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Smoke aerosols prevail throughout Amazonia because of widespread biomassburning during the dry season, and external mixing, low variability in theparticle size distribution and low particle hygroscopicity are typical. Therecan be profound effects on cloud properties. This study uses an adiabaticcloud model to simulate the activation of smoke particles as cloudcondensation nuclei (CCN) for three hypothetical case studies, chosen as toresemble biomass burning aerosol observations in Amazonia. The relativeimportance of variability in hygroscopicity, mixing state, and activationkinetics for the activated fraction and maximum supersaturation isassessed. For a population with κp = 0.04, an overestimationof the cloud droplet number concentration Nd for the three selected case studies between 22.4 ± 1.4 and 54.3 ± 3.7 % was obtainedwhen assuming a hygroscopicity parameter κp = 0.20. Assuminginternal mixing of the aerosol population led to overestimations of up to20 % of Nd when a group of particles with medium hygroscopicitywas present in the externally mixed population cases. However, theoverestimations were below 10 % for external mixtures between very lowand low-hygroscopicity particles, as seems to be the case for Amazon smokeparticles. Kinetic limitations were significant for medium- andhigh-hygroscopicity particles, and much lower for very low andlow-hygroscopicity particles. When particles were assumed to be atequilibrium and to respond instantly to changes in the air parcelsupersaturation, the overestimation of the droplet concentration was up to ∼  100 % in internally mixed populations, and up to ∼  250 % in externally mixed ones, being larger for the highervalues of hygroscopicity. In addition, a perceptible delay between the timeswhen maximum supersaturation and maximum aerosol activated fraction arereached was noticed and, for aerosol populations with effectivehygroscopicity κpeff higher than a certain thresholdvalue, the delay in particle activation was such that no particles wereactivated at the time of maximum supersaturation. Considering internallymixed populations, for an updraft velocity W = 0.5 m s−1 thisthreshold of no activation varied between κpeff = 0.35 and κpeff = 0.5 for the different casestudies. However, for low hygroscopicity, kinetic limitations played a weakerrole for CCN activation of particles, even when taking into account the largeaerosol mass and number concentrations. For the very low range ofhygroscopicities, the overestimation of the droplet concentration due to theequilibrium assumption was lowest and the delay between the times whenmaximum supersaturation and maximum activated fraction were reached wasgreatly reduced or no longer observed (depending on the case study). Thesefindings on uncertainties and sensitivities provide guidance on appropriatesimplifications that can be used for modeling of smoke aerosols withingeneral circulation models. The use of medium values of hygroscopicityrepresentative of smoke aerosols for other biomass burning regions on Earthcan lead to significant errors compared to the use of low hygroscopicity forAmazonia (between 0.05 and 0.13, according to available observations). Alsoin this region, consideration of the biomass burning population as internallymixed will lead to small errors in the droplet concentration, whilesignificantly increasing the computational burden. Regardless of the largesmoke aerosol loads in the region during the dry season, kinetic limitationsare expected to be low

Impact of mixing state and hygroscopicity on CCN activity of biomass burning aerosol in Amazonia