The International Panel on Climate Change (IPCC) has very recently revised the prediction of global average temperature increase during the next century from 1.0-3.5 to 1.4-5.8 K. The increase in the upper limit of the prediction is largely due to the role of aerosols in the climate of the Earth: it is believed that reduction of pollution will result in reduced direct and indirect (via clouds) scattering of sunlight back to the space. However, as can be seen from the large uncertainty of the estimated temperature increase, not enough is known about the role of natural and anthropogenic aerosols in climate processes. This is also reflected in the Key Action 2, under the RTD priority 2.1.1, calling for ”… quantification and prediction of … concentration of … aerosols, in particular the fine fraction of particles and their precursors”. The concentration of aerosols is controlled by their sources and sinks, and thus the prediction of particle concentration requires the quantification of aerosol source terms. The main objective of QUEST is to quantify the number of new secondary aerosol particles formed through homogeneous nucleation in the European boundary layer, and the relative contributions of natural and anthropogenic sources. The role of homogeneous nucleation in the formation of new atmospheric particles was realized in the 1990s, and considerable effort has been devoted to studies of aerosol formation in various parts of the Globe. The longest continuous data series of nucleation events has been obtained at a forest field station in Finland, where aerosol size distributions between 3 and 150 nm in diameter have been recorded in 10 minute intervals since the beginning of 1996 . Nucleation events occur in this rather clean Boreal area roughly 50-60 times per year, the highest event frequency taking place in the spring months (March-May). The concentration of new particles per cc of air formed during one event varies between roughly 100-10 000. Taking the average number to be one thousand, and assuming that the nucleation takes place in a well mixed boundary layer having a height of 1000 m, it can be estimated that the aerosol source term in the Boreal forest area is on the order of 51013 m-2 per year. This is on the same order as the global aerosol yield estimated from primary emissions . The number given here is very crude as we can at present only guess the vertical extent of the nucleation zone; however, it clearly shows that homogeneous nucleation events influence atmospheric particle concentrations at least at regional scales, and possibly also globally. Many features of the Boreal nucleation events have been revealed thus far. Necessary (but not sufficient) conditions include sunny weather, vertical mixing of air in the morning (prior to the detection of the event) , and a treshold value of a quantity that depends on radiation intensity (vapor source) and pre-existing aerosol size distribution (vapor sink) . The springtime events always seem to take place in Polar or Arctic air masses , but so far it is unclear whether the meteorology is similar during other seasons. Aerosol flux measurements  indicate that the particles are formed aloft, but the vertical extent of the nucleation layer is unknown. However, there is clear evidence from simultaneous measurements at various locations, that the horizontal extent of the areas in which the nucleation takes place can be hundreds and in some cases even thousands of kilometers . No direct correlation of nucleation events with SO2 concentrations has been found; however the product of SO2 concentration, ammonia concentration, and calculated OH concentration correlates with the events (personal communication). These results hint that the recently suggested ternary sulfuric acid-ammonia-water nucleation mechanism of small clusters, followed by the growth of the clusters due to condensation of other (possibly organic) vapors , may be operational in the Boreal forest area. Furthermore, there is experimental evidence that nucleation event particles in the 4-5 nm range are soluble in butanol (working fluid of condensation particle counters), which indicates organic composition. However, the confirmation of the ternary nucleation hypothesis requires simultaneous measurements of sulfuric acid vapor and ammonia, and further studies of the composition of the nucleated particles. Furthermore, to facilitate large-scale modelling studies, the vertical extent of the nucleation events, as well as the meteorological conditions during non-springtime events have to be investigated. Measurements of nucleation events at a more Central European location indicate that SO2 levels increase during the majority of nucleation events . It can be hypothesized that a part of observed nucleation events (minority in Central Europe, majority in the Boreal area) are ”natural” and a part are affected (or even caused) by pollution (majority in Central Europe, minority in the Boreal area). The confirmation of this hypothesis and implementation of the pollution type nucleation mechanism into a large-scale model requires carefully designed measurements from a location which is preferably Southern European as there is very little available nucleation data from this area. One of the few observations of new particles in Southern Europe  is from the Italian site where we plan to study the frequency, meteorology, vertical extent, and chemical precursors of nucleation events. Another type of nucleation events has been observed all along the western coast of Europe and have been studied more particularly at the west coast of Ireland . These events, which have a duration of the order of 4 hours and up to 8 hours, occur almost daily around low tide and under conditions of solar radiation, indicating photochemical source. Incredibly, the peak new particle concentrations often exceed 106 cm-3, making this the strongest natural source region of atmospheric particles. The exact chemical mechanisms leading to the production of coastal particles still remains an open question. As in other environments, there appears to be sufficient sulphuric acid vapour to participate in ternary nucleation with ammonia and water, however, there is insufficient sulphuric acid to grow these particles to detectable sizes . The most probable chemical species involved in the production or growth of these particles is Iodine, or an Iodine Oxide, produced photochemically from biogenic halocarbon emissions . The production of particles from the photolysis of CH2I2 in the presence of ozone has been confirmed by recent smog chamber experiments . While the concentration of new particles in this environment is extraordinarily high, its impact on background particle and CCN contribution remains unclear and needs to be quantified. A limited single study  has shown that the coastal aerosol plume is detectable up to several hunderds of km downwind and that the new coastal particles readily grow into CCN sizes (larger than 100 nm). An intensive campaign at the coast of Ireland will quantify the flux of both biogenic halocarbon precursor gases and the yield of new, and radiatively-active particles in the European coastal boundary layer. The objective of QUEST is to determine the source strength of new particle formation in the three above mentioned cases. The specific objectives are: 1) To fill in gaps that exist in the understanding of chemical and physical pathways leading to homogeneous nucleation of new aerosol particles; 2) To understand the meteorological conditions required for the events to take place and to be able to predict the horizontal and vertical extent of the events; 3) To implement parametrized representations of the nucleation mechanisms, based on the information from 1) and 2), to an European scale model in order to determine the source strength of homogeneous nucleation of aerosol particles in the European boundary layer.