Abstract. Droplet formation provides a direct microphysical link between aerosols and clouds (liquid or mixed phase), and its adequate description poses a major challenge for any atmospheric model. Observations are critical for evaluating and constraining the process. Towards this, aerosol size distributions, cloud condensation nuclei, hygroscopicity and lidar-derived vertical velocities were observed in Alpine mixed-phase clouds during the Role of Aerosols and Clouds Enhanced by Topography on Snow (RACLETS) field campaign in the Davos, Switzerland region during February and March 2019. Data from the mountain-top site of Weissfluhjoch (WFJ) and the valley site of Davos Wolfgang are studied. These observations are coupled with a state-of-the art droplet activation parameterization to investigate the aerosol-cloud droplet link in mixed-phase clouds. The mean CCN-derived hygroscopicity parameter, κ, at WFJ ranges between 0.2–0.3, consistent with expectations for continental aerosol. κ tends to decrease with size, possibly from an enrichment in organic material associated with the vertical transport of fresh ultrafine particle emissions (likely from biomass burning) from the valley floor in Davos. The parameterization provides droplet number that agrees with observations to within ~25 %. We also find that the susceptibility of droplet formation to aerosol concentration and vertical velocity variations can be appropriately described as a function of the standard deviation of the distribution of updraft velocities, σw, as the droplet number never exceeds a characteristic limit, termed limiting droplet number, of ~150–550 cm−3, which depends solely on σw. We also show that high aerosol levels in the valley, most likely from anthropogenic activities, increase cloud droplet number, reduce cloud supersaturation (<0.1 %) and shift the clouds to a state that is less susceptible to aerosol and become very sensitive to vertical velocity variations. The transition from aerosol to velocity-limited regime depends on the ratio of cloud droplet number to the limiting droplet number, as droplet formation becomes velocity-limited when this ratio exceeds 0.5. Under such conditions, droplet size tends to be minimal, reducing the likelihood that large drops are present that promote glaciation through rime splintering and droplet shattering. Identifying regimes where droplet number variability is dominated by dynamical – rather than aerosol – changes is key for interpreting and constraining when and which types of aerosol effects on clouds are active.

&lt;p&gt;&lt;strong&gt;Abstract.&lt;/strong&gt; A significant fraction of atmospheric particles that serve as cloud condensation nuclei (CCN), and furthermore as cloud droplets are thought to originate from the condensational growth of new particles formed from the gas phase. Here, particle number size distributions (&lt;&amp;#8201;850&amp;#8201;nm), aerosol chemical composition and meteorological parameters were studied during 7 years of continuous measurements (June 2008 to May 2015) at a remote background site of the eastern Mediterranean. 162 NPF episodes were recorded and analyzed to assess the impact of NPF on CCN and cloud droplet number concentration (CDNC) formation. A new metric is introduced to quantitatively determine the initiation and duration of the influence of NPF on the CCN spectrum. Annually, NPF days were found to increase CCN concentrations between 40 and 50&amp;#8201;% in the 0.2&amp;#8211;1.0&amp;#8201;% supersaturation range. CCN perturbations from NPF are found to occur in the afternoon, relatively later in the winter and autumn than in the summer. Introducing the observed aerosol size distributions together with chemical composition into an established cloud droplet parameterization showed that the supersaturations that develop however are much lower (below 0.1&amp;#8201;%) for typical boundary layer dynamics (width of the vertical velocity distribution ~&amp;#8201;0.3&amp;#8201;m&amp;#8201;s&lt;sup&gt;&amp;#8722;1&lt;/sup&gt;) and NPF is found to enhance CDNC by 7 to 12.5&amp;#8201;%. This considerable contrast between CCN and CDNC response is in part from the different supersaturation levels considered, but also because supersaturation drops from increasing CCN because of water vapor competition effects. The low cloud supersaturation further delays the appearance of NPF impacts on CDNC to clouds formed in the late evening and nighttime &amp;#8211; which carries important implications for the extend and types of indirect effects induced by NPF events. An analysis based on CCN concentrations using prescribed supersaturation can provide much different, and even misleading, conclusions and should be avoided. The proposed approach here offers a simple, yet highly effective way for a more realistic impact assessment of NPF events on cloud formation.&lt;/p&gt;

23The volatility distribution of the organic aerosol (OA) and its sources during the Southern 24Oxidant and Aerosol Study (SOAS; Centerville, Alabama) was constrained using 25 measurements from an Aerodyne High-Resolution Time-of-Flight Aerosol Mass 26Spectrometer (HR-ToF-AMS) and a thermodenuder. Positive Matrix Factorization (PMF) 27 analysis was applied on both the ambient and thermodenuded high resolution mass 28 spectra, leading to four factors: more oxidized oxygenated OA (MO-OOA), less oxidized 29 oxygenated OA (LO-OOA), an isoprene epoxydiols (IEPOX) related factor (Isoprene-30 OA) and biomass burning OA (BBOA). BBOA had the highest mass fraction remaining 31Atmos. Chem. Phys. Discuss., https://doi.org/10.5194/acp-2017-1010 Manuscript under review for journal Atmos. Chem. Phys. Introduction 46Population exposure to atmospheric particulate matter (PM) increases premature 47 mortality from cardiovascular and respiratory diseases (Pope et al., 2002; IARC, 2013; 48 Cohen et al., 2017). The same particles also modulate the planetary radiative balance and 49 hydrological cycle (IPCC, 2013; NASEM, 2016;Seinfeld et al., 2016). Organic aerosol 50 (OA) constitutes a significant part of submicron aerosol mass (Zhang et al., 2007) and it 51 is characterized by daunting chemical complexity (Kanakidou et al., 2005; Hallquist et al., 52 2009). OA is directly emitted from anthropogenic and natural sources, but it is also 53 produced by condensation of products formed during the oxidation of gas-phase organic 54 compounds with O3, NO3 and OH radicals (secondary organic aerosol, SOA; Kanakidou 55 et al., 2005). OA formation can be further promoted by the interactions of anthropogenic 56 and biogenic compounds; in the southeastern United States, anthropogenic sulfate 57 enhances OA formation through rapid reactive uptake of IEPOX to particles and aqueous 58 phase reactions (Xu et al., 2015a;Xu et al., 2016a; Budisulistiorini et al., 2017). 59Several approaches have been developed to unravel the sources and the degree of 60 atmospheric processing of aerosol sampled by the AMS. These include custom principal 61 component analysis (Zhang et al., 2005), multiple component analysis (Zhang et al., 62 Atmos. Chem. Phys. Discuss., https://doi

&lt;p&gt;&lt;strong&gt;Abstract.&lt;/strong&gt; Atmospheric New Particle Formation (NPF) is a common phenomenon all over the world. In this study we present the longest time series of NPF records in the eastern Mediterranean region by analyzing seven years of aerosol number size distribution data obtained with a mobility particle sizer. The measurements were performed at the Finokalia environmental research station on Crete, Greece during the period June 2008&amp;#8211;June 2015. We found that NPF took place 29&amp;#8201;% of the available days, undefined days were 26&amp;#8201;% and non-event days 45&amp;#8201;%. NPF is more frequent in April and May probably due to the biogenic activity and is less frequent in August and November. The NPF frequency increased during the measurement period, while particle growth rates showed a decreasing trend, indicating possible changes in the ambient sulfur dioxide concentrations in the area. Throughout the period under study, we frequently observed production of particles in the nucleation mode during night-time, a feature rarely observed in the ambient atmosphere. Nucleation mode particles had the highest concentration in winter, mainly because of the minimum sinks, and their average contribution to the total particle number concentration was 9&amp;#8201;%. Nucleation mode particle concentrations were low outside periods of active NPF and growth, so there are hardly any other local sources of sub-25&amp;#8201;nm particles. Additional atmospheric ion size distribution data simultaneously collected for more than two years period were also analyzed. Classification of NPF events based on ion measurements differed from the corresponding classification based on mobility spectrometer measurements, possibly indicating a different representation of local and regional NPF events between these two measurement data sets. We used MALTE-box model for a simulation case study of NPF in the eastern Mediterranean region. Monoterpenes contributing to NPF can explain a large fraction of the observed NPF events according to our model simulations. However the parametrization that resulted after sensitivity tests was significantly different from the one applied for the boreal environment.&lt;/p&gt;

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