A Novel Controlled Thermal Desorption Technique for Evaluation of Organic Aerosol Component Volatility and Absorptive Partitioning

Particulate material in the atmosphere has a major effect on both climate and human health; specifically the finer particulate material (below around one micron in diameter) being responsible for the majority of the radiative and air quality impacts. Whilst inorganic components are readily quantified, organic components comprise a large fraction of the particulate material, normally greater than 50% of the total mass. This fraction is very poorly quantified or described. Some of the organic material is known to be emitted directly into the atmosphere and is therefore known as primary material. This is normally a minority of the total organic aerosol mass, the rest comprising 'secondary' components (aerosol being the sum of the particulate and gaseous components). Secondary components may be defined in a number of ways, but a useful working definition is that they are the components that have entered the particles from the gas phase or have been formed in the particle from components that have entered the particles from the gas phase. The absorptive partitioning model of secondary organic aerosol (SOA) component formation has been widely applied and found to provide a useful framework for explanation of the process of gas to particle conversion. More recently there has been a postulation of significant pathways for condensed phase reaction and potential formation of these secondary components by reactive uptake which would impact on the reversibility of SOA formation and the ability to explain SOA formation by the absorptive partitioning alone. In the Manchester aerosol chamber, we have recently noticed some interesting results on dilution of SOA samples. Because of the predictions of absorptive partitioning, it would be expected that SOA mass would reduce more than the amount by which it has been diluted owing to evaporation of more volatile components. This has not been observed in our experiments in several biogenic precursor systems and has very significant atmospheric implications. If absorptive partitioning is demonstrably incapable of explaining the chamber results, then many models of atmospheric organic aerosol behaviour based on chamber yield data will have problems. Previous experiments have used thermal denuders to probe the SOA volatility and loosely infer reversibility of SOA formation. It is proposed to design and construct a novel thermal denuder system to probe, in combination with controlled dilution, the volatility of secondary organic aerosol components formed in the chamber photo-oxidation of biogenic and anthropogenic organic precursors. The denuder system will be characterised using particles of known composition and properties generated in the laboratory prior to coupling it to the Manchester aerosol chamber to establish the validity of the widely accepted absorptive partitioning model of aerosol formation. The anticipated superior performance of the denuder system will be suitable for quantifying component volatility and, by coupling it to the chamber in dilution experiments, for assessing the reversibility predicted by absorptive partitioning theory.