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Centre for Atmospheric Science

Aerosol Modelling

Aerosol particles influence climate directly by the scattering and absorption of solar radiation and indirectly through their role as cloud condensation nuclei, the latter effect comprising the largest uncertainty in radiative forcing. Atmospheric aerosol particles are also of concern for policymakers owing to the health effects related to human exposure to particulate matter. This is of particular concern in the urban environment where vehicular emissions are a major source of particles.

Our modelling activities aim to address properties and processes relevant to both climate and health effects of the atmospheric aerosol.

The aerosol modelling activities within CAS range from zero-dimensional time-invariant prediction of aerosol properties, through zero-dimensional time-evolving detailed process simulations through to 3-dimensional coupled aerosol and transport modelling across scales ranging from the street canyon to regional domains.

Multi-component aerosol particle properties.
Historically, it has been commonly assumed that aerosol particles comprised only inorganic components, such as seasalt, ammonium sulphate and crustal mineral compounds. However, in recent years, it has been found that organic components may constitute a substantial fraction of the aerosol composition, ranging from 20-60% of the fine particulate matter depending on the location. Model treatment of organic components is particularly challenging since, unlike the inorganic fraction, the atmospheric aerosol will contain thousands of individual organic compounds covering a wide range of chemical and physical properties. As a result of this diversity and variability, major uncertainties in aerosol modelling arise from a lack of understanding of the behaviour of the organic fraction and the conceptual difficulty of the coupling of this fraction with inorganic compounds.

The water uptake of such multicomponent particles in the moist atmosphere is significantly different from that of single-component particles. This, in turn, affects the size, cloud forming potential and optical properties of the ambient aerosol population. We have developed and used a suite of modelling tools to make predictions of aerosol properties and behaviour and to reconcile measurements of atmospheric aerosol using a number of instruments. These tools are also being used for the development of parameterisations for large-scale models.

Modelling the thermodynamic properties of multicomponent aerosol particles.

Reconciliation of measured aerosol properties.

Application of “closure” studies to marine aerosol

Prediction of the pure component and mixture properties of organic compounds


Detailed physical and chemical evolution of the atmospheric aerosol.
Aerosol particles will evolve by chemical and physical processes in the atmosphere. To accurately represent such atmospheric “aging” processes, it is necessary to develop coupled models of gaseous photochemistry, thermodynamics, aerosol microphysics and condensed phase reaction. Depending on the model framework, such models may be used i) for simulating processes in laboratory experiments, ii) as testbeds for the impacts of aerosol property predictions as described above and iii) to simulate atmospheric behaviour.

Owing to computational expense, zero-dimensional “box” models are able to carry greater complexity than larger-scale 3-dimensional models. We use a number of coupled box model approaches to investigate detailed atmospheric aerosol processes.

Reactive halogen cycling in the marine environment

Nucleation in the coastal environment

Box model testing of organic partitioning model

Modelling the impact of aerosol properties on cloud formation and development


Process model validation and evaluation.
The property and process modelling activities will benefit from the newly-commissioned NERC-funded chamber facility. The motivation for the chamber design was to investigate coupled multiphase processes in a controlled environment (McFiggans, 2006). The chamber will be used to test both aerosol property and evolution models.

Aerosol Chamber

Modelling evolving aerosol properties in 3-D transport models.
In order to realistically represent the loading and evolution of aerosol particles in the atmosphere for comparison with field measurements, it is necessary to incorporate the aerosol representation in transport frameworks. Within CAS, we are developing and conducting investigations with existing aerosol transport models on a range of scales.

Aerosol evolution at the urban street canyon scale

Regional aerosol transport modelling

A summary of the funded aerosol modelling activities within CAS may be found here