In the present paper, the microphysical processes of aerosol particles are investigated using the new coupled system of the aerosol HAM model and the Weather Research and Forecasting (WRF) modeling system. The WRF-HAM model uses a new "pseudomodal" approach for the representation of the aerosol size distribution. The aerosol population in this model is represented by the superposition of seven modes, assuming a lognormal distribution within each mode. The seven modes are grouped into four geometrical size classes, ranging from the nucleation, Aitken, and accumulation modes to coarse modes. Besides size, two categories of particles are distinguished, internally mixed and water soluble particles (four modes), and externally mixed and insoluble particles (three modes). This classification allows prediction of the hygroscopic properties of initially insoluble aerosol compounds, which controls their atmospheric lifetimes and also their interactions with clouds.
The WRF-HAM model includes the various microphysical processes of condensation of sulfuric acid, nucleation and new particle formation, coagulation of aerosol particles, and the thermodynamical equilibrium of aerosols with the water vapor. Gravitational sedimentation and dry deposition are considered the main removal processes for the aerosol particles in this coupled WRF-HAM model. The model also contains the in-cloud scavenging of aerosol particles by precipitation within convective cumulus clouds. The main global aerosol compounds including sulfate, black carbon, particulate organic carbon (POM), sea salt and mineral dust have been considered in this study. The emission fluxes of different aerosol compounds are prescribed based on the Emission Inventory for the Aerosol Model Inter-comparison Experiment B, AEROCOM representative for the year 2000.
The model simulations were conducted for a 24-hour period from 22 to 23 February 2006 in a domain with 30 km horizontal grid spacing, encompassing south-western Asia, North Africa and some parts of Europe. The model simulation results were investigated for different microphysical processes of condensation, nucleation and coagulation. For better understanding of the treatment of different microphysical processes in this study, all aerosol particles were assumed to have been emitted into the atmosphere only during the first time step.
The model simulation results show that the sulfuric acid concentration consumed by new particle formation in the nucleation mode is higher than that by condensation on aerosol particles. In addition, the secondary sulfate aerosol in nucleation mode formed at high altitudes in the cloudy regions of the domain. It has also been concluded that aerosol particles in both insoluble and mixed modes grow by condensation of sulfuric acid. Moreover, it is shown that the condensation of gaseous sulfate on aerosol particles causes the transfer of particles in insoluble modes to the corresponding mixed modes. Thus, the mass of different aerosol compounds in insoluble modes decreases after condensation of sulfuric acid over their surfaces, while that of the aerosol particles in mixed modes increases significantly due to both condensation of gaseous sulfate and transfer of condensed particles in insoluble modes. The coagulation process of aerosol particles was further observed to modify the aerosol mass within the modes and transfers the coagulating particles to the coarser modes. The aerosol mass modification by coagulation is smaller than that by the condensation process.