Real-time measurements of black carbon indoors and outdoors: A comparison of the photoelectric aerosol sensor and the aethalometer

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 R-2 = 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 (R-2 = 63%, N= 36,558, p < 0.0001; and R-2 = 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.