Air pollution: Sources and sinks of carbon monoxide in the atmosphere

Carbon monoxide (CO) is a major contributor to air pollution. It is produced by the burning of fossil fuels in motor vehicles and the burning of vegetation for fuel, as agricultural waste, or in wildfires. Many governments have enacted policies to reduce the emissions of CO. For example, CO emissions in the UK are estimated to have declined 80% between 1990 and 2017, largely attributed to EU-wide emission standards for motor vehicles.

This project will investigate the sources and sinks of CO by using radiocarbon as a tracer. Radiocarbon (14C) is a powerful tracer of fossil fuel sources of carbon because fossil fuels have been stored underground so long they have lost all their 14C to radioactive decay. By measuring 14C in CO (14CO), the fraction of fossil fuel vs biogenic sources of CO can be determined on regional scales. This is a new technique that will be developed as part of this studentship, which will potentially lead to methods for verification of policies for reducing air pollution.

Measurements of 14C can also provide information on the removals (sinks) of CO in the atmosphere, which occur through reaction with the hydroxyl radical (OH). Understanding variations in the reaction with OH is important not just for CO but for many other compounds that also react with OH including methane. OH has been called the "vacuum cleaner" or "detergent" of the atmosphere. 14CO can provide information on OH because 14CO is mainly produced via cosmogenic radiation at spatially- and temporally-varying rates which are reasonably well-known, particularly after recent improvements in cosmogenic production estimates. Therefore, observed variations in 14CO over large scales can provide information on the variation in OH, which is otherwise very difficult to observe. This technique has been used in the past [Manning et al. Nature 2005, doi: 10.1038/nature03900], but it needs updating with improved models.