Experimental study of the aerosol impact on fog microphysics

Abstract

Abstract. Comprehensive field campaigns dedicated to fog life cycle observation were conducted during the winters of 2010–2013 at the Instrumented Site for Atmospheric Remote Sensing Research (SIRTA) observatory in a suburb of Paris. In order to document their properties, in situ microphysical measurements collected during 23 fog events induced by both radiative cooling and stratus lowering 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 hygroscopicity 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. Our study reveals low values of the derived critical supersaturation occurring in fog with a median of 0.043 %. Consequently, the median dry and wet activation diameters are 0.39 and 3.79 µm, respectively, leading to a minor fraction of the aerosol population activated into droplets. The corresponding Nact values are low, with median concentrations of 53.5 and 111 cm−3 within the 75th percentile. The activated fraction of aerosols exhibits remarkably low correlation with κ values, which reflects the chemical composition of the aerosols. On the contrary, the activated fraction exhibits a strong correlation with the inferred critical diameter throughout the field campaigns. This suggests that the variability in the activated fraction is mostly driven by particle size, while variations in aerosol composition are of secondary importance. Moreover, our analysis suggests that the supersaturation reached in fog could be lowered by the aerosol number concentration, which could contribute to the sink term of water vapor during the radiative cooling. Although radiative fogs are usually associated with higher aerosol loading than stratus-lowering events, our analysis also reveals that the activated fraction at the beginning of the event is 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. This work demonstrates that an accurate calculation of supersaturation is required to provide a realistic representation of fog microphysical properties in numerical models.

Method to Estimate Water Vapor Supersaturation in the Ambient Activation Process Using Aerosol and Droplet Measurement Data

“…In previous studies, Gerber () found in a radiation fog that nongradient turbulent mixing can cause an average shape of supersaturated eddies with a median peak supersaturation of 0.049%. Mazoyer et al () also tried to obtain the supersaturation through aerosol and droplet measurements and derived a critical supersaturation with a median of 0.043% during the winters of 2010–2013 at the SIRTA observatory in the suburb of Paris. The results of our method do not deviate much from these measurements or calculations, and the relative low values can be explained by the heavy pollution in the NCP.…”

“…In previous studies, water vapor supersaturation is often involved in the interaction between aerosols and clouds or fogs (Gerber, ; Herich et al, ; Mazoyer et al, ; Su et al, ). Water droplets form by condensation of water vapor on the aerosol particles.…”

The evolution of cloud microphysics upon aerosol interaction at the summit of Mt. Tai, China

“…Asmi et al (2012) found that high NCCN/NC was accompanied with the elevated aerosol hygroscopicity parameter κ during winter when longrange transported polluted continental aerosol commonly occurred. At a given ss, Mazoyer et al (2019) also found the CCN…”

“…They pushed the minification of cloud droplet sizes (Rosenfeld et al, 2014a), decreased the ambient supersaturation, enhanced the evaporation of small droplets (Ackerman et al, 2004), and finally caused the cloud events to vanish (Mazoyer et al, 2019).…”

“…In situ measurements of cloud microphysics by aircraft or on high-altitude monitoring sites have provided some additional information for insight into the cloud processes (Allan et al, 2008;Li et al, 2017a;Padmakumari et al, 2017;Van Pinxteren et al, 2016;Reid et al, 1999). Furthermore, we 10 lack knowledge of the microphysical parameters of cloud at different stages, an analogous study by Mazoyer et al (2019) found that different fog stages own varied relationships among microphysical parameters.…”