Abstract
Measurements of cloud condensation nuclei (CCN) concentrations between 0.2-1.0% supersaturation and aerosol size distribution were performed at an urban background site of Athens during HygrA-CD. The site is affected by local and long-range transported emissions as portrayed by the external mixing of the particles, as the larger ones appear to be more hygroscopic and more CCN-active than smaller ones. Activation fractions at all supersaturations exhibit a diurnal variability with minimum values around noon, which are considerably lower than unity. This reinforces the conclusion that the aerosol is mostly externally mixed between “fresher”, less hygroscopic components with more aged, CCN active constituents.
Highlights
The international HygrA-CD field campaign took place between 15 May and 22 June, 2014, in the frame of the Initial Training on Atmospheric Remote Sensing (ITARS) project
It constitutes the theoretical basis for linking aerosol to cloud condensation nuclei (CCN) activity, by determining the characteristic level of ambient water vapor supersaturation (Sc), required for particles to activate into cloud droplets
CCN activity can be represented by the activation fraction (AF), which is the ratio between the measured CCN and the total condensation nuclei (CN), calculated by the integration of the aerosol size distribution
Summary
The international HygrA-CD field campaign took place between 15 May and 22 June, 2014, in the frame of the Initial Training on Atmospheric Remote Sensing (ITARS) project. Hygra-CD brought together different instruments and expertise aiming to enhance our understanding on the impact of aerosols and clouds on weather and climate (http://hygracd.impworks.gr). It was based on the synergy between remote sensing and in-situ instrumentation, making use of numerical weather prediction and atmospheric modeling. The main physico-chemical principles involved in the transformation (“activation”) of CCN into a cloud droplet involve the effects of curvature and solute on the equilibrium water vapor pressure, represented by the Köhler theory [3] It constitutes the theoretical basis for linking aerosol to CCN activity, by determining the characteristic level of ambient water vapor supersaturation (Sc), required for particles to activate into cloud droplets. For a given aerosol particle, S depends on the particle dry size and chemical composition
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