On the formation of radiation fogs under heavily polluted conditions

Abstract

Abstract.We have studied the effect of gaseous pollutants on fog droplet growth in heavily polluted air using a model that describes time-dependent sulfate production in the liquid phase and thermodynamical equilibrium between the droplets and the gas phase. Our research indicates that the oxidation of SO 2 to sulfate has a significant effect on fog droplet growth especially when hygroscopic trace gases, for example HNO 3 and NH 3 are present. The increased sulfate production by dissolution of hygroscopic gases results from increased pH (caused by absorption of ammonia) and from the increased size of the fog/smog droplets. Our results indicate that unactivated fogs may become optically very thick when the droplet concentrations are on the order of several thousand per cubic centimeter of air.

UCLALES–SALSA v1.0: a large-eddy model with interactive sectional microphysics for aerosol, clouds and precipitation

“…Since this goes beyond the practical range for the applications in this paper, where in general we aim towards a time step around 1 s, two sets of measures are taken. First, for small aerosol particles with ambient relative humidity (RH) below 98 %, the wet size of aerosol particles is determined as an equilibrium solution based on the molalities of different particle species (Stokes and Robinson, 1996;Kokkola et al, 2008), and the APC equations are solved only above 98 % RH. Second, a simple substepping method is applied with Eqs.…”

“…The Sectional Aerosol module for Large-Scale Applications (SALSA; Kokkola et al, 2008) is used as the basis for developing a unified sectional microphysical model for aerosols, clouds and precipitation. The SALSA module, previously employed in the ECHAM climate model family, discretises the aerosol size distribution into 10 size bins according to the dry particle diameter (Bergman et al, 2012) as shown in Fig.…”

“…Thus, there are also marked differences related to the dynamics of the fog layer and its life cycle as compared to clouds (Porson et al, 2011). Fog properties and their occurrence are strongly affected by aerosol properties and anthropogenic emissions (Bott, 1991;Kokkola et al, 2003;Stolaki et al, 2015;Maalick et al, 2016), although many of the details of these interactions remain poorly understood. Improved knowledge can be pursued through increasingly sophisticated microphysical schemes embedded in LES models.…”

“…We build and extend upon a state-of-the-art LES model and a sectional microphysical model (Stevens et al, 2005;Kokkola et al, 2008) to create a cloud-resolving framework, where the size distributions of aerosol, clouds and precipitation are all described with a detailed sectional approach. In particular, the model introduced in this work accurately preserves the characteristics of the aerosol size distribution both inside and outside of clouds, making it ideal for studying the impact of removal processes, cloud processing and evaporation on the particle size distribution, as well as the associated feedbacks on cloud properties, precipitation formation and boundary layer dynamics.…”

Modelling of HNO3-mediated laser-induced condensation: A parametric study

“…More specifically, we dropped the assumption of a dilute ionic solution which in particular imposes low trace gas concentrations in the atmosphere, as well as conditions close to water vapor saturation. 11 Instead, we considered a situation similar to that of stratospheric aerosols containing concentrated sulphuric acid in an atmosphere rich in gaseous HNO 3 , as treated by Kokkola et al 15 We checked that in spite of these modifications, our model converges back to the classical one at low HNO 3 concentrations.…”