/EC variability among the methods when compared with diesel-impacted aerosols (diesel and urban), which showed little to no “instrumentally calculated” pyrolyzed carbon (PyC). Thermal effects on the OC/EC ratios among the 3 TOT methods were significantly lower for diesel-impacted aerosols. Similar OC/EC findings were observed for the 3 TOR methods. Optical effects (TOT/TOR ratio) for the OC/EC ratio ranged from 1.37-1.71 (residential cookstoves), 1.63-2.23 (rural), 1.05-1.24 (diesel exhaust), and 0.80-1.12 (urban) for the 3 methods, with IMPROVE (TOT and TOR) always significantly lower when compared with NIST-EPA (TOT and TOR) and NIOSH (TOT and TOR) for all sample types. Thermal and optical effects on the EC/TC ratios were similar to those observed for the OC/EC ratios. Due to their distinct aerosol characteristics, different sample types behave differently under various thermal and optical conditions. Hence, use of a single TOA method to define OC/EC ratios for all aerosol types may not be feasible.