1,721,020 research outputs found
On the sensitivity decay of the cumarine targets for fast ozone measurements. Implications for the estimation of the instrumental zero and flux calculations
No full textFast ozone concentrations measurements are necessary in order to measure ozone fluxes with the eddy covariance technique. Since the development of the first instrument early in the 90s several other instruments, all based on a chemiluminescent reaction between ozone and a cumarine target, were developed but only in 2010 Mueller et al. recognized the importance of estimating the zero (i.e. the voltage at zero ozone concentration) which depends both on instrument and target performances. In this work we will show a new methodology to estimate the zero, this new methodology avoids some problems which were unsolved by the Mueller's one. Our first assumption wais that the sensitivity of the targets decays in an exponential way rather than a linear one, as proposed by Mueller et al. (2010). This assumption was in agreement with what proposed by Ermel et al. (2013) Similarly to the Mueller's approach, the first step we performed was plotting the instrument voltage output versus the ozone concentrations, but two main differences were introduced in our methodology: first of all we compared periods in which the target received a comparable ozone dose and then the estimation of the zero is extrapolated with an exponential fit of the data rather a linear one. In this way it was possible to avoid negative zeroes which were sometimes obtained, especially in the first 24/36 hours of the target life, by applying Mueller's methodology; negative zeroes lead to an underestimation of the ozone fluxes . After estimating the zero for some sub-periods of the target life, the evolution of the zero is modeled by interpolating the zero data as a function of the ozone dose received by the target. Moreover, with this approach the zero changes continuously with no abrupt change during the target life, avoiding remarkable discontinuities in the fluxes. Comparisons between the two methodologies will be showed
Size resolved aerosol fluxes above a deciduous forest: seasonal variability and underlying processes
No full textAtmospheric aerosol is a global concern for its detrimental effects on human health and climate. Vegetation has been identified as a possible sink for atmospheric particles. However, the interactions between vegetation and aerosols are not completely understood, and they are not limited to deposition only since also bidirectional aerosol exchanges can appear. Moreover, the knowledge of aerosol-vegetation interactions is quite limited for deciduous forests, as well as its seasonality which follows the appearance and disappearance of leaves on the crowns. Much of research, in fact, has been focused on conifers which have almost constant leaf area during the year.
For this sake, eddy covariance measurements of size-resolved aerosol fluxes were conducted in a broadleaf deciduous forest in the Po Valley, by means of a fast electric low-pressure impactor able to resolve 14 dimensional classes with cut-off diameters ranging from 6 nm to 10 μm. The measurements spanned approximately eight months and covered both a leaf-off period and a leaf on period, from leaf sprout to leaf senescence.
Overall, aerosol fluxes exhibited a distinct seasonality and variations in exchange direction depending on particle diameter. During the winter, when leaves were absent, most aerosol classes exhibited positive upward fluxes. In contrast, during the leaf-on season, ultrafine (dp<100 nm) and fine aerosols (100 nm<1 μm) displayed contrasting exchange patterns, with fine aerosol being predominantly deposited and ultrafine aerosol primarily emitted.
The vertical exchange of aerosols was found to be dependent on leaf area index, friction velocity and surface wetness. Vertical fluxes increased as LAI increased, both for fine aerosols -which were deposited- and for the ultrafine ones, which were emitted.
In dry conditions and in presence of leaves, at moderate friction velocities (u*<0.75 m/s) aerosol deposition increased as u* increased. However, above a u* value of 0.75 m/s upward fluxes appeared, and they increased as friction velocity increased becoming rapidly dominant. In wet conditions, leaf wetness did not enhance aerosol deposition and the latter started only when friction velocity increased above a certain threshold (0.4 m/s). No upward aerosol fluxes were observed in wet conditions.
An attempt to explain the observed deposition and resuspension processes will be provided
Biomass and physiological responses of Quercus robur (L.) young trees during 2 years of treatments with different levels of ozone and nitrogen wet deposition
No full textKey message: The root biomass of oak young trees significantly decreased after 2 years of exposure to high levels of ozone, but increased nitrogen wet deposition tended to partly contrast this effect. Abstract: A 2-year Open-Top Chamber (OTC) experiment with young Quercus robur trees that were exposed to different levels of ozone (O3) and nitrogen deposition was performed in Curno (Northern Italy) for the FP7 Project ÉCLAIRE. The plants were exposed to four levels of ozone (−40 % of ambient ozone in charcoal-filtered OTCs, −5 % in non-filtered OTCs, and +30 and +75 % in O3-enriched OTCs) and two levels of nitrogen wet deposition (tap water and tap water +70 kg N ha−1 year−1). The stomatal conductance and A/Ci response curves were measured during the two experimental seasons, and in October, the plant dry biomass partition between the roots and stem was assessed. Oak plants were moderately sensitive to O3. After the second year of treatments, the dose–response relationships based on the O3 stomatal flux indicated a 4.6 % of root biomass loss and a 12.1 % of reduction of the number of leaves per 10 mmol O3 m−2 absorbed by plants grown with no nitrogen addition. Ozone also decreased both the stomatal conductance and the maximum carboxylation rate allowed by Rubisco (Vcmax) during the first year of treatments. However, the effect on Vcmax was lost during the second year, and the plants showed an uncoupling between leaf-level physiological responses and plant-level biomass responses. Increased nitrogen deposition enhanced the growth of plants and partially mitigated the O3 impact on biomass and physiology, but no significant effect of the interaction between the two factors was found. The data that were collected could contribute to the definition of the O3 dose–response relationships based on biomass losses for deciduous trees in Southern Europe climatic conditions and could improve the O3 risk assessment models by providing new information about the effect of increased nitrogen deposition on the ozone impact
Evaluation of the uncertainty in the ozone flux effect modelling: from the experiments to the dose-response relationships
No full textThe paper is anattempt to assess theuncertainty involved in themodellingprocess ofderiving thefluxeeffects
relationships from the experimental evidences of a network of coordinated experiments on ozone and
vegetation (the LRTAP Convention’s ICP Vegetation).
Ten modelling steps have been described and analysed. Even if some steps were more critical than
others in introducing uncertainty, the whole modelling process seems to be quite robust.
The stomatal dose calculation is precise enough when performed on long time periods (e.g. growing
season). The main sources of inaccuracy for the stomatal dose calculation can be ascribed to the adoption
of different schemes for the calculation of ozone concentration at leaf level (7e10%). The estimation of
the ozone effects on vegetation, and particularly the derivation of the plant’s response at the theoretical
zero-ozone dose, proves to be the main source of uncertainty of the whole process (up to 17%). In order to
improve the precision of this step an increased number of ozone treatments and replicates in the
experiments is recommended, while a harmonization/standardization of the calculation algorithm is
required to improve the accuracy of the dose calculation
One year of ozone flux measurements and O3, NOx and CO2 profiles at the micrometeorological flux tower of Bosco Fontana (Mantua, Italy)
No full textIn the framework of the ÉCLAIRE Project of the 7th EU framework programme an intensive joint measuring campaign and a subsequent long-term campaign have been conducted at a oak-hornbeam mixed mature forest in the Po Valley (Bosco Fontana, Mantova, Italy).
Within the forest a 42 m tall micrometeorological tower was installed in order to assess the mass and energy exchange between the ecosystem and the surrounding air, with the focus of the campaign measurements having been the in-canopy chemistry and exchange of aerosols and the focus of the long-term measurements being the ozone and carbon emission/uptake.
During the joint measuring campaign a profile of ozone fluxes was obtained at four levels above and within the canopy, using the EC technique, with the ozone fluxes at the top level continuing during the long-term measurements, together with fluxes of water and CO2.
The forest represented a relevant sink of tropospheric ozone, both in summer and in winter, with an average monthly ozone removal of 4.4 kg ha-1 and a total annual deposition of more than 50 kg ha-1.. The amount of ozone removed by the forest after entering the leaves stomates was estimated by deriving the bulk stomatal resistance through the inversion of the Monteith equation and then using it to calculate the ozone stomatal fluxes. The stomatal sink resulted only a minor part of the ozone removed by the forest but it is meaningful for trees since ozone is a phototoxic agent. The maximum stomatal fraction was derived for June (40% of the total ozone flux) and the minimum for October (13%). The remaining part of the ozone fluxes, obtained as a residual among the total and the stomatal fluxes, is due to a complex of removal processes – either of physical and chemical nature- collectively named as “non-stomatal” deposition. The non-stomatal ozone deposition, not causing any relevant phototoxic effect, could be regarded a net ecosystem service offered by the forest to the urban community. On the contrary the net carbon sequestration by the forests was almost negligible on an annual basis, as it could be expected for a mature ecosystem.
The main results on ozone flux profiles, ozone flux partition and storage will be presented.
This kind of study highlights the importance of making continuous direct measurements in order to raise the knowledge and the awareness of the ecosystem services offered to the cities by the urban and periurban vegetation
Eddy covariance measurements of ozone fluxes at 4 levels above and within a forest canopy in the Po valley (Italy)
No full textIn June and July 2012, during the intensive field campaign of the ECLAIRE project, ozone fluxes as well as
sensible heat and momentum fluxes were measured at four different levels at the Bosco Fontana site, a 26 m tall
mixed oak-hornbeam forest in the Po valley (I). Each measuring level (41 m, 32 m, 24 m and 16 m) was equipped
with a sonic anemometer and a fast ozone analyser, absolute ozone concentrations were measured by means of
a UV photometer. Additional meteorological parameters were measured on the top and at each level and ozone
concentrantions were measured at ground level (0.15 m) by another UV photometer.
In order to compare measurements collected with different instruments, data were preliminary processed and
the following methodology were applied: despiking, instantaneous rotations, WPL corrections, frequency loss
corrections and calculation of the random error.
The main aims of this field campaign were the description of the deposition processes within and above canopy
and the test of the constant flux hypothesis. About this latter interesting features emerged: the conservation of
the flux was valid for the momentum only outside the canopy and no conservation was observed for the sensible
heat. On the contrary ozone fluxes showed a much more irregular behaviour: for the three upper levels, ozone
fluxes were nearly constant in the first hours of the day while an enhancement of the fluxes was observed at 24
m. This latter fact was strictly linked with the in-canopy dynamics: a greater heating of the canopy was observed
in the afternoon, leading to the formation of an inversion at this level. This inversion divided the in-canopy air
volume into two layers: the lower one with a stable stratification and the upper one with a turbulent regime; as a
consequence of this inversion a remarkable reduction of the ozone concentration was observed for the two lowest
levels in the afternoon, when in the lowest layers ozone is consumed by NO emissions from soil.
The ozone canopy uptake was estimated in two different ways: with the big-leaf approach and as a difference of
the above and below canopy fluxes. A comparison of the results of these two approaches will be showed
Plants in the city and their gaseous exchanges with the atmosphere. A possible way to estimate the air pollutant removal by plants and the related biological cost.
No full textThe basic concepts on gas exchange between plants and the atmosphere,
both at leaf and ecosystem level, are summarized and presented in this paper. After the
illustration of the diffusional processes and the introduction of the electric analogy for
the stomatal resistance, the turbulent processes are described and the Eddy Covariance
technique is presented with all its assumptions and the applicability criteria for meaningful
measurements. Then, the interpretation of measurement at ecosystem level is
described for a paradigmatic phytotoxical pollutant, ozone. The big-leaf concept and the
procedure of flux partition between stomatal and non-stomatal deposition are presented
and discussed, as well as the metric used to estimate the phytotoxical ozone dose and
the ozone risk assessment for vegetation. The non-stomatal deposition processes are also
discussed and examples of measurements performed in peri-urban reserves and other
ecosystems mainly under the TreeCity PRIN 2010-2011 project and the ECLAIRE FP7
Project, are presente
Turbulence in a Coastal Mediterranean Area: Surface Fluxes and Related Parameters at Castel Porziano, Italy
Full text linkMeasurement of turbulence above a Mediterranean maquis at Castelporziano, Rom
Measurements of Soil Carbon Dioxide Emissions from Two Maize Agroecosystems at Harvest under Different Tillage Conditions
Full text linkIn this study a comparison of the soil CO2 fluxes emitted from two maize (Zea mays L.) fields with the same soil type was performed. Each field was treated with a different tillage technique: conventional tillage (30 cm depth ploughing) and no-tillage. Measurements were performed in the Po Valley (Italy) from September to October 2012, covering both pre- and postharvesting conditions, by means of two identical systems based on automatic static soil chambers. Main results show that no-tillage technique caused higher CO2 emissions than conventional tillage (on average 2.78 and 0.79 μmol CO2 m−2 s−1, resp.). This result is likely due to decomposition of the organic litter left on the ground of the no-tillage site and thus to an increased microbial and invertebrate respiration. On the other hand, fuel consumption of conventional tillage technique is greater than no-tillage consumptions. For these reasons this result cannot be taken as general. More investigations are needed to take into account all the emissions related to the field management cycle
Size-resolved aerosol fluxes above a broadleaved deciduous forest
No full textIn order to understand the aerosol exchange dynamics between the atmosphere and a peri-urban forest ecosystem located in the Po Valley, a region characterized by high PM concentrations, Eddy Covariance (EC) aerosol fluxes were measured between September and December 2017. The aerosol sampling, performed with an Electrical Low Pressure Impactor (ELPI +, DEKATI), involved a wide range of particle sizes including both ultrafine and fine aerosol. The monitoring campaign comprised a period with leaves (PL) and a period without leaves (PNL) to assess their influence on the emission and deposition fluxes.
The diurnal profiles of PN (Particle Number) fluxes associated to the geometric mean diameters (GMD) of 0.02 μm and 0.48 μm were chosen as representative of the behaviour of ultrafine and fine particles, respectively. Fluxes of ultrafine particles showed a net emission pattern both in PL and PNL assuming values up to 5.6 ·106 m-2 s-1 and 4.5·106 m-2 s-1, respectively. Instead, fine particles fluxes showed a net deposition pattern in PL, assuming values up to -1.1 ·106 m-2 s-1, while in PNL a slight emission up to 4.1 ·105 m-2 s-1 occurred. The behaviour of the fluxes of the cumulative classes PM0.1 and PM1 was similar to the one of the PN fluxes of ultrafine and fine aerosol, respectively.
Deposition velocities were calculated for PL and PNL depending on the atmospheric stability class. The values emerged from this study (from -0.25 cm s-1 up to 0.12 cm s-1) evidenced that under stable and very stable atmospheric conditions all size classes presented negative or slightly positive deposition velocities both in PL and PNL. Instead under unstable conditions fine particles showed deposition velocities whose direction changed in the two periods (PL and PNL)
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