Investigating evidence of enhanced aerosol formation and growth due to autumnal moth larvae feeding on mountain birch at SMEAR I in northern Finland

Laboratory studies have shown that heibivory-induced biogenic volatile organic compound (BVOC) emissions might enhance aerosol formation and growth. To increase understanding of the atmospheric relevance of this enhancement, we analyzed 25 years of data from SMEAR I (Station for Measuring Ecosystem-Atmosphere Relations) in northern Finland, where autumnal moth (Epirrita autumnata) larvae are prominent defoliators of mountain birch. We did not find a direct correlation between the autumnal moth density and aerosol processes, nor between the total number concentration and temperature, and hence the basal BVOC emissions. Instead, there is some evidence that the total particle concentration is elevated even for a few years after the infestation due to delayed defense response of mountain birch. The low total biomass of the trees concomitantly with low autumnal moth densities during most of the years of our study, may have impacted our results, hindering the enhancement of aerosol processes.Peer reviewe

Atmospheric and ecosystem big data providing key contributions in reaching United Nations' sustainable development goals

Big open data comprising comprehensive, long-term atmospheric and ecosystem in-situ observations will give us tools to meet global grand challenges and to contribute towards sustainable development. United Nations' Sustainable Development Goals (UN SDGs) provide framework for the process. We present synthesis on how Station for Measuring Earth Surface-Atmosphere Relations (SMEAR) observation network can contribute to UN SDGs. We describe SMEAR II flagship station in Hyytiala, Finland. With more than 1200 variables measured in an integrated manner, we can understand interactions and feedbacks between biosphere and atmosphere. This contributes towards understanding impacts of climate change to natural ecosystems and feedbacks from ecosystems to climate. The benefits of SMEAR concept are highlighted through outreach project in Eastern Lapland utilizing SMEAR I observations from Varrio research station. In contrast to boreal environment, SMEAR concept was also deployed in Beijing. We underline the benefits of comprehensive observations to gain novel insights into complex interactions between densely populated urban environment and atmosphere. Such observations enable work towards solving air quality problems and improve the quality of life inside megacities. The network of comprehensive stations with various measurements will enable science-based decision making and support sustainable development by providing long-term view on spatio-temporal trends on atmospheric composition and ecosystem parameters.Peer reviewe

Long-term measurements of volatile organic compounds highlight the importance of sesquiterpenes for the atmospheric chemistry of a boreal forest

The concentrations of terpenoids (isoprene; monoterpenes, MTs; and sesquiterpenes, SQTs) and oxygenated volatile organic compounds (OVOCs; i.e. aldehydes, alcohols, acetates and volatile organic acids, VOAs) were investigated during 2 years at a boreal forest site in Hyytiala, Finland, using in situ gas chromatograph mass spectrometers (GC-MSs). Seasonal and diurnal variations of terpenoid and OVOC concentrations as well as their relationship with meteorological factors were studied. Of the VOCs examined, C-2-C-7 unbranched VOAs showed the highest concentrations, mainly due to their low reactivity. Of the terpenoids, MTs showed the highest concentrations at the site, but seven different highly reactive SQTs were also detected. The monthly and daily mean concentrations of most terpenoids, aldehydes and VOAs were highly dependent on the temperature. The highest exponential correlation with temperature was found for a SQT (beta-caryophyllene) in summer. The diurnal variations in the concentrations could be explained by sources, sinks and vertical mixing. The diurnal variations in MT concentrations were strongly affected by vertical mixing. Based on the temperature correlations and mixing layer height (MLH), simple proxies were developed for estimating the MT and SQT concentrations. To estimate the importance of different compound groups and compounds in local atmospheric chemistry, reactivity with main oxidants (hydroxyl radical, OH; nitrate radical, NO3; and ozone, O-3) and production rates of oxidation products (OxPRs) were calculated. The MTs dominated OH and NO3 radical chemistry, but the SQTs greatly impacted O-3 chemistry, even though the concentrations of SQT were 30 times lower than the MT concentrations. SQTs were also the most important for the production of oxidation products. Since the SQTs show high secondary organic aerosol (SOA) yields, the results clearly indicate the importance of SQTs for local SOA production.Peer reviewe

Measurement report : Long-term measurements of aerosol precursor concentrations in the Finnish subarctic boreal forest

Aerosol particles form in the atmosphere via the clustering of certain atmospheric vapors. After growing into larger particles by the condensation of low-volatility gases, they can affect the Earth's climate by scattering light and acting as cloud condensation nuclei (CCN). Observations of low-volatility aerosol precursor gases have been reported around the world, but longer-term measurement series and any Arctic data sets showing seasonal variation are close to nonexistent. Here, we present similar to 7 months of aerosol precursor gas measurements performed with a nitrate-based chemical ionization atmospheric pressure interface time-of-flight (CI-APi-TOF) mass spectrometer. We deployed our measurements similar to 150 km north of the Arctic Circle at the SMEAR I (Station for Measuring Ecosystem-Atmosphere Relations) continental Finnish subarctic field station, located in the Varrio strict nature reserve. We report concentration measurements of the most common compounds related to new particle formation (NPF): sulfuric acid (SA), methane sulfonic acid (MSA), iodic acid (IA) and the total concentration of highly oxygenated organic molecules (HOMs). At this remote measurement site, SA originates from both anthropogenic and biological sources and has a clear diurnal cycle but no significant seasonal variation. MSA shows a more distinct seasonal cycle, with concentrations peaking in the summer. Of the measured compounds, IA concentrations are the most stable throughout the measurement period, except in April during which time the concentration of IA is significantly higher than during the rest of the year. Otherwise, IA has almost identical daily maximum concentrations in spring, summer and autumn, and on NPF event or non-event days. HOMs are abundant during the summer months and low in the autumn months. Due to their low autumn concentrations and high correlation with ambient air temperature, we suggest that most HOMs are products of biogenic emissions, most probably monoterpene oxidation products. NPF events at SMEAR I happen under relatively low-temperature (1-8 degrees C) conditions, with a fast temperature rise in the early morning hours as well as lower and decreasing relative humidity (RH, 55% vs. 80 %) during NPF days compared with non-event days. NPF days have clearly higher global irradiance values (similar to 450 m(-2) vs. similar to 200 m(-2) and about 10 ppbv higher ozone concentrations than non-event days. During NPF days, we have, on average, higher SA concentrations, peaking at noon; higher MSA concentrations in the afternoon; and slightly higher IA concentration than during non-event days. In summary, these are the first long-term measurements of aerosol-forming vapors from SMEAR I in the subarctic region, and the results of this work will help develop an understanding of atmospheric chemical processes and aerosol formation in the rapidly changing Arctic.Peer reviewe

Multivariate model-based investigation of the temperature dependence of ozone concentration in Finnish boreal forest

Tropospheric ozone (O-3) concentrations are observed to increase with temperature in urban and rural locations. We investigated the apparent temperature dependency of daytime ozone concentration in the Finnish boreal forest in summertime based on long-term measurements. We used statistical mixed effects models to separate the direct effects of temperature from other factors influencing this dependency, such as weather conditions, long-range transport of precursors, and concentration of various hydrocarbons. The apparent temperature dependency of 1.16 ppb ?(-1) based on a simple linear regression was reduced to 0.87 ppb ?(-1) within the canopy for summer daytime data after considering these factors. In addition, our results indicated that small oxygenated volatile organic compounds may play an important role in the temperature dependence of O-3 concentrations in this dataset from a low-NOx environment. Summertime observations and daytime data were selected for this analysis to focus on an environment that is significantly affected by biogenic emissions. Despite limitations due to selection of the data, these results highlight the importance of considering factors contributing to the apparent temperature dependence of the O-3 concentration. In addition, our results show that a mixed effects model achieves relatively good predictive accuracy for this dataset without explicitly calculating all processes involved in O-3 formation and removal.Peer reviewe

Sources and sinks driving sulfuric acid concentrations in contrasting environments : implications on proxy calculations

Sulfuric acid has been shown to be a key driver for new particle formation and subsequent growth in various environments, mainly due to its low volatility. However, direct measurements of gas-phase sulfuric acid are oftentimes not available, and the current sulfuric acid proxies cannot predict, for example, its nighttime concentrations or result in significant discrepancies with measured values. Here, we define the sources and sinks of sulfuric acid in different environments and derive a new physical proxy for sulfuric acid to be utilized in locations and during periods when it is not measured. We used H2SO4 measurements from four different locations: Hyytiala, Finland; Agia Marina, Cyprus; Budapest, Hungary; and Beijing, China, representing semi-pristine boreal forest, rural environment in the Mediterranean area, urban environment and heavily polluted megacity, respectively. The new proxy takes into account the formation of sulfuric acid from SO2 via OH oxidation and other oxidation pathways, specifically via stabilized Criegee intermediates. The sulfuric acid sinks included in the proxy are its condensation sink (CS) and atmospheric clustering starting from H2SO4 dimer formation. Indeed, we found that the observed sulfuric acid concentration can be explained by the proposed sources and sinks with similar coefficients in the four contrasting environments where we have tested it. Thus, the new proxy is a more flexible and an important improvement over previous proxies. Following the recommendations in this paper, a proxy for a specific location can be derived.Peer reviewe

The Center of Excellence in Atmospheric Science (2002–2019) — from molecular and biological processes to the global climate

The study of atmospheric processes related to climate requires a multidisciplinary approach, encompassing physics, chemistry, meteorology, forest science, and environmental science. The Academy of Finland Centre of Excellence in atmospheric sciences (CoE ATM) responded to that need for 18 years and produced extensive research and eloquent results, which are summarized in this review. The work in the CoE ATM enhanced our understanding in biogeochemical cycles, ecosystem processes, dynamics of aerosols, ions and neutral clusters in the lower atmosphere, and cloud formation and their interactions and feedbacks. The CoE ATM combined continuous and comprehensive long-term in-situ observations in various environments, ecosystems and platforms, ground- and satellitebased remote sensing, targeted laboratory and field experiments, and advanced multi-scale modeling. This has enabled improved conceptual understanding and quantifications across relevant spatial and temporal scales. Overall, the CoE ATM served as a platform for the multidisciplinary research community to explore the interactions between the biosphere and atmosphere under a common and adaptive framework

The Effect of Autumnal Moth Induced Volatile Organic Compound Emissions to Aerosol Load in Subarctic Region

Volatile organic compounds (VOCs) are hydrocarbons that are emitted to the atmosphere from biogenic or anthropogenic sources. Plants emit VOCs as a part of normal metabolism, but the emissions are significantly increased under stressed conditions. For example heat wave, drought and herbivory cause stress for the plants. Laboratory studies have shown that VOCs emitted by herbivory infested boreal forest trees have enhanced secondary organic aerosol (SOA) production. In this study, 25 years (1992–2016) of atmospheric data from measurement site in eastern Finnish Lapland was analyzed to understand wheter the enhancement is atmospherically relevant. The knowledge is important, as aerosol particles cause changes in radiative forcing, and thus contribute to the climate change. At the study site autumnal moth (Epirrita autumnata) larvae are prominent defoliator of mountain birches (Betula pubescens spp. czerepanovii). Autumnal moths have cyclic population dynamics, and during the severe population outbreaks, they can consume all the leaves of mountain birches in vast regions. Despite the severity of the herbivory to the local ecosystem, the analysis did not show connection between the number of autumnal moths and aerosol processes. Also, no clear correlation between the total number concentration and temperature, and hence the basal VOC emissions from biogenic sources, was observed. Nor did sulfur dioxide or sulfuric acid concentration have strong correlation with total particle concentration which would have been expected. The results indicate that probably the total biomass of mountain birches is too small to cause detectable changes in atmospheric variables. Additionally, the study period had only one severe population outbreak during which the data availability of atmospheric variables was limited. However, climate change proceeds fast in the Arctic region. Hence, the basal VOC emissions from vegetation will increase. Also, both the mountain birches and new moth species will expand to the areas where they did not earlier succeed. In the future the enhancement of autumnal moth larvae feeding may be atmospherically relevant

Insights into carbon-based terrestrial climate feedback mechanisms

Feedback mechanisms between atmosphere and biosphere have a potential to mitigate climate change. The connection arises from the activation of photosynthesis due to rising CO2 concentration and air temperature, leading to enhanced emissions of biogenic volatile organic compounds (BVOC). In the atmosphere, BVOCs can contribute to formation of stabilized Criegee intermediates which, by oxidizing sulfur dioxide, can produce sulfuric acid. Sulfuric acid and oxidized BVOCs are key components for formation and growth of aerosol particles, which can act as seeds for cloud droplets. Cloud droplet number concentration, and hence the properties of clouds depend on the concentration of aerosol seeds. Moreover, scattering of solar radiation by aerosol particles and clouds can further intensify photosynthesis, since diffuse radiation penetrates more evenly inside the canopy, but also mitigate global warming, because the amount of incoming solar radiation is reduced. In this thesis processes behind formation and growth of aerosol particles were analyzed by determining the relevance of biotic stress induced emissions of BVOCs on atmospheric aerosol load, and deriving a proxy for sulfuric acid with revised source and sink terms. Additionally, the interactions between aerosol particles, clouds, and photosynthesis were studied by developing an algorithm for cloud type classification, and quantifying the effect of aerosol particles and clouds on radiation and photosynthesis. The main findings of the thesis are: 1) Though clear correlation between aerosol load and intensity of biotic stress could not be observed in field data from Finnish Lapland, we found indications of delayed induced stress effect, leading to elevated aerosol load for several years after the most intense stress periods. 2) Inclusion of a term describing oxidation of sulfur dioxide by stabilized Criegee intermediates in sulfuric acid proxies enables the estimation of night and wintertime sulfuric acid concentration. Dimer formation term is important in polluted environments where molecular clusters are formed rapidly. 3) The overall effectiveness of the cloud algorithm to distinguish different cloud types was nearly 70 %. The dominating cloud types in the measurement site in southern Finland are low level patchy and uniform clouds. 4) In the boreal region, the ecosystem scale photosynthesis can be up to 30 % larger in the presence of clouds compared to clear-sky and clean atmospheric conditions. The maximal enhancement in photosynthesis occurs when the fraction of diffuse radiation from global radiation is 0.4–0.5. The thesis covers several topics on aerosol–cloud–radiation interactions, shedding light on versatile and complex interactions between the atmosphere and boreal forest ecosystem. Furthermore, it provides tools for upcoming researches on atmospheric physics and chemistry.Ilmakehän ja ekosysteemin välillä on takaisinkytkentöjä, jotka voivat auttaa hillitsemään ilmaston lämpenemistä. Yhteys juontuu siitä, että ilman nouseva hiilidioksidipitoisuus ja lämpötila kiihdyttävät yhteyttämistä, mikä johtaa suurempiin kasvillisuuden hiilivetypäästöihin. Ilmakehässä hiilivedyt voivat muodostaa niin kutsuttuja Criegee-kaksoisradikaaleja, joiden on havaittu hapettavan rikkidioksidia muodostaen lopulta rikkihappoa. Sekä rikkihappo että hapettuneet hiilivety-yhdisteet voivat osallistua hiukkasmuodostukseen. Prosessissa kaasumaiset höyryt tiivistyvät ilmassa leijuviksi kiinteiksi tai nestemäisiksi hiukkasiksi, joista osa voi toimia pilvipisaroiden tiivistymisytiminä, minkä vuoksi niiden lukumäärä vaikuttaa muodostuvien pilvien ominaisuuksiin. Lisäksi sekä hiukkaset että pilvet voivat sirottaa auringon säteilyä kasvattaen hajasäteilyn määrää maapallolla, minkä on havaittu tehostavan yhteyttämistä entisestään, sillä hajasäteily imeytyy paremmin latvuskerroksen läpi. Säteilyn siroaminen vähentää myös maapallolle pääsevän säteilyn määrää, koska osa säteilystä siroaa takaisin avaruuteen, mikä auttaa hillitsemään ilmaston lämpenemistä. Tässä tutkimuksessa keskitytään eräisiin ilmakehän ja ekosysteemin välisiin vuorovaikutuksiin sekä kehitetään työkaluja tulevia tutkimuksia varten. Keskeisimmät tutkimuksen aiheet ja tulokset ovat: 1) Miten bioottisen stressin aiheuttamat puiden hiilivetypäästöt vaikuttavat hiukkasten muodostumiseen ja kasvuun Itä-Lapissa? Tutkimus on ensimmäinen kenttäaineistoon pohjautuva tutkimus aiheesta. Vaikka laboratorio- ja mallinnuskokeissa on havaittu bioottisten stressien nostavan hiukkaspitoisuutta, ei samaa vaikutusta kenttäkokeessa havaittu. Tulos voi johtua sekä matalasta stressitasosta tutkimuksen aikana että puiden pienestä biomassasta. 2) Muotoilimme mallin, jolla ilmakehän rikkihappopitoisuutta voidaan arvioida. Erityisesti rikkihapon lähteenä toimivien Criegee-kaksoisradikaalien lisääminen malliin paransi sitä merkittävästi. 3) Kehitimme algoritmin, jolla voidaan määrittää pilven alarajankorkeuden ja säteilyparametrien avulla mittausasemalla esiintyvät pilvityypit. Algoritmi määritti oikean pilvityypin 70 % tutkituista tapauksista. 4) Tutkimme, miten hiukkaset ja pilvet vaikuttavat hajasäteilyn syntymiseen sekä miten hajasäteily vaikuttaa yhteyttämiseen boreaalisella alueella. Havaitsimme, että yhteyttämisen kannalta optimaalinen hajasäteilyn osuus globaalista säteilystä on 0.4-0.5 ja että yhteyttäminen voi tehostua jopa 30 % hajasäteilyn takia verrattuna tilanteeseen, jossa taivas on selkeä ja hiukkasia on vähän

Clouds over Hyytiälä, Finland : an algorithm to classify clouds based on solar radiation and cloud base height measurements

We developed a simple algorithm to classify clouds based on global radiation and cloud base height measured by pyranometer and ceilometer, respectively. We separated clouds into seven different classes (stratus, stratocumulus, cumulus, nimbostratus, altocumulus + altostratus, cirrus + cirrocumulus + cirrostratus and clear sky + cirrus). We also included classes for cumulus and cirrus clouds causing global radiation enhancement, and we classified multilayered clouds, when captured by the ceilometer, based on their height and characteristics (transmittance, patchiness and uniformity). The overall performance of the algorithm was nearly 70% when compared with classification by an observer using total-sky images. The performance was best for clouds having well-distinguishable effects on solar radiation: nimbostratus clouds were classified correctly in 100% of the cases. The worst performance corresponds to cirriform clouds (50 %). Although the overall performance of the algorithm was good, it is likely to miss the occurrences of high and multilayered clouds. This is due to the technical limits of the instrumentation: the vertical detection range of the ceilometer and occultation of the laser pulse by the lowest cloud layer. We examined the use of clearness index, which is defined as a ratio between measured global radiation and modeled radiation at the top of the atmosphere, as an indicator of clear-sky conditions. Our results show that cumulus, altocumulus, altostratus and cirriform clouds can be present when the index indicates clear-sky conditions. Those conditions have previously been associated with enhanced aerosol formation under clear skies. This is an important finding especially in the case of low clouds coupled to the surface, which can influence aerosol population via aerosol-cloud interactions. Overall, caution is required when the clearness index is used in the analysis of processes affected by partitioning of radiation by clouds.Peer reviewe