Importance of Supersaturation in Arctic Black Carbon Simulations

Black carbon (BC) aerosol particles in the Arctic heat the atmosphere and snow/ice surfaces and may strengthen the snow-albedo feedback that amplifies Arctic warming. Model simulations of BC concentrations in the Arctic depend strongly on the representation of microphysical processes such as aging, activation, and wet removal. Most BC modeling studies have classified BC particles into hydrophobic BC, which cannot form cloud droplets, and hydrophilic BC, which can form cloud droplets, by assuming a globally constant critical supersaturation threshold value (S-thre), without considering its consistency with cloud maximum supersaturation (S-max). Here we show that it is essential to consider the consistency of S-thre with S-max in global model simulations to reduce uncertainties in near-surface ambient BC concentrations in the Arctic. Previous studies often obtained good agreement between simulated and observed near-surface Arctic BC mass concentrations when a low S-thre (similar to 0.1%) was assumed in their models. However, this S-thre may be too low (activation and wet removal of BC may be underestimated) for the Arctic, because some recent observations and our model simulations suggest that S-max may actually be higher (similar to 0.3%) there. We also demonstrate that spatially varying S-thre values and their consistency with S-max, which previous studies did not consider, must be represented in models for more accurate estimation of BC budget in the Arctic. Because both S-max and BC-aging speed depend on climatic conditions, our findings are an important step toward better simulations of BC impacts on past, present, and future Arctic climates.

北极的黑碳（BC）气溶胶粒子使大气和雪/冰表面升温，并可能强化雪 - 反照率反馈，从而加剧北极变暖。北极黑碳浓度的模型模拟在很大程度上取决于微观物理过程的表征，例如老化、活化和湿清除。大多数黑碳建模研究通过假设一个全球恒定的临界过饱和阈值（S - thre），将黑碳粒子分为不能形成云滴的疏水性黑碳和亲水性黑碳（能形成云滴），而没有考虑其与云最大过饱和度（S - max）的一致性。在此我们表明，在全球模型模拟中考虑S - thre与S - max的一致性对于减少北极近地表环境黑碳浓度的不确定性至关重要。先前的研究在其模型中假设一个较低的S - thre（约0.1%）时，常常能使模拟的和观测到的北极近地表黑碳质量浓度之间取得良好的一致性。然而，这个S - thre对于北极来说可能过低（黑碳的活化和湿清除可能被低估），因为一些近期的观测和我们的模型模拟表明，那里的S - max实际上可能更高（约0.3%）。我们还证明，先前研究未考虑的空间变化的S - thre值及其与S - max的一致性必须在模型中体现出来，以便更准确地估算北极的黑碳收支。由于S - max和黑碳老化速度都取决于气候条件，我们的研究结果是朝着更好地模拟黑碳对过去、现在和未来北极气候影响迈出的重要一步。