Browsing by Subject "DIOXIDE"

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  • Uudekll, Peep; Kozlova, Jekaterina; Mandar, Hugo; Link, Joosep; Sihtmae, Mariliis; Kaosaar, Sandra; Blinova, Irina; Kasemets, Kaja; Kahru, Anne; Stern, Raivo; Tatte, Tanel; Kukli, Kaupo; Tamm, Aile (2017)
    Spherical nickel particles with size in the range of 100-400 nm were synthesized by non-aqueous liquid phase benzyl alcohol method. Being developed for magnetically guided biomedical applications, the particles were coated by conformal and antimicrobial thin titanium oxide films by atomic layer deposition. The particles retained their size and crystal structure after the deposition of oxide films. The sensitivity of the coated particles to external magnetic fields was increased compared to that of the uncoated powder. Preliminary toxicological investigations on microbial cells and small aquatic crustaceans revealed non-toxic nature of the synthesized particles.
  • Kooijmans, Linda M. J.; Maseyk, Kadmiel; Seibt, Ulli; Sun, Wu; Vesala, Timo; Mammarella, Ivan; Kolari, Pasi; Aalto, Juho; Franchin, Alessandro; Vecchi, Roberta; Valli, Gianluigi; Chen, Huilin (2017)
    Nighttime vegetative uptake of carbonyl sulfide (COS) can exist due to the incomplete closure of stomata and the light independence of the enzyme carbonic anhydrase, which complicates the use of COS as a tracer for gross primary productivity (GPP). In this study we derived night-time COS fluxes in a boreal forest (the SMEAR II station in Hyytiala, Finland; 61 degrees 51 ' N, 24 degrees 17 ' E; 181ma.s.l.) from June to November 2015 using two different methods: eddy-covariance (EC) measurements (FCOS-EC) and the radon-tracer method (FCOS-Rn). The total night-time COS fluxes averaged over the whole measurement period were -6.8 +/- 2.2 and -7.9 +/- 3.8 pmolm (-2) s (-1) for FCOS-Rn and FCOS-EC, respectively, which is 33-38% of the average daytime fluxes and 21% of the total daily COS uptake. The correlation of Rn-222 (of which the source is the soil) with COS (average R-2 = 0.58) was lower than with CO2 (0.70), suggesting that the main sink of COS is not located at the ground. These observations are supported by soil chamber measurements that show that soil contributes to only 34-40% of the total night-time COS uptake. We found a decrease in COS uptake with decreasing nighttime stomatal conductance and increasing vapor-pressure deficit and air temperature, driven by stomatal closure in response to a warm and dry period in August. We also discuss the effect that canopy layer mixing can have on the radon-tracer method and the sensitivity of (FCOS-EC) to atmospheric turbulence. Our results suggest that the nighttime uptake of COS is mainly driven by the tree foliage and is significant in a boreal forest, such that it needs to be taken into account when using COS as a tracer for GPP.
  • Kauppi, Pekka E.; Ciais, Philippe; Hogberg, Peter; Nordin, Annika; Lappi, Juha; Wernick, Iddo K. (2020)
    The growth of the global terrestrial sink of carbon dioxide has puzzled scientists for decades. We propose that the role of land management practices-from intensive forestry to allowing passive afforestation of abandoned lands-have played a major role in the growth of the terrestrial carbon sink in the decades since the mid twentieth century. The Forest Transition, a historic transition from shrinking to expanding forests, and from sparser to denser forests, has seen an increase of biomass and carbon across large regions of the globe. We propose that the contribution of Forest Transitions to the terrestrial carbon sink has been underestimated. Because forest growth is slow and incremental, changes in the carbon density in forest biomass and soils often elude detection. Measurement technologies that rely on changes in two-dimensional ground cover can miss changes in forest density. In contrast, changes from abrupt and total losses of biomass in land clearing, forest fires and clear cuts are easy to measure. Land management improves over time providing important present contributions and future potential to climate change mitigation. Appreciating the contributions of Forest Transitions to the sequestering of atmospheric carbon will enable its potential to aid in climate change mitigation.
  • Miettinen, H.; Pumpanen, J.; Rantakari, M.; Ojala, A. (2020)
    We studied in 2013 and 2014 the spring carbon dynamics in a Boreal landscape consisting of a lake and 15 inflowing streams and an outlet. The first year had weather and a hydrological regime typical of past years with a distinct spring freshet connected with the thaw of the average snowpack. The latter year had higher air temperatures which did not permit snow accumulation, despite similar winter precipitation. As such, there was hardly any spring freshet in 2014, and stream discharge peaked in January, i.e., the conditions resembled those predicted in the future climate. Despite the hydrological differences between the years, there were only small interannual differences in the stream CO2 and DOC concentrations. The relationship between the concentrations and discharge was stronger in the typical year. CO2 concentrations in medium-sized streams correlated negatively with the discharge, indicating dilution effect of melting snowpacks, while in large-sized streams the correlation was positive, suggesting stronger groundwater influence. The DOC pathway to these streams was through the subsurface soil layers, not the groundwater. The total amount of carbon transported into the lake was ca. 1.5-fold higher in the typical year than in the year with warm winter. In 2013, most of the lateral inputs took place during spring freshet. In 2014, the majority of inputs occurred earlier, during the winter months. The lateral CO2 signal was visible in the lake at 1.5 m depth. DOC dominated the carbon transport, and in both years, 12% of the input C was in inorganic form.
  • Kulmala, Markku; Ezhova, Ekaterina; Kalliokoski, Tuomo; Noe, Steffen; Vesala, Timo; Lohila, Annalea; Liski, Jari; Makkonen, Risto; Bäck, Jaana; Petäjä, Tuukka; Kerminen, Veli-Matti (2020)
    Forests cool the climate system by acting as a sink for carbon dioxide (CO2) and by enhancing the atmospheric aerosol load. whereas the simultaneous decrease of the surface albedo tends to have a warming effect. Here, we present the concept of CarbonSink+. which considers these combined effects. Using the boreal forest environment as an illustrative example, we estimated that accounting for the CarbonSink+ enhances the forest CO2 uptake by 10-50% due to the combined effects of CO2 fertilization and aerosol-induced diffuse radiation enhancement on photosynthesis. We further estimated that with afforestation or reforestation, i.e., replacing grasslands with forests in a boreal environment, the radiative cooling due to forest aerosols cancels most of the radiative warming due to decreased surface albedos. These two forcing components have. however, relatively large uncertainty ranges. resulting in large uncertainties in the overall effect of CarbonSink+. We discuss shortly the potential future changes in the strength of CarbonSink+ in the boreal region, resulting from changes in atmospheric composition and climate.
  • Maignan, Fabienne; Abadie, Camille; Remaud, Marine; Kooijmans, Linda M. J.; Kohonen, Kukka-Maaria; Commane, Róisín; Wehr, Richard; Campbell, J. Elliott; Belviso, Sauveur; Montzka, Stephen A.; Raoult, Nina; Seibt, Ulli; Shiga, Yoichi P.; Vuichard, Nicolas; Whelan, Mary E.; Peylin, Philippe (2021)
    Land surface modellers need measurable proxies to constrain the quantity of carbon dioxide (CO2) assimilated by continental plants through photosynthesis, known as gross primary production (GPP). Carbonyl sulfide (COS), which is taken up by leaves through their stomates and then hydrolysed by photosynthetic enzymes, is a candidate GPP proxy. A former study with the ORCHIDEE land surface model used a fixed ratio of COS uptake to CO2 uptake normalised to respective ambient concentrations for each vegetation type (leaf relative uptake, LRU) to compute vegetation COS fluxes from GPP. The LRU approach is known to have limited accuracy since the LRU ratio changes with variables such as photosynthetically active radiation (PAR): while CO2 uptake slows under low light, COS uptake is not light limited. However, the LRU approach has been popular for COS-GPP proxy studies because of its ease of application and apparent low contribution to uncertainty for regional-scale applications. In this study we refined the COS-GPP relationship and implemented in ORCHIDEE a mechanistic model that describes COS uptake by continental vegetation. We compared the simulated COS fluxes against measured hourly COS fluxes at two sites and studied the model behaviour and links with environmental drivers. We performed simulations at a global scale, and we estimated the global COS uptake by vegetation to be -756 Gg S yr(-1) , in the middle range of former studies (-490 to -1335 Gg S yr(-1)). Based on monthly mean fluxes simulated by the mechanistic approach in ORCHIDEE, we derived new LRU values for the different vegetation types, ranging between 0.92 and 1.72, close to recently published averages for observed values of 1.21 for C-4 and 1.68 for C-3 plants. We transported the COS using the monthly vegetation COS fluxes derived from both the mechanistic and the LRU approaches, and we evaluated the simulated COS concentrations at NOAA sites. Although the mechanistic approach was more appropriate when comparing to high-temporal-resolution COS flux measurements, both approaches gave similar results when transporting with monthly COS fluxes and evaluating COS concentrations at stations. In our study, uncertainties between these two approaches are of secondary importance compared to the uncertainties in the COS global budget, which are currently a limiting factor to the potential of COS concentrations to constrain GPP simulated by land surface models on the global scale.
  • Yi, Chuixiang; Ricciuto, Daniel; Li, Runze; Wolbeck, John; Xu, Xiyan; Nilsson, Mats; Aires, Luis; Albertson, John D.; Ammann, Christof; Arain, M. Altaf; de Araujo, Alessandro C.; Aubinet, Marc; Aurela, Mika; Barcza, Zoltan; Barr, Alan; Berbigier, Paul; Beringer, Jason; Bernhofer, Christian; Black, Andrew T.; Bolstad, Paul V.; Bosveld, Fred C.; Broadmeadow, Mark S. J.; Buchmann, Nina; Burns, Sean P.; Cellier, Pierre; Chen, Jiquan; Ciais, Philippe; Clement, Robert; Cook, Bruce D.; Curtis, Peter S.; Dail, D. Bryan; Dellwik, Ebba; Delpierre, Nicolas; Desai, Ankur R.; Dore, Sabina; Dragoni, Danilo; Drake, Bert G.; Dufrene, Eric; Dunn, Allison; Elbers, Jan; Eugster, Werner; Falk, Matthias; Feigenwinter, Christian; Flanagan, Lawrence B.; Foken, Thomas; Frank, John; Fuhrer, Juerg; Gianelle, Damiano; Goldstein, Allen; Goulden, Mike; Granier, Andre; Gruenwald, Thomas; Gu, Lianhong; Guo, Haiqiang; Hammerle, Albin; Han, Shijie; Hanan, Niall P.; Haszpra, Laszlo; Heinesch, Bernard; Helfter, Carole; Hendriks, Dimmie; Hutley, Lindsay B.; Ibrom, Andreas; Jacobs, Cor; Johansson, Torbjoern; Jongen, Marjan; Katul, Gabriel; Kiely, Gerard; Klumpp, Katja; Knohl, Alexander; Kolb, Thomas; Kutsch, Werner L.; Lafleur, Peter; Laurila, Tuomas; Leuning, Ray; Lindroth, Anders; Liu, Heping; Loubet, Benjamin; Manca, Giovanni; Marek, Michal; Margolis, Hank A.; Martin, Timothy A.; Massman, William J.; Matamala, Roser; Matteucci, Giorgio; McCaughey, Harry; Merbold, Lutz; Meyers, Tilden; Migliavacca, Mirco; Miglietta, Franco; Misson, Laurent; Moelder, Meelis; Moncrieff, John; Monson, Russell K.; Montagnani, Leonardo; Montes-Helu, Mario; Moors, Eddy; Moureaux, Christine; Mukelabai, Mukufute M.; Munger, J. William; Myklebust, May; Nagy, Zoltan; Noormets, Asko; Oechel, Walter; Oren, Ram; Pallardy, Stephen G.; Kyaw, Tha Paw U.; Pereira, Joao S.; Pilegaard, Kim; Pinter, Krisztina; Pio, Casimiro; Pita, Gabriel; Powell, Thomas L.; Rambal, Serge; Randerson, James T.; von Randow, Celso; Rebmann, Corinna; Rinne, Janne; Rossi, Federica; Roulet, Nigel; Ryel, Ronald J.; Sagerfors, Jorgen; Saigusa, Nobuko; Sanz, Maria Jose; Mugnozza, Giuseppe-Scarascia; Schmid, Hans Peter; Seufert, Guenther; Siqueira, Mario; Soussana, Jean-Francois; Starr, Gregory; Sutton, Mark A.; Tenhunen, John; Tuba, Zoltan; Tuovinen, Juha-Pekka; Valentini, Riccardo; Vogel, Christoph S.; Wang, Shaoqiang; Wang, Weiguo; Welp, Lisa R.; Wen, Xuefa; Wharton, Sonia; Wilkinson, Matthew; Williams, Christopher A.; Wohlfahrt, Georg; Yamamoto, Susumu; Yu, Guirui; Zampedri, Roberto; Zhao, Bin; Zhao, Xinquan (2010)
  • Provenzale, Maria; Ojala, Anne; Heiskanen, Jouni; Erkkila, Kukka-Maaria; Mammarella, Ivan; Hari, Pertti; Vesala, Timo (2018)
    Lakes are important actors in biogeochemical cycles and a powerful natural source of CO2. However, they are not yet fully integrated in carbon global budgets, and the carbon cycle in the water is still poorly understood. In freshwater ecosystems, productivity studies have usually been carried out with traditional methods (bottle incubations, C-14 technique), which are imprecise and have a poor temporal resolution. Consequently, our ability to quantify and predict the net ecosystem productivity (NEP) is limited: the estimates are prone to errors and the NEP cannot be parameterised from environmental variables. Here we expand the testing of a free-water method based on the direct measurement of the CO2 concentration in the water. The approach was first proposed in 2008, but was tested on a very short data set (3 days) under specific conditions (autumn turnover); despite showing promising results, this method has been neglected by the scientific community. We tested the method under different conditions (summer stratification, typical summer conditions for boreal dark-water lakes) and on a much longer data set (40 days), and quantitatively validated it comparing our data and productivity models. We were able to evaluate the NEP with a high temporal resolution (minutes) and found a very good agreement (R-2 >= 0.71) with the models. We also estimated the parameters of the productivity-irradiance (PI) curves that allow the calculation of the NEP from irradiance and water temperature. Overall, our work shows that the approach is suitable for productivity studies under a wider range of conditions, and is an important step towards developing this method so that it becomes more widely used.
  • Larnimaa, Santeri; Halonen, Lauri; Karhu, Juho; Tomberg, Teemu; Metsälä, Markus; Genoud, Guillaume; Hieta, Tuomas; Bell, Steven; Vainio, Markku (2020)
    We present the first high-resolution measurements and rotational analysis of the fundamental asymmetric stretching vibrational band ν3(F2) of radiocarbon methane. A spectrometer consisting of a mid-infrared continuous-wave optical parametric oscillator and a cantilever-enhanced photoacoustic detector was employed to determine the wavenumbers of 43 rovibrational lines. A spectroscopic model could reproduce all the observed transition wavenumbers within the accuracy of the experiment. Our work contributes to the development of radiocarbon methane detectors based on laser spectroscopy.
  • Rödenbeck, C.; Zaehle, S.; Keeling, R.; Heimann, M. (2018)
    Interannual variations in the large-scale net ecosystem exchange (NEE) of CO2 between the terrestrial biosphere and the atmosphere were estimated for 1957-2017 from sustained measurements of atmospheric CO2 mixing ratios. As the observations are sparse in the early decades, available records were combined into a 'quasi-homogeneous' dataset based on similarity in their signals, to minimize spurious variations from beginning or ending data records. During El Nino events, CO2 is anomalously released from the tropical band, and a few months later also in the northern extratropical band. This behaviour can approximately be represented by a linear relationship of the NEE anomalies and local air temperature anomalies, with sensitivity coefficients depending on geographical location and season. The apparent climate sensitivity of global total NEE against variations in pan-tropically averaged annual air temperature slowly changed over time during the 1957-2017 period, first increasing (though less strongly than in previous studies) but then decreasing again. However, only part of this change can be attributed to actual changes in local physiological or ecosystem processes, the rest probably arising from shifts in the geographical area of dominating temperature variations. This article is part of a discussion meeting issue 'The impact of the 2015/2016 El Nino on the terrestrial tropical carbon cycle: patterns, mechanisms and implications'.
  • Rödenbeck, Christian; Zaehle, Sönke; Keeling, Ralph; Heimann, Martin (2018)
    The response of the terrestrial net ecosystem exchange (NEE) of CO2 to climate variations and trends may crucially determine the future climate trajectory. Here we directly quantify this response on inter-annual timescales by building a linear regression of inter-annual NEE anomalies against observed air temperature anomalies into an atmospheric inverse calculation based on long-term atmospheric CO2 observations. This allows us to estimate the sensitivity of NEE to inter-annual variations in temperature (seen as a climate proxy) resolved in space and with season. As this sensitivity comprises both direct temperature effects and the effects of other climate variables co-varying with temperature, we interpret it as "inter-annual climate sensitivity". We find distinct seasonal patterns of this sensitivity in the northern extratropics that are consistent with the expected seasonal responses of photosynthesis, respiration, and fire. Within uncertainties, these sensitivity patterns are consistent with independent inferences from eddy covariance data. On large spatial scales, northern extratropical and tropical interannual NEE variations inferred from the NEE-T regression are very similar to the estimates of an atmospheric inversion with explicit inter-annual degrees of freedom. The results of this study offer a way to benchmark ecosystem process models in more detail than existing effective global climate sensitivities. The results can also be used to gap-fill or extrapolate observational records or to separate inter-annual variations from longer-term trends.
  • Volpers, Michael; Claassens, Nico J.; Noor, Elad; van der Oost, John; de Vos, Willem M.; Kengen, Serve W. M.; dos Santos, Vitor A. P. Martins (2016)
    The strong advances in synthetic biology enable the engineering of novel functions and complex biological features in unprecedented ways, such as implementing synthetic autotrophic metabolism into heterotrophic hosts. A key challenge for the sustainable production of fuels and chemicals entails the engineering of synthetic autotrophic organisms that can effectively and efficiently fix carbon dioxide by using sustainable energy sources. This challenge involves the integration of carbon fixation and energy uptake systems. A variety of carbon fixation pathways and several types of photosystems and other energy uptake systems can be chosen and, potentially, modularly combined to design synthetic autotrophic metabolism. Prior to implementation, these designs can be evaluated by the combination of several computational pathway analysis techniques. Here we present a systematic, integrated in silico analysis of photo-electro-autotrophic pathway designs, consisting of natural and synthetic carbon fixation pathways, a proton-pumping rhodopsin photosystem for ATP regeneration and an electron uptake pathway. We integrated Flux Balance Analysis of the heterotrophic chassis Escherichia coli with kinetic pathway analysis and thermodynamic pathway analysis (Max-min Driving Force). The photo-electro-autotrophic designs are predicted to have a limited potential for anaerobic, autotrophic growth of E. coli, given the relatively low ATP regenerating capacity of the proton pumping rhodopsin photosystems and the high ATP maintenance of E. coli. If these factors can be tackled, our analysis indicates the highest growth potential for the natural reductive tricarboxylic acid cycle and the synthetic pyruvate synthase-pyruvate carboxylate - glyoxylate bicycle. Both carbon fixation cycles are very ATP efficient, while maintaining fast kinetics, which also results in relatively low estimated protein costs for these pathways. Furthermore, the synthetic bicycles are highly thermodynamic favorable under conditions analysed. However, the most important challenge identified for improving photo-electro-autotrophic growth is increasing the proton-pumping rate of the rhodopsin photosystems, allowing for higher ATP regeneration. Alternatively, other designs of autotrophy may be considered, therefore the herein presented integrated modeling approach allows synthetic biologists to evaluate and compare complex pathway designs before experimental implementation.
  • Marshall, John D.; Laudon, Hjalmar; Makela, Annikki; Peichl, Matthias; Hasselquist, Niles; Nasholm, Torgny (2021)
    Forests pass water and carbon through while converting portions to streamflow, soil organic matter, wood production, and other ecosystem services. The efficiencies of these transfers are but poorly quantified. New theory and new instruments have made it possible to use stable isotope composition to provide this quantification of efficiencies wherever there is a measurable difference between the branches of a branchpoint. We present a linked conceptual model that relies on isotopes of hydrogen, carbon, and oxygen to describe these branchpoints along the pathway from precipitation to soil and biomass carbon sequestration and illustrate how it can be tested and generalized. Plain Language Summary The way a forest works can be described in terms of carbon and water budgets, which describe the ways that carbon and water flow through the forest. The paths of such flows are frequently branched and the branches are often different in their stable isotope composition. This means that stable isotopes can be used to describe the branching events. We present isotopic methods of quantifying several such events, then link them in a chain that begins with the evaporation of water and ends with biomass production.
  • Genoud, Guillaume; Lehmuskoski, Johannes; Bell, Steven; Palonen, Vesa; Oinonen, Markku; Koskinen-Soivi, Mari-Leena; Reinikainen, Matti (2019)
    In-situ monitoring of radiocarbon emissions is challenging due to the lack of a suitable method for sensitive online detection of this isotope. Here we report on a complete system for automatized continuous on-site monitoring of radiocarbon gaseous emissions from nuclear facilities. By combining radiocarbon detection using mid-infrared cavity ringdown spectroscopy and an advanced sampling system, an elevated amount of radiocarbon in an atmospheric-like gas matrix was detected. Radiocarbon was detected in the form of (CO2)-C-14 after extraction of the carbon dioxide from the air sample. The system is also able to discriminate between radiocarbon in organic or inorganic molecular form by converting (CH4)-C-14 into (CO2)-C-14. This work lays the groundwork for further use of this technology in nuclear facilities for online on-site monitoring of radioactive gaseous emissions as well as future work on in-situ monitoring of atmospheric radiocarbon.
  • Pihlatie, Mari; Rannik, Ullar; Haapanala, Sami; Peltola, Olli; Shurpali, Narasinha; Martikainen, Pertti J.; Lind, Saara; Hyvönen, Niina; Virkajärvi, Perttu; Zahniser, Mark; Mammarella, Ivan (2016)
    Carbon monoxide (CO) is an important reactive trace gas in the atmosphere, while its sources and sinks in the biosphere are poorly understood. Soils are generally considered as a sink of CO due to microbial oxidation processes, while emissions of CO have been reported from a wide range of soil-plant systems. We measured CO fluxes using the micrometeorological eddy covariance method from a bioenergy crop (reed canary grass) in eastern Finland from April to November 2011. Continuous flux measurements allowed us to assess the seasonal and diurnal variability and to compare the CO fluxes to simultaneously measured net ecosystem exchange of CO2, N2O and heat fluxes as well as to relevant meteorological, soil and plant variables in order to investigate factors driving the CO exchange. The reed canary grass (RCG) crop was a net source of CO from mid-April to mid-June and a net sink throughout the rest of the measurement period from mid-June to November 2011, excluding a measurement break in July. CO fluxes had a distinct diurnal pattern with a net CO uptake in the night and a net CO emission during the daytime with a maximum emission at noon. This pattern was most pronounced in spring and early summer. During this period the most significant relationships were found between CO fluxes and global radiation, net radiation, sensible heat flux, soil heat flux, relative humidity, N2O flux and net ecosystem exchange. The strong positive correlation between CO fluxes and radiation suggests abiotic CO production processes, whereas the relationship between CO fluxes and net ecosystem exchange of CO2, and night-time CO fluxes and N2O emissions indicate biotic CO formation and microbial CO uptake respectively. The study shows a clear need for detailed process studies accompanied by continuous flux measurements of CO exchange to improve the understanding of the processes associated with CO exchange.
  • Palonen, Vesa; Pumpanen, Jukka; Kulmala, Liisa-Maija; Levin, Ingeborg; Heinonsalo, Jussi; Vesala, Timo (2018)
    We present a radiocarbon (C-14) dataset of tropospheric air CO2 forest soil air CO2, and soil CO2 emissions over the course of one growing season in a Scots pine forest in southern Finland. The CO2 collection for C-14 accelerator mass spectrometry (AMS) analysis was done with a portable, suitcase-sized system, using molecular sieve cartridges to selectively trap CO2 The piloting measurements aimed to quantify the spatial, seasonal and diurnal changes in the C-14 content of CO2 in a northern forest site. The atmospheric samples collected above the canopy showed a large seasonal variation and an 11 parts per thousand difference between day and nighttime profiles in August. The higher Delta C-14 values during night are partly explained by a higher contribution of C-14-elevated soil CO2, accumulating in the nocturnal boundary layer when vertical mixing is weak. We observed significant seasonal trends in Delta C-14-CO2 at different soil depths that reflected changes in the shares of autotrophic and heterotrophic respiration. Also the observed diurnal variation in the Delta C-14 values in soil CO2 highlighted the changes in the origin of CO2, with root activity decreasing more for the night than decomposition.
  • Foken, Thomas; Babel, Wolfgang; Munger, J. William; Gronholm, Tiia; Vesala, Timo; Knohl, Alexander (2021)
    Extensive studies are available that analyse time series of carbon dioxide and water flux measurements of FLUXNET sites over many years and link these results to climate change such as changes in atmospheric carbon dioxide concentration, air temperature and growing season length and other factors. Many of the sites show trends to a larger carbon uptake. Here we analyse time series of net ecosystem exchange, gross primary production, respiration, and evapotranspiration of four forest sites with particularly long measurement periods of about 20 years. The regular trends shown are interrupted by periods with higher or lower increases of carbon uptake. These breakpoints can be of very different origin and include forest decline, increased vegetation period, drought effects, heat waves, and changes in site heterogeneity. The influence of such breakpoints should be included in long-term studies of land-atmosphere exchange processes.
  • Tomberg, Teemu; Vainio, Markku; Hieta, Tuomas; Halonen, Lauri (2018)
    An exceptional property of photo-acoustic spectroscopy is the zero-background in wavelength modulation configuration while the signal varies linearly as a function of absorbed laser power. Here, we make use of this property by combining a highly sensitive cantilever-enhanced photo-acoustic detector, a particularly stable high-power narrow-linewidth mid-infrared continuous-wave optical parametric oscillator, and a strong absorption cross-section of hydrogen fluoride to demonstrate the ability of cantilever-enhanced photo-acoustic spectroscopy to reach sub-parts-per-trillion level sensitivity in trace gas detection. The high stability of the experimental setup allows long averaging times. A noise equivalent concentration of 650 parts-per-quadrillion is reached in 32 minutes.