Measurement of Light Absorption Properties in Ambient Aerosols

Abstract:

Black Carbon (BC) is one of climate change relevant species, which is abundant in ambient PM2.5. BC can be measured by its light absorption properties. A range of measurement equipments and methods are available for black carbon in aerosols. Reflectometer is one of the cheaper BC measuring devices, which determines the darkness of deposited particles on a filter using a principle of light reflectance. However uncertainty exists in readings of light reflection coefficient that should be reduced using an established QA/QC procedure. In this study a simple calibration experiment between reflection coefficient and BC mass from candle smoke was conducted. A calibration curve and BC estimation formula have been derived and applied for ambient air samples, PM10-2.5 and PM2.5 collected by dichotomous samplers at different sites, i.e. remote, sub-urban, urban traffic, and urban industry areas. Reflectometer optical BC estimated using the candle calibration formula (BCR) were compared with elemental carbon (EC) determined by using National Institute of Occupational Safety and Heath (NIOSH) protocol with correction by thermal-optical transmission (TOT). High correlation between BCR and EC was observed in remote and sub-urban areas with different the BCR/EC slope. Better estimation of BCR to EC at sub-urban area (BCR/EC slope almost closed to 1) was observed than the one in remote area. Too thick particle deposit layer on a filter may possibly be a main reason of low correlation presented in the urban traffic area. BCR and EC in the PM from diesel emission also show a good correlation when the filters were not too dark. The sampling protocol should be developed to collect a thin layer of the particles for the optical BC measurement.

Keywords: black carbon, elemental carbon, climate change, calibration, candle smoke