Cloud droplet activation of organic–salt mixtures predicted from two model treatments of the droplet surface

A new monolayer model predicts the bulk-surface partitioning, surface composition, and thickness of droplets comprising chemically unresolved, atmospherically relevant organic aerosols.

where NAFA is the mass fraction of NAFA in dry particle, is used. Similar sensitivity analysis for the Gibbs model has recently been given elsewhere. 2 In both cases, we consider dry particle size of 150 nm.  ( c ) Figure S1: (a) Critical supersaturation, (b) surface tension at activation, and (c) NAFA surface fraction and normalized surface thickness calculated with alternative surface tension parameters for 150 nm NAFA particles as a function of NAFA mass fraction.
As can be seen from Figs. 8, 9, 10 and S1, at least for the 150-nm dry size, the form of surface tension parametrisation used strongly affects the shape of the NAFA surface fraction vs. NAFA mass fraction curve, but does not induce any significant changes. This further confirms the conclusions presented in the main text and shows that although details of predicted surface/bulk partitioning are sensitive to the assumed form of the surface tension function, predicted properties relevant for CCN activation are less so, as long as the surface tension parametrisation has a sensible form.   Figure S2: (a) Critical supersaturation, (b) surface tension at activation, and (c) SDS surface fraction and normalized surface thickness calculated with different different σ CMC for 150 nm SDS particles as a function of SDS mass fraction.
In Fig. S2, model results for the SDS-NaCl mixture with reduced surface tension at CMC are given for a 150-nm dry particle. Comparison with Figs. 5, 6, and 7 shows only slight quantitative changes when SDS is small. For higher SDS mass fractions in dry particle, though, differences are even qualitative, as concentration in droplet bulk does not reach CMC. This behaviour is, however, expected on basis of eqn. (1), as x s SDS < 1 results in the modelled droplet surface tension. It can be concluded that the predictions of the monolayer model even for the activating droplets are sensitive to the assumed values of CMC and σ CMC at high surfactant mass fractions.

S2 Growth factor at activation
Growth factor plots corresponding to Fig. 3 in the main text are shown here for SDS-NaCl, NAFA-NaCl, ragweed pollenkitt-(NH 4 ) 2 SO 4 , and poplar pollenkitt-(NH 4 ) 2 SO 4 .     Figure S5: Growth factor at activation calculated by the monolayer and Gibbsian model as a function of dry particle size for ragweed pollenkitt mass fractions (a) 0.05; (b) 0.5; (c) 1 and as a function of pollenkitt mass fraction for dry particle sizes (d) 50 nm; (e) 100 nm; and (f) 150 nm.

S3 Poplar pollenkitt
In this section, the figures for poplar pollenkitt analogous to those for ragweed pollenkitt are shown. We refer the reader to the discussion in section 3.2.2 in the main text.    Figure S9: Droplet poplar pollenkitt surface fraction on the left axes calculated by the monolayer and Gibbsian model and surface thickness from the monolayer model normalized to the thickness of one poplar pollenkitt monolayer on the right axes as a function of dry particle size for poplar pollenkitt mass fractions (a) 0.05; (b) 0.5; (c) 1 and as a function of pollenkitt mass fraction for dry particle sizes (d) 50 nm; (e) 100 nm; and (f) 150 nm.