الفهرس 
Introduction
Organic vaporsOnly 14 pages are availabe for public view
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Abstract
Atmospheric aerosol particles have a significant impact on air quality, human health and global climate. A better understanding of these aerosol-related effects, especially with respect to long-term climate projections, requires more comprehensive knowledge on aerosol sources and their atmospheric transformation processes. An important process controlling the number concentration of atmospheric particles is the formation of new ultrafine particles typically 1–2 nm in size, through gas-to-particle conversion. Once thermodynamically stable, the new particles can grow through condensation and coagulation to sizes of 50 –100 nm where they can become cloud condensation nuclei (CCN). In this thesis we studied the formation of nanometer sized atmospheric aerosol particles and their subsequent growth to the CCN size range (i.e. 50 nm and larger) in different atmospheric environments. Our study was based on analyzing long time series of aerosol particle size distributions from different field measurements sites (San Pietro Capofiume (SPC), Italy; Melpitz, Germany; Hyytiälä, Finland). Additionally, we used theoretical approaches and modeling studies to support our detailed characterization of the formation and growth processes using field observations. Based on our study, we came to 7 main conclusions: 1) new particle formation (NPF) events are very intense and frequent in PoValley and, surprisingly, they indeed act as an important source of CCN in spite of the polluted nature and the strong anthropogenic activities of the region; 2) secondary particle formation from vapors is occurring in a rural polluted area (SPC, Italy) and in a rural area (Melpitz, Germany) as frequently as in a clean area (Hyytiälä, Finland); 3) between 1996/1997 and 2003/2006 a significant decrease of the anthropogenic SO2 (-65%) in the Melpitz station shows a strong connection to the decrease in the frequency of new particle formation events (-45%) and in formation rates (-68%), and was in contrast to the increased growth rates of nucleated particles (+22%); 4) the production of CCN following NPF events in Melpitz between the two previously mentioned periods appears to have increased by tens of percent and this is most likely due to the increased particle growth rate, possibly due to organics; 5) field data from the Hyytiälä measurement station show that the maximum observed gas-phase sulphuric acid concentrations are limited to relative humidity (RH) values below 60% and that this is likely due to low hydroxyl radical (OH) concentrations at high RH; 6) theoretical calculations indicate that the increase of coagulational scavenging at elevated RH probably has a relatively small effect in masking the appearance of new particle formation events; 7) the model simulation results show that the decreased source term at high RH limits H2SO4 levels in the air, and therefore high new particle formation rates (above ~1 cm-3s-1) rarely occur above 80% RH. In this thesis a method for identifying NPF is presented, as are basic characteristics such as growth and formation rates, condensational and coagulation sinks. Also the possible effects of meteorological parameters and gas phase concentrations on new particle formation at different atmospheric environments are summarized. Methodology to estimate the production of CCN following NPF events is introduced for field measurements and is supported by modeling studies. The results are particularly useful when estimating, for instance, the role of SO2, the main precursor of gaseous sulphuric acid (H2SO4), which in turn is a main precursor for atmospheric particle nucleation, on particle formation and the growth process. This work gives a better understanding of the role of RH in inhibiting new particle formation. .