Long term model studies of aerosol formation and growth in borea forest

Natural and anthropogenic aerosols may have a great impact on climate as they can directly interact with solar radiation and indirectly affect the Earth's radiation balance and precipitation by modifying clouds. In order to quantify the direct and indirect effect, it is essential to understand the complex processes that connect an aerosol particle to a cloud droplet. However, while modern measurement techniques are able to detect particle sizes down to nanometer all the way from ground up to the stratosphere, the data does not serve for all of our needs for understanding the processes. Hence we will demonstrate a modeling approach to investigate the complex processes of aerosols in the atmospheric boundary layer (ABL). SOSAA (model to Simulate the concentration of Organic vapors, Sulphuric Acid, and Aerosols) is a 1D chemical-transport model with detailed aerosol dynamics. It was constructed to study the emissions, transport, chemistry, as well as aerosols in the ABL in and above a vegetation canopy [Boy et al., 2011]. As a first application of the model, we tested different nucleation theories by simulating the new particle formation events in year 2010 at a boreal forest site in Finland. Since condensible organic vapors are the dominant contributors to the aerosol particle growth, particularly in regions where biogenic volatile organic compound emissions are high, we also simulated the concentrations of a set of organic compounds and their first reaction products from oxidation [e.g. Kerminen et al. (2000); Sellegri et al. (2005); Boy et al. (2005); Allan et al. (2006); Laaksonen et al. (2008)]. The results have showed the ability of SOSAA to reconstruct the general behavior of atmospheric trace gases and new particle formation in a boreal forest environment with reasonable uncertainties. The underestimation in sulfuric acid concentration during nighttime supports the view that other production mechanisms of sulfuric acid exist. It also suggests that the missing mechanisms are not likely to be photochemically driven. The simulations about particle nucleation emphasizes the complexity of the phenomenon since there is no single nucleation theory that works all the time. The model also confirmed the importance of organic vapors to particle growth. As a column model, SOSAA has turned out to be a good tool to study the vertical distribution of particles. Scavenging of background particles before a nucleation event has been observed regularly in the simulations. The deposition velocity has been estimated as strong in the model. Measured (upper plot) and modeled (lower plot) particle number size distribution for March 2011.