Consequences of Incomplete Surface Energy Balance Closure for CO2 Fluxes from Open-Path CO2/H2O Infrared Gas Analysers

We present an approach for assessing the impact of systematic biases in measured energy fluxes on CO₂ flux estimates obtained from open-path eddy-covariance systems. In our analysis, we present equations to analyse the propagation of errors through the Webb, Pearman, and Leuning (WPL) algorithm [Quart. J. Roy. Meteorol. Soc. 106, 85–100, 1980] that is widely used to account for density fluctuations on CO₂ flux measurements. Our results suggest that incomplete energy balance closure does not necessarily lead to an underestimation of CO₂ fluxes despite the existence of surface energy imbalance; either an overestimation or underestimation of CO₂ fluxes is possible depending on local atmospheric conditions and measurement errors in the sensible heat, latent heat, and CO₂ fluxes. We use open-path eddy-covariance fluxes measured over a black spruce forest in interior Alaska to explore several energy imbalance scenarios and their consequences for CO₂ fluxes.     

The correct form of the Webb,Pearman and Leuning equation for eddy fluxes of trace gases in steady and non-steady state,horizontally homogeneous flows

The original density corrections proposed by Webb et al. [Webb EK, Pearman GI, Leuning R (1980) Quart J Roy Meteorol Soc 106:85–100] for calculating the eddy fluxes of trace gases are shown to be correct for both steady and non-steady state, horizontally homogeneous flows. The revised theory replaces the original assumption of zero vertical flux of dry air with the requirement of no sources or sinks of dry air in the layer below the height of measurement.     

On the Significance of the Webb Correction to Fluxes

For establishing correct mass or energy balances at the Earth's surface, detailed and correct measurements of air constituent fluxes are needed. Flux measurements obtained from the eddy covariance technique have to pass several corrections of different relevance in order to give correct flux data. One of these corrections, the Webb correction, is analysed herein from latent heat flux and CO₂ flux data recorded during two field experiments. The significance of this correction for the latent heat flux data varies with the air humidity and the Bowen ratio. The correction changes the latent heat flux values only a little, but significantly (by 2 to 3%). For other air constituents (like CO₂), the Webb correction is much more important (20 to 30% of the flux).     

A Re-examination of Density Effects in Eddy Covariance Measurements of CO2 Fluxes

Corrections of density effects resulting from air-parcel expansion/compression are important in interpreting eddy covariance fluxes of water vapor and CO2 when open-path systems are used. To account for these effects, mean vertical velocity and perturbation of the density of dry air are two critical parameters in treating those physical processes responsible for density variations. Based on various underlying assumptions, different studies have obtained different formulas for the mean vertical velocity and perturbation of the density of dry air, leading to a number of approaches to correct density effects. In this study, we re-examine physical processes related to different assumptions that are made to formulate the density effects. Specifically, we re-examine the assumptions of a zero dry air flux and a zero moist air flux in the surface layer, used for treating density variations, and their implications for correcting density effects. It is found that physical processes in relation to the assumption of a zero dry air flux account for the influence of dry air expansion/compression on density variations. Meanwhile, physical processes in relation to the assumption of a zero moist air flux account for the influence of moist air expansion/compression on density variations. In this study, we also re-examine mixing ratio issues. Our results indicate that the assumption of a zero dry air flux favors the use of the mixing ratio relative to dry air, while the assumption of a zero moist air flux favors the use of the mixing ratio relative to the total moist air. Additionally, we compare different formula for the mean vertical velocity, generated by air-parcel expansion/compression, and for density effect corrections using eddy covariance data measured over three boreal ecosystems.     

Gap Filling and Quality Assessment of CO2 and Water Vapour Fluxes above an Urban Area with Radial Basis Function Neural Networks

Vertical turbulent fluxes of water vapour, carbon dioxide, and sensible heat were measured from 16 August to the 28 September 2006 near the city centre of Münster in north-west Germany. In comparison to results of measurements above homogeneous ecosystem sites, the CO₂ fluxes above the urban investigation area showed more peaks and higher variances during the course of a day, probably caused by traffic and other varying, anthropogenic sources. The main goal of this study is the introduction and establishment of a new gap filling procedure using radial basis function (RBF) neural networks, which is also applicable under complex environmental conditions. We applied adapted RBF neural networks within a combined modular expert system of neural networks as an innovative approach to fill data gaps in micrometeorological flux time series. We found that RBF networks are superior to multi-layer perceptron (MLP) neural networks in the reproduction of the highly variable turbulent fluxes. In addition, we enhanced the methodology in the field of quality assessment for eddy covariance data. An RBF neural network mapping system was used to identify conditions of a turbulence regime that allows reliable quantification of turbulent fluxes through finding an acceptable minimum of the friction velocity. For the data analysed in this study, the minimum acceptable friction velocity was found to be 0.15 m s⁻¹. The obtained CO₂ fluxes, measured on a tower at 65 m a.g.l., reached average values of 12 μmol m⁻² s⁻¹ and fell to nighttime minimum values of 3 μmol m ⁻² s⁻¹. Mean daily CO₂ emissions of 21 g CO₂ m⁻²d ⁻¹ were obtained during our 6-week experiment. Hence, the city centre of Münster appeared to be a significant source of CO₂. The half-hourly average values of water vapour fluxes ranged between 0.062 and 0.989 mmol m⁻² s⁻¹and showed lower variances than the simultaneously measured fluxes of CO₂.     

A Re-examination of Density Effects in Eddy Covariance Measurements of CO_2 Fluxes

Corrections of density effects resulting from air-parcel expansion/compression are important in interpreting eddy covariance fluxes of water vapor and CO2 when open-path systems are used. To account for these effects, mean vertical velocity and perturbation of the density of dry air are two critical parameters in treating those physical processes responsible for density variations. Based on various underlying assumptions, different studies have obtained different formulas for the mean vertical velocity and ...     

Influence of the surface microlayer on the flux of nonconservative trace gases (CO,H2, CH4, N2O) across the ocean-atmosphere interface

Gas exchange experiments were conducted in the tropical Atlantic Ocean during a ship expedition with FS Meteor using a small rubber raft. The temporal change of the mixing ratios of CO, H₂, CH₄ and N₂O in the headspace of a floating glass box and the concentrations of these gases in the water phase were measured to determine their transfer velocities across the ocean-atmosphere interface. The ocean acted as a sink for these gases when the water was undersaturated with respect to the mixing ratio in the headspace. The transfer velocities were different for the individual gases and showed still large differences even when normalized for diffusivity. Applying the laminar film model, film thicknesses of 20 to 70 m were calculated for the observed flux rates of the different gas species. When the water was supersaturated with respect to atmospheric CO, H₂, CH₄ and N₂O, the transfer velocities of the emission process were smaller than those determined for the deposition process. In case of H₂ and CH₄, emission was even not calculable although, based on the observed gradient, the laminar film model predicted significant fluxes at the air-sea interface. The results are interpreted by destruction processes active within the surface microlayer.     

Flux-Variance Method for Latent Heat and Carbon Dioxide Fluxes in Unstable Conditions

Applied previously to momentum and heat fluxes, the present study extends the flux-variance method to latent heat and CO₂ fluxes in unstable conditions. Scalar similarity is also examined among temperature (θ), water vapour (q), and CO₂ (c). Temperature is adopted as the reference scalar, leading to two feasible strategies to estimate latent heat and CO₂ fluxes: the first one relies on flux-variance similarity relations for scalars, while the second is based on the parameterization of relative transport efficiency in terms of scalar correlation coefficient and a non-dimensional quantity. The relationship between the θ-to-q transport efficiency (λ _{θ q} ) and θ-q correlation coefficient (R _{θ q} ) is used to describe the intermediate hydrological conditions. We also parameterize the θ-to-c transport efficiency (λ _{θ c} ) as a function of the θ-c correlation coefficient (R _{θ c} ) by introducing a new non-dimensional ratio (α). The flux-variance method is a viable technique for flux gap-filling, when turbulence measurements of wind velocity are not available. It is worth noting that the extended method is not exempt from a correction for density effects when used for estimating water or carbon exchange.     

On the scavenging of gaseous nitrogen compounds by large and small rain drops: II. Wind tunnel and theoretical studies of the simultaneous uptake of NH3, SO2 and CO2 by water drops

An experimental investigation of the simultaneous absorption of NH₃ and SO₂ from the ambient atmosphere by freely falling water drops has been carried out in the Mainz vertical wind tunnel. The experimental results were found to be in good agreement with the results derived from computations with the Kronig-Brink convective diffusion model and also with a model which assumes a drop to be well mixed at all times. Encouraged by this agreement, these computation schemes for the uptake of gas by single drops where incorporated in a pollution washout model with realistic SO₂, NH₃ and CO₂ gas profiles. This model allows an entire raindrop size distribution to fall through a gas layer. The results of this plume-model show that the SO₂ uptake is strongly dependent on the NH₃ concentration in the atmosphere and on the rainrate. We also find that the small drops contribute more towards the washout of these gases. In the case of simultaneous presence of NH₃ and SO₂, desorption of these gases is negligible.     

Ly(α) absorption cross-section of H2O and O2

The absorption cross-sections of water vapor and oxygen were measured, using a low-pressure radio frequency discharge through traces of hydrogen in argon as a light source for Ly() radiation. The cross-sections are H₂O = 1.59 × 10^–17 cm² and O₂ = 1.13 × 10^–20 + 1.72 × 10^–23 for water and oxygen, respectively, where P is the oxygen pressure in units of Torr. Ly() lamps, such as used for this work, are important light sources for photochemical laboratory work and find applications for trace-gas detection in the atmosphere. For the latter application, accurate cross-sections of water vapor and oxygen are needed.