Abstract
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.
Highlights
Aerosol hygroscopicity is defined as the ability of aerosol particles to take up water
During the ParisFog 2012/13 campaign, 17 developed fog events were detected at the SIRTA site
One fog event of the type Precipitation fog (PREC) was detected, neither the scanning mobility particle sizer (SMPS) nor the WELAS measured the latter fog type, no results are presented for this event
Summary
Aerosol hygroscopicity is defined as the ability of aerosol particles to take up water. This water uptake plays an important role in the Earth’s radiative budget, as it changes the particle’s light scattering and absorption properties. E. Hammer et al.: Size-dependent particle activation properties in fog Aerosols influencing fog are a major factor in visibility reduction at surface levels, which can cause fatalities in traffic (Rosenfeld, 1996). When the relative humidity increases, particles can grow substantially and if the atmosphere becomes supersaturated with respect to water vapor, particles can activate to form fog droplets (e.g., Bergot, 2007), depending on their size and hygroscopic properties. Studies in Europe showed that during developed fog, the visibility can decrease down to 50 m or lower (e.g., David et al, 2013; Elias et al, 2009)
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