Field validation of a semi-continuous method for aerosol black carbon (aethalometer) and temporal patterns of summertime hourly black carbon measurements in southwestern PA

Two methods for measuring aerosol elemental carbon (EC) are compared. Three -hour integrated carbon samples were collected on quartz filters during the summer of 1990 in Uniontown, PA, primarily during episodes of elevated par ticulate pollution levels. These samples were analyzed for EC and organic c arbon (OC) using a thermo/optical reflectance (TOR) method. Aerosol black c arbon (BC) was measured using an Aethalometer, a semi-continuous optical ab sorption method. The optical attenuation factor for ambient BC was supplied by the instrument manufacturer. Three-hour average concentrations were cal culated from the semi-continuous BC measurements to temporally match the EC /OC integrated quartz filter samples. BC and EC concentrations are highly c orrelated over the study period (R-2 = 0.925). The regression equation is B C (mu g m(-3)) = 0.95 (+/- 0.04) EC - 0.2 (+/- 0.4). The means of 3 h avera ge measurements for EC and BC are 2.3 and 2.0 mu g m(-3), respectively, ave rage concentrations of EC and BC ranged from 0.6 to 9.4 and 0.5 to 9.0 mu g m(-3) respectively. TOR OC and EC concentrations were not highly correlate d (R-2 = 0.22). The mean OC/EC ratio was 1.85. The 10-week Aethalometer hourly dataset was analyzed for daily and weekly t emporal patterns. A strong diurnal BC pattern was observed, with peaks occu rring between 7 a.m. and 9 a.m. local time. This is consistent with the inc rease in emissions from ground level combustion sources in the morning, cou pled with poor dispersion before daytime vertical mixing is established. Th ere was also some indication of a day-of-week effect on BC concentrations, attributed to activity of local ground level anthropogenic sources. Compari son of BC concentrations with co-located measurements of coefficient of haz e in a separate field study in Philadelphia, PA, during the summer of 1992 showed good correlation between the two measurements (R-2 = 0.82). (C) 1999 Published by Elsevier Science Ltd. All rights reserved.