Contrasting Stratospheric Smoke Mass and Lifetime from 2017 Canadian and 2019/2020 Australian Megafires: Global Simulations and Satellite Observations

Stratospheric injections of carbonaceous aerosols and combustion gases by extreme wildfires have become increasingly common. Recent "megafires", particularly large and intense fires, delivered particulate burdens to the lower stratosphere comparable to those of moderate volcanic eruptions. The 2017 Canadian megafire generated four large Pyrocumulonimbi (pyroCbs), injecting up to approximately 0.3 Tg of smoke in the lower stratosphere. Even more extreme, the 2019/2020 Australian event produced a pyroCb activity resulting in stratospheric smoke intrusions of approximately 1 Tg. To understand their contrasting behavior, we present global climate simulations of the atmospheric response to these events, applying smoke burdens informed by remote observations. Model outcomes, compared to satellite data of smoke transport, reproduce reasonably well the initial plume rise, at 0.2-0.3 km/day, attaining heights of approximately 20 km in Canada and above 30 km in Australia. Global dispersal of the plume occurs within about three weeks in both cases, consistent with observations. Smoke removal timescales, approximately 5 months for the Canadian megafire, agree with remote measurements. During the Australian megafire, observations indicate stratospheric injections three times larger, and models predict comparatively longer smoke lifetimes, approximately 16 months. After the latter event, atmospheric optical depths and radiative cooling achieved values close to those measured following the Pinatubo eruption. Sensitivity tests of model assumptions indicate, in accord with prior studies, that smoke burden, injection heights, and black carbon content can determine plume evolution and possible climate impacts. An empirical relation between peak heights of stratospheric plumes and lifetimes is derived that can help assess megafire impacts on the stratosphere, climate and the Earth system.

极端野火向平流层注入含碳气溶胶与燃烧废气的现象正愈发频发。近年的巨型野火（megafires），尤其是规模庞大、强度极高的林火，向平流层下层输送的颗粒物载荷，可与中等规模火山喷发的颗粒物载荷相当。2017年加拿大巨型野火生成了4个大型火积云（Pyrocumulonimbi，下文简称pyroCb），向平流层下层注入了约0.3太克（teragram，Tg）的烟雾。更为极端的2019/2020年澳大利亚林火事件则产生了火积云活动，导致平流层烟雾侵入量约达1太克（Tg）。为探究二者行为差异的成因，我们基于遥感观测得到的烟雾载荷数据，开展了针对上述两类事件的大气响应全球气候模拟。将模拟结果与烟雾输送的卫星观测数据对比后发现，模拟较好地重现了初始烟羽抬升过程：抬升速率为0.2~0.3千米/天，加拿大事件的烟羽最终可达约20千米高度，澳大利亚事件则超过30千米。两类事件的烟羽均在约三周内完成全球扩散，与观测结果一致。加拿大巨型野火的烟雾移除时长远约5个月，与遥感测量结果相符。澳大利亚巨型野火的观测数据显示其平流层注入量为加拿大事件的3倍，而模型预测其烟雾寿命相对更长，约为16个月。该事件发生后，大气光学厚度与辐射冷却值接近皮纳图博火山（Pinatubo）喷发后的观测水平。针对模型假设的敏感性试验表明，与既往研究结论一致，烟雾载荷、注入高度以及黑碳含量，均可决定烟羽演化过程及其潜在气候影响。我们推导得到了平流层烟羽峰值高度与烟雾寿命之间的经验关系式，该关系式可用于评估巨型野火对平流层、气候及地球系统的影响。