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Primary biological aerosol particles (PBAP) play an important role in the climate system, facilitating the formation of ice within clouds, consequently PBAP may be important in understanding the rapidly changing Arctic. Within this work, we use single-particle fluorescence spectroscopy to identify and quantify PBAP at an Arctic mountain site, with transmission electronic microscopy analysis supporting the presence of PBAP. We find that PBAP concentrations range between 10−3–10−1 L−1 and peak in summer. Evidences suggest that the terrestrial Arctic biosphere is an important regional source of PBAP, given the high correlation to air temperature, surface albedo, surface vegetation and PBAP tracers. PBAP clearly correlate with high-temperature ice nucleating particles (INP) (>-15 °C), of which a high a fraction (>90%) are proteinaceous in summer, implying biological origin. These findings will contribute to an improved understanding of sources and characteristics of Arctic PBAP and their links to INP.

Ice-nucleating particles (INPs) initiate ice formation in supercooled clouds, typically starting in western Europe at a few kilometres above the ground. However, little is known about the concentration and composition of INPs in the lower free troposphere (FT). Here, we analysed INPs active at −10 ∘C (INP−10) and −15 ∘C (INP−15) that were collected under FT conditions at the high-altitude observatory Jungfraujoch between January 2019 and March 2021. We relied on continuous radon measurements to distinguish FT conditions from those influenced by the planetary boundary layer. Median concentrations in the FT were 2.4 INP−10 m−3 and 9.8 INP−15 m−3, with a multiplicative standard deviation of 2.0 and 1.6 respectively. A majority of INPs were deactivated after exposure to 60 ∘C; thus, they probably originated from certain epiphytic bacteria or fungi. Subsequent heating to 95 ∘C deactivated another 15 % to 20 % of the initial INPs, which were likely other types of fungal INPs that might have been associated with soil organic matter or with decaying leaves. Very few INP−10 withstood heating to 95 ∘C, but on average 20 % of INP−15 in FT samples did so. This percentage doubled during Saharan dust intrusions, which had practically no influence on INP−10. Overall, the results suggest that aerosolised epiphytic microorganisms, or parts thereof, are responsible for the majority of primary ice formation in moderately supercooled clouds above western Europe.

Soil dust is a major driver of ice nucleation in clouds leading to precipitation. It consists largely of mineral particles with a small fraction of organic matter constituted mainly of remains of micro-organisms that participated in degrading plant debris before their own decay. Some micro-organisms have been shown to be much better ice nuclei than the most efficient soil mineral. Yet, current aerosol schemes in global climate models do not consider a difference between soil dust and mineral dust in terms of ice nucleation activity. Here, we show that particles from the clay and silt size fraction of four different soils naturally associated with 0.7 to 11.8 % organic carbon (w/w) can have up to four orders of magnitude more ice nucleation sites per unit mass active in the immersion freezing mode at −12 °C than montmorillonite, the nucleation properties of which are often used to represent those of mineral dusts in modelling studies. Most of this activity was lost after heat treatment. Removal of biological residues reduced ice nucleation activity to, or below that of montmorillonite. Desert soils, inherently low in organic content, are a large natural source of dust in the atmosphere. In contrast, agricultural land use is concentrated on fertile soils with much larger organic matter contents than found in deserts. It is currently estimated that the contribution of agricultural soils to the global dust burden is less than 20 %. Yet, these disturbed soils can contribute ice nuclei to the atmosphere of a very different and much more potent kind than mineral dusts.

We investigated Hg accumulation in maize (Zea mays) plants grown in non-contaminated conditions on a farm in Switzerland throughout a growing season. Concentrations of Hg in leaves and husk followed the same temporal pattern as the mass growth of these parts. In contrast, silk and tassel accumulated Hg almost linearly over time until harvest. At the end of the growing season Hg concentration was highest in tassel (10.4 ng g−1), followed by leaves (7.3 ng g−1) and silk (5.7 ng g−1). Silk and tassel had accumulated 5–10 times more Hg per unit dry mass than all aboveground parts of the plant on average. Cob and kernels contained only very small amounts of Hg. Greater exposure of a plant part to the atmosphere was clearly associated with higher rates of Hg accumulation.

Ice nucleation in cold clouds is a decisive step in the formation of rain and snow. Observations and modelling suggest that variations in the concentrations of ice nucleating particles (INPs) affect timing, location and amount of precipitation. A quantitative description of the abundance and variability of INPs is crucial to assess and predict their influence on precipitation. Here we used the hydrological indicator δ 18 O to derive the fraction of water vapour lost from precipitating clouds and correlated it with the abundance of INPs in freshly fallen snow. Results show that the number of INPs active at temperatures ≥ −10 °C (INPs −10 ) halves for every 10% of vapour lost through precipitation. Particles of similar size (>0.5 μm) halve in number for only every 20% of vapour lost, suggesting effective microphysical processing of INPs during precipitation. We show that INPs active at moderate supercooling are rapidly depleted by precipitating clouds, limiting their impact on subsequent rainfall development in time and space.

The state of a slightly supercooled ephemeral cloud can be changed by the presence of a few particles capable of catalysing freezing, and potentially result in precipitation. We investigated the atmospheric abundance of particles active as ice nuclei at −8°C (IN −8 ) over the course of a year at the high-alpine station Jungfraujoch (3580 m.a.s.l., Switzerland) through the use of immersion freezing assays of particles collected on quartz micro-fibre filters. In addition, we determined IN −8 on a hill in the planetary boundary layer 95 km northwest of Jungfraujoch and in the dust laden Saharan Air Layer reaching Tenerife. Results indicate a strong seasonality of IN −8 at Jungfraujoch. Values were largest during summer (between 1 and 10 m −3 ) and about two orders of magnitude smaller during winter. Sahara dust events had a negligible influence on IN −8 at Jungfraujoch. Seasonality in the boundary layer was not observed in the upper, but in the lower bound of IN −8 values. Values<1 m −3 were only found on cold winter days, when IN −8 were more likely to have already been activated and deposited than on warmer days. A good correlation between IN −8 and maximum daily temperature at Jungfraujoch (R 2 =0.54) suggests IN −8 abundance at Jungfraujoch may be limited most of the year by microphysical processing related to IN activation in approaching air masses.

One year of radon, benzene and carbon monoxide (CO) concentrations were analysed to characterise the combined influences of variations in traffic density and meteorological conditions on urban air quality in Bern, Switzerland. A recently developed radon-based stability categorisation technique was adapted to account for seasonal changes in day length and reduction in the local radon flux due to snow/ice cover and high soil moisture. Diurnal pollutant cycles were shown to result from an interplay between variations in surface emissions (traffic density), the depth of the nocturnal atmospheric mixing layer (dilution) and local horizontal advection of cleaner air from outside the central urban/industrial area of this small compact inland city. Substantial seasonal differences in the timing and duration of peak pollutant concentrations in the diurnal cycle were attributable to changes in day length and the switching to/from daylight-savings time in relation to traffic patterns. In summer, average peak benzene concentrations (0.62 ppb) occurred in the morning and remained above 0.5 ppb for 2 hours, whereas in winter average peak concentrations (0.85 ppb) occurred in the evening and remained above 0.5 ppb for 9 hours. Under stable conditions in winter, average peak benzene concentrations (1.1 ppb) were 120% higher than for well-mixed conditions (0.5 ppb). By comparison, summertime peak benzene concentrations increased by 53% from well-mixed (0.45 ppb) to stable nocturnal conditions (0.7 ppb). An idealised box model incorporating a simple advection term was used to derive a nocturnal mixing length scale based on radon, and then inverted to simulate diurnal benzene and CO emission variations at the city centre. This method effectively removes the influences of local horizontal advection and stability-related vertical dilution from the emissions signal, enabling a direct comparison with hourly traffic density. With the advection term calibrated appropriately, excellent results were obtained, with high regression coefficients in spring and summer for both benzene (r 2 ~0.90-0.96) and CO (r 2 ~0.88-0.98) in the two highest stability categories. Weaker regressions in winter likely indicate additional contributions from combustion sources unrelated to vehicular emissions. Average vehicular emissions during daylight hours were estimated to be around 0.503 (542) kg km −2 h −1 for benzene (CO) in the Bern city centre.

Atmospheric ice nucleating particles (INPs) contribute to initiate precipitation. In particular, biological INPs act at warmer temperatures than other types of particles (>−10 °C) therefore potentially defining precipitation distribution. Here, in order to identify potential environmental drivers in the distribution and fate of biological INPs in the atmosphere, we conducted a mid-term study of the freezing characteristics of precipitation. A total of 121 samples were collected during a period of >1.5 years at the rural site of Opme (680 m a.s.l. (above sea level), France). INP concentration ranged over two orders of magnitude at a given temperature depending on the sample; there were <1 INPs mL−1 at ≥−5 °C, ~0.1 to 10 mL−1 between −5 °C and −8 °C, and ~1 to 100 mL−1 at colder temperatures. The data support the existence of an intimate natural link between biological INPs and hydrological cycles. In addition, acidification was strongly correlated with a decrease of the freezing characteristics of the samples, suggesting that human activities impact the role of INPs as triggers of precipitation. Water isotope ratio measurements and statistical comparison with aerosol and cloud water data confirmed some extent of INP partitioning in the atmosphere, with the INPs active at the warmest temperatures tending to be more efficiently precipitated.

Ice nucleating particles active at −8 °C or warmer (INP−8) are produced by plants and by microorganisms living from and on them. Laboratory studies have shown that large numbers of INP−8 are produced by decaying leaves. At three widely dispersed locations in Northwestern Eurasia, we saw, from an analysis of PM10 filter samples, that seasonal median concentrations of INP−8 in the boundary layer doubled from summer to autumn. Concentrations of INP−8 increased in autumn soon after the normalized differential vegetation index had started to decrease. Whether the large-scale phenological event of leaf senescence and shedding in autumn has an impact on ice formation in clouds is a justified question.

Plant canopies are an important source of biological particles aerosolized into the atmosphere. Certain aerosolized microorganisms are able to freeze slightly supercooled cloud droplets and therefore affect mixed-phase cloud development. Still, spatiotemporal variability of such biological ice-nucleating particles (INPs) is currently poorly understood. Here, we study this variability between late summer and leaf shedding on the scale of individual leaves collected about fortnightly from four temperate broadleaf tree species (Fagus sylvatica, Juglans regia, Prunus avium and Tilia platyphyllos) on a hillside (Gempen, 650 m a.s.l. (metres above sea level)) and in a vertical canopy profile of one Fagus sylvatica (Hölstein, 550 m a.s.l.) in north-western Switzerland. The cumulative concentration of INPs active at ≥-10 °C (INPs−10) did not vary significantly between the investigated tree species but, as inferred from leaf mass per area and leaf carbon isotopic ratios, seemed to be lower on sun leaves as compared with shade leaves. Between August and mid-November, the median INP concentration increased from 4 to 38 INP−10 cm−2 of leaf area and was positively correlated with mean relative humidity throughout 24 h prior to sampling (Spearman's r=0.52, p<0.0001, n=64). In 53 of the total 64 samples collected at the Gempen site, differential INP spectra between −3 and −10 °C exhibited clearly discriminable patterns: in 53 % of the spectra, the number of additionally activated INPs increased persistently with each 1 °C decrease in temperature; the remaining spectra displayed significant peaks in differential INP concentration above −9 °C, most frequently in the temperature interval between −8 and −9 °C (21 %) and between −7 and −8 °C (17 %). Interestingly, the three most frequent patterns in differential INP spectra on leaves in Gempen were also prevalent in similar fractions in air samples with clearly discriminable patterns at the high-altitude Jungfraujoch site (3580 m a.s.l., Switzerland) collected during summer in the previous year. These findings corroborate the idea that a large fraction of the airborne biological INP population above the Alps during summer originates from plant surfaces. Which parameter or set of parameters could affect biological INP populations on both scales – upwind airsheds of high-altitude sites as well as individual leaves – is an intriguing question for further exploration. A first guess is that leaf wetness duration plays a role.