Modelling organic aerosol: sensitivities and simplifications

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• Prof Gordon McFiggans, University of Manchester
• Tuesday 07 June 2011, 14:15-15:15
• Unilever Lecture Theatre, Department of Chemistry.

If you have a question about this talk, please contact Dr Alex Archibald.

Note unusual day

Organic material is a significant and variable component of tropospheric aerosol, accounting for between 10 and 90 % of fine particle mass and having the potential to substantially impact on climate through direct and indirect radiative effects.

In this talk, I’ll present the findings from a wide range of sensitivity studies we’ve conducted in Manchester to investigate the representation of the organic fraction of atmospheric particulates. The conventional way to predict organic aerosol formation from condensable gaseous material is by an equilibrium absorptive partitioning consideration. In this approach, it is necessary to use predictions of the individual partitioning component properties (their vapour pressures and activity coefficients in the multicomponent mixtures). We demonstrate that the predicted aerosol loading and composition has large and variable sensitivity to the predictive methods and make recommendations for the treatments. In addition, the impacts on predicted physical behaviour of the particles is substantial and in conflict with observations. I’ll discuss the substantial questions that these findings raise about both the implicit assumptions in the modelling approach and field measurements of aerosol properties. Owing to its complexity, large-scale models must represent the organic fraction and its interactions with inorganic aerosol components using highly simplified methods. Bottom-up representations usually underestimate organic particulate mass and more simplified bottom-up treatments may be expected to perform even more poorly. It is difficult to evaluate the reasons for such behaviour, in part because of the lack of traceability of the representation to more complex and realistic mechanistic approaches. We have coupled the reduced Common Representative Intermediates (CRIv2-R5) scheme describing tropospheric degradation of methane and 22 emitted non-methane hydrocarbons and oxygenated volatile organic compounds (comprising 220 species and 609 reactions), and traceable to the Master Chemical Mechanism (v3.1), to a sectional aerosol microphysics representation within the WRF -Chem model to enable prediction of the regional transformation of multicomponent aerosol in the oxidising atmosphere. The criterion for selection of partitioning components and the estimation of their properties is based on the sensitivity analyses conducted using the absorptive partitioning model and the aerosol treatment uses the hyprid Partial Derivative Fitted Taylor Expansion (PD-FiTE) coupled to the MTEM inorganic reprentation in WRF -Chem to provide computationally efficient calculation of the multicomponent gas / liquid equilibria in the particles. The model can be used to investigate the regional evolution of aerosol in the moist atmosphere and the organic contribution to aerosol mass predicted by a mechanism traceable to a more realistic representation of tropospheric VOC oxidation. If they’re ready in time, I will present results of the first simulations investigating the formation and transformation of tropospheric aerosols in the UK under a range of changing emission profiles and oxidant conditions.

This talk is part of the Centre for Atmospheric Science seminars, Chemistry Dept. series.

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