对“云内黑碳向云顶聚集”的检验

Black carbon (BC)-cloud interactions play a key role in climate system, but the mechanism is very complex and even unknown so far. BC behavior in the processes of cloud-precipitation, including serving as cloud condensation nuclei (CCN) and/or ice nuclei (IN), changing cloud microphysical properties, and movement in the cloud etc., and its impact on climate and environment are sparsely studied. Based on a new observation of BC and scattering particle concentrations in snow/firn from a high elevation region, we find that the trend of BC concentration along the elevation changes clearly at 7000 m a.s.l., showing a decrease below 7000 m a.s.l. while an increase above 7000 m a.s.l. Meanwhile, the ratio of BC and scattering particle concentrations shows a constantly increasing trend along the whole elevation profile. On the basis of this phenomenon we propose a hypothesis, which is “enrichment of black carbon into the cloud-top”. In this project, we propose to launch field observations and then to explore (1) spatial distribution of BC concentration in ice-phrase precipitation along the elevation, (2) spatial-temporal correlation between the optical properties of aerosol and clouds, and (3) BC movement with the evolution of cloud microphysical properties within the cloud, and separation with other types of aerosol, and consequently test the hypothesis of “enrichment of black carbon into the cloud-top”. This project will provide theoretical foundation for BC movement within the cloud and assess the influences of BC on climate.

黑碳-云的相互作用在气候系统中扮演着重要角色，但因作用过程极其复杂而研究不够全面。目前，关于黑碳在云-降水过程中的行为（包括黑碳作为云凝结核或大气冰核的能力、对云微物理场的改变和在云内迁移的规律等方面）及其气候环境影响鲜有研究。基于高海拔地区垂直梯度雪冰样品分析发现，雪冰黑碳含量在海拔7000米发生趋势性转变，即在海拔7000米以下随海拔升高而降低，而在7000米以上则随海拔升高而升高；同时发现黑碳与散射颗粒含量比值在整个垂直剖面随海拔升高而升高。基于这一现象，提出了“云内黑碳向云顶聚集”的假说。为检验这一假说，本项目拟在青藏高原开展以下观测和研究：①冰相降水中黑碳含量沿海拔垂直梯度分布的特征；②大气气溶胶和云光学特性的时空关联；③黑碳在云微物理特性演变过程中的迁移以及与其它气溶胶的分异。本项目的执行将为进一步厘清黑碳在云中迁移的机制和正确评估黑碳的气候环境效应提供科学依据。

黑碳粒子在大气-云-降雪中的行为是理解其对气候系统影响的关键。黑碳在大气中的垂直分布具有什么样的特征？黑碳在云内的迁移，对云微物理特性和光学性质的影响如何？黑碳作为冰核，在降雪云系中的高度分布是否具有不同于散射粒子的分布特征？对这些问题探索是准确评估黑碳产生的直接辐射强迫、间接辐射强迫和冰雪反照率的关键。本项目在青藏高原典型西风气候区（慕士塔格冰川区）和季风气候区（珠穆朗玛峰绒布冰川-峡谷区）开展大气-雪冰黑碳海拔梯度观测，利用系留气球/浮空艇实施大气黑碳垂直观测，并借助地面激光雷达开展大气粒子光学特性垂直廓线的研究。项目主要获得以下认识：（1）在西风气候区的降雪云系中，黑碳作为冰核随海拔升高，其浓度越低，粒径分布更小；（2）珠峰地区大气黑碳的日变化受大气边界层和下沉风共同作用；（3）大气黑碳垂直廓线显示：自由大气具有比边界层更低的黑碳浓度；携大量黑碳粒子的气团，可在自由大气中通过水平输送进入青藏高原上空；大气边界层内，受湍流作用影响，黑碳粒子可在边界层顶聚集。本项目所取得的研究成果，增加了黑碳在大气-云-降雪中行为的认识角度，丰富了黑碳在自由大气中传输的认识，为准确评估青藏高原大气-雪冰黑碳的气候环境影响提供了坚实的依据。