Real-Time Measurements of Black Carbon Indoors and Outdoors: A Comparison of the Photoelectric Aerosol Sensor and the Aethalometer

Abstract

A real-time instrument employing photoelectric emission has been suggested as a semiquantitative tracer of black carbon (BC). The instrument is known as the Photoelectric Aerosol Sensor (PAS) and has been manufactured in Europe since the 1980s. As a test of this relationship, real-time measurements were made using two models of this instrument side by side with two Aethalometers for one year (1998) and for an additional six months (December 1999–May 2000) inside and outside an occupied house in Reston, VA. Four sources, two outdoors and two indoors, were investigated. The outdoor sources included automobile traffic and woodburning; the indoor sources included cooking and candle burning. Correlations between the Aethalometer and both models of PAS instruments for three of the four sources ranged from R2 = 72% to 85%. For cooking, the earlier PAS Model 1001i using mercury vapor as a UV source was correlated with the Aethalometer for broiled foods, but the later PAS Model 2000 using a krypton chloride excimer lamp showed almost no response. When all sources were combined, both the outdoor PAS 2000 and the indoor PAS 1001i correlated with the corresponding Aethalometers (R2 = 63%, N = 36,558, p < 0.0001; and R2 = 68%, N = 34,954, p < 0.0001, respectively). Although the precision of the Aethalometer and PAS 2000 was high (4.5% and 5.4%, respectively), and correlations between them fairly good for some sources, the accuracy of both instruments is essentially unknown, due in part to the lack of a standard capable of providing calibrations in the field. There is also an unexplained difference of about a factor of 10 between the PAS 1001i and PAS 2000 models used in this study. The PAS/Aethalometer ratio varies widely across studies, suggesting that both instruments have site-specific and/or source-specific responses. Nonetheless, for certain uses, such as identifying sources, determining indoor-outdoor relationships, mapping diurnal variation, identifying peaks, and measuring personal exposures, the PAS has important advantages.