西安冬季大气棕碳光学特征及辐射强迫

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.

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

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 15^th December 2015 to 31^st January 2016, the absorption Ångström exponent(AAE) and radiation forcing relative to black carbon (BC) of BrC_pri and BrC_secwere 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^-1in 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_absat 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.