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

Lead Research Organisation: University of Manchester
Department Name: Earth Atmospheric and Env Sciences


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.
Description The high-level objective of the project was to develop a technique to investigate the theoretical basis of organic aerosol formation. This was to be achieved by the development of a thermal denuder with several advantages over existing techniques, The denuder would then be coupled to a photochemical "smog" chamber to investigate the mechanisms of particle mass formation using a number of reference chemical systems.

The first activity was the design of the denuder system. This initially progressed in line with the proposed design and was conducted in parallel with the construction of a numerical model of coupled fluid flow, heat and mass transport and particle evaporation and condensational growth, aiming to evaluate the performance of existing designs along with the new design. A large number of flaws in existing designs were identified beyond those written into the original proposal. These related to the attainment of thermal equilibrium in the sample residence time of the denuder and the degree of re-condensation on cooling, depending on the temperature profiles and flow rates in the instrument. Existing designs and interpretation of particle properties was revisited and the investigation was published in Fuentes and McFiggans [2012]. The denuder was constructed to the original design, but the flow rate and hence dimensions were adjusted using the theoretical results. The fully operational prototype was then characterised using test aerosol, but found to have very low transmission efficiency of the particles. A transparent housing was constructed for flow visualisation using coloured smoke and it was found that the flow was too slack for the sheath air design to carry the sample in a controlled laminar fashion, given the magnitude of the buoyancy forces with the imposed temperature gradients. The instrument was redesigned with an adequate residence time, but larger diameter to reduce wall effects to an acceptable level. The instrument was rebuilt and characterised, with demonstrated adequate performance.

The denuder was then operated on the outflow from the photochemical chamber for the following photo-oxidation secondary organic aerosol (SOA) test systems: _-pinene, _-caryophyllene, toluene, heptadecane and 1,3,5-trimethylbenzene within the framework of the ACID-PRUF programme (NERC, NE/I020121/1). This is substantially more extensive than was envisaged in the original proposal. Figure 1 shows the performance on the _-pinene system. Dilution experiments have been performed to evaluate the theory of absorptive partitioning and data analysis is ongoing.
Exploitation Route The ongoing use of the thermal denuder in funded NERC and EU projects will ensure publication of the output from the instrument built within the project. An intercomparison of the denuder with existing instrumentation operated across the community will ensure that the favourable performance of the instrument is broadly disseminated.
Sectors Environment

Title Thermodenuder 
Description A methodology and instrument for removing non-refractory material from aerosol particles by heating and removal of the evaporated vapour 
Type Of Material Improvements to research infrastructure 
Provided To Others? No  
Impact Used extensively and routinely in our work to investigate aerosol component properties as a function of volatility