An Experiment Relating Black Carbon Content to Reduction of Snow Albedo

冰雪中的黑碳(BC)已被认为是全球气候变化辐射强迫估计中最大的不确定性之一。在这个项目中，人工积雪将通过在开阔的田野上冻结造雪机产生的水滴来制作，使用添加和不添加商业煤烟的水，浓度约为自然煤烟水平的100倍，以获得关于反照率的大信号。之所以在室外而不是在实验室进行实验，是为了获得均匀的照明，避免消除边缘效应，并能够测量反照率而不是窄视场反射率。光学有效的雪粒尺寸将根据测量的近红外反照率确定；然后，匹配测量的可见光反照率将需要在辐射传输模型中添加BC。人造积雪的BC含量将通过过滤融化的水来测量；过滤器将通过实验室分光光度计进行分析，就像对天然雪样本的过滤器所做的那样。滤光片测量的BC将与反照率测量得出的BC进行比较。将进行一系列实验，包括(A)煤烟类型，以调查不同的粒度分布；(B)煤烟量，以调查反照率降低的非线性；以及(C)人造雪的颗粒大小，因为预测在粗粒雪中，特定浓度的煤烟对反照率的降低比在细粒雪中更大。结合实验，将开发用于气候模型的参数，使用与实验结果一致的烟尘特性。对于解析地表雪层的模型，将对三个光谱波段的单次散射量进行参数化。对于雪反照率是大气模式指定下限条件的模式，对于大气环流模式中常用的两个宽带，平均反照率将计算为雪粒尺寸和BC浓度的函数，以及在晴空和多云天空下入射太阳光谱的太阳总反照率。简单的解析函数将适用于模型结果。这项工作的广泛影响在于有可能促进我们对BC在雪中的辐射强迫及其气候后果的理解，并改进这一过程在用于气候和缓解研究的模型中的表示。

Black carbon (BC) in snow and ice has been identified as one of the largest uncertainties in estimates of global radiative forcing for climate change. In this project artificial snowpacks will be made by freezing of water droplets produced by a snowmaking machine in an open field, using water with and without added quantities of a commercial soot, in concentrations about 100 times natural soot levels so as to obtain a large signal on albedo. The reason for conducting the experiment outdoors rather than in a laboratory is to obtain uniform illumination, to avoid elimination of edge-effects, and to be able to measure albedo rather than narrow-field-of-view reflectance. The optically effective snow grain size will be determined from the measured near-infrared albedo; matching the measured visible albedo will then require addition of BC to the radiative transfer model. The BC content of the artificial snowpacks will be measured by filtering the meltwater; the filters will be analyzed by a laboratory spectrophotometer as is done for filters from samples of natural snow. The BC from the filter measurement will be compared to that inferred from the albedo measurement.A series of experiments will be carried out, varying (a) the type of soot, to investigate different size distributions; (b) the amount of soot, to investigate the nonlinearity of albedo reduction; and (c) the grain size of the artificial snow, because the albedo reduction by a given concentration of soot is predicted to be greater in coarse-grained snow than in fine-grained snow. In conjunction with the experiments, parameterizations will be developed for use in climate models, using soot properties consistent with the experimental results. For models that resolve layers of the surface snow, single-scattering quantities will be parameterized for three spectral bands. For models in which snow albedo is a specified lower boundary condition for the atmospheric model, average albedos will be computed as functions of snow grain size and BC concentration, for the two broad bands commonly used in general circulation models (GCMs), as well as the total solar albedo, for incident solar spectra under both clear sky and cloudy sky. Simple analytical functions will be fit to the model results.Broader impacts of this work are in the potential to advance our understanding of the radiative forcing of BC in snow and its climatic consequences and to improve the representation of this process in models used for climate and mitigation studies.