Arctic aerosol
A Leeds-Met Office Hadley Centre collaboration.
Climate warming is proceeding faster in the Arctic than in any other region of the world, with IPCC predicting a 5 ºC temperature increase by the end of this century. Non greenhouse gas pollutants have an important influence on Arctic climate. In a phenomenon known as Arctic haze high concentrations of aged particles consisting mainly of sulphate, black carbon (BC), organic carbon, and nitrate enter the Arctic atmosphere in winter and spring [Stohl, 2006] and accumulate due to strong surface temperature inversions and low scavenging rates [Shaw, 1995]. The summer Arctic is cleaner but is episodically perturbed by boreal forest fires. The very low natural cloud condensation nuclei sources mean that the microstructure of Arctic clouds is particularly sensitive to pollution [Garrett et al., 2004], and multi-year observations suggest changes in short and long-wave radiation of several Wm-2 [Lubin and Vogelmann, 2006; 2007]. In addition, deposition of BC on snow and ice reduces the albedo. Flanner et al. [2007] have shown that the “efficacy’’ of BC/snow forcing is more than three times greater than for CO2 and that the Arctic annual mean 2m air temperature can increase as much as 1.6 K due to BC, primarily from forest fires.
There is a pressing need to understand changes in the Arctic climate, which are already apparent and predicted to accelerate. Warming of the Arctic is likely to alter the sources, properties and magnitude of aerosol pollution [Law and Stohl, 2007] and dry summers could mean more frequent high latitude forest fires, which are already the main source of BC in summer [Stohl, 2006]. The shrinking of the Arctic sea ice cover in the biologically active season will also significantly enhance the annual fluxes of primary particles and DMS, the latter by as much as 80% by 2080 [Gabric et al., 2005]. Observations show that the downward trend in BC concentrations in the North American Arctic has stopped and may have reversed in the first half of this decade [Quinn et al. 2007]. The effects of these changes on Arctic climate are likely to be substantial but at present are very uncertain.
In this Leeds-Met Office cooperative studentship we will:
1. Substantially improve our understanding of processes controlling anthropogenic and natural aerosol pollution and deposition in the Arctic.
2. Identify the critical climate model components controlling Arctic aerosol, including the seasonal variations and long term changes, and thereby develop new or improved aerosol model schemes.
3. Use new models to study the impact of climate change on Arctic aerosol and the contribution of pollution to long term changes in aerosol radiative forcing and cryospheric change (through the effect of pollution on snow melting rates).
The main focus of the project is on the transport of aerosol to the Arctic and its deposition and impact on snow cover, as well as the local aerosol production and loss processes. The principal challenge is to make accurate global model simulations of size and composition-resolved aerosol and related quantities like aerosol optical depth, evaluated against observations. We are interested in improving the ability of climate models to capture episodic transport events, seasonal variations, and long term trends.