Abstract：
Brown carbon (BrC) is a class of light-absorbing organic aerosols (OA) and has significant influence on atmospheric radiative forcing. However, the current limited understanding of the physicochemical properties of BrC restricts the accurate evaluation of its environmental effects. Here the optical characteristics and chemical composition of BrC during wintertime in the Yangtze River Delta (YRD) region, China were measured by using high-resolution aerosol mass spectrometry (HR-AMS) and UV-vis spectrometry. Our results showed that BrC in PM2.5 during the campaign was dominated by water-soluble organics, which consist of less oxidized oxygenated OA (LO-OOA), more oxidized oxygenated OA (MO-OOA), fossil fuel OA (FFOA) and biomass burning OA (BBOA). MO-OOA and BBOA were the strongest light absorbing BrC at 365 nm (Abs365), followed by LO-OOA and FFOA with a mass absorption coefficient (MAC) being 0.74 ± 0.04, 0.73 ± 0.03, 0.48 ± 0.04 and 0.39 ± 0.06 m2 g-1 during the campaign, respectively. In the low relative humidity (RH < 80 %) haze periods Abs365 of LO-OOA contributed to 44 % of the total light absorption at 365 nm, followed by MO-OOA (31 %), FFOA (21 %) and BBOA (4 %). In contrast, in the high-RH (RH > 80 %) haze periods Abs365 was dominated by MO-OOA, which accounted for 62 % of the total Abs365, followed by LO-OOA (17 %), BBOA (13 %) and FFOA (8 %). Chemical composition analysis further showed that LO-OOA and MO-OOA are produced from gas-phase photooxidation of VOCs and aerosol aqueous reactions, respectively, in which ammonia significantly enhanced the formation and light absorption of BrC in the high RH haze period. On average, >75 % of the total Abs365nm in the YRD region during the haze events was contributed by LO-OOA and MO-OOA, suggesting that atmospheric BrC in China haze periods is predominantly formed by secondary reactions.
摘要：
褐碳(BrC)是一类光吸收性有机气溶胶(OA),对大气辐射强迫有显著影响。然而,目前对BrC理化性质的了解有限，限制了对其环境影响的准确评估。这里介绍了长江三角洲（YRD）地区冬季BrC的光学特征和化学成分，使用高分辨率气溶胶质谱（HR-AMS）和紫外-可见光谱法对中国进行了测量。我们的结果表明，在竞选期间，PM2.5中的BrC以水溶性有机物为主，由较少氧化的含氧OA(LO-OOA)组成，更多氧化的含氧OA（MO-OOA），化石燃料OA(FFOA)和生物质燃烧OA(BBOA)。MO-OOA和BBOA是365nm处吸收光最强的BrC(Abs365),其次是LO-OOA和FFOA，其质量吸收系数（MAC）为0.74±0.04、0.73±0.03、0.48±0.04和0.39±0.06m2g-1，分别。在低相对湿度（RH<80％）雾度期间，LO-OOA的Abs365占365nm处总光吸收的44％。其次是MO-OOA(31%)，FFOA（21%）和BBOA（4%）。相比之下,在高RH(RH>80%)雾霾期Abs365以MO-OOA为主，占Abs365总数的62%，其次是LO-OOA（17%），BBOA（13%）和FFOA（8%）。化学成分分析进一步表明，LO-OOA和MO-OOA是由VOCs的气相光氧化和气溶胶水反应产生的，分别,其中氨显著增强了高RH霾期BrC的形成和光吸收。平均而言,在雾霾事件期间，YRD区域中总Abs365nm的>75%是由LO-OOA和MO-OOA贡献的，这表明中国雾霾期的大气BRC主要是由二次反应形成的。
