Changes in aerosol size- and phase distributions due to physical and chemical processes in fog

Abstract

Measurements of the scavenging efficiency of aerosol particles in fog are presented. The scavenging efficiency as a function of size for accumulation-mode particles is presented, along with efficiencies for the total number, accumulation-mode number, and accumulation-mode volume. Particles below ca. 0.3 µm diameter were not efficiently scavenged in the fogs. The scavenging efficiency for accumulation-mode particles showed two steps, indicating that the hygroscopic/ hydrophobic nature of the aerosol appeared to have been a controlling factor in determining scavenging efficiencies. Observed changes in the aerosol size distributions are discussed in reference to the processes (i.e., in-cloud scavenging, aqueous-phase reactions) potentially influencing them.

Parisfog

“…The N aer was largest under easterly conditions (25% of occurrences), as shown in Table 6, when the Paris area is exposed to continental air masses and the SIRTA observatory is in the Paris plume (Chazette et al 2005). In 25% of easterly conditions, N aer at SIRTA ranged between 15,000 and 22,000 cm −3 , similar to levels observed in the Po Valley in 1989 and 1994 by Noone et al (1992) and Yuskiewicz et al (1998), respectively. The N aer were significantly less in westerly flow conditions (50% of occurrence) advecting oceanic air masses found upstream of Paris.…”

“…Variable aerosol conditions were also encountered at the onset of fog events, ranging from 2,000 cm −3 (07/01) to 25,000 cm −3 (08/03). In the presence of fog, however, 75% of the N aer distribution ranged between 4,000 and 8,000 cm −3 because of collision of aerosols with hydrated aerosols and fog droplets and, to a lesser extent, activation of aerosols into fog droplets (e.g., Noone et al 1992). Aerosol samples were collected during ParisFog to monitor chemical composition of aerosols before, during, and after six major fog events.…”

Experimental study of the aerosol impact on fog microphysics

“…A fixed value can be used as a rough estimate as described in previous studies by Noone et al (1992); Hoag et al (1999);Elias et al (2009) among others, with thresholds ranging from 2.5 to 5 µm. To take into account case to case variability, Elias et al (2015) used WELAS 20 measurements.…”

“…They can grow by water vapour diffusion or by coalescence with other droplets due to gravitational and turbulent motions and they can also evaporate in case of mixing with clear air or changes in temperature. Following Noone et al (1992), the mixing with clear air has similar consequences as of external mixture for aerosols, that means that some particles can deactivate and their diameter can be under 20 the critical wet diameter because of evaporation. In convective clouds, the activation occurs mainly within the first tenth of meters above the cloud base and it is thus possible to directly sample the resulting droplet spectra with instrumented aircraft.…”

“…But supersaturation values in the chamber are greater than 0.1 % and it is well known that SS peak in fog is generally lower than 0.1 % (Hammer et al, 2014;Ming and Russell, 2004;Svenningsson et al, 1992;Hudson, 1980). Therefore assumptions must be made to extrapolate κ at lower supersaturation.…”