Properties of light-absorbing aerosols in the Nagoya urban area, Japan, in August 2011 and January 2012: Contributions of brown carbon and lensing effect

Abstract

The optical properties of aerosols at 405 and 781 nm were measured in an urban site in Nagoya, Japan, in August 2011 and in January 2012 using a photoacoustic spectrometer. Comparison of the absorption coefficient at 781 nm of aerosols that did and did not pass through a thermo-denuder showed that an increase in black carbon (BC) light absorption due to the coating of non-refractory materials (i.e., the lensing effect) was small (on average, 10%) in August and negligible in January. The effective density distributions for the particles that did and did not pass through the thermo-denuder, which were measured simultaneously in August, suggested that the majority of BC particles sampled had a minimal coating. The small lensing effect observed can be explained partly by assuming that a large portion of non-refractory materials was mixed externally with BC. The contribution of direct light absorption by organic matter (OM) that vaporized at temperatures below 300°C to the total light absorption at 405 nm was negligible in August, but those by OM that vaporized below 300 and 400°C averaged 11 and 17%, respectively, in January. The larger contribution of light-absorbing OM in January is likely due to the greater contribution of OM originating from the burning of biomass, including biofuel and agricultural residue, in Japan, northern China, or Siberia, during the winter.