西安冬季大气棕碳光学特征及辐射强迫

Optical properties and radiative forcing contribution of brown carbon in Xi’an during winter

为探讨西安冬季不同大气污染状况(污染天和清洁天)大气细粒子(PM_(2.5))及其一次棕碳(BrC_(pri))和二次棕碳(BrC_(sec))的光学特征及辐射效应,开展了高分辨率多波段光学参数观测,获得PM_(2.5)多波段光学吸收系数(b_(abs)),通过进一步数据分析得到BrC_(pri)和BrC_(sec)的光谱依赖指数(AAE)及其相对黑碳(BC)的辐射强迫.结果显示,污染天时段370nm光学吸收(b_(abs)(370))和880nm光学吸收(b_(abs)(880))均值分别为(733±311)Mm^(-1)和(185±80)Mm^(-1),分别高出清洁天约5.9倍和6.2倍.清洁天PM_(2.5)的AAE1.08~2.09,变化幅度大于污染天(1.28~1.79).清洁天棕碳光学吸收(b_(abs)(BrC))在370nm波长对总吸收占比高于污染天,均超过30%.一次棕碳光学吸收(b_(abs)(BrC_(pri)))在清洁天和污染天均对b_(abs)(BrC)呈现高贡献,占比范围分别为76%~86%和82%~91%,说明一次排放仍然是造成西安冬季污染的重要原因.清洁天BrC、BrC_(pri)和BrC_(sec)的AAE均值分别为4.42、4.31和4.78,均高于污染天,说明清洁天粒子等效直径相对较小,老化程度较高导致BrC的高光谱依赖性.b_(abs)(BrC_(sec))日间变化表明污染天凌晨高湿条件下的液相反应可能是BrC_(sec)形成的主要机制,比清洁天更为强烈,而污染天日间BrC_(sec)受光漂白的影响较大.最后估算了BrC_(pri)和BrC_(sec)的辐射强迫效应,在紫外波段(300~400nm),污染天BrC_(pri)和BrC_(sec)相对BC的辐射强迫分别为62%和16%,而清洁天分别为59%和23%,表明BrC_(pri)和BrC_(sec)在西安冬季的辐射强迫效应不容忽视.

To investigate the light absorption coefficient(b_(abs))and radiative forcing of primary and secondary brown carbon(BrC_(pri) and BrC_(sec))during winter haze and clean days in Xi’an,Aethalometer(AE33)was used to obtain the b_(abs) of PM_(2.5) from 15th December 2015 to 31st January 2016,the absorptionÅngström exponent(AAE)and radiation forcing relative to black carbon(BC)of BrC_(pri) and BrC_(sec) were further analyzed.The results showed that the mean values of b_(abs)(370)and b_(abs)(880)were(733±311)Mm^(-1) and(185±80)Mm^(-1) in haze days,those were 5.9 and 6.2 times higher than that in clean days,respectively.The AAEs of PM_(2.5) varied from1.08 to 2.09 in clean days,which showed higher amplitude than those in haze days(1.28~1.79).The contributions of b_(abs)(BrC)to total b_(abs) at 370 nm in clean days were higher than those in haze days(>30%).High contributions of b_(abs)(BrC_(pri))to b_(abs)(BrC)in clean days(76%~86%)and haze days(82%~91%)indicated that the primary emissions were the important causes of severe air pollution during wintertime in Xi’an.The AAEs values of BrC(4.42),BrC_(pri)(4.31)and BrC_(sec)(4.78)in clean days were higher than those in haze days,which may be attributed to the smaller equivalent diameter and higher aging of PM_(2.5) in clean days.The results indicated the dominance of the aqueous reactions mechanism for BrC_(sec) formation before the sunrise during polluted days.After sunrise,the bleaching of BrC_(sec) chromophores through oxidative processes was of more importance in polluted days than that in clean days.The relative radiative forcing of BrC_(pri) and BrC_(sec) to BC(300~400nm)were 59%and 23%in clean days,while those were 62%and 16%in haze days,respectively.The results underlined the radiative forcing of BrC_(pri) and BrC_(sec) in winter.