Real-Time Measurements of the Size Fractionation of Ambient Black Carbon Aerosols at Elevated Humidities

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Abstract
The aethalometer is an instrument that responds to the concentration of aerosol black carbon in real time. Recent versions constructed at Lawrence Berkeley Laboratory (LBL) contain two or three channels that may be operated simultaneously. When preceded by size-segregating inlets of differing cut points, this configuration allows for a real-time determination of the size fractionation of the black carbon component of the aerosol. In field measurements at LBL, we studied the incorporation of black carbon into marine fog droplets by comparing the total concentration with the “interstitial aerosol” concentration measured downstream of an interception mesh of 1-μm cutpoint. The data show that up to 80% of the carbon may be occluded by droplets. In separate experiments during the SCAQS program in southern California, a three-channel instrument, equipped with impactors, measured total, < 1 μm, and < 0.3 μm concentrations, and showed diurnal variations of size fractionation of the ambient carbonaceous aerosol. These results imply that a significant fraction of the aerosol black carbon particles may exhibit hygroscopic behavior.