Abstract
Ground-based measurements of low stratus or fog at a mountain station in the Czech Republic have been conducted to describe the cloud nucleus activation process from the perspectives of size dependence, effectiveness, and speed. The activated particles were estimated from the difference in measured aerosol concentration between the whole-air inlet (WAI) and PM2.5 interstitial inlet.
Both inlets were connected via a switching valve to a scanning mobility particle sizer (SMPS) and aerodynamic particle sizer (APS), thereby measuring aerosol particles from 15 nm to 20 mu m in size. Episodes of fog, freezing fog, and mist were studied; however, during the mist episodes, no activation occurred.
Most of the differences between fog and freezing fog were comparable to the measurement uncertainty. The activated fraction reached over 90 % for particles larger than 450 and 510 nm for fog and freezing fog, respectively.
The smallest activated diameter, 130 nm, was identical for fog and freezing fog. The activation process speed, derived from the time evolution of the size-dependent activated fraction function (AFF), was also similar for fog and freezing fog.
The AFF becomes steeper, and the inflection point moves to smaller particles. For most episodes, 120 min after initiation, the AFF reached a steady state.
The effectiveness, speed, and size dependence of the activation process were found to be connected with the air mass history, a proxy for the air mass composition and liquid water content, resp. supersaturation. For continental air masses, the activated fraction was independent of the air mass history, and the activated particle mode position only ranged from 230 to 260 nm.
For maritime air masses, smaller particles were activated with the mode shifted to 120 nm.