The sensitivity of Secondary Organic Aerosol component partitioning to the predictions of component properties – Part 2: Determination of particle hygroscopicity and its dependence on "apparent" volatility 1Centre for Atmospheric Sciences, School of Earth, Atmospheric & Environmental Sciences, University of Manchester, Manchester, M13 9PL, UK
03 Aug 2011
2National Centre for Atmospheric Science (NCAS), University of Manchester, Manchester, M13 9PL, UK
Received: 27 January 2011 – Published in Atmos. Chem. Phys. Discuss.: 16 March 2011 Abstract. A large number of calculations of absorptive partitioning of organic
compounds have been conducted, making use of several methods to estimate pure
component vapour pressures and activity coefficients (p0 and γi).
The sensitivities of the predicted particle properties (density,
hygroscopicity, CCN activation potential) to the choice of p0 and
γi models and to the number of components used to represent the
organic mixture have been systematically compared.
Revised: 05 July 2011 – Accepted: 11 July 2011 – Published: 03 August 2011
The variability in theoretical hygroscopic growth factor attributable to the
choice of estimation technique increases with decreasing mixture complexity.
Generally there is low sensitivity to the choice of vapour pressure
predictive technique. The inclusion of non-ideality is responsible for a
larger difference in predicted growth factor, though still relatively minor.
Assuming instantaneous equilibration of all semi-volatile on drying the
aerosol to 0 % RH massively increases the sensitivity. Without such
re-equilibration, the calculated growth factors are comparable to the low
hygroscopicity of organic material widely measured in the laboratory and
atmosphere. Allowing re-equilibration on drying produces a calculated
hygroscopicity greater than measured for ambient organic material, and
frequently close to those of common inorganic salts. Such a result has
substantial implications on aerosol behaviour in instruments designed to
measure hygroscopicity and on the degree of equilibration of semi-volatile
components in the ambient atmosphere.
The impacts of this variability on behaviour of particles as cloud
condensation nuclei, on predicted cloud droplet number and uncertainty in
radiative forcing are explored. When it is assumed only water evaporates on
drying, the sensitivity in radiative forcing, "ΔF" to choice of p0
and γi estimation technique is low when the particle organic volume
fraction is less than 55 %. Sensitivities increase with decreasing
component complexity. If all components re-equilibrate on drying, the
sensitivity of ΔF increases substantially for organic volume
fractions as low as between 16 and 22 % depending on the complexity of the
organic composition and assumed aerosol size distribution. The current study
ignores the impact of predicted changes in particle size which will increase
uncertainty in droplet number and forcing.
Citation: Topping, D. O., Barley, M. H., and McFiggans, G.: The sensitivity of Secondary Organic Aerosol component partitioning to the predictions of component properties – Part 2: Determination of particle hygroscopicity and its dependence on "apparent" volatility, Atmos. Chem. Phys., 11, 7767-7779, doi:10.5194/acp-11-7767-2011, 2011.