Sami Haapanala

Annual cycle of methane emission from a boreal fen measured by the eddy covariance technique

The northern wetlands are one of the major sources of methane into the atmosphere. We measured annual methane emission from a boreal minerotrophic fen, Siikaneva, by the eddy covariance method. The average wintertime emissions were below 1 mg m-2 h-1, and the summertime emissions about 3.5 mg m-2 h-1. The water table depth did have any clear effect on methane emissions. During most of the year the emission depended on the temperature of peat below the water table. However, during the high and late summer the emission was independent on peat temperature as well. No diurnal cycle of methane flux was found. The total annual emission from the Siikaneva site was 12.6 g m-2. The emissions of the snow free period contributed 91% to the annual emission. The emission pulse during the snow melting period was clearly detectable but of minor importance adding only less than 3% to the annual emission. Over 20% of the carbon assimilated during the year as carbon dioxide was emitted as methane. Thus methane emission is an important component of the carbon balance of the Siikaneva fen. This indicates need of taking methane into account when studying carbon balances of northern fen ecosystems.

CO2 exchange of a sedge fen in southern Finland—the impact of a drought period

Eddy covariance (EC) measurements of net ecosystem CO2 exchange (NEE) were conducted on a boreal sedge fen in southern Finland (61°50’N, 24.12’E) during a 1.5-yr period covering two summers in 2004.2005. The EC data were complemented by chamber measurements, which enabled the partition of the daytime NEE into respiration and photosynthesis. A special emphasis was put on the hydrometeorological responses of CO2 exchange during a drought period in July 2005. A mean CO2 efflux of 0.009 mg CO2 m-2 s-1 was observed during mid-winter (January.February), while the night-time respiration during the two Julys averaged 0.09 mgCO2 m-2 s-1. During both years the mean midday uptake in late July was about -0.16 mgCO2 m-2 s-1. An annual CO2 balance of -188 g CO2 m-2 was observed in 2005.Aslightly higher net sink of -219 gCO2 m-2 was estimated for 2004. The drought period experienced in July 2005 caused a clear depression in the daily NEE values. From the combined analysis of EC and chamber measurements it was concluded that this was mainly due to increased respiration, but evidence was also found of suppressed photosynthesis due to a high VPD.

Spatial variation in plant community functions regulates carbon gas dynamics in a boreal fen ecosystem

The aim of this study was to asses how the variability in carbon gas exchange at the plant community scale affected the C gas exchange estimates at the ecosystem scale in a fen that was homogeneous in a micrometeorological sense, that is, had an even surface topography and plant cover. CO2 and CH4 exchange was measured at the plant community scale with chambers and at the ecosystem scale with the eddy covariance (EC) technique. Community-scale measurements were upscaled to the ecosystem scale by weighting the community-specific estimates by the area of the community. All communities were net CO2 sinks and CH4 sources during the growing season, but net ecosystem production (NEP) and CH4 emissions ranged from 21 to 190 g CO2-C m-2 and from 4.3 to 13 g CH4-C m-2, respectively, between the communities. The seasonal estimates of NEP and CH4, upscaled to the 200 m radius from the EC tower, were 82 and 7.9 g CH4-C m-2, which agreed well with the EC measurements. As the communities differed markedly in their C gas dynamics, their proportions controlled the ecosystem scale estimates. Successful upscaling required detailed knowledge on the proportions and leaf area of the communities.

Effects of cooling and internal wave motions on gas transfer coefficients in a boreal lake

Lakes and other inland waters contribute significantly to regional and global carbon budgets. Emissions from lakes are often computed as the product of a gas transfer coefficient, k 600 , and the difference in concentration across the diffusive boundary layer at the air–water interface. Eddy covariance (EC) techniques are increasingly being used in lacustrine gas flux studies and tend to report higher values for derived k 600 than other approaches. Using results from an EC study of a small, boreal lake, we modelled k 600 using a boundary-layer approach that included wind shear and cooling. During stratification, fluxes estimated by EC occasionally were higher than those obtained by our models. The high fluxes co-occurred with winds strong enough to induce deflections of the thermocline. We attribute the higher measured fluxes to upwelling-induced spatial variability in surface concentrations of CO2 within the EC footprint. We modelled the increased gas concentrations due to the upwelling and corrected our k 600 values using these higher CO2 concentrations. This approach led to greater congruence between measured and modelled k values during the stratified period. k 600 has a well-resolved and ~cubic relationship with wind speed when the water column is unstratified and the dissolved gases well mixed. During stratification and using the corrected k 600 , the same pattern is evident at higher winds, but k 600 has a median value of ~7 cm h−1 when winds are less than 6 m s−1, similar to observations in recent oceanographic studies. Our models for k 600 provide estimates of gas evasion at least 200% higher than earlier wind-based models. Our improved k 600 estimates emphasize the need for integrating within lake physics into models of greenhouse gas evasion.

Carbon dioxide and energy fluxes over a small boreal lake in Southern Finland

Dynamics of carbon dioxide and energy exchange over a small boreal lake were investigated. Flux measurements have been carried out by the eddy covariance technique during two open-water periods (June–October) at Lake Kuivajarvi in Finland. Sensible heat (H) flux peaked in the early morning, and upward sensible heat flux at night results in unstable stratification over the lake. Minimum H was measured in the late afternoon, often resulting in adiabatic conditions or slightly stable stratification over the lake. The latent heat flux (LE) showed a different pattern, peaking in the afternoon and having a minimum at night. High correlation (r2 = 0.75) between H and water-air temperature difference multiplied by wind speed (U) was found, while LE strongly correlated with the water vapor pressure deficit multiplied by U (r2 = 0.78). Monthly average values of energy balance closure ranged between 70 and 99%. The lake acted as net source of carbon dioxide, and the measured flux (FCO2) averaged over the two open-water periods (0.7 µmol m−2 s−1) was up to 3 times higher than those reported in other studies. Furthermore, it was found that during period of high wind speed (>3 m s−1) shear-induced water turbulence controls the water-air gas transfer efficiency. However, under calm nighttime conditions, FCO2 was poorly correlated with the difference between the water and the equilibrium CO2 concentrations multiplied by U. Nighttime cooling of surface water enhances the gas transfer efficiency through buoyancy-driven turbulent mixing, and simple wind speed-based transfer velocity models strongly underestimate FCO2.

An Overview of the Urban Boundary Layer Atmosphere Network in Helsinki

The Helsinki Urban Boundary-Layer Atmosphere Network (UrBAN: http://urban.fmi.fi) is a dedicated research-grade observational network where the physical processes in the atmosphere above the city are studied. Helsinki UrBAN is the most poleward intensive urban research observation network in the world and thus will allow studying some unique features such as strong seasonality. The networks key purpose is for the understanding of the physical processes in the urban boundary layer and associated fluxes of heat, momentum, moisture, and other gases. A further purpose is to secure a research-grade database, which can be used internationally to validate and develop numerical models of air quality and weather prediction. Scintillometers, a scanning Doppler lidar, ceilometers, a sodar, eddy-covariance stations, and radiometers are used. This equipment is supplemented by auxiliary measurements, which were primarily set up for general weather and/or air-quality mandatory purposes, such as vertical soundings and t...

Neglecting diurnal variations leads to uncertainties in terrestrial nitrous oxide emissions

Nitrous oxide (N2O) is an important greenhouse gas produced in soil and aquatic ecosystems. Its warming potential is 296 times higher than that of CO2. Most N2O emission measurements made so far are limited in temporal and spatial resolution causing uncertainties in the global N2O budget. Recent advances in laser spectroscopic techniques provide an excellent tool for area-integrated, direct and continuous field measurements of N2O fluxes using the eddy covariance method. By employing this technique on an agricultural site with four laser-based analysers, we show here that N2O exchange exhibits contrasting diurnal behaviour depending upon soil nitrogen availability. When soil N was high due to fertilizer application, N2O emissions were higher during daytime than during the night. However, when soil N became limited, emissions were higher during the night than during the day. These reverse diurnal patterns supported by isotopic analyses may indicate a dominant role of plants on microbial processes associated with N2O exchange. This study highlights the potential of new technologies in improving estimates of global N2O sources.

Hot-air Balloon Measurements of Vertical Variation of Boundary Layer New Particle Formation

In this study, we used a hot-air balloon as a platform for boundary layer particle and cluster measurements. We did altogether 11 flights during the spring of 2005 and 2006. During the spring of 2006, we observed five new particle formation days. During all days, new particle formation took place in the mixed boundary layer. During one of the days, we observed particle formation in the free troposphere, separate from that of the mixed layer. The observations showed that the concentration of freshly-formed 1.5-2 nm negative ions was several times higher than the concentration of positive ions. We also clearly observed that nucleation during one of the days, 13 March 2006, was a combination of neutral and ion-induced nucleation. During some of the days, particle growth stopped at around 3 nm, probably due to lack of condensable organic vapours. Simulations of boundary layer dynamics showed that particles are formed either throughout the mixed layer or in the lower part of it, not at the top of the layer.

Temporal Variation of Ecosystem Scale Methane Emission From a Boreal Fen in Relation to Temperature, Water Table Position, and Carbon Dioxide Fluxes

We have analyzed decade-long methane flux data set from a boreal fen, Siikaneva, together with data on environmental parameters and carbon dioxide exchange. The methane flux showed seasonal cycle but no systematic diel cycle. The highest fluxes were observed in July–August with average value of 73 nmol m−2 s−1. Wintertime fluxes were small but positive, with January–March average of 6.7 nmol m−2 s−1. Daily average methane emission correlated best with peat temperatures at 20–35 cm depths. The second highest correlation was with gross primary production (GPP). The best correspondence between emission algorithm and measured fluxes was found for a variable-slope generalized linear model (r2 = 0.89) with peat temperature at 35 cm depth and GPP as explanatory variables, slopes varying between years. The homogeneity of slope approach indicated that seasonal variation explained 79% of the sum of squares variation of daily average methane emission, the interannual variation in explanatory factors 7.0%, functional change 5.3%, and random variation 9.1%. Significant correlation between interannual variability of growing season methane emission and that of GPP indicates that on interannual time scales GPP controls methane emission variability, crucially for development of process-based methane emission models. Annual methane emission ranged from 6.0 to 14 gC m−2 and was 2.7 ± 0.4% of annual GPP. Over 10-year period methane emission was 18% of net ecosystem exchange as carbon. The weak relation of methane emission to water table position indicates that space-to-time analogy, used to extrapolate spatial chamber data in time, may not be applicable in seasonal time scales. (Less)