Measuring the Size Distribution of Atmospheric Organic and Black Carbon Using Impactor Sampling Coupled with Thermal Carbon Analysis: Method Development and Uncertainties

A method for determining the mass size distribution of organic and black carbon (OC and BC) in atmospheric aerosols is introduced. The method relies on a particle sampling with 2 parallel size-segregating devices, a 12-stage Small Deposit area low pressure impactor (SDI) and a virtual impactor (VI), and the subsequent analysis of the samples with thermal and thermal-optical methods, respectively. The method development revealed that SDI is, like other sampling methods, susceptible to serious sampling artifacts and OC pyrolysis during thermal analysis. However, some of the SDI's limitations can be overcome by parallel VI measurements. The good correlation between the SDI and the VI data for most of the samples collected here indicates that under most conditions, the lack of the pyrolysis correction for the SDI samples does not cause significant errors in the OC/BC split. Valuable features of this method are that it offers a good size resolution in both sub- and supermicron size fractions, indicates if there has been serious positive or negative artifacts for OC during sampling, reveals if the samples have been affected by OC pyrolysis during thermal analysis, and provides semiquantitative means by which the OC and BC size distributions can be corrected for the samples being affected by OC pyrolysis. Application of the method to real atmospheric samples is demonstrated, and the major areas requiring further research and/or method development are identified.