Measuring Organic Carbon and Black Carbon in Rainwater: Evaluation of Methods

Measuring wet deposition of organic carbon (OC) and black carbon (BC) is crucial for the complete understanding of the global circulation, lifetime, and radiative forcing of these aerosols. There is currently no accepted standard analytical method for measuring OC and BC concentration in precipitation. Different analytical methods have been employed for this purpose, but their feasibility has yet to be assessed. This manuscript evaluates the use of thermal-optical analysis (TOA), single-particle soot photometry (SP2), and ultraviolet-visible (UV/VIS) spectrophotometry for measuring BC in precipitation. In addition, total organic carbon (TOC) analysis was evaluated for the measurement of dissolved organic carbon (DOC) in precipitation. Potential interferences and sources of bias were assessed for each method. Precipitation samples and reference materials containing carbon particles generated from wood combustion and a natural gas diffusion flame were used in this study. The UV/VIS spectrophotometer, despite showing linearity with BC concentration, had inadequate sensitivity (18g/L) to measure the low concentrations expected in precipitation. The SP2 analysis was adequate to measure refractory BC in precipitation in terms of precision and detection limit; however, systematic loss was estimated to be 34% (+/- 3%). Sample filtration followed by TOA was inefficient for measuring particulate carbon in rainwater, as the quartz fiber filter captured less than 38% of the BC mass. Filtration was improved by adding salts and acids into the water samples, and ammonium dihydrogen phosphate, (NH4)H2PO4, was determined to be the best additive by increasing the collection efficiency of quartz fiber filters up to 95% (+/- 5%). The TOC analyzer proved to be precise in the expected concentration range (200-5000g-C/L) for measuring DOC and total carbon (TC), including particulate OC and 94% (+/- 2%) of the refractory BC in solution. Copyright 2014 American Association for Aerosol Research