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The static chamber method (non-flow-through-non-steady-state chambers) is the most common method to measure fluxes of methane (CH4) from soils. Laboratory comparisons to quantify errors resulting from chamber design, operation and flux calculation methods are rare. We tested fifteen chambers against four flux levels (FL) ranging from 200 to 2300 g CH4m−2 h−1. The measurements were conducted on a calibration tank using three quartz sand types with soil porosities of 53% (dry fine sand, S1), 47% (dry coarse sand, S2), and 33% (wetted fine sand, S3). The chambers tested ranged from 0.06 to 1.8 m in height, and 0.02 to 0.195 m3 in volume, 7 of them were equipped with a fan, and 1 with a vent-tube. We applied linear and exponential flux calculation methods to the chamber data and compared these chamber fluxes to the reference fluxes from the calibration tank. The chambers underestimated the reference fluxes by on average 33% by the linear flux calculation method (Rlin), whereas the chamber fluxes calculated by the exponential flux calculation method (Rexp) did not significantly differ from the reference fluxes (p < 0.05). The flux under- or overestimations were chamber specific and independent of flux level. Increasing chamber height, area and volume significantly reduced the flux underestimation (p < 0.05). Also, the use of non-linear flux calculation method significantly improved the flux estimation; however, simultaneously the uncertainty in the fluxes was increased. We provide correction factors, which can be used to correct the under- or overestimation of the fluxes by the chambers in the experiment.

Eddy covariance (EC) measurements of turbulent fluxes of momentum, sensible heat and latent heat—in addition to net radiation measurements—were conducted for three consecutive years in an urban environment: Helsinki, Finland. The aims were to: (i) quantify the detection limit and random uncertainty of turbulent fluxes, (ii) assess the systematic error caused by EC calculation-procedure choices on the energy balance residual, and (iii) report the energy balance of the world’s northernmost urban flux station. The mean detection limits were about 10% of the observed flux, and the random uncertainty was 9–16%. Of all fluxes, the latent heat flux— as measured with a closed-path gas analyzer—was most prone to systematic calculation errors due to water vapor interactions with tube walls: using a lag window that is too small can cause a 15% lack of data (due to the dependency of lag time on relative humidity) and omitting spectral corrections can cause on average a 26% underestimation of the flux. The systematic errors in EC calculation propagate into the energy balance residual and can be larger than the residual itself: for example, omitting spectral corrections overestimates the residual by 13% or 18% on average, depending on the analyzer.

Surface-atmosphere exchange of momentum, energy and atmospheric constituents affects the atmosphere--from alterations in local microclimates and mesoscale weather to climate modification. These exchange processes can be studied using direct eddy-covariance (EC) measurements of vertical turbulent transport, but the technique has not yet readily been applied in non-prevailing ecosystems. Thus, the aim of this thesis is to extend the applicability of the EC technique in two ways: to non-standard sites and by further developing the technique itself. To reach the aim, EC measurements over a boreal lake and three urban sites in Helsinki were performed. Long-term measurements over a lake revealed that the water below the thermocline was decoupled from the atmosphere and thus not important for atmospheric vertical turbulent fluxes. The energy exchange between the lake and the atmosphere departs from vegetated surfaces especially due to large nocturnal evaporation fuelled by lake-water heat storage. Long-term measurements at a semi-urban site in Helsinki showed that the surface-atmosphere exchange is altered by anthropogenic activity: changes in surface-cover and an additional anthropogenic heat release (13 W m-2) led to an altered surface energy balance, and anthropogenic CO2 emissions led to a large positive annual CO2 balance (1.8 kg C m-2). Intra-site and intra-city variation in surface-cover led to differences in atmospheric stability and CO2 emissions. The EC technique evaluation demonstrated that (i) the 'energy imbalance problem' in EC measurements is not primarily surface-cover dependent, and that (ii) common calculation errors in EC calculations can be almost 30% of the flux. Water vapour flux measurements with a closed-path analyser were affected by sorption: the signal's arrival is delayed and it is attenuated. A new spectral-correction method based on wavelet analysis was developed to automatically correct for this signal attenuation of constituents. The conclus, Pyörrekovarianssitekniikalla maanpinnan ja ilmakehän välisen vuorovaikutuksen jäljillä Maanpinta ja ilmakehä ovat vuorovaikutuksessa keskenään erilaisten energian ja materian pystyvirtausten, eli voiden, kautta. Turbulenttiset pystysuuntaiset vuot kuljettavat muun muassa lämpöä, vesihöyryä ja hiilidioksidia. Turbulenttisten voiden suuruus riippuu pinnan tyypistä sekä ilmakehän ominaisuuksista, ja vuot vaikuttavat sekä paikallissäähän että ilmastoon. Ihmisten aiheuttamat lämpöpäästöt voivat esimerkiksi aiheuttaa korkeampia lämpötiloja kaupungeissa ja hiilidioksidipäästöt kiihdyttävät ilmastonmuutosta. Näitä turbulenttisia pystyvoita voidaan mitata suoralla pyörrekovarianssi-mittaustekniikalla. Se perustuu korkeataajuisiin tuulen, lämpötilan ja kaasujen pitoisuusmittauksiin, jotka tehdään korkeassa mastossa, tutkittavan ekosysteemin yläpuolella. Tämän väitöstyön tavoitteena oli laajentaa pyörrekovarianssi-tekniikan soveltuvuutta uusiin ympäristöihin (järvi ja kaupunki) sekä kehittää uusia vuonlaskentamenetelmiä. Tutkimus perustui useamman vuoden mittauksiin eteläsuomalaisella järvellä sekä kolmella mittausasemalla Helsingissä. Työn uusia ympäristöjä käsittelevässä osiossa osoitettiin, että järven pinnan energiatase eroaa suuresti maanpinnan taseesta, koska järvi haihduttaa vettä myös läpi yön ja sillä on suuri lämmönvarastointikapasiteetti. Nämä tulokset painottavat kuinka tärkeää on huomioida järvien olemassaolo säänennustusmalleissa. Kumpulan kampuksella Helsingissä tehdyt mittaukset osoittivat, että ihmisperäinen lämmönpäästö on suurta (13 W m 2), ja että hiilidioksidipäätöt aiheuttavat positiivisen vuositaseen (1.8 kg C m 2), vaikka kasvillisuus onkin hiilidioksidinielu. Kaupungin sisäisen vaihtelun analyysi osoitti, että Helsingin keskustan suuret lämpöpäästöt pitävät meteorologiset inversiotilanteet loitolla myös talvisin, ja että keskustassa on myös jatkuvasti suuremmat hiilidioksidipäästöt kuin Kumpulassa, koska kasvillisuutta on vähemmän. Näitä tuloksia voida

Understanding seasonal variations in the evaporation of inland waters (e.g., lakes and reservoirs) is important for water resource management as well as the prediction of the hydrological cycles in response to climate change. We analyzed eddy covariance-based evaporation measurements from the Ross Barnett Reservoir (32◦26N, 90◦02W; which is always ice-free) in central Mississippi during the cool months (i.e., September–March) of 2007 and 2008, and found that the variability in cold front activities (i.e., passages of cold fronts and cold/dry air masses behind cold fronts) played an important role in modulating the exchange of sensible (H) and latent (λE) heat fluxes. Our analysis showed that 2007’s warmer cool season had smaller mean H and λE than 2008’s cooler cool season. This implies that the warmer cool season did not accelerate evaporation and heat exchange between the water surface and the atmosphere. Instead, more frequent cold fronts and longer periods of cold/dry air masses behind the cold fronts in 2008 resulted in overall larger H and λE as compared with 2007, this primarily taking the form of sporadic short-term rapid ‘pulses’ of H and λE losses from the water’s surface. These results suggest that future climate-induced changes in frequency of cold fronts and the meteorological properties of the air masses behind cold fronts (e.g., wind speeds, temperature, and humidity), rather than other factors of climate change, would produce significant variations in the water surface’s energy fluxes and subsequent evaporation rates.