Formation and growth of atmospheric nanoparticles in the eastern Mediterranean: results from long-term measurements and process simulations

Abstract

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 10 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-June 2018. We found that NPF took place on 27 % of the available days, undefined days were 23 % and non-event days 50 %. NPF is more frequent in April and May probably due to the terrestrial biogenic activity and is less frequent in August. Throughout the period under study, nucleation was observed also during the night. Nucleation mode particles had the highest concentration in winter and early spring, mainly because of the minimum sinks, and their average contribution to the total particle number concentration was 8 %. Nucleation mode particle concentra-tions were low outside periods of active NPF and growth, so there are hardly any other local sources of sub-25 nm particles. Additional atmospheric ion size distribution data simultaneously collected for more than 2 years were also analyzed. Classification of NPF events based on ion spectrometer measurements differed from the corresponding classification based on a mobility spectrometer, possibly indicating a different representation of local and regional NPF events between these two measurement data sets. We used the MALTE-Box model for simulating a 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 adjusted parameterization resulting from our sensitivity tests was significantly different from the initial one that had been determined for the boreal environment.Published by Copernicus Publications on behalf of the European Geosciences Union.

Regional new particle formation as modulators of cloud condensation nuclei and cloud droplet number in the eastern Mediterranean

“…For the conversion of organic carbon to matter needed for the application of Köhler theory and the calculation of the organics volume fraction, a ratio of OM/OC of 2.1 was used, based on other studies from this site (Sciare et al, 2005;Hildebrandt et al, 2010). Any CCN prediction uncertainty from using bulk, daily chemical composition is further reduced when used to compute droplet number (e.g., Sotiropoulou et al, 2007;Kalkavouras et al, 2017).…”

“…In our case, where past experience has shown that the composition displays remarkably consistent behavior (Bougiatioti et al, 2009(Bougiatioti et al, , 2011, the successful CCN closure shows that the used approach is indeed sufficient in calculating effectively the d c and not using a prescribed value. CCN number concentrations are then taken as being equal to the concentration of particles with a diameter above d c (Kalkavouras et al, 2017). The aerosol hygroscopicity parameter, κ, is calculated assuming that it is a mixture of an organic and inorganic component with volume fraction ε org , ε inorg and characteristic hygroscopicity κ org , κ inorg , respectively (κ = ε inorg κ inorg + ε org κ org ).…”

“…Furthermore, NPF can be triggered by the rapid dimethylsulfide (DMS) oxidation above clouds (Bates et al, 1987;Kreidenweis et al, 1991;Katoshevski et al, 1999) and cloud outflow regions associated with convection (e.g., Hermann et al, 2003). NPF within marine boundary layers can strongly affect CCN number concentrations at all cloud-relevant supersaturations (e.g., Kalivitis et al, 2015;Kalkavouras et al, 2017;Debevec et al, 2018). When these small particles, however, are mixed within the boundary layer, they may subsequently grow to CCN-relevant sizes, or even act as CCN in strongly convective clouds (Fan et al, 2013;Wang et al, 2016).…”

“…Using cloud droplet parameterizations to interpret observed aerosol size distribution data, however, may allow one to address this issue in a simple yet effective manner. Kalkavouras et al (2017) illustrated this issue by using a "conventional" approach to quantify CCN enhancement, using the critical diameter (d c ) at which all particles act as CCN depending on observed composition and a prescribed supersaturation. They reported much higher CCN number enhancement (∼ 87 %) for two sites in the eastern Mediterranean (Santorini and Finokalia) than in cloud droplet number concentration, N d , (∼ 12 %) during two consecutive NPF episodes in summer.…”

“…The current study follows up on the initial work of Kalkavouras et al (2017) and quantifies the impact of NPF on CCN levels and cloud droplet number concentrations in the eastern Mediterranean atmosphere over 7 years of field measurements (June 2008 to May 2015) of aerosol number size distributions and chemical composition. From this data, we aim to (i) quantify the seasonality and contribution of atmospheric NPF to the production of newly CCN in the eastern Mediterranean marine atmosphere, (ii) determine the timing properties of newly formed particles from the beginning of NPF events (i.e., starting time (t start ) and duration) throughout their activation into cloud droplets and their relative contribution to the CCN budget, and (iii) investigate the NPF impacts on CDNC (N d ) and on maximum supersaturation (s max ) formed in clouds in the vicinity of Finokalia.…”