1. Modeling the dynamic behavior of Cloud Condensation Nuclei: case study comparing clean (LBA/CLAIRE 2001) and polluted (LBA/SMOCC 2002) air conditions in Amazonia. Theotonio Pauliquevis 1, Paulo Artaxo 1, Luciana V Rizzo 1, Meinrat O Andreae 2, Göran Frank 2, Olga L. Mayol-Bracero 3, Susimar Gonzalez 3 [1] Institute of Physics, University of São Paulo, Rua do Matão, Travessa R, 187 São Paulo, SP, CEP 05508-900, Brazil., theo@if.usp.br,theo@if.usp.br [2] Biogeochemistry Department, Max Planck Institute for Chemistry, Mainz, Germany [3] Institute for Tropical Ecosystem Studies, University of Puerto Rico, Puerto Rico 3 rd Scientific LBA conference – Brasilia, 2004 1) INTRODUCTION The influence of Cloud Condensation Nuclei (CCN) population over cloud properties in Amazonia is not yet completely understood. There are several issues concerning which consequences in cloud properties will take place in a future scenario of a deforested Amazonia, with a higher aerosol loading concentration. Under a natural condition, CCN concentration is very low (~200#/cm3, SS = 1%), and particles have an hygroscopical behavior. The soluble fraction of these particles is about 20%. In such a situation, with the typical high amounts of available water vapor, the formation of clouds is extremely efficient. In Amazonia, the occurrence of rain is usually linked to warm clouds, with small lightning activity. If the the aerosol concentrations rise up due to antropic changes in land use, like biomass burning, urbanization, etc, this process changes. The higher the concentration of particles, the higher the CCN concentration, yielding a higher cloud droplet concentration too. Given that the total amount of available water vapor is constant, the average size of droplets diminish too, changing droplet size distribution with consequences to rain formation processes. In this study we modeled the dynamic behavior of droplets under two different situations: a clean situation, represented by the atmosphere of Balbina during the LBA/CLAIRE 2001 Experiment, and a polluted situation, represented by the atmospheric conditions of Rondônia during dry season, measured during the LBA/SMOCC 2002 Experiment. 2) MAPS – Model for Aerosol Processes Studies To model the behavior of an air parcel rising in a free atmosphere we used MAPS, a box model (zero dimension, figure 1) developed at NCAR-USA. We simulated a rising air parcel by the control of temperature and humidity inside the box at each time step. The initial conditions of the model are mass size distribution of aerosols (from MOUDI cascade impactor in situ measurements), chemical composition of aerosols and solubility of organics. The model has 64 bins for aerosol mass size distribution, and a hybrid growth structure that treats separately soluble and insoluble particles. As a representative chemical composition of aerosol, we used results from Particle Induced X-ray Emission (PIXE) analysis of Stacked Filter Units (SFU) that we operated during CLAIRE and SMOCC campaigns, and from these results we derived the initial concentrations of Na, NH 4, NO3, Cl, SO4, Elemental Carbon, crustal sources and Organic Carbon inside the box. Figure 1: zero-dimension box model 3) Model initial conditions from in-situ ground measurements during CLAIRE and SMOCC campaigns Figures 2 and 3 show aerosol mass size distributions measured by a cascade impactor during CLAIRE (clean) and SMOCC (polluted) experiment. Those distributions were used as initial conditions to run the model. For the clean (polluted) conditions one can note the predominance of mass in the course (fine) mode. This is due to the fact that for pristine (disturbed) areas, most of aerosols are characterized by biogenic (biomass burning) emissions, which are predominantly coarse (fine). Tables 1 and 2 show chemical composition for fine (d < 2.5  m) and coarse ( 2.5  m < d < 10  m). Figures 2 (above) and 3 (below): aerosol mass size distribution for clean (CLAIRE 2001 Experiment) and polluted consitions (SMOCC 2002 Experiment) Tables 1 (above) and 2 (below): aerosol chemical composition for clean (CLAIRE 2001) and polluted conditions (SMOCC 2002) 4) Results and discussion of MAPS Simulations Acknowledgments: We acknowledge the support of FAPESP and CNPq/Instituto do Milênio of LBA for this project. figure 5: From up to down, results from MAPS simulations for: air parcel temperature, supersaturation, Liquid Water Content and time evolution of particle diameter for each of the 64 model bins, for clean (LEFT) and polluted (RIGHT) conditions in Amazonia. The results of MAPS simulations are shown in figure 5, and important conclusions can be took from those. Considering that the thermodynamic conditions for both air parcels were identical (humidity and cooling rate = -2.25 C/hour) the differences in the results are only due to the different aerosol initial conditions. The results show that the super saturation magnitudes obtained are distinct, activating different particle population fractions. For the clean situation (CLAIRE conditions), the simulated maximum super saturation reaches 0.32%, activating particles with sizes > 0.22  m. Some smaller particles get activated in the beginning, but as supersaturation diminishes, they reduce their size. For the polluted situation, the maximum super saturation reaches 0.19 %, activating only particles greater than 0.53  m. Table 3: cooling rate (at which air parcels were subject to during the simulations), number and mass concentration of aerosols for both simulated situations 5) Conclusions When one considers the hypothesis that a higher amount of available CCN may affect the cloud microphysics, mainly due to the formation of more droplets, and consequently diminishing the efficiency of coalescence process, it should be precisely quantified how many particles are really acting as CCN. Typical values of in-cloud super saturation are in the range 0.10-0.15%, and in this simulation we observed that maximum reached super saturation was smaller for the higher aerosol concentration situation. It is due to the fact that small particles are faster to absorb water vapor, due to thermodynamical reasons, in opposition to bigger particles which are slower. The consequence of this process is that reaching smaller super saturation, results in bigger minimum activation diameter, and a smaller fraction of aerosols is activated from the entire population. As a consequence, the number of activated droplets should not grow linearly with CCN concentration at the same super saturation. It suggests that there is a mechanism working in a compensatory way. There may be a particle concentration range where the changes in cloud microphysics are not so sensible. It is important specially if we consider that background particle concentrations (300/#cc) in Amazon are rising due to anthropogenic influence, and the understanding of the mechanisms that can therefore affect cloud microphysics is extremely important.