Abstract.We have investigated the formation and early growth of atmospheric secondary aerosol particles building on atmospheric measurements. The measurements were part of the QUEST 2 campaign which took place in spring 2003 in Hyytiälä (Finland). During the campaign numerous aerosol particle formation events occurred of which 15 were accompanied by gaseous sulphuric acid measurements. Our detailed analysis of these 15 events is focussed on nucleation and early growth (to a diameter of 3 nm) of fresh particles. It revealed that new particle formation seems to be a function of the gaseous sulphuric acid concentration to the power from one to two when the time delay between the sulphuric acid and particle number concentration is taken into account. From the time delay the growth rates of freshly nucleated particles from 1 nm to 3 nm were determined. The mean growth rate was 1.2 nm/h and it was clearly correlated with the gaseous sulphuric acid concentration. We tested two nucleation mechanisms -recently proposed cluster activation and kinetic type nucleation -as possible candidates to explain the observed dependences, and determined experimental nucleation coefficients. We found that some events are dominated by the activation mechanism and some by the kinetic mechanism. Inferred coefficients for the two nucleation mechanisms are the same order of magnitude as chemical re-action coefficients in the gas phase and they correlate with the product of gaseous sulphuric acid and ammonia concentrations. This indicates that besides gaseous sulphuric acid also ammonia has a role in nucleation.

Abstract. This paper synthesizes the available scientific information connecting atmospheric nucleation with subsequent cloud condensation nuclei (CCN) formation. We review both observations and model studies related to this topic, and discuss the potential climatic implications. We conclude that CCN production associated with atmospheric nucleation is both frequent and widespread phenomenon in many types of continental boundary layers, and probably also over a large fraction of the free troposphere. The contribution of nucleation to the global CCN budget spans a relatively large uncertainty range, which, together with our poor understanding of aerosol-cloud interactions, results in major uncertainties in the radiative forcing by atmospheric aerosols. In order to better quantify the role of atmospheric nucleation in CCN formation and Earth System behavior, more information is needed on (i) the factors controlling atmospheric CCN production and (ii) the properties of both primary and secondary CCN and their interconnections. In future investigations, more emphasis should be put on combining field measurements with regional and large-scale model studies.

[1] We use a global chemical transport model with size-resolved aerosol microphysics to investigate the sources of cloud condensation nuclei (CCN) in the Southern Hemisphere remote marine boundary layer (MBL). Long-term observations of CCN number at Cape Grim (40°41 0 S, 144°41 0 E) show a clear seasonal cycle with a 2-3 times higher concentration in summer than in winter, which has been attributed to seasonal changes in the dimethyl sulfide (DMS) ocean-to-atmosphere flux. We find that this cycle at Cape Grim and also throughout the 30°-45°S latitude band is caused mostly by changes in the regional-scale DMS ocean water concentration. In this latitude band, DMS emissions increase the simulated CCN concentrations from November to April, with a maximum effect of 46% in January (calculated at 0.23% supersaturation). Farther south, the impact of DMS on CCN is apparent only from December to February and increases the CCN concentration at most by 18% at 45°-60°S and by 40% at 60°-75°S. These modelderived contributions of DMS to Southern Ocean summertime CCN are smaller than the 80% derived from correlations between satellite-observed chlorophyll and column CCN, which we explain in terms of nonlinear behavior of CCN from the free troposphere (FT). We show that the main microphysical pathway of DMS influence on CCN number is nucleation of DMS-derived H 2 SO 4 in the FT and subsequent growth of formed particles by condensation and coagulation during entrainment into the MBL. Our simulations suggest that >90% of the increase in MBL CCN when DMS is added to the model is formed in this way. The growth of ultrafine sea spray particles to CCN sizes due to condensation of DMS-derived H 2 SO 4 in the MBL affects the simulated CCN concentrations by less than 6%. Overall, entrainment of nucleated sulfate aerosol into the MBL from the FT accounts for 43-65% of the summer zonal mean CCN concentrations but only 7-20% of the winter CCN over the Southern Hemisphere oceans.

Abstract. Gaseous sulphuric acid is a key precursor for new particle formation in the atmosphere. Previous experimental studies have confirmed a strong correlation between the number concentrations of freshly formed particles and the ambient concentrations of sulphuric acid. This study evaluates a body of experimental gas phase sulphuric acid concentrations, as measured by Chemical Ionization Mass Spectrometry (CIMS) during six intensive measurement campaigns and one long-term observational period. The campaign datasets were measured in Hyytiälä,

Abstract.A new sea spray source function (SSSF), termed Oceanflux Sea Spray Aerosol or OSSA, was derived based on in-situ sea spray aerosol measurements along with meteorological/physical parameters. Submicron sea spray aerosol fluxes derived from particle number concentration measurements at the Mace Head coastal station, on the west coast of Ireland, were used together with open-ocean eddy correlation flux measurements from the Eastern Atlantic Sea Spray, Gas Flux, and Whitecap (SEASAW) project cruise. In the overlapping size range, the data for Mace Head and SEASAW were found to be in a good agreement, which allowed deriving the new SSSF from the combined dataset spanning the dry diameter range from 15 nm to 6 µm. The OSSA source function has been parameterized in terms of five lognormal modes and the Reynolds number instead of the more commonly used wind speed, thereby encapsulating important influences of wave height, wind history, friction velocity, and viscosity. This formulation accounts for the different flux relationships associated with rising and waning wind speeds since these are included in the Reynolds number. Furthermore, the Reynolds number incorporates the kinematic viscosity of water, thus the SSSF inherently includes dependences on sea surface temperature and salinity. The temperature dependence of the resulting SSSF is similar to that of other in-situ derived source functions and results in lower production fluxes for cold waters and enhanced fluxes from warm waters as compared with SSSF formulations that do not include temperature effects.