In this project the LES (Large-Eddy Simulation) model ASAM (All Scale Atmospheric Model) is used to simulate these processes and will help us understand the mechanisms behind. ASAM is a developing research code developed and maintained by TROPOS (Leibniz Institute for Tropospheric Research, Leipzig, Germany) (Jähn et al., 2014, accepted). Recently, Zhou Putian has implemented the BVOC (Biogenic Volatile Organic Compoud) emission model MEGAN (Model of Emissions of Gases and Aerosols from Nature, version 2.0, Guenther et al., 2006) into ASAM.
The current version of ASAM contains four main parts (Fig. 1): the dynamic part, the energy balance part, the emissions from canopy and the chemistry part. The aerosol part will be implemented in future. The dynamics, mainly refering to the wind field, is calculated by dynamic modules. The canopy drag is computed simultaneously for a given LAD (Leaf Area Density) profile. In the energy balance part the solar radiation and long wave radiation are considered, as well as the heat fluxes from both canopy and soil. The BVOC emissions from canopy are obtained from MEGAN subroutines. The list of chemical reactions are mainly from SOSA (model to Simulate the concentration of Organic vapours and Sulphuric Acid, Boy et al., 2011). The ELVOCs (Extremely Low-Volatility Organic Compounds) will also be considered in future due to its significant role in aerosol growth process (Ehn et al., 2014). The aerosol dynamic processes (nucleation, coagulation, condensation and dry deposition) will be calculated by the modules of the latest version of UHMA. This version of UHMA has included a new condensation mechanism, the organic salt formation, the acid catalysed oligomerization and a particle phase module for the growth of manometer sized particles into CCN.
Figure 2 shows an animation of how isoprene is emitted from the canopy and then transported by the eddies in the planetary boundary layer. Here only a part of the isosurface of isoprene is shown in the simulation domain for clarity. First, the emission is nearly horizontally homogeneous with small disturbances. But then it is stretched and twisted by the developing turbulence. Finally, the 1 pptv isoprene can be transported to the top of the boundary layer.
Figure 1: ASAM diagram illustration
Figure 2: Animation of how isoprene is emitted and transported in the planetary boundary layer. The two backsides and the bottom side of the simulation domain are shown along with the isosurface of 1 pptv isoprene (gray surface).The upper color bar shows the concentration of isoprene on the sides with the unit of [pptv]. The velocity is represented by the colored arrows. Its magnitudes shown by the lower color bar with the unit of [m/s].