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SUMMARY:Modelling organic aerosol: sensitivities and simplifications - Pro
 f Gordon McFiggans\, University of Manchester
DTSTART:20110607T131500Z
DTEND:20110607T141500Z
UID:TALK30536@talks.cam.ac.uk
CONTACT:Dr Alex Archibald
DESCRIPTION:Organic material is a significant and variable component of tr
 opospheric aerosol\, accounting for between 10 and 90 % of fine particle m
 ass and having the potential to substantially impact on climate through di
 rect and indirect radiative effects.  \n\nIn this talk\, I'll present the 
 findings from a wide range of sensitivity studies we've conducted in Manch
 ester to investigate the representation of the organic fraction of atmosph
 eric particulates. The conventional way to predict organic aerosol formati
 on from condensable gaseous material is by an equilibrium absorptive parti
 tioning consideration. In this approach\, it is necessary to use predictio
 ns of the individual partitioning component properties (their vapour press
 ures and activity coefficients in the multicomponent mixtures). We demonst
 rate that the predicted aerosol loading and composition has large and vari
 able sensitivity to the predictive methods and make recommendations for th
 e treatments. In addition\, the impacts on predicted physical behaviour of
  the particles is substantial and in conflict with observations. I'll disc
 uss the substantial questions that these findings raise about both the imp
 licit assumptions in the modelling approach and field measurements of aero
 sol properties.\nOwing to its complexity\, large-scale models must represe
 nt the organic fraction and its interactions with inorganic aerosol compon
 ents using highly simplified methods. Bottom-up representations usually un
 derestimate organic particulate mass and more simplified bottom-up treatme
 nts may be expected to perform even more poorly. It is difficult to evalua
 te the reasons for such behaviour\, in part because of the lack of traceab
 ility of the representation to more complex and realistic mechanistic appr
 oaches. \nWe have coupled the reduced Common Representative Intermediates 
 (CRIv2-R5) scheme describing tropospheric degradation of methane and 22 em
 itted non-methane hydrocarbons and oxygenated volatile organic compounds (
 comprising 220 species and 609 reactions)\, and traceable to the Master Ch
 emical Mechanism (v3.1)\, to a sectional aerosol microphysics representati
 on within the WRF-Chem model to enable prediction of the regional transfor
 mation of multicomponent aerosol in the oxidising atmosphere. The criterio
 n for selection of partitioning components and the estimation of their pro
 perties is based on the sensitivity analyses conducted using the absorptiv
 e partitioning model and the aerosol treatment uses the hyprid Partial Der
 ivative Fitted Taylor Expansion (PD-FiTE) coupled to the MTEM inorganic re
 prentation in WRF-Chem to provide computationally efficient calculation of
  the multicomponent gas / liquid equilibria in the particles. The model ca
 n be used to investigate the regional evolution of aerosol in the moist at
 mosphere and the organic contribution to aerosol mass predicted by a mecha
 nism traceable to a more realistic representation of tropospheric VOC oxid
 ation. \nIf they're ready in time\, I will present results of the first si
 mulations investigating the formation and transformation of tropospheric a
 erosols in the UK under a range of changing emission profiles and oxidant 
 conditions.
LOCATION:Unilever Lecture Theatre\, Department of Chemistry
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