Quality analysis applied on eddy covariance measurements at complex forest sites using footprint modelling

Summary Measuring turbulent fluxes with the eddy covariance method has become a widely accepted and powerful tool for the determination of long term data sets for the exchange of momentum, sensible and latent heat, and trace gases such as CO₂ between the atmosphere and the underlying surface. Several flux networks developed continuous measurements above complex terrain, e.g. AmeriFlux and EUROFLUX, with a strong focus on the net exchange of CO₂ between the atmosphere and the underlying surface. Under many conditions basic assumptions for the eddy covariance method in its simplified form, such as stationarity of the flow, homogeneity of the surface and fully developed turbulence of the flow field, are not fulfilled. To deal with non-ideal conditions which are common at many FLUXNET sites, quality tests have been developed to check if these basic theoretical assumptions are valid.In the framework of the CARBOEUROFLUX project, we combined quality tests described by Foken and Wichura (1996) with the analytical footprint model of Schmid (1997). The aim was to identify suitable wind sectors and meteorological conditions for flux measurements. These tools were used on data of 18 participating sites. Quality tests were applied on the fluxes of momentum, sensible and latent heat, and on the CO₂-flux, respectively. The influence of the topography on the vertical wind component was also checked. At many sites the land use around the flux towers is not homogeneous or the fetch may not be large enough. So the relative contribution of the land use type intended to be measured was also investigated. Thus the developed tool allows comparative investigations of the measured turbulent fluxes at different sites if using the same technique and algorithms for the determination of the fluxes as well as analyses of potential problems caused by influences of the surrounding land use patterns.     

The effect of water vapour broadening on methane eddy correlation flux measurements

A high resolution tunable diode laser absorption spectrometer (TDLAS) was used to measure the broadening effect of water vapor and other gases (dry air, nitrogen, oxygen, hydrogen and helium) on three methane lines in the v₄ fundamental. The effects on methane eddy correlation flux measurements amount to a few percent for the least broadened line for expected H₂O fluxes, to 10% for the most broadened line for higher H₂O and lower CH₄ fluxes likely to be encountered. The broadening coefficients of methane measured for air, N₂, O₂, and He are in good agreement with recently published values.     

A comparison of dynamic and static chambers for methane emission measurements from subarctic fens

Abstract

Methane emissions measured at three subarctic fen sites by dynamic and static chambers were compared; in the dynamic chambers, the air was circulated at a wind speed of 1.9 m s^?1. Emissions ranged from 7 to 214 mg CH₄ m^?2 d^?1 and measurements from the two types of chamber were strongly correlated (r² = 0.72), with no overall difference between the means (paired t‐test, p = 0.34) and with only 4 of the 14 comparisons showing statistically different means (t‐test, p < 0.25). The overall ratio of dynamic: static chamber fluxes was 1.24, but was highest (1.68) at the wettest, central site and lowest (0.74) at the driest, edge site. The coefficients of variation of chamber flux measurements at each site ranged from 0.13 to 1.77, with an overall average of 0.53; sampling with over 30 static chambers revealed approximately normal distributions at the edge and middle sites and a positively skewed distribution at the central site. Within both static and dynamic chambers, methane concentrations increased linearly through 24 h. These inexpensive, portable static chambers can be used to replicate methane emission measurements within a wide range of wetland sites.     

Field studies of methane emission from termite nests into the atmosphere and measurements of methane uptake by tropical soils

The flux of CH₄ and CO₂ from termite nests into the atmosphere has been measured in a broad-leafed-type savannah in South Africa. Measurements were carried out on nests of species of six genera, i.e., Hodotermes, Macrotermes, Odontotermes, Trinervitermes, Cubitermes, and Amitermes. The flux rates of CH₄ relative to the flux rate of CO₂ in terms of carbon obtained for the individual species showed ratios of 2.9×10^-3, 7.0×10^-4, 6.7×10^-5, 8.7×10^-3, 2.0×10^-3 and 4.2×10^-3, respectively. Using data published on the assimulation efficiencies of termites, the flux of carbon as CH₄ accounts for 6.0×10^-5 to 2.6×10^-3 of the carbon ingested which results in a global CH₄ emission by termites of 2 to 5×10¹² g/yr. Methane is decomposed in the soil with average decomposition rates of 52 g/m²/h. The annual CH₄ consumption in the tropics and subtropics is estimated to be 21×10¹² g which exceeds the CH₄ emission rate by termites.     

石油与天然气生产过程甲烷逃逸排放检测与核算研究现状及建议

甲烷作为油气生产中的主要气态污染物和增温潜势较强的一类温室气体,其逃逸排放检测与核算是中国油气行业温室气体控制与减排过程中的首要问题。通过比较国内外油气行业逃逸甲烷排放核算方法,以及总结当前国内外油气田现场开展的甲烷排放检测研究,对油气行业企业级温室气体清单编制提出了建议。在甲烷逃逸排放核算方面,由于生产过程、工艺等出现的变化,建议加快建立符合我国油气生产实际情况的排放因子,纳入放空气燃烧的周期变化、绿色完井措施使用、陆上/海上油气生产等内容,鼓励油气生产企业通过设备组件统计、现场实测等方式进行排放因子更新。针对逃逸甲烷现场检测数据不足,影响核算结果对比的现状,在开展油气生产过程现场检测时,建议自下而上与自上而下检测方法相结合,保证检测结果的可验证性、可重复性,并谨慎看待针对部分油气生产区块的检测结果进行大尺度区域甲烷逃逸排放量的推算结果。     

Annual variation of surface roughness obtained from wind profile measurements

Summary Profile measurements of wind and temperature have been performed at the Agricultural University of Norway on a routine basis since 1997. 10-min. averages are stored in a database together with other relevant meteorological parameters. The database can be used to determine the seasonal variation of surface aerodynamic roughness, showing the growth of grass between cutting during the growing season, the effect of snowfall and the melting of snow etc. However, careful screening of the data must be conducted before reliable estimates can be made. The main objective of this study is to establish simple practical rules for filtering out unreliable datasets for the evaluation of the surface roughness parameter z₀, and to present its annual variation. The resulting values for the summer period agree well with values found in the literature for homogenous grass covered surfaces. In the transition periods during autumn and spring, and during wintertime in mild weather conditions, the surface is generally non-homogenous with a mixture of snow patches, ponds of melting water and shrubs of withered grass. The results show that the mechanical interaction between a non-homogeneous land surface and the boundary layer flow can be described by one roughness parameter, with a numeric value somewhere in between the ideal values for the different surface characteristics. Another use of the database is to investigate drainage flow and the relationship between drainage flow, prevailing wind direction and the mean vertical velocity of the air. Most micrometeorological studies of the fluxes of heat and water vapour in the surface layer, assume the mean vertical velocity to be zero, focusing on eddy fluxes and thereby excluding any transport in the mean flow. In certain situations, this may lead to serious errors. This work shows that convergence of horizontal flow leads to an upward movement of air, which is enhanced if the prevailing direction of the wind opposes the outflow of the cold drainage winds from the area.     

The subcloud-layer eddy kinetic energy budget of a microburst-producing thunderstorm determined from jaws dual-Doppler measurements

Dual-Doppler data collected from 1646 to 1648 MDT on 14 July, 1982 in Colorado were employed to study the eddy kinetic energy budget in the subcloud layer of a microburst-producing thunderstorm during its mature stage. Each term in the budget equation was computed from the Doppler-derived winds and retrieved thermodynamic fields within the 10 by 10 km horizontal domain. Results show that in the atmospheric boundary layer (ABL) where the microburst dominates, the turbulent flow extracts energy from the mean flow in order for the microburst to maintain its strong diverging outflow at low levels. The vertical transport of eddy kinetic energy is predominantly downward in the low layer due to the organized downdrafts in the microburst area. The horizontal flux convergence (divergence) of eddy kinetic energy by the mean and eddying motions is approximately balanced by that of the vertical flux divergence (convergence). Similarly, the contributions from the pressure and buoyancy production terms are nearly in balance. As a result, a net change of the eddy kinetic energy generation in the subcloud layer is relatively small in comparison with the individual term in the budget equation.     

Model for Methane Emission from Rice Fields and Its Application in Southern China

Ｍｏｄｅｌ　ｆｏｒ　Ｍｅｔｈａｎｅ　Ｅｍｉｓｓｉｏｎ　ｆｒｏｍ　Ｒｉｃｅ　Ｆｉｅｌｄｓ　ａｎｄ　Ｉｔｓ　Ａｐｐｌｉｃａｔｉｏｎ　ｉｎ　Ｓｏｕｔｈｅｒｎ　ＣｈｉｎａＤｉｎｇＡｉｊｕａｎｄＷａｎｇＭｉｎｇｘｉｎｇ（ＩｎｓｔｉｔｕｔｅｏｆＡｔｍｏｓｐｈｅｒ...     

Inferring the thermal-infrared hemispheric emission from a sparsely-vegetated surface by directional measurements

The thermal-infrared (longwave) emission from a vegetated terrain is generally anisotropic, i.e., the emission temperature varies with the view direction. If a directional measurement of temperature is considered to be equal to the effective temperature of the hemispheric emission, then the estimate of the latter can be significantly in error. The view-direction (zenith angle _eq ) at which the emission equivalence does hold is determined in our modeling study. In a two-temperature field-of-view (soil and plants), _eq falls in a narrow range depending on plant density and canopy architecture. _eq does not depend on soil and (uniform) plant temperatures nor on their ratio, even though the pattern of emission vs. the view direction depends crucially on this ratio. For a sparse canopy represented as thin, vertical cylindrical stalks (or vertical blades uniformly distributed in azimuth) with horizontal facets, _eq ranges from 48 to 53° depending on the optical density of the vertical elements alone. When plant elements are modeled as small spheres, _eq lies between 53 to 57° (for the same values of the canopy optical density). Only for horizontal leaves (a truly planophile canopy) is the temperature measured from any direction equal to the temperature of the hemispheric emission. When the emission temperature changes with optical depth within the canopy at a specified rate, _eq depends to some extent on that rate. For practically any sparsely vegetated surface, a directional measurement at the zenith angle of 50° offers an appropriate evaluation of the hemispheric emission, since the error in the estimate will, at most, only slightly exceed 1% (around 4 W m^–2). Estimates of the hemispheric emission through a nadir measurement, on the other hand, can be in error in some cases by about 10%, i.e., on the order of 40 W m^–2....by a small sample we may judge the whole piece... Miguel de Cervantes, 1605, inDon Quixote de la Mancha, Pt. I, Ch. 4     

The effect of ozone and aerosols on the surface erythemal UV radiation estimated from OMI measurements

Surface erythemal UV radiation is mainly affected by total column ozone, aerosols, clouds, and solar zenith angle. The effect of ozone on the surface UV radiation has been explored many times in the previous studies due to the decrease of ozone layer. In this study, we calculated the effect of aerosols on the surface UV radiation as well as that of ozone using data acquired from Ozone Monitoring Instrument (OMI). First, ozone, aerosol optical depth (AOD), and surface erythemal UVB radiation measured from satellite are compared with those from ground measurements. The results showed that the comparison for ozone was good with r ² of 0.92. For aerosol, there was difference between satellite measurements and surface measurements due to the insufficient information on aerosol in the retrieval algorithm. The r ² for surface erythemal UV radiation was high (～0.94) but satellite measurements showed about 30% larger values than surface measurements on average by not considering the effect of absorbing aerosols in the retrieval process from satellite measurements. Radiative amplification factor (RAF) is used to access the effect of ozone and aerosol quantitatively. RAF for ozone was 0.97～1.49 with solar zenith angle. To evaluate the effect of aerosol on the surface UV radiation, only clear-sky pixel data were used and solar zenith angle and total column amount of ozone were fixed. Also, RAF for aerosol was assessed according to the single scattering albedo (SSA) of aerosols. The results showed that RAF for aerosol with smaller SSA (< 0.90) was larger than that for with larger SSA (> 0.90). The RAF for aerosol was 0.09～0.22 for the given conditions which was relatively small compared to that for ozone. However, considering the fact that aerosol optical depth can change largely in time and space while the total column amount of ozone does not change very much, it needs to include the effect of aerosol to predict the variations of surface UV radiation more correctly.     

Applicability of an eddy covariance system based on a close-path quantum cascade laser spectrometer for measuring nitrous oxide fluxes from subtropical vegetable fields

亚热带蔬菜地是氧化亚氮(N_2O)的重要排放源,本研究首次采用基于闭路量子级联激光吸收光谱仪的涡动相关(QCLAS-EC)法观测亚热带蔬菜地秋冬季非施肥阶段的N_2O排放通量,以评估QCLAS-EC法测量亚热带蔬菜地N_2O通量的适用性。结果表明,QCLAS-EC观测系统在野外条件下能长期稳定运行,可观测到N_2O排放的季节变化趋势,其检测限为18.5μg N m~(-2) h~(-1)(95%置信水平),试验期间获得的97.5%的N_2O通量大于此检测限,表明QCLAS-EC观测系统可有效并准确测量亚热带蔬菜地的N_2O排放通量,仪器本身的灵敏度不是通量检测的限制因素。     

Comparison of manual and automatic methods for measurement of methane emission from rice paddy fields

The methane emission flux from rice paddies was simultaneously measured with automatic and manual methods in the suburban of Suzhou. Both methods were based on the static chamber/ GC-FID techniques. Detail analysis of the experimental results indicates: a) The data of methane emission measured with the automatic method is reliable. b) About 11 or 19 o’clock of local time is recommended as the optimum sampling time for the manual spot measurement of methane emission from rice paddies. The methane emission fluxes measured by manual sampling at local time other than the optimum time have to be corrected. The correction coefficient may be determined by automatic and continuous measurement. c) In order to get a more accurate result, an empirical correction factor, such as 18%, is recommended to correct the seasonally total amount of measured methane emission by enlarging the au-tomatically measured data or reducing the manually measured ones.