Publications (25)22.17 Total impact
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Article: On the temporal upscaling of evapotranspiration from instantaneous remote sensing measurements to 8-day mean daily-sums
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ABSTRACT: The regular monitoring of evapotranspiration from satellites has been limited because of discontinuous temporal coverage, resulting in snapshots at a particular point in space and time. We developed a temporal upscaling scheme using satellite-derived instantaneous estimates of evapotranspiration to produce a daily-sum evapotranspiration averaged over an 8-day interval. We tested this scheme against measured evapotranspiration data from 34 eddy covariance flux towers covering seven plant functional types from boreal to tropical climatic zones. We found that the ratio of a half-hourly-sum of potential solar radiation (extraterrestrial solar irradiance on a plane parallel to the Earth's surface) between 10:00 hh and 14:00 hh to a daily-sum of potential solar radiation provides a robust scaling factor to convert a half-hourly measured evapotranspiration to an estimate of a daily-sum; the estimated and measured daily sum evapotranspiration showed strong linear relation (r(2) = 0.92) and small bias (-2.7%). By comparison, assuming a constant evaporative fraction (the ratio of evapotranspiration to available energy) during the daytime, although commonly used for temporal upscaling, caused 13% underestimation of evapotranspiration on an annual scale. The proposed temporal upscaling scheme requires only latitude, longitude and time as input. Thus it will be useful for developing continuous evapotranspiration estimates in space and time, which will improve continuous monitoring of hydrological cycle from local to global scales. (C) 2011 Elsevier B.V. All rights reserved.Agricultural and Forest Meteorology 01/2012; 152:212-222. · 3.39 Impact Factor -
Article: Boreal pine forest floor biogenic volatile organic compound emissions peak in early summer and autumn
Agricultural and Forest Meteorology 01/2011; · 3.39 Impact Factor -
Article: Boreal pine forest floor biogenic volatile organic compound emissions peak in early summer and autumn
Agricultural and Forest Meteorology 01/2011; 151:682-691. · 3.39 Impact Factor -
Article: Water balance of a boreal Scots pine forest
Boreal Environment Research 08/2010; 15:375-396. · 1.80 Impact Factor -
Article: Greenhouse gas fluxes in a drained peatland forest during spring frost-thaw event
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ABSTRACT: Fluxes of greenhouse gases (GHG) carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) were measured during a two month campaign at a drained peatland forest in Finland by the eddy covariance (EC) technique (CO2 and N2O), and automatic and manual chambers (CO2, CH4 and N2O). In addition, GHG concentrations and soil parameters (mineral nitrogen, temperature, moisture content) in the peat profile were measured. The aim of the measurement campaign was to quantify the GHG fluxes during freezing and thawing of the top-soil, a time period with potentially high GHG fluxes, and to compare different flux measurement methods. The forest was a net CO2 sink during the two months and the fluxes of CO2 dominated the GHG exchange. The peat soil was a small sink of atmospheric CH4 and a small source of N2O. Both CH4 oxidation and N2O production took place in the top-soil whereas CH4 was produced in the deeper layers of the peat, which were unfrozen throughout the measurement period. During the frost-thaw events of the litter layer distinct peaks in CO2 and N2O emissions were observed. The CO2 peak followed tightly the increase in soil temperature, whereas the N2O peak occurred with a delay after the thawing of the litter layer. CH4 fluxes did not respond to the thawing of the peat soil. The CO2 and N2O emission peaks were not captured by the manual chambers and hence we conclude that high time-resolution measurements with automatic chambers or EC are necessary to quantify fluxes during peak emission periods. Sub-canopy EC measurements and chamber-based fluxes of CO2 and N2O were comparable, although the fluxes of N2O measured by EC were close to the detection limit of the system. We conclude that if fluxes are high enough, i.e. greater than 5–10 μg N m−2 h−1, the EC method is a good alternative to measure N2O and CO2 fluxes at ecosystem scale, thereby minimizing problems with chamber enclosures and spatial representativeness of the measurements.Biogeosciences. 01/2010; -
Article: Autumn temperature and carbon balance of a boreal Scots pine forest in Southern Finland
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ABSTRACT: We analyzed the dynamics of carbon balance components: gross primary production (GPP) and total ecosystem respiration (TER), of a boreal Scots pine forest in Southern Finland. The main focus is on investigations of environmental drivers of GPP and TER and how they affect the inter-annual variation in the carbon balance in autumn (September–December). We used standard climate data and CO2 exchange measurements collected by the eddy covariance (EC) technique over 11 years. EC data revealed that increasing autumn temperature significantly enhances TER: the temperature sensitivity was 9.5 gC m−2 °C−1 for the period September–October (early autumn when high radiation levels still occur) and 3.8 gC m−2 °C−1 for November–December (late autumn with suppressed radiation level). The cumulative GPP was practically independent of the temperature in early autumn. In late autumn, air temperature could explain part of the variation in GPP but the temperature sensitivity was very weak, less than 1 gC m−2 °C−1. Two models, a stand photosynthesis model (COCA) and a global vegetation model (ORCHIDEE), were used for estimating stand GPP and its sensitivity to the temperature. The ORCHIDEE model was tested against the observations of GPP derived from EC data. The stand photosynthesis model COCA predicted that under a predescribed 3–6 °C temperature increase, the temperature sensitivity of 4–5 gC m−2 °C−1 in GPP may appear in early autumn. The analysis by the ORCHIDEE model revealed the model sensitivity to the temporal treatment of meteorological forcing. The model predictions were similar to observed ones when the site level 1/2-hourly time step was applied, but the results calculated by using daily meteorological forcing, interpolated to 1/2-hourly time step, were biased. This is due to the nonlinear relationship between the processes and the environmental factors.Biogeosciences. 01/2010; -
Article: Controls on winter ecosystem respiration at mid- and high-latitudes
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ABSTRACT: Winter CO2 fluxes represent an important component of the annual carbon budget in northern ecosystems. Understanding winter respiration processes and their responses to climate change is also central to our ability to assess terrestrial carbon cycle and climate feedbacks in the future. The factors influencing the spatial and temporal pattern of winter respiration (RECO) of northern ecosystems are poorly understood. For this reason, we analyzed eddy covariance flux data sets from 57 ecosystem sites ranging from ~35° N to ~70° N. Deciduous forests carry the highest winter RECO ratios (9.7–10.5 g C m−2 d−1), when winter is defined as the period during which air temperature remained below 0 °C. By contrast, wetland ecosystems had the lowest winter RECO (2.1–2.3 g C m−2 d−1). Evergreen needle-leaved forests, grasslands and croplands were characterized by intermediate winter RECO values of 7.4–7.9 g C m−2 d−1, 5.8–6.0 g C m−2 d−1, and 5.2–5.3 g C m−2 d−1, respectively. Cross site analysis showed that winter air or soil temperature, and the seasonal amplitude of the leaf area index inferred from satellite observation, which is a proxy for the amount of litter available for RECO in the subsequent winter, are the two main factors determining spatial pattern of daily mean winter RECO. Together, these two factors can explain 71% (Tair, ΔLAI) or 69% (Tsoil, ΔLAI) of the spatial variance of winter RECO across the 57 sites. The spatial temperature sensitivity of daily winter RECO was determined empirically by fitting an Arrhenius relationship to the data. The activation energy parameter of this relationship was found to decrease at increasing soil temperature at a rate of 83.1 KJ ° C-1 (r = −0.32, p < 0.05), which implies a possible dampening of the increase in winter RECO due to global warming. The interannual variability of winter RECO is better explained by soil temperature than by air temperature, likely due to the insulating effects of snow cover. The increase in winter RECO with a 1 °C warming based calculated from the spatial analysis was almost that double that calculated from the temporal analysis. Thus, models that calculate the effects of warming on RECO based only on spatial analyses could be over-estimating the impact.Biogeosciences Discussions 01/2010; -
Article: Temporal variations in surface water CO2 concentration in a boreal humic lake based on high-frequency measurements
Boreal Environment Research 04/2009; 14:48-60. · 1.80 Impact Factor -
Article: Autumn warming and carbon balance of a boreal Scots pine forest in Southern Finland
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ABSTRACT: We analyzed the dynamics of carbon balance components: gross primary production (GPP) and total ecosystem respiration (TER), of a boreal Scots pine forest in Southern Finland. Our aim was to study how these dynamics are related to different environmental conditions and how they affect the inter-annual variation in the carbon balance in autumn (September–December). We used standard micrometeorological data and CO2 exchange measurements collected by the eddy covariance (EC) technique over 11 years. The intra-annual relationships between the carbon balance components and the environmental factors were studied by the correlation analysis. Two models, a stand photosynthesis model and a generic dynamic vegetation model (ORCHIDEE), were also applied in the analysis. EC data revealed that increasing autumn temperature significantly enhances TER: the temperature sensitivity was 9.5 gC m−2 °C−1 for the period September–October (early autumn when high radiation levels still occur) and 3.8 gC m−2 °C−1 for November–December (late autumn with suppressed radiation level). The cumulative GPP was practically independent of the temperature in early autumn. In late autumn, air temperature could explain part of the variation in GPP but the temperature sensitivity was very weak, less than 1 g C m−2 °C−1. The stand photosynthesis model predicted that under a predescribed 3–6°C temperature increase, the temperature sensitivity of 4–5 gC m−2 °C−1 in GPP may appear in early autumn. The TER and GPP sensitivities, produced by the ORCHIDEE model, were similar to observed ones when the site level ½ h time step was applied, but the results calculated by using daily meteorological forcing, interpolated to ½ h time step, were biased stemming from the nonlinear relationship between the processes and the environmental factors.Biogeosciences Discussions. 01/2009; -
Article: A case study of eddy covariance flux of N<sub>2</sub>O measured within forest ecosystems: quality control and flux error analysis
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ABSTRACT: Eddy covariance (EC) flux measurements of nitrous oxide obtained by using a 3-D sonic anemometer and a tunable diode laser gas analyzer for N2O were investigated. Two datasets (Sorø, Denmark and Kalevansuo, Finland) from different measurement campaigns including sub-canopy flux measurements of energy and carbon dioxide are discussed with a focus on selected quality control aspects and flux error analysis. Although fast response trace gas analyzers based on spectroscopic techniques are increasingly used in ecosystem research, their suitability for reliable estimates of eddy covariance fluxes is still limited, and some assumptions have to be made for filtering and processing data. The N2O concentration signal was frequently dominated by offset drifts (fringe effect), which can give an artificial extra contribution to the fluxes when the resulting concentration fluctuations are correlated with the fluctuations of the vertical wind velocity. Based on Allan variance analysis of the N2O signal, we found that a recursive running mean filter with a time constant equal to 50~s was suitable to damp the influence of the periodic drift. Although the net N2O fluxes over the whole campaign periods were quite small at both sites (~5 μg N m−2 h−1 for Kalevansuo and ~10 μg N m−2 h−1 for Sorø), the calculated sub-canopy EC fluxes were in good agreement with those estimated by automatic soil chambers. However EC N2O flux measurements show larger random uncertainty than the sensible heat fluxes, and classification according to statistical significance of single flux values indicates that downward N2O fluxes have larger random error.Biogeosciences Discussions. 01/2009; -
Article: Photosynthesis of boreal ground vegetation after a forest clear-cut
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ABSTRACT: Heather (Calluna vulgaris), rosebay willowherb (Epilobium angustifolium), wavy hair-grass (Deschampsia flexuosa) and raspberry (Rubus idaeus) are typical species at boreal clear-cut sites. In this study, we measured their photosynthesis separately in the growing season of 2005 using a manual chamber. All measured species showed clear and species-specific seasonal cycles of photosynthetic activity (Pmax). The maxima of C. vulgaris and E. angustifolium occurred around June and July, while that of R. idaeus occurred as late as August. A simple model of photosynthetic activity is presented, addressing the photosynthesis of C. vulgaris was mainly explained by temperature history when the soil moisture is high. The activity of deciduous D. flexuosa also followed the temperature history, unlike the activities of E. angustifolium and R. idaeus. During a short drought, some shoots decreased their Pmax levels but none of the species showed similar reactions between individuals. We also observed that the comparison of the whole-plant Pmax or respiration of different-sized individuals were less scattered than the results based on full-grown leaf mass, implying that species-specific rates of photosynthesis at ground level are rather similar regardless of the plant size. Using species composition and continuous temperature and light measurements, we upscaled the species-specific process rates and integrated fixed and respired CO2 of ground vegetation for the entire 2005 growing season. The photosynthetic production per surface area of soil was 760 g C m−2 y−1 at the fertile site and 300 g C m−2 y−1 at the infertile site. During the snow-free period (18 April–21 November), the above ground parts of measured species released 75 g C m−2 y−1 at the infertile site. At the fertile site, E. angustifolium and R. idaeus respired 22 and 12 g C m−2 y−1, respectively.Biogeosciences. 01/2009; -
Article: Vertical advection and nocturnal deposition of ozone over a boreal pine forest
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ABSTRACT: Night-time ozone deposition for a Scots pine forest in Southern Finland was studied at the SMEAR II measurement station by evaluating the turbulent eddy covariance (EC), storage change and vertical advection fluxes. Similarly to night-time carbon dioxide flux, the eddy-covariance flux of ozone was decreasing with turbulence intensity (friction velocity), and storage change of the compound did not compensate the reduction (well-known night-time measurement problem). Accounting for vertical advection resulted in invariance of ozone deposition rate on turbulence intensity. This was also demonstrated for carbon dioxide, verified by independent measurements of NEE by chamber systems. The result highlights the importance of advection when considering the exchange measurements of any scalar. Analysis of aerodynamic and laminar boundary layer resistances by the model approach indicated that the surface resistance and/or chemical sink strength was limiting ozone deposition. The possible aerial ozone sink by known fast chemical reactions with sesquiterpenes and NO explain only a minor fraction of ozone sink. Thus the deposition is controlled either by stomatal uptake or surface reactions or both of them, the mechanisms not affected by turbulence intensity. Therefore invariance of deposition flux on turbulence intensity is expected also from resistance and chemical sink analysis.Atmospheric Chemistry and Physics. 01/2009; -
Article: Annual particle flux observations over a heterogeneous urban area
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ABSTRACT: Long-term eddy covariance (EC) particle number flux measurements for the size range 6 nm to 5 μm were performed at the SMEAR III station over urban area in Helsinki, Finland. Heterogeneous urban environment allowed us to study the effect of different land-use classes in different wind directions on the measured fluxes. The particle fluxes were observed to be the highest from the road direction during weekdays with day-time median flux 0.8×109 m−2 s−1. Particle fluxes showed a~clear dependence on traffic rates and mixing conditions of the boundary layer. In the direction of road, the larger particle fluxes were dominated by smaller sizes. Footprint analysis was performed by using numerical modeling and emission rate of particles from road was estimated to be 0.8×1012 s−1 m−1 during day-time. With typical traffic rate of 2500 vehicles per hour this corresponds to average emission rate of 1.2×1015 vehicles−1 km−1. The particle fluxes from vegetated area were the lowest with daytime median fluxes below 0.2×109 m−2 s−1. During weekends and nights the particle fluxes were low from all land use sectors being in the order of 0.02–0.1×109 m−2 s−1. On annual scale, the highest fluxes were measured in winter when emissions from stationary combustion sources are higher.Atmospheric Chemistry and Physics Discussions. 01/2009; -
Article: Photosynthetic production of boreal ground vegetation after a forest clear-cut
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ABSTRACT: Heather (Calluna vulgaris), rosebay willowherb (Epilobium angustifolium), wavy hair-grass (Deschampsia flexuosa) and raspberry (Rubus idaeus) are typical species on boreal clear-cut sites. According to our study, they all had clear and species-specific annual cycles of photosynthetic activity (Pmax). The maxima of C. vulgaris and E. angustifolium occurred around June and July, while that of R. idaeus occurred as late as in August. The photosynthetic activity of C. vulgaris followed the temperature history closely when the soil moisture was high. Deciduous D. flexuosa also followed the temperature history, unlike E. angustifolium and R. idaeus. During a short drought, some shoots decreased their Pmax levels but none of the species showed congruent reactions within individuals. In general, we noticed that the comparison of Pmax or respiration of different shoots caused less discrepancy when based on ground area than on leaf mass. Using species composition and continuous temperature and light measurements, we upscaled the species-specific process rates and integrated fixed and respired CO2 of ground vegetation during an entire growing season 2005. The photosynthetic production of ground vegetation was 760 g C m−2 y−1 at the fertile site and 300 g C m−2 y−1 at the infertile site. During the snow-free period (18 April–21 November), C. vulgaris respired 68 g C m−2 y−1 and E. angustifolium 7 g C m−2 y−1 at the infertile site. At the fertile site, E. angustifolium and R. idaeus respired 22 and 12 g C m−2 y−1, respectively.Biogeosciences Discussions. 01/2009; -
Article: Assessing seasonality of boreal coniferous forest CO<sub>2</sub> exchange by estimating biochemical model parameters from micrometeorological flux observations
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ABSTRACT: The biochemical seasonality of the northern boreal coniferous forests was investigated by means of inversion modelling using eddy covariance data. Eddy covariance data was used to optimize the biochemical model parameters. Our study sites consisted of three Scots pine (l. Pinus sylvestris ) forests and one Norway spruce (l. Picea abies ) forest that were located in Finland and Sweden. We obtained temperature and seasonal dependence for the biochemical model parameters: the maximum rate of carboxylation ( V <sub>c(max)</sub> and the maximum rate of electron transport ( J <sub>max</sub>). Both of the parameters were optimized without assumptions about their mutual magnitude. The values obtained for the biochemical model parameters were similar at all the sites during summer time. To describe seasonality, different temperature fits were made for the spring, summer and autumn periods. During summer, average J <sub>max</sub> across the sites was 54.0 μmol m<sup>−2</sup> s<sup>−1</sup> (variance 31.2 μmol m<sup>−2</sup> s<sup>-1</sup>) and V <sub>c(max)</sub> was 12.0 μmol m<sup>−2</sup> s<sup>−1</sup> (variance 6.6 μmol m<sup>−2</sup> s<sup>-1</sup>) at 17°C. The sensitivity of the model to LAI was also studied. Simulation runs were done to study the effect of the seasonality implemented in the model using different temperature fits. The impact of seasonality on annual GPP was 15%, which corresponded to an increase of 2°C in air temperature.Biogeosciences Discussions. 01/2008; -
Article: Quality control of CarboEurope flux data – Part 1: Coupling footprint analyses with flux data quality assessment to evaluate sites in forest ecosystems
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ABSTRACT: We applied a site evaluation approach combining Lagrangian Stochastic footprint modeling with a quality assessment approach for eddy-covariance data to 25 forested sites of the CarboEurope-IP network. The analysis addresses the spatial representativeness of the flux measurements, instrumental effects on data quality, spatial patterns in the data quality, and the performance of the coordinate rotation method. Our findings demonstrate that application of a footprint filter could strengthen the CarboEurope-IP flux database, since only one third of the sites is situated in truly homogeneous terrain. Almost half of the sites experience a significant reduction in eddy-covariance data quality under certain conditions, though these effects are mostly constricted to a small portion of the dataset. Reductions in data quality of the sensible heat flux are mostly induced by characteristics of the surrounding terrain, while the latent heat flux is subject to instrumentation-related problems. The Planar-Fit coordinate rotation proved to be a reliable tool for the majority of the sites using only a single set of rotation angles. Overall, we found a high average data quality for the CarboEurope-IP network, with good representativeness of the measurement data for the specified target land cover types.Biogeosciences. 01/2008; -
Article: Characterisation of ecosystem water-use efficiency of european forests from eddy covariance measurements
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ABSTRACT: Water-use efficiency (WUE) has been recognized as an important characteristic of vegetation productivity in various natural scientific disciplines for decades, but only recently at the ecosystem level, where different ways exist to characterize water-use efficiency. Hence, the objective of this research was (a) to systematically compare different ways of calculating ecosystem water-use efficiency (WUEe) from eddy-covariance measurements, (b) quantify the diurnal, seasonal and interannual variability of WUEe in relation to meteorological conditions, and (c) analyse between-site variability of WUEe as affected by vegetation type and climatic conditions, across sites in European forest ecosystems. Day-to-day variability of gross primary productivity (GPP) and evapotranspiration (ET) were more strongly coupled than net ecosystem production (NEP) and ET, obviously because NEP also depends on the respiration that is not heavily coupled to water fluxes. However, the slope of daytime NEP versus ET (mNEP) from half-hourly measurements of a single day may also be used as a WUEe-estimate giving very similar results to those of the GPP-ET slope (mGPP), since the diurnal variation is dominated by GPP. Since ET is the sum of transpiration (linked to GPP) and evaporation from wet vegetation and soil surfaces (not linked to GPP) we expected that WUEe is increasing when days after rain are excluded from the analysis. However only very minor changes were found, justifying an analysis of WUEe related to vegetation type. In most of the studied ecosystems the instantaneous WUEGPP was quite sensitive to diurnally varying meteorological conditions and tended to decline from the morning to the afternoon by more than 50% because of increasing vapour pressure deficits (VPD). Seasonally, WUEGPP increased with a rising monthly precipitation sum and rising average monthly temperatures up to a threshold of 11, 14 and 18°C in boreal, temperate and Mediterranean ecosystems, respectively. Across all sites, the highest monthly WUEGPP-values were detected at times of positive anomalies of summer-precipitation. During drought periods with high temperatures, high VPD, little precipitation and low soil water content, the water-use efficiency of gross carbon uptake (WUEGPP) tended to decrease in all forest types because of a stronger decline of GPP compared to ET. However the largest variation of growing season WUEGPP was found between-sites and significantly related to vegetation type: WUEGPP was highest in ecosystems dominated by deciduous trees ranging from 5.0 g CO2 kg H2O−1 for temperate broad-leaved deciduous forests (TD), to 4.5 for temperate mixed forests (TM), 3.5 for temperate evergreen conifers (TC), 3.4 for Mediterranean broad-leaved deciduous forests (MD), 3.3 for Mediterranean broad-leaved evergreen forests (Mbeg), 3.1 for Mediterranean evergreen conifers (MC), 2.9 for boreal evergreen conifers (BC) and only 1.2 g CO2 kg H2O−1 for a boreal wetland site (BT). Although vegetation type and meteorology co-vary, the WUEGPP variation was hardly related to meteorology, as we could show by comparing similar meteorological conditions only. Furthermore we compared across-site WUEGPP only under conditions when the 10% high GPP rates were exhibited. The between site differences remained, and at all sites ecosystem reached higher WUEGPP levels under this condition. This means when vegetation is most productive usually it also maximises the amount of carbon gained per water lost. Overall our results show that water-use efficiency exhibits a strong time-scale dependency in the sense that at longer time-scale meteorological conditions play a smaller role compared to shorter time scale. Moreover, we highlight the role of vegetation in determining carbon-water relation at ecosystem level. Consequently, all predictions of changing carbon-water cycle under changing climate should take into this role and the differences between vegetation types. These results show the strong time-scale dependency of water-use efficiencyBiogeosciences Discussions. 01/2008; -
Article: High-frequency measurements of productivity of planktonic algae using rugged nondispersive infrared carbon dioxide probes
Limnology and oceanography, methods 01/2008; 6:347-354. · 1.53 Impact Factor -
Article: The CO2-balance of boreal, temperate and tropical forests derived from a global database
Global Change Biology 01/2007; · 6.86 Impact Factor -
Article: Hydrocarbon fluxes above a Scots pine forest canopy: Measurements and modeling
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ABSTRACT: We measured the fluxes of several hydrocarbon species above a Scots pine (Pinus sylvestris) stand using disjunct eddy covariance technique with proton transfer reaction – mass spectrometry. The measurements were conducted during four days in July at SMEAR II research station in Hyytiälä, Finland. Compounds which showed significant emission fluxes were methanol, acetaldehyde, acetone, and monoterpenes. A stochastic Lagrangian transport model with simple chemical degradation was applied to assess the sensitivity of the above canopy fluxes to chemistry. According to the model, the chemical degradation had a minor effect on the fluxes measured in this study but has a major effect on the vertical flux profiles of more reactive compounds, such as sesquiterpenes. The monoterpene fluxes followed the traditional exponential temperature dependent emission algorithm but were considerably higher than the fluxes measured before at the same site. The normalized emission potential (30°C) was 2.5 μg gdw–1 h−1 obtained using the temperature dependence coefficient of 0.09°C−1.Atmospheric Chemistry and Physics Discussions. 01/2007;
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2006–2012
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University of Helsinki
- • Department of Forest Ecology
- • Department of Physical Sciences
Helsinki, Province of Southern Finland, Finland
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