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Discussion papers
https://doi.org/10.5194/acp-2018-875
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2018-875
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 08 Oct 2018

Research article | 08 Oct 2018

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Experimental study of the aerosol impact on fog microphysics

Marie Mazoyer1, Frederic Burnet1, Greg Roberts1, Martial Haeffelin2, Jean-Charles Dupont2, and Thierry Elias3 Marie Mazoyer et al.
  • 1Meteo-France/CNRS – CNRM/GAME, Toulouse, France
  • 2Institut Pierre Simon Laplace/UVSQ, Palaiseau, France
  • 3HYGEOS, Lille, France

Abstract. Comprehensive field campaigns dedicated to fog life cycle observation were conducted during the winters of 2010–2013 at the SIRTA observatory in the suburb of Paris. In order to document their properties, in situ microphysical measurements collected during 23 fog events are examined here. They reveal large variability in number, concentration and size of both aerosol background before the fog onset and fog droplets according to the different cases. The objective of this paper is to evaluate the impact of aerosol particles on the fog microphysics.

To derive an accurate estimation of the actual activated fog droplet number concentration Nact we determine the hygro-scopicity parameter κ, the dry and the wet critical diameter and the critical supersaturation for each case by using an iterative procedure based on the κ-Köhler theory that combines cloud condensation nuclei (CCN), dry particle and droplet size distribution measurements. Resulting values of κ=0.17±0.05 were found typical of continental aerosols. Our study reveals low values of the derived critical supersaturation with a median of 0.043% and large values for both wet and dry activation diameters. Consequently, the corresponding Nact values are low with median concentrations of 53.5cm−3 and 111cm−3 within the percentile 75th.

No detectable trend between the concentration of aerosol particles with diameter>200nm and Nact was observed. In contrast the CCN data at 0.1% supersaturation exhibits a strong correlation with these aerosol concentrations. We therefore conclude that the actual supersaturations reached during these fog episodes are too low and no simultaneous increase of aerosols>200nm and droplet concentrations can be observed. Moreover our analysis suggests that a high aerosol loading limits the supersaturation values. It is also found that the activated fraction mainly depends on the aerosol size while the hygroscopicity appears to be of a secondary importance.

Although radiative fogs are usually associated with higher aerosol loading rather than to stratus lowering events, our analysis reveals that the activated particle concentrations at the beginning of the event are similar for both types of fog. However the evolution of the droplet concentration during the fog life cycle shows significant differences between both types of fog.

Marie Mazoyer et al.
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Marie Mazoyer et al.
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Short summary
In situ microphysical measurements collected during 23 fog events at the SIRTA (south of Paris) are examined here. A original iterative method based on the κ-Köhler theory have been used to compute statistics of their activation properties. Useful information are provided to constrain and validate numerical simulations. The paper demonstrates that supersaturation encountered in these fogs is too low to observe a correlation between concentrations of aerosols > 200 nm and droplet concentrations.
In situ microphysical measurements collected during 23 fog events at the SIRTA (south of Paris)...
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