The comparison between thermal-optical transmittance elemental carbon and Aethalometer black carbon measured at multiple monitoring sites

Continuous and semi-continuous measurements of organic carbon (OC), elemental carbon (EC) and PM2.5 were performed during the summer of 2002 in Rochester, NY and Philadelphia, PA. During the study period in Philadelphia, high concentrations of wood smoke from a Canadian forest fire were transported to the monitoring site. Two-hour integrated thermal EC (EC)/optical EC (BCS) using a Sunset Lab OC/EC analyzer and continuous Aethalometer optical EC (BCA) using a two-wavelength Aethalometer were compared in various environments. The weekdays diurnal average for EC peaked during the morning rush-hour and was much higher than the value during weekends, whereas the OC results showed no diurnal variation of OC during either weekdays or weekends at both sites. The diurnal variations of BCA were also strongly correlated with the rush-hour traffic. The correlation coefficient between EC and BCA in Rochester (r2=0.84) was higher than in Philadelphia (r2=0.60) while the BCA/EC slopes were 3.3 and 2.7 in Rochester and Philadelphia, respectively. The difference in the slopes indicates that the specific attenuation cross-section used for the optical carbon analysis depends on the physical and chemical characteristics of elemental carbon. During the Canadian forest fire, the BCA/EC slope dramatically dropped to 0.4 with a correlation coefficient of 0.60. The decrease of the proportionality between EC and BCA demonstrates the variability of the absorption coefficient. The level of UV absorbing organic compounds significantly increased during the fire aerosol episode suggesting the presence of abundant organic compounds in the forest fire smoke particles.