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Cambridge NERC Doctoral Training Partnerships

Graduate Research Opportunities
Brief summary: 
How are reactive halogens and nitrogen cycled through polar snowpacks and what can they tell us about the oxidation capacity of the atmosphere?
Importance of the area of research concerned: 
The polar snowpack is a reservoir of natural and man-made chemical compounds, some of which are chemically reactive. Bromide and nitrate are two such compounds that play a critical role in polar atmospheric chemistry. Reactive bromine compounds (i.e. BrO and Br) are strong oxidants; they cause severe ozone depletion, oxidise elemental mercury, and also strongly influence the nitrogen cycle by converting gaseous nitrogen oxides NOx (NO and NO2) to nitrate (NO3-). Research suggests that bromine can induce net NOx loss of up to 60-80% at high latitudes [1]. However, it is far from clear and subject of an on-going debate, what the sources and source processes of reactive bromine are within the sea ice zone, where the largest bromine loading was observed. A better understanding of the air-snow exchange of reactive bromine and nitrogen and their effect on atmospheric composition is critical in terms of (i) estimating the role of the snowpack as a source of reactive bromine, in particular relative to that of blowing snow [2]; (ii) quantifying bromide and nitrate preservation in snow and firn; (iii) assessing changes in atmospheric oxidation capacity (i.e. O3 and OH) in a warming climate.
Project summary : 
The saline snowpack on sea ice has been proposed as a source of reactive bromine and nitrogen via heterogeneous reactions under sunlight. It has been found that reactive bromine can efficiently accelerate NOx to nitrate conversion - an even more important channel to produce nitrate than the known process via N2O5 hydrolysis. Given that sea ice zone has the highest reactive bromine loading in the Earth’s atmosphere, the Br-N cross-reactions are particularly important in terms of determining polar N-cycling and oxidising capacity. However, we do not know many details of the role that the snowpack plays. This project will implement a simplified snow physical-chemistry model into a global chemistry model to explore the role that snowpack plays, with the aims of quantifying the emission flux of reactive bromine and nitrogen and assessing their impacts on polar atmospheric oxidation capacity.
What will the student do?: 
The student will begin by modifying a physical snow-nitrate-NOx model [3] by adding a simplified NOx-halogen-chemistry scheme to the skin layer of snowpack where air-snow exchange, deposition, photochemistry and heterogeneous reactions are strongest. Key parameters will be constrained by in-situ snowpack data. Model performance will be evaluated using various field data obtained from both hemispheres, including bromide and nitrate concentrations in surface snow as well as gaseous phase NOx and BrO above the snowpack. The student will then implement this 1-layer snow model into a global chemistry model (UKCA or p-TOMCAT) [2] for a large-scale investigation. The project should produce a model will the ability to (i) output emission fluxes of reactive bromine and nitrogen under different snow conditions (i.e. snow salinity, bromide and nitrate, as well as gaseous reactive bromine and NOx in the air); (ii) reproduce snowpack bromide and nitrate profiles at selected sampling locations in the polar regions; (iii) quantify the snowpack’s impact on polar atmospheric oxidation capacity.
References - references should provide further reading about the project: 
[1] Yang, X., Cox, R. A., Warwick, N. J., Pyle, J. A., Carver, G. D., O'Connor, F. M., and Savage, N.H.: Tropospheric bromine chemistry and its impacts on ozone: A model study. J. Geophys. Res., 110, D23311, doi:10.1029/2005JD006244, 2005.
[2] Yang, X., Blechschmidt, A.-M., Bognar, K., McClure–Begley, A., Morris, S., Petropavlovskikh, I., Richter, A., Skov, H., Strong, K., Tarasick, D., Uttal, T., Vestenius, M., and Zhao, X.: Pan-Arctic surface ozone: modelling vs measurements, Atmos. Chem. Phys. Discuss.,, in review, 2020.
[3] Chan, H. G., Frey, M. M., and King, M. D.: Modelling the physical multiphase interactions of HNO3 between snow and air on the Antarctic Plateau (Dome C) and coast (Halley), Atmos. Chem. Phys., 18, 1507–1534,, 2018.
You can find out about applying for this project on the British Antarctic Survey (BAS) page.
Dr Xin Yang
British Antarctic Survey Graduate Administrator