Comprehensive analysis of relative dispersion of the particle size distribution and its relationships to key physical fog processes under different aerosol conditions and evolution stages

Cloud/fog droplet relative dispersion has important impacts on aerosol indirect effect, radiative transfer, and microphysical processes. However, previous studies have been mostly concerned with clouds, with limited studies on fog, esp., on the combined influences of all the key physical processes and during fog evolution. This study aims for such a comprehensive investigation by examining the relationships between relative dispersion and other microphysical variables, and the underlying microphysical and dynamic processes, based on the field fog campaigns in polluted and clean conditions. In the polluted fog, droplet concentrations are higher, leading to smaller droplets and increased dispersion. The correlation between dispersion and droplet volume-mean radius is positive in the polluted fog, but shifts to negative in clean fog. We attribute the difference to various microphysical processes like aerosol activation, condensation, collision-coalescence, and entrainment-mixing. In the polluted fog, the high aerosol concentration, low supersaturation and strong turbulence (entrainment-mixing) provide suitable conditions for the concurrent occurrence of droplet condensation and aerosol activation, resulting in the positive correlation between dispersion and volume-mean radius, especially during fog formation stage. In contrast, during the mature stage in clean fog, condensation is dominant with weak aerosol activation leading to a negative correlation between relative dispersion and volume-mean radius. The collision-coalescence process is more active in mature stage, increasing radius and leading to the negative correlation between dispersion and volume-mean radius in the clean fog. The result sheds new light on understanding the relative dispersion and mechanisms in fog under different aerosol backgrounds.