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One year of ground-based night-time Raman lidar observations has been analysed under the Optimization of Aerosol Seeding In rain enhancement Strategies (OASIS) project, in order to characterize the aerosol particle properties over a rural site in the United Arab Emirates. In total, 1130 aerosol particle layers were detected during the 1-year measurement campaign which took place between March 2018 and February 2019. Several subsequent aerosol layers could be observed simultaneously in the atmosphere up to 11 km. The observations indicate that the measurement site is a receptor of frequent dust events, but predominantly the dust is mixed with aerosols of anthropogenic and/or marine origin. The mean aerosol optical depth over the measurement site ranged at 0.37±0.12 and 0.21±0.11 for 355 and 532 nm, respectively. Moreover, mean lidar ratios of 43±11 sr at a wavelength of 355 nm and 39±10 sr at 532 nm were found. The average linear particle depolarization ratio measured over the course of the campaign was 15±6% and 19±7% at the 355 and 532 nm wavelengths, respectively. Since the region is both a source and a receptor of mineral dust, we have also explored the properties of Arabian mineral dust of the greater area of the United Arab of Emirates and the Arabian Peninsula. The observed Arabian dust particle properties were 45±5 (42±5) sr at 355 (532) nm for the lidar ratio, 25±2% (31±2 %) for the linear particle depolarization ratio at 355 (532) nm, and 0.3±0.2 (0.2±0.2) for the extinction-related Angstrom exponent (backscatterrelated Angstrom exponent) between 355 and 532 nm. This study is the first to report comprehensive optical properties of the Arabian dust particles based on 1-year long observations, using to their fullest the capabilities of a multi-wavelength Raman lidar instrument. The results suggest that the mineral dust properties over the Middle East and western Asia, including the observation site, are comparable to those of African mineral dust with regard to the particle depolarization ratios, but not for lidar ratios. The smaller lidar ratio values in this study compared to the reference studies are attributed to the difference in the geochemical characteristics of the soil originating in the study region compared to northern Africa. © 2020 Royal Society of Chemistry. All rights reserved.

SSCI-VIDE+ATARI+BNO:LFI:CFE:DSG; International audience; The importance of the surface tension term of the Köhler equation in the activation of sub-micron particles into cloud droplets in the atmosphere has been evidenced by several works. But information on the compounds actually lowering the surface tension sub-micron particles (= “surfactants”) in the atmosphere remains scarce. This presentation reports the characterization of the surfactants present in PM1 aerosols in different regions of Europe: Lyon, France, Rogoznica, Croatia, and Pallas, Finland: concentrations in the air and in the particle volume, adsorption isotherms and Critical Micelle Concentration (CMC). At the remote site of Pallas, Finland, the surfactant concentrations and their correlations with the PM1 population and with other PM1 chemical components (inorganic ions) indicated that these compounds were present in the entire particle size distribution, not only in a few large particles, as it has been hypothesized. At all sites, and in nearly all samples, the CMC obtained from the surfactant adsorption isotherms was low (≤ mM) so that, combined with the observed concentrations, the surfactants found in these PM1 samples should strongly depress the surface tension of the sub-micron particles until activation (Growth Factor = 5 - 10), thus enhance their CCN ability.In Pallas, the potential role of surfactants on cloud formation was also explored by performing statistical analyses between the surfactants properties in PM1 aerosols and cloud occurrence over 9 months (237 aerosol samples and 257 cloud events). The results will be presented.

This study presents analysis of in situ measurements conducted over the period 2005–2014 in the Indian Himalayas to give a thorough overview of the factors and causes that drive aerosol properties. Aerosol extensive properties (namely, particle number concentration, scattering coefficient, equivalent black carbon, PM2.5, and PM10) have 1.5–2 times higher values in the early to late afternoon than during the night, and a strong seasonality. The interannual variability is ±20% for both PM2.5 and total particle number concentration. Analysis of the data shows statistically significant decreasing trends of −2.3 μg m−3 year−1 and −2.7 μg m−3 year−1 for PM2.5 and PM10, respectively, over the study period. The mountainous terrain site (Mukteshwar, MUK) is primarily under the influence of air from the plains. This is due to convective transport processes that are enhanced by local and mesoscale topography, leading to pronounced valley/mountain winds and consequently to atmospheric boundary layer air lifting from the plains below. The transport from plains is evident in seasonal‐diurnal patterns observed at MUK. The timing of the patterns corresponds with changes in turbulence and water vapor (q). According to our analysis, using these as proxies is a viable method for examining boundary layer influence in the absence of direct atmospheric boundary layer height measurements. Comparing the measurements with climate models shows that even regional climate models have problems capturing the orographic influence accurately at MUK, highlighting the importance of long‐term direct measurements at multiple points to understand aerosol behavior in mountainous areas.

The authors regret that the grant number in the acknowledgement section is incorrect and it should be as follows: I-122-430. The authors would like to apologise for any inconvenience caused.

The northern Eurasian regions and Arctic Ocean will very likely undergo substantial changes during the next decades. The Arctic-boreal natural environments play a crucial role in the global climate via albedo change, carbon sources and sinks as well as atmospheric aerosol production from biogenic volatile organic compounds. Furthermore, it is expected that global trade activities, demographic movement, and use of natural resources will be increasing in the Arctic regions. There is a need for a novel research approach, which not only identifies and tackles the relevant multi-disciplinary research questions, but also is able to make a holistic system analysis of the expected feedbacks. In this paper, we introduce the research agenda of the Pan-Eurasian Experiment (PEEX), a multi-scale, multi-disciplinary and international program started in 2012 (https://www.atm.helsinki.fi/peex/). PEEX sets a research approach by which large-scale research topics are investigated from a system perspective and which aims to fill the key gaps in our understanding of the feedbacks and interactions between the land-Atmosphere-Aquatic-society continuum in the northern Eurasian region. We introduce here the state of the art for the key topics in the PEEX research agenda and present the future prospects of the research, which we see relevant in this context.

SSCI-VIDE+ATARI+VGE:BNO:LFI; International audience; Clouds are essential components of the Earth’s hydrological system and the main cooling factor of the climate budget. But some aspects of their formation are still not completely understood, in particular the role of surfactants (Boucher et al., 2013). Although Köhler theory predicts that surfactants should enhance cloud droplet activation, current models consider their role to be negligible (Farmer et al., 2015), and the surface tension of growing droplets is systematically assumed to be that of pure water.Recent studies are starting to bring the evidence of the contrary, especially in laboratory experiments (Asa-Awuku et al., 2008; Ruehl et al., 2012; Giordano et al., 2013: Ruehl et al., 2014; 2016). In addition, the presence of strong surfactants, corresponding to surface tensions below 40 mN m-1, has been evidenced in atmospheric aerosols from many different regions (Ekström et al., 2010; Baduel et al., 2012). In a recent study, the concentration of surfactants in PM2.5 aerosols from a coastal region was found to be large enough to decrease the surface tension of the droplets until they reach activation (Gérard et al., 2016).It is thus now important to look for evidence of the role of surfactants on cloud droplet formation directly in the atmosphere (Nozière, 2016). For this, the surfactants present in PM1 aerosols at the GAW station Sammaltunturi, in Northern Finland (67°58'N 24°07'E, 565 m asl), a site directly impacted by the formation of orographic clouds (Lihavainen et al., 2008; Anttila et al., 2012), were extracted and analyzed from April to November 2015. The PM1 aerosols were collected with a frequency of 24 hours on quartz-filters. The total surfactant fraction was extracted with a double extraction method (Gérard et al. 2016): a water extraction followed by a Solid Phase Extraction (SPE) removing interferents and concentrating the surfactants. The surface tension curves of the aerosol surfactants were determined with the pendant drop technique and their absolute concentrations by a colorimetric method (complexation with a dye, organic phase extraction, and UV-vis spectrophotometry detection).In total, samples were collected over 223 days and included 70 cloud events, representing 25 % of the sampling time. The evolution of the surfactant concentration and properties, and of the surface tension of the PM1 aerosols during this campaign will be discussed and compared with meteorological parameters and other data measured simultaneously at the site.This work was supported by the French Agence Nationale de la Recherche (ANR) and US National Science Foundation (NSF) on a joint ANR-NSF project (SONATA). ReferencesAnttila, T. et al. (2012) Atmos. Chem. Phys., 12, 11435Asa-Awuku, A. et al. (2008) Atmos. Chem. Phys., 8, 799Baduel, C. et al. (2012) Atmos. Environ., 47, 413Boucher, O. et al. (2013) Clouds and Aerosols. In Climate Change 2013: The Physical Science Basis., Cambridge University Press.Ekström, S. et al. (2010) Biogeosciences, 7, 387Farmer, D. K. et al. (2015) Chem. Rev., 115, 4199Gérard, V. et al. (2016) Environ. Sci. Technol., in pressGiordano, M. R. et al. (2013) Environ. Sci. Technol., 47, 10980 Lihavainen, H. et al. (2008) Atmos. Chem. Phys., 8, 6925Nozière, B. (2016) Science, in pressRuehl, C. R. et al. (2012) Geophys. Res. Lett., 39, L23801Ruehl, C. R. et al. (2014) J. Phys. Chem. A, 118, 3952Ruehl C. R. et al. (2016) Science, in press

In this study we introduce a new flow tube suitable for binary and ternary homogeneous nucleation studies. The production of sulfuric acid and water vapor mixture, the experimental setup and the method of sulfuric acid concentration determination are discussed in detail. Wall losses were estimated from the measured sulfuric acid concentration profiles along the flow tube and compared to a theoretical prediction. In this investigation the experimental evidence of new particle formation was observed at a concentration of 109 molecules cm−3 of sulfuric acid and the nucleation rates measured at three relative humidities (RH) 10, 30 and 50%, cover six orders of magnitude, from 10−3 to 103 particles cm−3. Particle free air was used as a carrier gas. Our initial results are compared to the theoretical prediction of binary homogeneous nucleation, to results obtained by other investigators, and to atmospheric nucleation.

The Finnish Meteorological Institute, in collaboration with the University of Helsinki, has established a new ground-based remote-sensing network in Finland. The network consists of five topographically, ecologically and climatically different sites distributed from southern to northern Finland. The main goal of the network is to monitor air pollution and boundary layer properties in near real time, with a Doppler lidar and ceilometer at each site. In addition to these operational tasks, two sites are members of the Aerosols, Clouds and Trace gases Research InfraStructure Network (ACTRIS); a Ka band cloud radar at Sodankylä will provide cloud retrievals within CloudNet, and a multi-wavelength Raman lidar, PollyXT (POrtabLe Lidar sYstem eXTended), in Kuopio provides optical and microphysical aerosol properties through EARLINET (the European Aerosol Research Lidar Network). Three C-band weather radars are located in the Helsinki metropolitan area and are deployed for operational and research applications. We performed two inter-comparison campaigns to investigate the Doppler lidar performance, compare the backscatter signal and wind profiles, and to optimize the lidar sensitivity through adjusting the telescope focus length and data-integration time to ensure sufficient signal-to-noise ratio (SNR) in low-aerosol-content environments. In terms of statistical characterization, the wind-profile comparison showed good agreement between different lidars. Initially, there was a discrepancy in the SNR and attenuated backscatter coefficient profiles which arose from an incorrectly reported telescope focus setting from one instrument, together with the need to calibrate. After diagnosing the true telescope focus length, calculating a new attenuated backscatter coefficient profile with the new telescope function and taking into account calibration, the resulting attenuated backscatter profiles all showed good agreement with each other. It was thought that harsh Finnish winters could pose problems, but, due to the built-in heating systems, low ambient temperatures had no, or only a minor, impact on the lidar operation – including scanning-head motion. However, accumulation of snow and ice on the lens has been observed, which can lead to the formation of a water/ice layer thus attenuating the signal inconsistently. Thus, care must be taken to ensure continuous snow removal.

We have analysed the trends of total aerosol particle number concentrations (N) measured at long-term measurement stations involved either in the Global Atmosphere Watch (GAW) and/or EU infrastructure project ACTRIS. The sites are located in Europe, North America, Antarctica, and on Pacific Ocean islands. The majority of the sites showed clear decreasing trends both in the full-length time series, and in the intra-site comparison period of 2001–2010, especially during the winter months. Several potential driving processes for the observed trends were studied, and even though there are some similarities between N trends and air temperature changes, the most likely cause of many northern hemisphere trends was found to be decreases in the anthropogenic emissions of primary particles, SO2 or some coemitted species. We could not find a consistent agreement between the trends of N and particle optical properties in the few stations with long time series of all of these properties. The trends of N and the proxies for cloud condensation nuclei (CCN) were generally consistent in the few European stations where the measurements were available. This work provides a useful comparison analysis for modelling studies of trends in aerosol number concentrations.