北京秋冬季有机碳和元素碳(黑碳)测试结果的细节研究

长期以来，对碳气溶胶的定量研究主要关注OC（有机碳）、EC（元素碳）或BC（黑碳）的整体测定结果，很少有对测定结果细节特征的深入解读.为全面掌握和利用仪器分析结果包含的科学信息，使用热光法IMPROVE_A协议（model 2001A）测定了2015年10月（秋季）和2016年1月（冬季）北京市PM_2.5中的ρ（OC）和ρ（EC），使用光学法（黑碳仪AE31）测定了相应的ρ（BC）.结果表明:① ρ（OC）和ρ（EC）的秋季平均值分别为8.59、3.89 μg/m3，冬季分别为16.45和6.19 μg/m3，冬季明显高于秋季；② 热光法测定结果显示，秋季样品中ρ（OC₁）/ρ（OC）的平均值为0.08±0.04，而冬季则升至0.22±0.05，这可能与冬季较高的挥发性有机物（VOCs）排放及低温带来的冷凝效应有关；③ 七波长黑碳仪测定结果显示，在秋季，紫外波段（370 nm）测定的BC当量ρ（BC370）]与红外波段（880 nm）测定的BC标准量ρ（BC880）]的比值ρ（BC370）/ρ（BC880）]为1.05±0.11，说明棕色碳（BrC）的吸光影响非常弱，而冬季该比值升至1.47±0.11，升幅达40%；④ 结合两种方法对强吸光碳的测定结果，发现ρ（BC）/ρ（EC）与ρ（PM_2.5）的变化趋势一致，证明污染程度加重会带来EC内混合比例上升，因而提高其吸光能力，使黑碳仪测得的ρ（BC）上升.然而，进一步考察表明，这种上升是有限度的，当ρ（PM_2.5）达到50~70 μg/m3时，ρ（BC）/ρ（EC）进入"平台状态"，秋季"平台值"约为1.05，冬季约为0.55.研究显示，仪器的测定结果包含大量被忽略的信息，对其细节的深入解读有利于更好地了解碳气溶胶的综合特征. 

Study of the Details of Aerosol Carbon Measurements in Autumn and Winter in Beijing

Quantitative studies on carbonaceous aerosols are based on individual bulk values of organic carbon (OC) and elemental carbon (EC) or black carbon (BC), yet few cases of in-depth interpretation of the minutiae of the measurement results are available, making full understanding and application of the information involved in instrument analysis results difficult. In October 2015 (autumn) and January 2016 (winter), ρ(OC) and ρ(EC) in PM_2.5 of Beijing were measured using the thermal/optical IMPROVE_A protocol (model 2001A), and meantime ρ(BC) was measured using the optical aethalometer method (AE31). The results showed that:(1) The ρ(OC) and ρ(EC) in autumn were 8.59 and 3.89 μg/m3, respectively, and those in winter were 16.45 and 6.19 μg/m3, respectively. The values in winter were significantly higher than those in autumn. (2) The average value of ρ(OC₁)/ρ(OC) ratio in autumn samples analyzed by thermal/optical method was 0.08±0.04, and in winter samples, the ratio rose to 0.22±0.05, which may be a result of higher emissions of volatile organic compounds (VOCs) and subsequent condensation in lower ambient temperatures in winter. (3) Regarding the determination results of the 7-wavelength aethalometer, the ratio of BC equivalent measured at 370 nm (ρ(BC370)) to the standard BC measured at 880 nm (ρ(BC880)), i.e., ρ (BC370)/ρ(BC880), was 1.05±0.11 in autumn, indicative of weak optical absorption by brown carbon (BrC) in that season, but jumped to 1.47±0.11 in winter, 40% higher than that of autumn. (4) The variation trends of ρ(BC)/ρ(EC) and ρ(PM_2.5) were very similar, implying an increased chance of core-shell internal mixing and a consequent enhancement of EC's mass absorption efficiency (MAE). ρ(BC) values reported by the aethalometer were thus magnified, leading to a bigger ρ(BC)/ρ(EC) ratio. However, further investigation showed that the increase reached a 'platform' when ρ(PM_2.5) reached 50-70 μg/m3, with an 'autumn platform value' at around 1.05 and a 'winter platform value' at around 0.55. It could be seen that the measurement results of the instruments contain a large amount of information, some of which has usually been ignored in routine practice. The in-depth interpretation of the details of the information helps better understand the comprehensive characteristics of carbonaceous aerosols.