Articles | Volume 14, issue 19
Atmos. Chem. Phys., 14, 10517–10533, 2014
Atmos. Chem. Phys., 14, 10517–10533, 2014

Research article 07 Oct 2014

Research article | 07 Oct 2014

Size-dependent particle activation properties in fog during the ParisFog 2012/13 field campaign

E. Hammer1, M. Gysel1, G. C. Roberts2,3, T. Elias4, J. Hofer1,**, C. R. Hoyle1,5, N. Bukowiecki1, J.-C. Dupont6, F. Burnet2, U. Baltensperger1, and E. Weingartner1,* E. Hammer et al.
  • 1Laboratory of Atmospheric Chemistry, Paul Scherrer Institute, 5232 Villigen PSI, Switzerland
  • 2CNRM/GAME – Meteo-France/CNRS, 42 av. Gaspard Coriolis, 31057 Toulouse, France
  • 3Scripps Institution of Oceanography, 9500 Gilman Dr., La Jolla CA 92093, USA
  • 4Hygeos, 165 Avenue de Bretagne, 59000 Lille, France
  • 5Swiss Federal Institute for Forest Snow and Landscape Research (WSL)-Institute for Snow and Avalanche Research (SLF), 7270 Davos, Switzerland
  • 6Institut Pierre-Simon Laplace, Université Versailles Saint Quentin, 78280 Guyancourt, France
  • *now at: Institute for Aerosol and Sensor Technology, University of Applied Sciences, 5210 Windisch, Switzerland
  • **now at: Leibniz Institute for Tropospheric Research (TROPOS), 04318 Leipzig, Germany

Abstract. Fog-induced visibility reduction is responsible for a variety of hazards in the transport sector. Therefore there is a large demand for an improved understanding of fog formation and thus improved forecasts. Improved fog forecasts require a better understanding of the numerous complex mechanisms during the fog life cycle. During winter 2012/13 a field campaign called ParisFog aiming at fog research took place at SIRTA (Instrumented Site for Atmospheric Remote Sensing Research). SIRTA is located about 20 km southwest of the Paris city center, France, in a semi-urban environment. In situ activation properties of the prevailing fog were investigated by measuring (1) total and interstitial (non-activated) dry particle number size distributions behind two different inlet systems; (2) interstitial hydrated aerosol and fog droplet size distributions at ambient conditions; and (3) cloud condensation nuclei (CCN) number concentration at different supersaturations (SS) with a CCN counter. The aerosol particles were characterized regarding their hygroscopic properties, fog droplet activation behavior and contribution to light scattering for 17 developed fog events. Low particle hygroscopicity with an overall median of the hygroscopicity parameter, κ, of 0.14 was found, likely caused by substantial influence from local traffic and wood burning emissions. Measurements of the aerosol size distribution at ambient RH revealed that the critical wet diameter, above which the hydrated aerosols activate to fog droplets, is rather large (with a median value of 2.6μm) and is highly variable (ranging from 1 to 5μm) between the different fog events. Thus, the number of activated fog droplets was very small and the non-activated hydrated particles were found to contribute significantly to the observed light scattering and thus to the reduction in visibility. Combining all experimental data, the effective peak supersaturation, SSpeak, a measure of the peak supersaturation during the fog formation, was determined. The median SSpeak value was estimated to be in the range from 0.031 to 0.046% (upper and lower limit estimations), which is in good agreement with previous experimental and modeling studies of fog.

Final-revised paper