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Boreal forests cover vast areas of land in the high latitudes of the Northern Hemisphere, which are under amplified climate warming. The interactions between the forests and the atmosphere are known to generate a complex set of feedback processes. One feedback process, potentially producing a cooling effect, is associated with an increased reflectance of clouds due to aerosol–cloud interactions. Here, we investigate the effect that the boreal forest environment can have on cloud-related properties during the growing season. The site investigated was the SMEAR II station in Hyytiälä, Finland. Air mass back trajectories were the basis of the analysis and were used to estimate the time each air mass had spent over land prior to its arrival at the station. This enabled tracking the changes occurring in originally marine air masses as they travelled across the forested land. Only air masses arriving from the northwestern sector were investigated, as these areas have a relatively uniform forest cover and relatively little anthropogenic interference. We connected the air mass analysis with comprehensive in situ and remote-sensing data sets covering up to 11 growing seasons. We found that the properties of air masses with short land transport times, thereby less influenced by the forest, differed from those exposed to the forest environment for a longer period. The fraction of air masses with cloud condensation nuclei concentrations (at 0.2 % supersaturation) above the median value of 180 cm -3 of the analysed air masses increased from approximately 10 % to 80 % after 55 h of exposure to boreal forest, while the fraction of air masses with specific humidity above the median value of 5 g kg -1 increased from roughly 25 % to 65 %. Signs of possible resulting changes in the cloud layer were also observed from satellite measurements. Lastly, precipitation frequency increased from the average of approximately 7 % to about 12 % after a threshold of 50 h of land transport. Most of the variables showed an increase with an increasing land transport time until approximately 50–55 h, after which a balance with little further variation seemed to have been reached. This appears to be the approximate timescale in which the forest–cloud interactions take effect and the air masses adjust to the local forest environment. [ABSTRACT FROM AUTHOR]

Boreal forests cover vast areas of land in the high latitudes of the northern hemisphere, which are under amplified climate warming. The interaction between the forests and the atmosphere are known to generate a complex set of feedback processes. One feedback process, potentially producing a cooling effect, is associated with an increased reflectance of clouds due to aerosol-cloud interactions. Here, we investigate the effect that the boreal forest environment can have on cloud-related properties during the growing season. The site investigated was in Hyytiälä, Finland, in a region near the edge of the biome. Air mass back trajectories were the basis of the analysis and were used to determine the time each air mass had spent over land prior to its arrival at the station. This enabled tracking the changes occurring in originally marine air masses as they travelled across the forested land. Only air masses arriving from the north-western sector were investigated, as these areas have a relatively uniform forest cover and relatively little anthropogenic interference. We connected the air mass analysis with comprehensive in-situ and remote-sensing data sets covering up to eleven growing seasons. We found that the properties of air masses with short land transport times, thereby less influenced by the forest, differed from those exposed to the forest environment for a longer period. The latter were associated with higher number concentrations of cloud condensation nuclei and increased water vapour content. Indications of corresponding transformations in the cloud layer were observed from satellite measurements. Lastly, longer transport times over forest seem also to slightly enhance the observed precipitation frequency. Most of the variables showed an increase with an increasing land transport time until approximately 60 hours, after which a balanced state with little variation seemed to have been reached. This appears to be the approximate time scale in which the forest-cloud interactions take effect, and the air masses adjust to the local forest environment. [ABSTRACT FROM AUTHOR]

Long-term high-quality aerosol particulate matter (PM) concentration measurements have been conducted in southern Finland at Station for Measuring Ecosystem-Atmosphere Relations (SMEAR II, Hyytiälä) with different, yet comparable measurement equipment since 1995. In this paper, the mass concentrations measured between 2005 and 2017 using three different independent methods: i.e. 1. the cascade impactor, 2. Differential Mobility Particle Sizer (DMPS) and Aerosol Particle Sizer (APS) and 3. Synchronized Hybrid Ambient Real-time Particulate Monitor (SHARP) are compared and analysed. First, the mass concentrations of the different size classes, i.e. PM1 (PM within sub-micrometer particle diameter size range), PM2.5 (sub-2.5 µm) and PM10 (sub-10 µm), are reported. These data were further cross-compared through a bivariate fitting method. The comparison revealed an excellent equivalence among the three methods with slopes approaching unity and reasonable intercepts (≤ 1 µg m-3). An analysis of the seasonal variability of PM concentrations revealed that the mass concentrations were generally highest in summer in different size classes. The mean mass concentrations were 5.3, 5.4, and 6.5 µg m-3 for PM1, PM2.5 and PM10, respectively. The 2nd highest loadings were attained in spring, which were ca. 80-88 % of those in summer.The lowest loadings were measured in autumn and winter, when the mass concentrations were ca. 74-78 % of those in summer. Temperature had strong influence on the measured concentrations. While the high late spring and summertime temperatures promote secondary organic aerosol (SOA) formation and pollen emissions, the lowest wintertime temperatures enhance the need of residential heating processes yielding anthropogenic aerosol emissions (e.g. from traffic/industry/wood burning). The wintertime concentrations can also be expected to be influenced by boundary layer dynamics, which keep the PM emissions concentrated near Earth surface especially in winter. It is noteworthy that the mass concentrations were lower than those reported prior to 2005 (at SMEAR II). The descending trend (~-0.1-0.2 µg m3 y-1) was clearly visible here for all PM size classes in spring, summer and winter, while the trend in autumn remained statistically insignificant. This might have resulted at least partly from more stringent EU air quality legislation. [ABSTRACT FROM AUTHOR]

Interaction of water with fine particulate matter leads to the formation of reactive species (RS) that may influence the aging, properties, and health effects of atmospheric aerosols. In this study, we explore the RS yields of fine PM from remote forest (Hyytiälä, Finland) and polluted urban air (Mainz, Germany and Beijing, China) and relate these yields to different chemical constituents and reaction mechanisms. Ultrahigh-resolution mass spectrometry was used to characterize organic aerosol composition, electron paramagnetic resonance (EPR) spectroscopy with a spin-trapping technique was used to determine the concentrations ·OH, O2·-, and carbon- or oxygen-centered organic radicals, and a fluorometric assay was used to quantify H2O2 concentration. The mass-specific yields of radicals were lower for sampling sites with higher concentration of ambient PM2.5 (particles with a diameter < 2.5 µm), whereas the H2O2 yields exhibited no clear trend. The abundances of water-soluble transition metals and aromatics in ambient PM2.5 were positively correlated with the relative fraction of ·OH to the totally detected radicals, but negatively correlated with the relative fraction of carbon-centered radicals. Moreover, we found that the relative fractions of different types of radicals formed by ambient PM2.5 were comparable to the surrogate mixtures comprising transition metals, organic hydroperoxide, H2O2, and humic or fulvic acids. Therein humic and fulvic acids exhibited strong radical scavenging effect to substantially decrease the radical yield of mixtures comprising cumene hydroperoxide and Fe2+. The interplay of transition metals (e.g., iron), highly oxidized compounds (e.g., organic hydroperoxides), and complexing agents (e.g., humic or fulvic acids), leads to non-linear concentration dependencies of production and yields of different types of RS. Our findings show that how the composition of PM2.5 influences the amount and nature of RS produced upon interaction with water, which may explain differences in the chemical reactivity and health effects of particulate matter in clean and polluted air. [ABSTRACT FROM AUTHOR]