The Future of Carbonaceous Aerosol Measurement in the IMPROVE Monitoring Program

Carbonaceous aerosols are a signification fraction of fine particulate matter, often accounting for more than 50% of the mass in urban and rural environments. In the Interagency Monitoring of Protected Visual Environments (IMPROVE) program, thermal optical reflectance (TOR) is used to determine the total carbon (TC) concentration, separated into organic carbon (OC) and light absorbing carbon (LAC) fractions. TOR is an operationally defined analysis that cannot be referenced to traceable standards that reflect ambient compositions, so it is challenging to prevent potential instrument drift over long periods. The OC-LAC split can depend on the aerosol composition, which has spatial and long-term trends. These issues can introduce patterns and trends in the data records, complicating assessments of the spatial and temporal patterns of carbonaceous aerosols. In addition, TOR analysis is relatively expensive, and recent decreases in ambient concentrations are challenging the TOR detection limits, particularly for LAC. Due to these issues, two alternative carbonaceous measurements are being explored to supplement or replace TOR analysis in the IMPROVE network. The first is the use of Fourier transform infrared spectroscopy (FTIR), where FTIR spectra are calibrated against a subset of the measured TOR OC and LAC data. The second is a system using TC derived from a thermal method and filter absorption (fabs) measured using a hybrid integrating plate and sphere (HIPS) system. Estimates of LAC are then derived from the fabs data, and OC is estimated from the difference between TC and LAC. These methods have different benefits and drawbacks. FTIR is cost effective, but being calibrated to TOR, has the same inherent problems as an operationally defined method. TC and fabs measurements are also cost effective and known to produce stable long-term trends. However, fabs is subject to filter loading artifacts and relating TC and fabs to TOR OC and LAC is challenging. These three carbonaceous measurement methods are reviewed and contrasted with final recommendations for monitoring in the IMPROVE network.