Soot Aerodynamic Size Selection for Optical properties (SASSO)

Atmospheric soot is a pollutant that contains black carbon (BC) and potentially also brown carbon (BrC) and is produced from combustion sources such as diesel engines, wildfires, agricultural waste burning and the burning of solid fuels such as wood and coal. Because BC and BrC absorb sunlight, they can have a warming effect on climate, in particular on local scales; there is evidence to suggest that the increase in absorbing aerosols associated with pollution has been responsible for the weakening of the South Asian Monsoon. However, while BC and BrC are very important for climate, they are currently very poorly represented in the models used to study and predict these effects. Comparison exercises between the various models in use around the world tend to highlight strong disagreements and comparisons against observations of absorbing aerosols are consistently very poor. This indicates a strong need to improve the treatment of soot and its processes within models, however this has so far been limited by deficiencies in the instrumentation and laboratory techniques available. SASSO will capitalise on the timely development of new methodologies, facilities and instruments at the Universities of Manchester and Exeter to use a novel and unique combination of tools to study soot on a level of detail previously not possible. This data will be used to develop and test new models of soot optical properties and this will be implemented in the UK's main climate model (hadGEM3), to test what effects this new, improved understanding has on predictions of climate responses to changes in soot emissions.
The microphysical properties of soot are complex and traditionally extremely difficult to constrain and quantify on the level of detail desired to generate the detailed models and parameterisations needed. A particular complication is caused when BC co-exists with another substance (a 'coating') in a particle, which is the usual configuration in the atmosphere. The presence of a coating can increase the per-mass absorption of the BC through a phenomenon known as 'lensing', although data on this has so far proved inconsistent. There are also technical difficulties in the study of soot particles, such as the need to be able to strictly isolate particles of a single size while remaining aerosolised and the need to be able to separate the BC from the BrC in biomass burning emissions. The new technical developments at the Universities of Manchester and Exeter that will finally address these are as follows:
1. The Aerodynamic Aerosol Classifier, a centrifuge-based instrument capable of selecting particles by size and free of the charging artefacts that affect previous techniques
2. A new experimental technique for temporally separating the BC and BrC emissions from a burning wood sample using commercial fire testing equipment
3. The new EXSCALABAR aerosol optical instrument developed by the Met Office
4. The new Exeter wildFIRE facility for combustion studies
5. The Manchester Aerosol Chamber coupled to a light-duty diesel engine rig.
The combination of these will be used to generate data capable of probing BC and BrC components on a level of detail not previously possible. We will authoritatively quantify fundamental parameters of their physical properties (primarily their refractive indexes) and objectively test approaches to modelling lensing and other microphysical effects, suitable for use in climate models. The new modelling framework and parameters will be tested against ambient data (in situ and remote sensing) and implemented within hadGEM3. The effects of this improved scheme will be thoroughly tested on various levels including radiative transfer, climate forcing and local climate trends. The modifications will also be incorporated into the core hadGEM3 model for future work using this tool

The insights into the climate responses to soot will also have impacts beyond purely academic sector and into national and international policymaking. Because hadGEM3 is one of the main models in use internationally, the results of the studies looking at the effect of model improvement and the generally improved outputs will feed directly into activities such as CMIP and AEROCOM, and in turn the IPCC and international policymaking. Domestically, hadGEM3 represents the backbone of the UK's main climate and earth system modelling capability, so the work here will serve the national interests that this feeds into government decision-making relating to energy strategy, climate resilience (including extreme weather events) and the response to other global impacts of climate change.
There may be additional benefits from the technical development aspects, in terms of the novel applications of new, UK-developed instruments (the AAC and EXSCALABAR) and developments of methodologies of studying combustion processes. This may have further impacts within the instrumentation field.
There will be further benefits to the UK skills base through the training of PDRAs in the use of equipment and models that they would not otherwise have used, e.g. through Manchester staff using the facilities at Exeter. The general cross-disciplinary nature of the work will mean that scientists will be exposed to new facilities, instruments and ways of working.
In terms of outreach and public engagement, we believe that there is much scope, given the interest in climate change and also the very visible forms that soot and smoke can take. We will see to exploit various outreach channels available at both universities and the Met Office, such as science fairs, blogs, press releases, school visits and social media.