Quantifying the impact of anthropogenic and natural aerosol on Arctic climate

This project brings together aerosol modelling expertise at the University of Leeds with climate modelling expertise at the Met Office to tackle the pressing problem of air pollition-climate interactions in the Arctic. The project addresses the following questions: 1. What is the effect of anthropogenic and natural aerosol on the present-day Arctic climate? High concentrations of particles enter the Arctic atmosphere in winter and spring and accumulate due to strong surface temperature inversions and low scavenging rates, and in summer the Arctic is episodically and substantially perturbed by boreal forest fires. These sources of pollution are known to have a considerable effect on Arctic clouds and the albedo of snow pack. Observations and models suggest that long and short wave perturbations to clouds could be as much as several Watts per sq m and that changes in snow albedo could increase summer temperatures by between 0.5 and 1.6K. This project will further develop and then apply advanced aerosol transport and climate models to improve our understanding of the factors that control the input of these pollutants to the Arctic. 2. What are the limitations of current models and how can they be improved? Previous studies of Arctic aerosol have demonstrated that global models struggle to capture even the basic properties of aerosol in this region. In particular, they often greatly underpredict the amount of black carbon (BC) transported in spring time, which is critical given the importance of BC in snow pack albedo change and melting. The models also struggle to capture the seasonal cycle of aerosol properties, suggesting that they don't adequately simulate the transport and removal processes that vary strongly with season. However, the number of global model studies focused on Arctic aerosol (including burdens, microphysical properties, variability and trends) is very limited. There is much to be learned about the processes and about the weaknesses of the models. This project will comprehensively compare observed aerosol with models and establish the causes of previous poor model performance. Improved aerosol model components will be developed and then applied in climate simulations. 3. How might the impact of aerosol pollution change in a future climate? The Arctic climate is changing rapidly, with recession of summer sea ice and perturbed meteorology, which may be leading to alteration in the transport of pollutants to the region. Recent observations show that the long term downward trend in BC at N American Arctic sites may be reversing, and the causes are not clear. This project will use improved models of Arctic aerosol in a full climate simulation to quantify the long term changes in Arctic aerosol over the coming decades. These simulations will take into account altered sea ice (which changes DMS emissions and aerosol nucleation in summer), changes in the transport of aerosol, and the coupling of BC and snow albedo. The expertise of Leeds is in detailed aerosol transport models. The Global Model of Aerosol Processes (GLOMAP) has been developed over the last 5 years and will be used by the student to understand the processes controlling Arctic aerosol changes. Extensive existing observations and new data from IPY will be used to evaluate the model. The understanding gained from these studies will be used to develop improved aerosol schemes in the Met Office climate model. The climate model includes the interactions between the aerosol, clouds and snow pack that is not treated in the chemical transport model used at Leeds and will enable long term changes to be simulated. The student will become proficient in the use of an advanced aerosol transport model, in the analysis of observations, the development of new model routines, and in the running of climate models.