An Experiment Relating Black Carbon Content to Reduction of Snow Albedo

冰雪中的黑碳（BC）已被确定为全球辐射强迫对气候变化的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场地上使用有或没有添加商业烟灰的水来冻结造雪机产生的水滴，其浓度约为自然烟灰水平的100倍，从而获得反照率的大信号。在室外而不是在实验室进行实验的原因是为了获得均匀的照明，避免消除边缘效应，并且能够测量反照率而不是窄视场反射率。光学有效雪粒大小将由测量的近红外反照率确定；匹配测量到的可见光反照率需要在辐射传输模型中加入BC。人工积雪的BC含量将通过过滤融水来测定；滤光片将用实验室分光光度计进行分析，就像对天然雪样品的滤光片所做的那样。将过滤器测量得到的BC与反照率测量得到的BC进行比较。将进行一系列实验，改变(A)煤烟的类型，以研究不同的尺寸分布；(b)烟灰量，考察反照率降低的非线性；(c)人造雪的颗粒大小，因为根据预测，在给定浓度的烟灰下，粗颗粒雪的反照率降低要大于细颗粒雪。结合实验，将开发用于气候模式的参数化，使用与实验结果一致的煤烟特性。对于分解表层积雪层的模式，将参数化三个光谱波段的单次散射量。对于将雪反照率作为大气模式的下边界条件的模式，对于一般环流模式（GCMs）中常用的两个宽频带，将计算平均反照率作为雪粒度和BC浓度的函数，以及晴天和阴天入射太阳光谱的太阳总反照率。简单的解析函数将适合于模型结果。这项工作的更广泛影响在于有可能促进我们对雪中BC的辐射强迫及其气候后果的理解，并改进用于气候和减缓研究的模式中这一过程的代表性。

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.