A size-segregation method for monitoring the diurnal characteristics of atmospheric black carbon size distribution at urban traffic sites

摘要

Understanding the characteristics of the size distribution of ambient black carbon (BC) in distinct environments is critical because the influence of BC aerosols on climate, visibility, and human health depends strongly on the distribution of BC aerosols over the particle size spectrum. In this study, a novel method for determining the mass size distribution of BC in atmospheric aerosols was developed. This size-segregation method relies on measuring BC in parallel using two aethalometers, one of which is used to measure the total BC (BC_T) mass concentration as a reference level and the other is used to measure the BC (BC_i) mass concentration for BC sizes below specific particle sizes that are selected using a size cut-off inlet. In this study, this method was applied to measure BC in atmospheric samples at an urban traffic site. The aethalometers were operated continually from December 15, 2012 to January 31, 2013, and from February 15, 2013 to March 31, 2013. The measurement results presented in this paper are for the diurnal variation patterns, average concentrations, mass fractions, and size distributions of BC aerosols. The results indicate that BC_T mass concentration is approximately 2.8 μg m~(-3) in the Taipei urban area. The levels of BC at this sampling site were affected markedly by traffic emission levels and local wind speed. At the sampling site, the average BC_(2.5)/BC_T, BC_(1.0)/BC_T, BC_(0.5)/BC_T, BC_(0.25)/BC_T, and BC_(0.1)/BC_T were 0.96 ± 0.04, 0.92 ± 0.07, 0.89 ± 0.04, 0.73 ± 0.10, and 0.18 ± 0.08, respectively. The results indicate that approximately 90% of the BC aerosols were smaller than 0.5 μm, that most of the BC aerosols (55%) were in the size range of 0.1-0.25 μm, and that approximately 18% of the BC aerosols were ultrafine. Moreover, the daily average mass size distribution of BC exhibited a single accumulation mode at 0.16 μm at this sampling site. The mode of the BC mass size distribution at rush hour (9 AM) was only 0.14 μm, which is smaller than the daily average. Moreover, the mode of the BC mass size distribution at an early morning hour (3 AM) was 0.18 μm, and a minor coarse mode was also observed during this period. The results further revealed that the geometric diameter (D_(pg)) of the BC aerosols varied between 0.14 and 0.22 μm and the geometric standard deviation (σ_g) of these BC aerosols ranged between 1.4 and 2.3 during the sampling period. Because BC exhibits extremely low chemical reactivity; the size distribution of BC in the atmosphere does not change substantially except through coagulation.