Com-Part: Combustion Particles in the Atmosphere: Properties, Transformations, Fate & Impacts

Combustion particles have major impacts on air quality and climate. As well as primary particles emitted directly, secondary particles are formed through the atmospheric oxidation of vapour phase organic compounds that are also present in the exhaust. Traditionally, primary and secondary particles have been quantified in terms of the total mass of particulate, however the human health and climate impacts are increasingly recognised to be highly dependent on the exact size and mixing state of particles. Engine exhaust emissions, the dominant particle source in urban environments, are complicated by the fact that primary particles comprise a mixture of pure organic and mixed organic and elemental carbon particles, with the organic fraction near equilibrium with the vapour phase. The exact mixture is dependent on the engine speed, load and temperature. The complexity of the exhaust emission dependence on combustion conditions heavily influences the composition and properties of the primary particles and determines the magnitude and manner of secondary particle production.

Com-Part will use new developments in aerosol instrumentation to comprehensively characterise the complex exhaust emitted from a diesel engine under various operating conditions, representative of real-world usage, and subsequently subjected to dilution and photochemical processing using an aerosol reaction chamber. Particular attention will be paid to the mixing state of the refractory black carbon with respect to the primary and secondary organic matter and the fractional contributions of these to the particle mass. In addition to the diesel exhaust, emissions from small appliance engines (including 2-stroke) will be investigated, as these are currently under-characterised from a secondary organic aerosols perspective.

In addition to the composition measurements, the wavelength-dependent optical absorption, optical extinction and cloud condensation nuclei (CCN) will be studied, as these are the parameters key to determining the climate impacts of the aerosol. Various model treatments can be used to predict the optical properties based on knowledge of the particulate composition and various assumptions concerning particle morphology. The performance of these models relevant to engine emissions will be critically evaluated and parameterisations will be derived that are suitable for inclusion in radiative transfer models. Similarly, parameterisations of the CCN behaviour of the particles (using simplified single parameter effective "kappa" approaches or similar) will also be derived.

To place the measurements in an ambient context, the profiles obtained over a range of operating conditions will be scaled according to typical urban driving conditions, using the standard EURO emissions testing cycles as a basis. This will then be used to generate a typical urban emission and associated SOA formation profile, which can be in turn compared with ambient measurements. These ambient measurements will be made using the same instrumentation as used in the laboratory studies and can also be compared to archived data from intensive measurement campaigns such as ClearfLo and CalNex. Timescales for transformation of the primary particles and of the secondary material that is formed will be derived, based on the "photochemical age" through the chamber experiments. This will be derived by direct measurement of the decay rate of volatile organic compounds of known reactivity towards the major atmospheric oxidant, the OH radical.

i) Who may benefit from or make use of the research?
The objectives of this project are concerned with understanding and quantifying the contributions of combustion exhaust emissions to airborne concentrations of primary and secondary aerosol as they evolve from the urban to background regional atmosphere. The directly observed quantified transformation behaviour and derived relationships between particle composition, physical properties and optical properties with emission, oxidative degradation and dilution, will inform policymakers as to links between combustion emissions and airborne particulate concentrations. Such relationships can be used in economic models to calculate the costs of abatement through the control of different combustion emission categories. Subsequent controls will require legislation, and consequently those with the greatest need for this information are in government.

Responsibility for air pollution control in the UK operates at a number of levels. The overall framework is set by the European Commission, which sets the over-arching regulations such as the National Emissions Ceilings Directive and the air quality limit values with which member states must comply. At the next level, it is the responsibility of national governments to make national regulations designed to bring air quality into compliance with EU Directives and limit values. The application of such regulations is in the case of heavy industry, the responsibility of the environment agencies. Consequently, the key players in relation to combustion-derived particulate, which for the UK has both national and trans-boundary origins, are the European Commission and national governments as represented by DEFRA in the UK. Other agencies/ organisations such as the Health Protection Agency (which advises UK government on the health effects of air pollutant exposures) and the National Atmospheric Emission Inventory (which is funded by DEFRA to quantify emissions from different source categories and locations) also have a significant interest in this topic. Scientists working on this and related topics will also have a strong interest in the outcomes of the work.

Climate science is also high on the political agenda, and knowledge of aerosol properties is a key input to global and regional climate models. In the UK, this is an area of responsibility of the Department for Energy and Climate Change (DECC), with the Met. Office Hadley Centre being the primary practitioner in climate modelling.

ii) How might they benefit or make use of the research?
Once robust relationships have been developed to describe combustion particle transformations, they can be used in abatement model scenarios in which primary and secondary particulate emissions from categories of combustion are either transformed on timescales dependent on the predicted oxidant distribution or the particle properties linked to the predicted secondary organic aerosol mass, dependent on the suitability of the host model. Model outputs would then be able to subject the results to an economic analysis in which the lowest cost pathways to particulate matter abatement could be elucidated. Ultimately this would lead to tightened regulations on those emissions that could be abated with the greatest cost-benefit effectiveness.
In the case of climate impacts, abatement of secondary organic aerosol may have a net surface warming effect due to reduced albedo, whilst primary BC and secondary modified BC abatement may have net cooling impacts. Both are highly uncertain and important to quantify.
It must be acknowledged that both air quality and climate models that are structurally capable of representing the detail of the processes quantified within Com-Part are very much at the cutting edge of research. This drives the proposed attempts to package the relationships in two forms most adequate for simplified treatments at the appropriate scale.