利用黑碳仪获取北京市冬季气溶胶吸光系数的方法研究

黑碳仪是最常用的气溶胶吸光碳测试仪器，但其测得的吸光性能受到滤膜多重散射效应、颗粒物散射效应、颗粒物遮蔽效应的影响，只能算是b_ATN（光衰减系数），而不是b_in-situ（原位吸光系数）.虽然已有学者提出几种将b_ATN转换为b_cor（校正后吸光系数）的算法，但多是基于实验室模拟或高海拔地区气溶胶而开展的，与我国城市的气溶胶在理化性质方面有明显差别.为探索我国城市气溶胶将b_ATN转换为b_cor的问题，于2016—2017年冬季在北京市城区使用黑碳仪及光声消光仪分别测量b_ATN和b_in-situ，在分析多种算法的基础上，提出了面向城市气溶胶的校正方法.结果显示:在我国北方城市（以北京市为例），适应城市气溶胶的f值（遮蔽因子，为计算遮蔽校正系数的一个参数）为1.13，C（综合散射效应系数）为5.44；使用这些校正系数，将观测点由黑碳仪获取的b_ATN转换为b_cor.与光声消光仪测定的b_in-situ对比发现，无论是滤膜样点周期内还是长期观测时间内，b_cor与b_in-situ均呈一致性，其中，长期观测时间内二者的相关性表达为y=0.954x+0.829（r2为0.996），证明了校正方法的有效性和可信性.研究显示，b_cor与b_in-situ整体相关性较高，并且获得了本地化的校正因子，实现了仅通过黑碳仪获取准确颗粒物吸光系数的目的. 

Methodology of Deriving Absorption Coefficient of Winter Aerosol Using Aethalometer Measured Attenuation Coefficient in Beijing

Aethalometer is the most popular equipment for the measurement of light absorbing carbon. However, what it measures is the light attenuation coefficient (b_ATN) rather than the in-situ light absorption coefficient of aerosols (b_in-situ) due to the influences of filter multiple scattering, particle scattering and particle shadowing. Although a few correction algorithms have been proposed by some scholars to compensate for the artifacts so as to convert the b_ATN to the aerosol absorption coefficient (b_cor), most of them are based on the aerosols from laboratory generators or from high altitudes, which are usually different in physical and chemical properties from the urban aerosols in China. To explore the methodology of conversion from b_ATN to b_cor for the urban aerosols in China, an aethalometer was used to monitor the b_ATN and a photoacoustic Extinctiometer was used in parallel to monitor the b_in-situ during the winter season of 2016-2017 in Beijing. Based on the study of the available algorithms, a method to correct the artifacts existing in aethalometer measurements of urban aerosols was proposed. It is found that the shadowing factor f (a parameter used for calculating shadowing effect coefficient) and the total scattering correction coefficient (C) fitting for the investigated city are 1.13 and 5.44, respectively. With these correctors, the observed b_ATN values by the aethalometer were successfully converted into b_cor. A comparison between PAX-measured b_in-situ and aethalometer-converted b_cor shows that the two absorption coefficients are similar in value and are significantly correlated both for a filter spot duration and a long-term duration (among which, the correlation of long-term duration can be represented by y=0.954x+0.829, r²=0.996), which demonstrates the effectiveness and reliability of the above correction scheme. This lays a good foundation for deriving b_in-situ from aethalometer-measured b_ATN.