Source apportionment of carbonaceous aerosols
The most ubiquitous particles in the atmosphere - and hence the potentially most detrimental ones - are those containing carbonaceous species. The latter are traditionally divided into two fractions: whereas Organic Carbon (OC) contains the most volatile, least light-absorbing carbonaceous components, Elemental Carbon (EC) encompasses the most refractory and most light-absorbing ones (see Figure 1). Due to their different physical and chemical properties,these two fractions influence the environment in very different ways. Therefore, gaining knowledge about the specific origins of each fraction is required to implement efficient abatement strategies and understand more precisely how the different sources of carbonaceous particles may influence climate.
Several techniques are addressed to achieve this goal: Radiocarbon (also called 14C) is a powerful tool to carry out such source apportionment studies. With a half-life of 5730 years, this radioactive isotopeof carbon has totally decayed in fossil fuels, which means that sources like vehicle exhausts or industry releases emit 14C-free carbonaceous aerosols. On the contrary, non-fossil sources (such as wood burning or biogenic emissions) contain radiocarbon on levels that can be estimated. Therefore, measuring the 14C content of OC and EC provides useful information about the origins of each fraction.
Levoglucosan is an anhydro sugar molecule that is formed during combustion of cellulose above 300 °C. It is fairly stable in the atmosphere and can be measured with gas chromatography – mass spectrometry. Levoglucosan is thus a suitable tracer for biomass combustion aerosol. Biomass combustion aerosols are also thought to be strongly light absorbing in the ultraviolet region of the electromagnetic spectrum. Vehicle generated aerosols are on the other hand not expected to show this characteristics. By measuring light absorption in several wavelengths it is thus possible to separate biomass combustion aerosols from vehicle generated aerosols.
This project addresses three questions:
- How to separate optimally EC and OC for individual 14C measurements of each fraction? Separation techniques can rely on the difference in thermal or chemical refractivity of each fraction. However, two pitfalls have to be avoided, namely the charring of OC to EC during thermal treatments, as well as the early removal of EC with OC occurring with some thermal or chemical procedures. Minimising these two artefacts is of crucial importance to achieve reliable 14C-based source apportionment.
- Can the above presented 14C source apportionment technique be biased by releases of 14C-enriched aerosols from "hot" sources such as research laboratories, medical industries or nuclear power plants? To address this question, aerosol samplings will be performed around potential radiocarbon-emitting installations.
- What is the relative contribution of biomass combustion and fossil fuel combustion to the carbonaceous aerosol? In this project we will utilize 14C, levoglucosan and the light absorbing properties of the aerosols.