Seasonal Variation of Wet Deposition of Black Carbon in Arctic Alaska

Black carbon (BC) aerosol deposited in and onto Arctic snow increases the snow's absorption of sunlight and accelerates snowmelt. Wet removal of BC from the atmosphere plays a key role in determining its abundance in the Arctic atmosphere and in Arctic snow. However, this process is poorly understood, mainly due to the scarcity of relevant measurements. To reveal characteristic features of the wet deposition of BC, we made measurements of mass concentration of BC in snow and rain (C_MBC) with relatively high (16%) accuracy and mass concentration of BC in air (M_BC) at the Barrow Atmospheric Baseline Observatory, Alaska, from July 2013 to August 2017 and analyzed them along with routinely measured meteorological parameters from Barrow. Monthly mean M_BC near the surface and C_MBC were poorly correlated from midwinter to early spring, when C_MBC was close to the annual median while M_BC was at its annual peak. Seasonal variations in the altitude distribution of M_BC may lead to these differences in seasonal variation of M_BC near the surface and C_MBC. About 50% of the annual wet deposition of BC occurred in the three months of summer, associated with high values of total precipitation and BC originating from biomass burning. Size distributions of BC in snow and rain were stable throughout the year, suggesting that the size distribution of BC in the lower troposphere was similarly stable. Calculations by two global models reproduced the observed seasonal variations of C_MBC and showed that BC from biomass burning dominated C_MBC in summer.沉积于北极积雪内部及表面的黑碳（Black carbon, BC）气溶胶会增强积雪对太阳辐射的吸收，进而加速积雪消融。大气中黑碳的湿清除过程，是决定北极大气及北极积雪中黑碳含量的关键因素。然而，目前对该过程的认知仍较为匮乏，主要原因在于相关观测数据的稀缺。为揭示黑碳湿沉降的特征规律，本研究于2013年7月至2017年8月期间，在阿拉斯加巴罗大气基准观测站（Barrow Atmospheric Baseline Observatory）开展了观测：以16%的相对较高测量精度，测量了积雪与雨水中的黑碳质量浓度（C_MBC）以及大气中的黑碳质量浓度（M_BC），并结合该站点的常规气象观测参数对数据进行了分析。仲冬至早春时段，近地表月均M_BC与C_MBC的相关性较弱；此时C_MBC接近全年中位数水平，而M_BC则处于年度峰值。M_BC的垂直分布季节变化，可能是造成近地表M_BC与C_MBC季节变化差异的原因。全年约50%的黑碳湿沉降量集中在夏季三个月，这与该时段较高的总降水量以及源自生物质燃烧的黑碳密切相关。积雪与雨水中黑碳的粒径分布全年保持稳定，这表明对流层低层的黑碳粒径分布同样较为稳定。两个全球模式的模拟结果重现了观测到的C_MBC季节变化特征，并证实夏季C_MBC主要由生物质燃烧排放的黑碳主导。