Size-dependent activation of aerosols into cloud droplets at a subarctic background site during the second Pallas Cloud Experiment (2nd PaCE): method development and data evaluation

摘要

In situ measurements of aerosol water uptake andactivation of aerosols into cloud droplets provide information on howaerosols influence the microphysical properties of clouds. Here we present acomputational scheme that can be used in connection with such measurementsto assess the influence of the particle hygroscopicity and mixing state (interms of the water uptake) on the cloud nucleating ability of particles.Additionally, it provides an estimate for the peak supersaturation of watervapour reached during the formation of the observed cloud(s). The method wasapplied in interpreting results of a measurement campaign that focused onaerosol-cloud interactions taking place at a subarctic background sitelocated in Northern Finland (second Pallas Cloud Experiment, 2nd PaCE).A set of case studies was conducted, and the observed activation behaviorcould be successfully explained by a maximum supersaturation that variedbetween 0.18 and 0.26% depending on the case. In these cases, thediameter corresponding to the activated fraction of 50% was in the rangeof 110–140 nm, and the particles were only moderately water soluble withhygroscopic growth factors varying between 1.1 and 1.4. The conductedanalysis showed that the activated fractions and the total number ofparticles acting as CCN are expected to be highly sensitive to the particlehygroscopic growth properties. For example, the latter quantity varied overa factor between 1.8 and 3.1, depending on the case, when the meanhygroscopic growth factors were varied by 10%. Another importantconclusion is that size-dependent activation profiles carries information onthe mixing state of particles.