[1] Bacteria cultivated from aerosol and cloud water samples collected at a remote Austrian mountain site under wintry conditions were tested for their ability to act as cloud condensation nuclei (CCN). The experiment was carried out with a cloud condensation nuclei counter (CCNC) operating on the principle of a static thermal diffusion chamber. Average concentrations of cultivable airborne bacteria amounted to 8 colony forming units (CFU) m À3 in aerosol samples and to 79 CFU mL À1 in cloud water. The set of tested bacteria comprised Gram positive and Gram negative but no known ice nucleating species. At supersaturations between 0.07 and 0.11% all types of bacteria were activated as CCN. As the sizes of the bacteria were smaller than the Kelvin diameters for the respective supersaturations, the physico-chemical properties of their outer cell walls must have enhanced their CCN activity.

We investigated the interactions of air and snow over one entire winter accumulation period as well as the importance of chemical markers in a pristine free-tropospheric environment to explain variation in a microbiological dataset. To overcome the limitations of short term bioaerosol sampling, we sampled the atmosphere continuously onto quartzfiber air filters using a DIGITEL high volume PM10 sampler. The bacterial and fungal communities, sequenced using Illumina MiSeq, as well as the chemical components of the atmosphere were compared to those of a late season snow profile. Results reveal strong dynamics in the composition of bacterial and fungal communities in air and snow. In fall the two compartments were similar, suggesting a strong interaction between them. The overlap diminished as the season progressed due to an evolution within the snowpack throughout winter and spring. Certain bacterial and fungal genera were only detected in air samples, which implies that a distinct air microbiome might exist. These organisms are likely not incorporated in clouds and thus not precipitated or scavenged in snow. Although snow appears to be seeded by the atmosphere, both air and snow showed differing bacterial and fungal communities and chemical composition. Season and alpha diversity were major drivers for microbial variability in snow and air, and only a few chemical markers were identified as important in explaining microbial diversity. Air microbial community variation was more related to chemical markers than snow microbial composition. For air microbial communities Cl − , TC/OC, SO 4 2− , Mg 2+ , and Fe/Al, all compounds related to dust or anthropogenic activities, were identified as related to bacterial variability while dust related Ca 2+ was significant in snow. The only common driver for snow and air was SO 4 2− , a tracer for anthropogenic sources. The occurrence of chemical compounds was coupled with boundary layer injections in the free troposphere (FT). Boundary layer injections also caused the observed variations in community composition and chemistry between the two compartments. Long-term monitoring is required for a more valid insight in post-depositional selection in snow.

Abstract. Aerosol particles are essential constituents of the Earth's atmosphere, impacting the earth radiation balance directly by scattering and absorbing solar radiation, and indirectly by acting as cloud condensation nuclei. In contrast to most greenhouse gases, aerosol particles have short atmospheric residence times, resulting in a highly heterogeneous distribution in space and time. There is a clear need to document this variability at regional scale through observations involving, in particular, the in situ near-surface segment of the atmospheric observation system. This paper will provide the widest effort so far to document variability of climate-relevant in situ aerosol properties (namely wavelength dependent particle light scattering and absorption coefficients, particle number concentration and particle number size distribution) from all sites connected to the Global Atmosphere Watch network. High-quality data from almost 90 stations worldwide have been collected and controlled for quality and are reported for a reference year in 2017, providing a very extended and robust view of the variability of these variables worldwide. The range of variability observed worldwide for light scattering and absorption coefficients, single-scattering albedo, and particle number concentration are presented together with preliminary information on their long-term trends and comparison with model simulation for the different stations. The scope of the present paper is also to provide the necessary suite of information, including data provision procedures, quality control and analysis, data policy, and usage of the ground-based aerosol measurement network. It delivers to users of the World Data Centre on Aerosol, the required confidence in data products in the form of a fully characterized value chain, including uncertainty estimation and requirements for contributing to the global climate monitoring system.

A three-week long field campaign was carried out under autumnal meteorological conditions at four valley floor sites in the industrialised Swiss Rhone Valley. For one week of stable meteorological conditions, particulate matter with an aerodynamic diameter below 10 &mu;m (PM₁₀) was analysed from daily filters using ion chromatography, X-ray fluorescence, anhydrosugars and radiocarbon analysis of the organic and elemental matter (OM and EM, respectively). Furthermore, PM₁ composition along the whole campaign was monitored in Massongex (a site near industries) by a seven-wavelength aethalometer and a quadrupole aerosol mass spectrometer (Q-AMS). At all sites, PM₁₀ secondary inorganics and non-fossil EM and OM exhibited relatively stable concentrations over the selected days. On the contrary, PM₁₀ fossil carbonaceous fractions, mineral dust components and several trace elements showed a significant decrease on Sunday, compared to the analysed working days. Their concentrations were also highly correlated. This evidenced the role of exhaust and resuspension emissions by heavy-duty vehicle traffic to the PM₁₀ concentrations along the valley. <br><br> In Massongex, organic matter and black carbon (BC) were the main contributors to PM₁ over the campaign (accounting for 45% and 18% of PM₁, respectively). An optical discrimination of BC highlighted the prevalence of fossil over wood-burning sources. Three types of PM₁ organics could be identified by factor analysis: primary wood-burning organic aerosol (P-WBOA) dominated the PM₁ carbonaceous fraction, followed by oxygenated organics (OOA) mostly representing secondary organics, and by traffic or possibly industry-related hydrocarbon-like organics (HOA) as the smallest carbonaceous contribution. <br><br> Furthermore, unusually high contributions of fine chloride were detected at all sites. They were attributed to ammonium chloride (NH₄Cl) in Massongex and represented the only significant component exclusively attributable to industrial emissions

[1] During an intensive field campaign at a midlevel (1640 m above sea level) mountain in central Europe, cloud water samples were collected using a cloud water sampler and subsequently analyzed for organic and inorganic ions, total carbon (TC), and black carbon (BC). Cloud water surface tension (s) was measured using the ring method. Generally, the cloud water samples had surface tensions lower than that of pure water. The average value of s was 95.2% of the s of water, while the lowest value was 83.3%. A trend of lower s with higher concentrations of organic material (OC, obtained as TC-BC) was found at lower OC concentrations than those previously described [Charlson et al., 2001;Facchini et al., 1999a]. The effect of the lower s on cloud droplet activation was modeled using Köhler theory. For an assumed dry sulphate nucleus with 0.05 mm diameter, the critical supersaturation would be decreased by 7% (average s) and by 22% for minimum s.