29 July 2016

Quantifying Arctic Soot: What Drives the Climate-Changing Particle

Scientists investigate individual processes in global models to understand what makes a key difference to Arctic black carbon.

Science

The strong heating potential and short atmospheric residence time of black carbon (BC), commonly known as soot, offers rapid global warming mitigation opportunities. However, a substantial challenge is the large uncertainty range associated with BC, both near source regions and in the Arctic. A multi-model approach is needed to examine BC processes in the Arctic, such as transport seasonality, emissions, and wet and dry deposition.

Impact

This study developed a new framework for diagnosing relationships between key processes for Arctic BC burdens, which is directly relevant to studies of Arctic pollution and climate. Four models show that transport of BC to the Arctic from lower latitudes is the major BC source for this region. All models simulated a similar annual cycle of BC transport from lower latitudes to the Arctic, with maximum transport occurring in July. The research found substantial differences in simulated BC burdens and vertical distributions, and considerable differences in wet deposition efficiencies in the models are a leading cause of differences in simulated BC burdens.

Summary

The research team, including a Department of Energy scientist at Pacific Northwest National Laboratory, conducted a series of sensitivity experiments with models that were used for a recent assessment of Arctic aerosols and climate by the Arctic Monitoring and Assessment Programme (AMAP). As an extension to AMAP assessment activities, this research focused on the seasonality of BC transport, emissions, wet and dry deposition in the Arctic, with each individual process switched on or off. The results from these experiments indicate that differences in BC in the upper troposphere can largely be attributed to differences in the efficiency of BC scavenging in convective clouds outside the Arctic. The team found that versions of the models that did not account for scavenging of aerosols in convective clouds produced much higher concentrations with relatively weak seasonal variations in the upper troposphere and stratosphere, where the largest model differences occur. In addition, the efficiency of wet deposition in layer (stratiform) clouds partly explains some of the differences in lower tropospheric BC concentrations in the models. The researchers also found that convective wet removal outside the Arctic reduces the mean altitude of BC residing in the Arctic, making it more susceptible to scavenging by layer clouds in the Arctic. Consequently, scavenging of BC in convective clouds outside the Arctic acts to substantially increase the overall efficiency of BC wet deposition in the Arctic, which leads to low BC burdens and a more pronounced seasonal cycle compared to simulations without convective BC scavenging. Overall, the study highlighted key differences in simulated BC burdens in the Arctic; in particular, parameterizations of convective wet deposition in models were shown to have large impacts on simulated BC burdens. Conceptually, very different parameterizations of convective processes are used in the models considered here and in other climate models. Future studies should, therefore, focus on these parameterizations to improve the modeling of Arctic BC and climate.

Contact
Philip Rasch
Pacific Northwest National Laboratory (PNNL)