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Fennoscandian boreal forest is a region with commonly occurring particle formation, which benefits from the abundance of biogenic volatile organic compounds emitted by the vegetation. The same vegetation also regulates the exchange of water vapour between the ecosystem and the atmosphere. Thus, as the forest has the potential to provide the two components needed in cloud formation, i.e. condensation nuclei and humidity, there is reason to suspect consequent changes in air masses that are influenced by the forest below.We investigated the link between boreal forest air mass transport and cloud related properties in air masses that arrived to the SMEAR II station (61°10’N, 24°17’E, 170m a.s.l.), Finland, from between western and norther directions. These selected air masses were originally marine and travelled only across a land area with relatively minor anthropogenic emissions sources, allowing us to focus on biogenic influences. The source region and the time each air mass spent above land before arrival, were determined from 96-hour long air mass back trajectories. We used a long-term comprehensive data sets, spanning up to 11 growing seasons (April-September, 2006-2016).Air masses with short transport times over the forest, often coincided with measurements of particles in smaller size ranges. Higher numbers of larger cloud condensation nuclei sized particles became more common in air masses with longer transport times over the forest. Similarly, air masses that spent little time over land, were often relatively cool and carried less water vapour. Whereas, higher specific humidities were more likely in air masses with longer times spent over land, as associated warming had most likely facilitated an increased uptake of water vapour from plant evapotranspiration. We also observed corresponding moderate increases in satellite observed cloud optical thickness and in-situ measured precipitation. Air masses with very short transport times over land were an exception, as these fast-moving air masses are likely to be connected to weather fronts and therefore also have a high probability for clouds and precipitation. The reported differences between air masses more or less disappeared when the transport time over land reached approximately 60 hours, and any further increase in land transport time no longer caused a substantial change. This appears to be the time scale in which most of the forest environment’s influence on these cloud related properties is realised and a balance is reached.

The Station for Measuring Ecosystem–Atmosphere Relations (SMEAR) II, located within the boreal forest of Finland, is a unique station in the world due to the wide range of long-term measurements tracking the Earth–atmosphere interface. In this study, we characterize the composition of organic aerosol (OA) at SMEAR II by quantifying its driving constituents. We utilize a multi-year data set of OA mass spectra measured in situ with an Aerosol Chemical Speciation Monitor (ACSM) at the station. To our knowledge, this mass spectral time series is the longest of its kind published to date. Similarly to other previously reported efforts in OA source apportionment from multi-seasonal or multi-annual data sets, we approached the OA characterization challenge through positive matrix factorization (PMF) using a rolling window approach. However, the existing methods for extracting minor OA components were found to be insufficient for our rather remote site. To overcome this issue, we tested a new statistical analysis framework. This included unsupervised feature extraction and classification stages to explore a large number of unconstrained PMF runs conducted on the measured OA mass spectra. Anchored by these results, we finally constructed a relaxed chemical mass balance (CMB) run that resolved different OA components from our observations. The presented combination of statistical tools provided a data-driven analysis methodology, which in our case achieved robust solutions with minimal subjectivity. Following the extensive statistical analyses, we were able to divide the 2012–2019 SMEAR II OA data (mass concentration interquartile range (IQR): 0.7, 1.3, and 2.6 µg m−3) into three sub-categories – low-volatility oxygenated OA (LV-OOA), semi-volatile oxygenated OA (SV-OOA), and primary OA (POA) – proving that the tested methodology was able to provide results consistent with literature. LV-OOA was the most dominant OA type (organic mass fraction IQR: 49 %, 62 %, and 73 %). The seasonal cycle of LV-OOA was bimodal, with peaks both in summer and in February. We associated the wintertime LV-OOA with anthropogenic sources and assumed biogenic influence in LV-OOA formation in summer. Through a brief trajectory analysis, we estimated summertime natural LV-OOA formation of tens of ng m−3 h−1 over the boreal forest. SV-OOA was the second highest contributor to OA mass (organic mass fraction IQR: 19 %, 31 %, and 43 %). Due to SV-OOA's clear peak in summer, we estimate biogenic processes as the main drivers in its formation. Unlike for LV-OOA, the highest SV-OOA concentrations were detected in stable summertime nocturnal surface layers. Two nearby sawmills also played a significant role in SV-OOA production as also exemplified by previous studies at SMEAR II. POA, taken as a mix of two different OA types reported previously, hydrocarbon-like OA (HOA) and biomass burning OA (BBOA), made up a minimal OA mass fraction (IQR: 2 %, 6 %, and 13 %). Notably, the quantification of POA at SMEAR II using ACSM data was not possible following existing rolling PMF methodologies. Both POA organic mass fraction and mass concentration peaked in winter. Its appearance at SMEAR II was linked to strong southerly winds. Similar wind direction and speed dependence was not observed among other OA types. The high wind speeds probably enabled the POA transport to SMEAR II from faraway sources in a relatively fresh state. In the event of slower wind speeds, POA likely evaporated and/or aged into oxidized organic aerosol before detection. The POA organic mass fraction was significantly lower than reported by aerosol mass spectrometer (AMS) measurements 2 to 4 years prior to the ACSM measurements. While the co-located long-term measurements of black carbon supported the hypothesis of higher POA loadings prior to year 2012, it is also possible that short-term (POA) pollution plumes were averaged out due to the slow time resolution of the ACSM combined with the further 3 h data averaging needed to ensure good signal-to-noise ratios (SNRs). Despite the length of the ACSM data set, we did not focus on quantifying long-term trends of POA (nor other components) due to the high sensitivity of OA composition to meteorological anomalies, the occurrence of which is likely not normally distributed over the 8-year measurement period. Due to the unique and realistic seasonal cycles and meteorology dependences of the independent OA subtypes complemented by the reasonably low degree of unexplained OA variability, we believe that the presented data analysis approach performs well. Therefore, we hope that these results encourage also other researchers possessing several-year-long time series of similar data to tackle the data analysis via similar semi- or unsupervised machine-learning approaches. This way the presented method could be further optimized and its usability explored and evaluated also in other environments., Atmospheric Chemistry and Physics, 21 (13), ISSN:1680-7375, ISSN:1680-7367

Cyclohexene (C6H10) is commonly used as a proxy for biogenic monoterpenes, when studying their oxidation mechanisms and secondary organic aerosol (SOA) formation. The ozonolysis of cyclohexene has been shown to be effective at producing highly oxygenated organic molecules (HOM), a group of molecules known to be important in the formation of SOA. Here, we provide an in depth look at how, on a molecular level, the HOM formation and fate changed with perturbations from NOx and seed particles. HOM were produced in a chamber from cyclohexene ozonolysis, and measured with a chemical ionisation mass spectrometer (CIMS) using nitrate (NO3−) as reagent ion. As high-resolution CIMS instruments provide mass spectra with numerous ion signals and a wealth of information that can be hard to manage, we employed a primarily statistical approach for the data analysis. To utilise as many individual HOM signals as possible, each compound was assigned a parameter describing the quality of the observed signal. These parameters were then used as weights or to determine the inclusion of a given signal in further analyses. Under unperturbed ozonolysis conditions, known HOM peaks were observed in the chamber, including C6H8O9 as the largest HOM signal, and C12H20O9 as the largest dimer product. With the addition of nitric oxide (NO) into the chamber, the spectrum changed considerably, as expected. Dimer product signals decreased overall, but an increase in dimers with nitrate functionalities was seen, as a result of NO3 radical oxidation. The response of monomer signals to NO addition varied, and while nitrate-containing monomers increased, non-nitrate signals either increased or decreased, depending on the individual molecules. The addition of seed aerosol increased the condensation sink, which markedly decreased the signals of all low-volatility compounds. Larger molecules were seen to have a higher affinity for condensation, but a more detailed analysis showed that the uptake was controlled mainly by the number of oxygen atoms in each molecule. All non-nitrate compounds with at least 7 oxygen atoms were observed to condense onto the seed aerosol at close to equal rates. Nitrates required higher mass and higher oxygen content to condense at similar rates as the non-nitrate HOM. A comparison to experiments with alpha-pinene reported earlier, showed quite a similar relationship between elemental composition and volatility, although products from alpha-pinene ozonolysis appeared to require slightly higher oxygen numbers for the same decrease in volatility. In addition, two models developed for predicting volatilities of volatile organic compound (VOC) oxidation products were tested on the ozonolysis products of cyclohexene.

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.

Boreal forests are an important source of trace gases and atmospheric aerosols, as well as a crucial carbon sink. As such, they form a strongly interconnected coupled system with the atmosphere. The SMEAR II station is located in a boreal Scots pine forest in Hyytiala, Finland, and has over 25 years of continuous measurements of atmospheric and ecosystem variables. In this study, we analyse the seasonal variations of trace gases, atmospheric aerosols, greenhouse gases, and meteorological variables, measured at the SMEAR II sta-tion during the past two and a half decades. Several ecosystem and atmospheric variables show seasonal correlations with each other, which suggests seasonal interactions within the climate system that links together ecosystem processes, greenhouse gases, trace gases and atmospheric aerosols. For instance, increased global radiation in summer increases air temperature and consequently affects the plant phenology, which promotes the ecosystem carbon exchange and biogenic volatile organic compound (biogenic VOC) release. This further affects the ambient concentrations of highly oxygenated organic molecules (HOMs) as well as the formation and growth of atmospheric organic aerosols. Organic aerosols subsequently influence aerosol optical properties and, through increased scattering, have the potential to cool the climate. We also discuss the impacts of the warm and dry summers of 2010 and 2018 on the studied variables. For these years, we find a higher-than-average ecosystem primary production especially in June, leading to an increased VOC flux from the forest. The increased VOC flux in turn leads to higher HOM and secondary aerosol concentration in the atmosphere. The latter increases light scattering by atmospheric aero-sol particles and thus leads to climate cooling. The results obtained in this study improve our understanding of how boreal forests respond to climate change.