Water vapor correction functions for CO2 and CH4 in cavity ring-down spectroscopy

Cavity ring-down spectroscopy is a laser absorption technique based on the principle of measuring the rate of exponential decay of light intensity inside the ring-down cavity. When the absorption spectrum of a gas is known, it is possible to determine the mole fraction of this gas by measuring the height of the absorption peak, which can be acquired from the rate of decay of light. This technique is used in G1301, G2301 and G2401 (Picarro Inc.) gas analyzers which measure carbon dioxide (CO2), methane (CH4) and water vapor. However, measured gas mole fractions are diluted from their actual value; mostly due to variations in atmospheric water vapor. This effect causes large errors and it has to be corrected either by drying the sample or applying a water vapor correction. A default water vapor correction is included in Picarro gas analyzers, but it might not be accurate enough for use in some measurements. In this study, determination of water vapor correction coefficients was carried out by doing several droplet tests for seven different gas analyzers, which included one G2401, two G1301, four G2301gas analyzers. Mean correction functions determined for the analyzers were compared to the Picarro default correction. In addition, the comparison was made with time series data for one of the analyzers. Also, the water vapor measurement of the gas analyzers was calibrated to acquire the actual water vapor mole fraction. As a result, the factory correction for CO2 was proved sufficient for high accuracy measurements only up to 0.7 % water vapor mole fraction. For CH4, the factory coefficient was enough up to 2.0 %, which corresponds to dew point temperature of 18 °C. In conclusion, neither of factory corrections is enough for use all year round. So, the water vapor correction should be made for each gas analyzer when making high accuracy measurements. Due to cyclic drift of water vapor measurement, the correction should remain stable over time, but this needs further verification. Currently, the correction should be made at least once per year.Ontelovaimenemisspektroskopia on absorptiomenetelmä, jossa mitataan valon intensiteetin eksponentiaalista vaimenemista useita voimakkaasti heijastavia peilejä sisältävässä ontelossa. Kun tiedetään kaasun absorptiospektri, niin pystytään määrittämään kaasun mooliosuus mittaamalla absorptiopiikin korkeutta, mikä saadaan valon intensiteetin vaimenemisnopeudesta. Tätä tekniikkaa käytetään G1301, G2301 and G2401 -kaasuanalysaattoreissa (Picarro Inc.), jotka mittaavat hiilidioksidin, metaanin ja vesihöyryn mooliosuuksia. Mittausten ongelmana on kaasujen mooliosuuksien laimeneminen, joka johtuu pääosin ilmakehän vesihöyryn määrän vaihtelusta. Tämä ilmiö aiheuttaa suuria virheitä ja se täytyy korjata joko kuivaamalla näyte ennen mittausta tai tekemällä vesihöyrykorjaus. Korjaus on sisäänrakennettu Picarron valmistamissa kaasuanalysaattoreissa, mutta tämä tehdaskorjaus ei välttämättä ole riittävä mittauksissa, joissa tarvitaan suurta tarkkuutta. Tässä tutkielmassa vesihöyrykorjaus suoritettiin kahdelle G1301, neljälle G2301 ja yhdelle G2401-kaasuanalysaattorille tekemällä jokaiselle muutama pisarakoe. Pisarakokeista saaduista korjausfunktioista laskettiin keskiarvot eri analysaattoreille, mitä verrattiin tehdaskorjauksen arvoihin. Lisäksi vertailu tehtiin yhden kaasuanalysaattorin kohdalla myös aikasarja-analyysin avulla. Kaikille kaasuanalysaattoreille tehtiin myös vesihöyrykalibrointi, jotta saatiin selville todellinen vesihöyryn mooliosuus. Hiilidioksidin tehdaskorjaus osoittautui riittäväksi, mikäli vesihöyryn mooliosuus on alle 0.7 %. Metaanille tehdaskorjaus oli riittävä 2.0 % mooliosuuteen asti, mikä vastaa 18 °C kastepistelämpötilaa. Kumpikaan näistä tehdaskorjauksista ei ole riittävä ympärivuotiseen mittaamiseen, joten on suositeltavaa tehdä vesihöyrykorjaus erikseen jokaiselle kaasuanalysaattorille, jotta mittauksen tarkkuus olisi mahdollisimman hyvä. Vesihöyrymittauksen ryömintä on laitetekniikasta johtuen jaksottainen, joten vesihöyrykorjauksen pitäisi pysyä stabiilina ajan kanssa, mutta tätä ei ole vielä vahvistettu. Tällä hetkellä vesihöyrykorjaus tulisi tehdä vähintään kerran vuodessa

Excess soil moisture and fresh carbon input are prerequisites for methane production in podzolic soil

Boreal upland forests are generally considered methane (CH4) sinks due to the predominance of CH4 oxidizing bacteria over the methanogenic archaea. However, boreal upland forests can temporarily act as CH4 sources during wet seasons or years. From a landscape perspective and in annual terms, this source can be significant as weather conditions may cause flooding, which can last a considerable proportion of the active season and because often, the forest coverage within a typical boreal catchment is much higher than that of wetlands. Processes and conditions which change mineral soils from acting as a weak sink to a strong source are not well understood. We measured soil CH4 fluxes from 20 different points from regularly irrigated and control plots during two growing seasons. We also estimated potential CH4 production and oxidation rates in different soil layers and performed a laboratory experiment, where soil microcosms were subjected to different moisture levels and glucose addition simulating the fresh labile carbon (C) source from root exudates. The aim was to find the key controlling factors and conditions for boreal upland soil CH4 production. Probably due to long dry periods in both summers, we did not find occasions of CH4 production following the excess irrigation, with one exception in July 2019 with emission of 18 200 µg CH4 m−2 h−1. Otherwise, the soil was always a CH4 sink (median CH4 uptake rate of 260–290 and 150–170 µg CH4 m−2 h−1, in control and irrigated plots, respectively). The median soil CH4 uptake rates at the irrigated plot were 88 % and 50 % lower than at the control plot in 2018 and 2019, respectively. Potential CH4 production rates were highest in the organic layer (0.2–0.6 nmol CH4 g−1 d−1), but some production was also observed in the leaching layer, whereas in other soil layers, the rates were negligible. Potential CH4 oxidation rates varied mainly within 10–40 nmol CH4 g−1 d−1, except in deep soil and the organic layer in 2019, where potential oxidation rates were almost zero. The laboratory experiment revealed that high soil moisture alone does not turn upland forest soil into a CH4 source. However, a simple C source, e.g., substrates coming from root exudates with high moisture, switched the soil into a CH4 source. Our unique study provides new insights into the processes and controlling factors on CH4 production and oxidation, and the resulting net efflux that should be incorporated in process models describing global CH4 cycling

Methane exchange at the peatland forest floor - automatic chamber system exposes the dynamics of small fluxes

We measured methane (CH4) exchange rates with automatic chambers at the forest floor of a nutrient-rich drained peatland in 2011-2013. The fen, located in southern Finland, was drained for forestry in 1969 and the tree stand is now a mixture of Scots pine, Norway spruce, and pubescent birch. Our measurement system consisted of six transparent chambers and stainless steel frames, positioned on a number of different field and moss layer compositions. Gas concentrations were measured with an online cavity ring-down spectroscopy gas analyzer. Fluxes were calculated with both linear and exponential regression. The use of linear regression resulted in systematically smaller CH4 fluxes by 10-45% as compared to exponential regression. However, the use of exponential regression with small fluxes (Peer reviewe

Impact of partial harvest on CH4 and N2O balances of a drained boreal peatland forest

Rotation forestry including clearcutting is a common method of practising forestry in Fennoscandia. Clearcutting in peatland forests markedly increases environmental loading: leaching of nutrients and methane (CH4) and nitrous oxide (N2O) fluxes from soil. Continuous cover forestry has been suggested as an alternative because it does not include clearcutting but partial harvesting. However, impacts of partial harvesting on greenhouse gas fluxes are not well understood and in peatlands have not been studied at all. We conducted a partial harvest by removing 70% of the total stem volume in a mature nutrient-rich peatland forest in Southern Finland. The aim was to investigate how partial harvesting a peatland forest affects CH4 and N2O balances, and how much different surface types contribute to the balances. We used automatic and manual chamber methods to measure fluxes from both harvest and uncut control site. Fluxes were measured from the forest floor, logging trails, and ditches. Fluxes from these surface types were upscaled to obtain net ecosystem-level fluxes during two postharvest summers (June-August 2016 and 2017). After the harvest, forest floor CH4 fluxes did not change significantly at the harvested site compared to the control site. However, fluxes at logging trails increased significantly. N2O fluxes increased at the harvest site in the post-harvest years, but so did those at the control site as well. Upscaling CH4 fluxes to ecosystem-level indicated that despite their small area (2.4%), emissions from ditches could be large on ecosystem-scale, but their uncertainty was high, while the logging trail CH4 fluxes (20% of the total area) were small. In contrast, N2O fluxes from ditches were low, but the logging trail fluxes comprised 35-38% of the total surface balance. The overall conclusion is that partial harvesting did not cause considerable changes in CH4 and N2O fluxes from a forestry-drained peatland.Peer reviewe

Vegetation controls of water and energy balance of a drained peatland forest: Responses to alternative harvesting practices

We quantified the response of peatland water table level (WTL) and energy fluxes to harvesting of a drained peatland forest. Two alternative harvests (clear-cut and partial harvest) were carried out in a mixed-species ditch-drained peatland forest in southern Finland, where water and energy balance components were monitored for six pre-treatment and three post-treatment growing seasons. To explore the responses caused by harvestings, we applied a mechanistic multi-layer soil-plant-atmosphere transfer model. At the clear-cut site, the mean growing season WTL rose by 0.18 +/- 0.02 m (error estimate based on measurement uncertainty), while net radiation, and sensible and latent heat fluxes decreased after harvest. On the contrary, we observed only minor changes in energy fluxes and mean WTL (0.05 +/- 0.03 m increase) at the partial harvest site, although as much as 70% of the stand basal area was removed and leaf-area index was reduced to half. The small changes were mainly explained by increased water use of spruce undergrowth and field layer vegetation, as well as increased forest floor evaporation. The rapid establishment of field layer vegetation had a significant role in energy balance recovery at the clear-cut site. At partial harvest, chlorophyll fluorescence measurements and model-data comparison suggested the shade-adapted spruce undergrowth was suffering from light stress during the first post-harvest growing season. We conclude that in addition to stand basal area, species composition and stand structure need to be considered when controlling WTL in peatland forests with partial harvesting. Our results have important implications on the operational use of continuous cover forestry on drained peatlands. A continuously maintained tree cover with significant evapotranspiration capacity could enable optimizing WTL from both tree growth and environmental perspectives.Peer reviewe

Very High Spatial Resolution Soil Moisture Observation of Heterogeneous Subarctic Catchment Using Nonlocal Averaging and Multitemporal SAR Data

A soil moisture estimation method was developed for Sentinel-1 synthetic aperture radar (SAR) ground range detected high resolution (GRDH) data to analyze moisture conditions in a gently undulating and heterogeneous subarctic area containing forests, wetlands, and open orographic tundra. In order to preserve the original 10-m pixel spacing, PIMSAR (pixel-based multitemporal nonlocal averaging) nonlocal mean filtering was applied. It was guided by multitemporal statistics of SAR images in the area. The gradient boosted trees (GBT) machine learning method was used for the soil moisture algorithm development. Discrete and continuous in situ soil moisture values were used for training and validation of the algorithm. For surface soil moisture, the root mean square error (RMSE) of the method was 6.5% and 8.8% for morning and evening images, respectively. The corresponding maximum errors were 34.1% and 33.8%. The pixelwise sensitivity to the training set and method choice was estimated as the variance of the soil moisture values derived using the algorithms for the three best methods with respect to the criteria: the smallest maximum error, the smallest RMSE value, and the highest coefficient of determination (R-2) value. It was, on average, 6.3% with a standard deviation of 5.7%. Our approach successfully produced instantaneous high-resolution soil moisture estimates on daily basis for the subarctic landscape and can further be applied to various hydrological, biogeochemical, and management purposes.Peer reviewe

Very High Spatial Resolution Soil Moisture Observation of Heterogeneous Subarctic Catchment Using Nonlocal Averaging and Multitemporal SAR Data

A soil moisture estimation method was developed for Sentinel-1 synthetic aperture radar (SAR) ground range detected high resolution (GRDH) data to analyze moisture conditions in a gently undulating and heterogeneous subarctic area containing forests, wetlands, and open orographic tundra. In order to preserve the original 10-m pixel spacing, PIMSAR (pixel-based multitemporal nonlocal averaging) nonlocal mean filtering was applied. It was guided by multitemporal statistics of SAR images in the area. The gradient boosted trees (GBT) machine learning method was used for the soil moisture algorithm development. Discrete and continuous in situ soil moisture values were used for training and validation of the algorithm. For surface soil moisture, the root mean square error (RMSE) of the method was 6.5% and 8.8% for morning and evening images, respectively. The corresponding maximum errors were 34.1% and 33.8%. The pixelwise sensitivity to the training set and method choice was estimated as the variance of the soil moisture values derived using the algorithms for the three best methods with respect to the criteria: the smallest maximum error, the smallest RMSE value, and the highest coefficient of determination (R-2) value. It was, on average, 6.3% with a standard deviation of 5.7%. Our approach successfully produced instantaneous high-resolution soil moisture estimates on daily basis for the subarctic landscape and can further be applied to various hydrological, biogeochemical, and management purposes.Peer reviewe

Fast recovery of suppressed Norway spruce trees after selection harvesting on a drained peatland forest site

Continuous cover forestry (CCF) has been promoted as an environmentally sustainable option for drained peatlands. The CCF management has been challenged due to potentially lower tree growth compared to tradi-tional even-aged management, especially with suppressed trees that are released during a selection harvesting under CCF management. Our objective was to quantify the time lag of stem diameter growth response of suppressed Norway spruce trees (Picea abies Karst.) after a selection harvesting compared to that of dominant trees. We also tested if the carbon assimilation of the trees increased immediately after selection harvesting. We used radial increment cores from suppressed Norway spruce trees to estimate the impact of selection harvesting on the diameter growth and intrinsic water use efficiency (iWUE). We measured carbon isotope composition (delta 13C) of wood, to quantify how the reduced competition between trees altered iWUE and its components, the photosynthetic rate (A) and sto-matal conductance (g). The study was conducted in the Lettosuo experimental site on fertile forestry drained peatland area in southern Finland. Approximately 70 % of the initial stand area (18.5 ha) was harvested according to CCF principles by applying selection harvesting, and the rest of the area was divided to intact control area and to clear-cut area. In the study site, by selection harvest, trees were removed from multiple age classes, but especially mature trees individually or in a small groups were taken away to maintain uneven-aged structure of the forest. All the target trees grew in the similar competitive position before selection harvesting. Our results show that there was a delay with the diameter growth of the suppressed trees to selection har-vesting, whereas the most significant growth-enhancing effect occurred three-four years after selection har-vesting. In contrast to the delay in the increment, the photosynthetic rate relative to stomatal conductance increased immediately after selection harvesting, as shown by the instant 2.5 parts per thousand increase in delta 13C to a post-harvest level. Our results show that carbon uptake increased immediately for suppressed Norway spruce trees after selection harvesting, but the harvest did not induce a clear increase in stem diameter growth during the first years after the harvest.Peer reviewe

Greenhouse gas and energy fluxes in a boreal peatland forest after clear-cutting

The most common forest management method in Fennoscandia is rotation forestry, including clear-cutting and forest regeneration. In clear-cutting, stem wood is removed and the logging residues are either removed or left on site. Clear-cutting changes the microclimate and vegetation structure at the site, both of which affect the site's carbon balance. Peat soils with poor aeration and high carbon densities are especially prone to such changes, and significant changes in greenhouse gas exchange can be expected. We measured carbon dioxide (CO2) and energy fluxes with the eddy covariance method for 2 years (April 2016-March 2018) after clear-cutting a drained peatland forest. We observed a significant rise (23 cm) in the water table level and a large CO2 source (first year: 3086 +/- 148 g CO2 m(-2) yr(-1); second year: 2072 +/- 124 g CO2 m(-2) yr(-1)). These large CO2 emissions resulted from the very low gross primary production (GPP) following the removal of photosynthesizing trees and the decline of ground vegetation, unable to compensate for the decomposition of logging residues and peat. During the second summer (June-August) after the clear-cutting, GPP had already increased by 96% and total ecosystem respiration decreased by 14% from the previous summer. The mean daytime ratio of sensible to latent heat flux decreased after harvesting from 2.6 in May 2016 to 1.0 in August 2016, and in 2017 it varied mostly within 0.6-1.0. In April-September, the mean daytime sensible heat flux was 33% lower and latent heat flux 40% higher in 2017, probably due to the recovery of ground vegetation that increased evapotranspiration and albedo of the site. In addition to CO2 and energy fluxes, we measured methane (CH4) and nitrous oxide (N2O) fluxes with manual chambers. After the clear-cutting, the site turned from a small CH4 sink into a small source and from N2O neutral to a significant N2O source. Compared to the large CO2 emissions, the 100-year global warming potential (GWP100) of the CH4 emissions was negligible. Also, the GWP100 due to increased N2O emissions was less than 10% of that of the CO2 emission change.Peer reviewe