An Experiment Relating Black Carbon Content to Reduction of Snow Albedo

雪和冰中的黑碳（BC）被认为是估计全球气候变化辐射强迫的最大不确定性之一。在这个项目中，人工积雪将通过在开阔的场地上冷冻造雪机产生的水滴来制造，使用含有或不含有添加量的商业烟尘的水，浓度约为自然烟尘水平的100倍，以便在Alpendo上获得大信号。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，并且能够测量反射率而不是窄视场反射率。光学上有效的雪颗粒大小将从测量的近红外线散射来确定;然后匹配测量的可见光散射将需要将BC添加到辐射传输模型中。人工积雪的BC含量将通过过滤融水来测量;过滤器将通过实验室分光光度计进行分析，就像对天然雪样品中的过滤器所做的那样。将进行一系列实验，改变（a）烟灰的类型，以研究不同的尺寸分布;（B）烟灰的量，以研究烟灰减少的非线性;以及（c）人造雪的颗粒尺寸，因为预测在粗颗粒雪中由给定浓度的煤烟引起的碳烟还原比在细粒雪中更大。结合这些实验，将利用与实验结果一致的煤烟特性，制定用于气候模型的参数。对于解析地表雪层的模型，将对三个光谱带的单次散射量进行参数化。对于大气模式中雪覆盖度是指定的下边界条件的模式，对于大气环流模式（GCM）中常用的两个宽波段，平均覆盖度将作为雪颗粒大小和BC浓度的函数计算，对于晴空和多云天空下的入射太阳光谱，将作为总的太阳覆盖度计算。简单的分析函数将适合模式的结果。更广泛的影响，这项工作是在潜在的，以促进我们的理解BC在雪中的辐射强迫及其气候后果，并提高这一过程中的代表性用于气候和缓解研究的模式。

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.