Japan Geoscience Union Meeting 2014

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Oral

Symbol A (Atmospheric, Ocean, and Environmental Sciences) » A-AS Atmospheric Sciences, Meteorology & Atmospheric Environment

[A-AS22_1PM2] Atmospheric Chemistry

Thu. May 1, 2014 4:15 PM - 6:00 PM 511 (5F)

Convener:*Nobuyuki Takegawa(Research Center for Advanced Science and Technology, University of Tokyo), Yousuke Sawa(Geochemical Research Department, Meteorological Research Institute), Yugo Kanaya(Research Institute for Global Change, Japan Agency for Marine-Earth Science and Technology), Kenshi Takahashi(Research Institute for Sustainable Humanosphere, Kyoto University), Hiroshi Tanimoto(National Institute for Environmental Studies), Chair:Nobuyuki Takegawa(Research Center for Advanced Science and Technology, University of Tokyo)

4:15 PM - 4:30 PM

[AAS22-P08_PG] An Empirical Correction Factor for Filter-based Photo-absorption Black Carbon Measurements

3-min talk in an oral session

*Martin IRWIN1, Yutaka KONDO1, Nobuhiro MOTEKI1 (1.The University of Tokyo)

Keywords:aerosol, black carbon, filter-based measurements, absorption

Long-term observations of black carbon (BC) aerosol provide important information regarding seasonal variations, emission source attribution, and regional distribution & transport. Filter-based BC measurement techniques such as the Continuous Soot Monitoring System (COSMOS) are particularly well suited to this application, due to their relative robustness and reliability. However, caution is required when determining the threshold transmittance, Trthresh (proportional to the time interval between filter changes), in order to ensure that acceptable measurement accuracy is maintained throughout the sampling period. We present a new, empirically derived transmittance-dependent correction factor used to interpret the response characteristics of filter-based aerosol absorption measurements performed by COSMOS. Simultaneous measurements of ambient BC aerosol mass (MBC) were conducted in Tokyo, Japan, using two identical COSMOS instruments operated with different threshold transmittance, Trthresh, values, of 0.95 and 0.6. The derived values for MBC were consistently underestimated by the COSMOS operating at lower Trthresh, as a function of decreasing filter transmittance. The 1-hour averaged values of MBC were underestimated by around 10 %, incorporating measurements across the entire range of filter transmittance (1 - 0.6), with a maximum underestimation at around 17 % immediately preceding filter advancement (i.e. Tr = 〜0.60), and a minimum of 〜1 % immediately following filter advancement (i.e. Tr = 〜1). An empirical second-order correction factor was derived from these ambient measurements, and was applied to MBC as a function of filter transmittance, resolving the instruments to within 2 %. Furthermore, the operational performance of COSMOS was tested for a new quartz fibre filter (HEPA). A comparison of different filter types demonstrated a systematic overestimation of MBC of around 6 - 8 % when using HEPA filters. A sensitivity study of a radiative transfer model indicated that this enhanced absorption was primarily a result of the increased thickness of the HEPA filter.