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1.
The most direct method for flux estimation uses eddy covariance, which is also the most commonly used method for land-based measurements of surface fluxes. Moving platforms are frequently used to make measurements over the sea, in which case motion can disturb the measurements. An alternative method for flux estimation should be considered if the effects of platform motion cannot be properly corrected for. Three methods for estimating CO 2 fluxes are studied here: the eddy-covariance, the inertial-dissipation, and the cospectral-peak methods. High-frequency measurements made at the land-based Östergarnsholm marine station in the Baltic Sea and measurements made from a ship during the Galathea 3 expedition are used. The Kolmogorov constant for CO 2, used in the inertial-dissipation method, is estimated to be 0.68 and is determined using direct flux measurements made at the Östergarnsholm site. The cospectral-peak method, originally developed for neutral stratification, is modified to be applicable in all stratifications. With these modifications, the CO 2 fluxes estimated using the three methods agree well. Using data from the Östergarnsholm site, the mean absolute error between the eddy-covariance and inertial-dissipation methods is 0.25 μmol m ?2 s ?1. The corresponding mean absolute error between the eddy-covariance and cospectral-peak methods is 0.26 μmol m ?2 s ?1, while between the inertial-dissipation and cospectral-peak methods it is 0.14 μmol m ?2 s ?1. 相似文献
2.
Turf-grass lawns are ubiquitous in the United States. However direct measurements of land–atmosphere fluxes using the eddy-covariance
method above lawn ecosystems are challenging due to the typically small dimensions of lawns and the heterogeneity of land
use in an urbanised landscape. Given their typically small patch sizes, there is the potential that CO 2 fluxes measured above turf-grass lawns may be influenced by nearby CO 2 sources such as passing traffic. In this study, we report on two years of eddy-covariance flux measurements above a 1.5 ha
turf-grass lawn in which we assess the contribution of nearby traffic emissions to the measured CO 2 flux. We use winter data when the vegetation was dormant to develop an empirical estimate of the traffic effect on the measured
CO 2 fluxes, based on a parametrised version of a three-dimensional Lagrangian footprint model and continuous traffic count data.
The CO 2 budget of the ecosystem was adjusted by 135gCm −2 in 2007 and by 134gCm −2 in 2008 to determine the natural flux, even though the road crossed the footprint only at its far edge. We show that bottom-up
flux estimates based on CO 2 emission factors of the passing vehicles, combined with the crosswind-integrated footprint at the distance of the road, agreed
very well with the empirical estimate of the traffic contribution that we derived from the eddy-covariance measurements. The
approach we developed may be useful for other sites where investigators plan to make eddy-covariance measurements on small
patches within heterogeneous landscapes where there are significant contrasts in flux rates. However, we caution that the
modelling approach is empirical and will need to be adapted individually to each site. 相似文献
3.
We present an approach for assessing the impact of systematic biases in measured energy fluxes on CO 2 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 CO 2 flux measurements. Our results suggest that incomplete energy balance closure does not necessarily lead to an underestimation of CO 2 fluxes despite the existence of surface energy imbalance; either an overestimation or underestimation of CO 2 fluxes is possible depending on local atmospheric conditions and measurement errors in the sensible heat, latent heat, and CO 2 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 CO 2 fluxes. 相似文献
4.
A simple aerodynamic-variance method is proposed to fill gaps in continuous CO 2 flux measurements in rainy conditions, when open-path analysers do not function. The method requires turbulent conditions
(friction velocity greater than 0.1 ms –1), and uses measurements of mean wind speed, and standard deviations of temperature and CO 2 concentration fluctuations to complement, and at times replace, eddy-covariance measurements of friction velocity, sensible
heat flux and CO 2 flux. Friction velocity is estimated from the mean wind speed with a flux-gradient relationship modified for the roughness
sublayer. Since normalised standard deviations do not follow Monin-Obukhov similarity theory in the roughness sublayer, a
simple classification scheme according to the scalar turbulence scale was used. This scheme is shown to produce sensible heat
and CO 2 flux estimates that are well correlated with the measured values. 相似文献
5.
The objective of this study is to produce two-dimensional maps of the sensible and the latent heat fluxes from airborne measurements,
based on the analysis of a flight pattern, called grid flights. A footprint model with along-wind and cross-wind components
was used to project the measured fluxes onto the surface map. The method was applied to measurements over Arctic tundra during
the Mackenzie Area GEWEX (Global Energy and Water Cycle Experiment) Study (MAGS) 1999. The resulting flux estimates were computed
by integration of a wavelet transform, and corrected for long wavelength losses using information from 100 km long regional
runs that were conducted close to the grid flights. The random flux error was estimated based on the flight length that is
represented in each map element, and a map resolution of 3 × 3 km was chosen in order to keep the average random error of
the latent heat flux below 25%. The random error of the sensible heat flux was smaller by a factor of 1.4 on average. An analysis
of airborne flux measurements at different altitudes showed no significant increase of flux estimates for measurement heights
below 90 m. Thus, the fluxes measured at heights between 48 and 64 m were not corrected for vertical flux divergence. The
resulting flux maps provide quantitative two-dimensional estimates of the energy exchange between the surface and the atmosphere
during the snow melt period in an Arctic environment, which are well-suited for calibration and validation of numerical models. 相似文献
6.
A new version of an atmospheric pressure chemical ionisation mass spectrometer has been developed for ground based in situ
atmospheric measurements of OH and total peroxy (HO 2 + organic peroxy) radicals. Based on the previously developed principle of chemical conversion of OH radicals to H 2SO 4 in reaction with SO 2 and detection of H 2SO 4 using an ion molecule reaction with NO 3─, the new instrument is equipped with a turbulent chemical conversion reactor allowing for measurements in moderately polluted
atmosphere at NO concentrations up to several ppb. Unlike other similar devices, where the primary NO 3─ ions are produced using radioactive ion sources, the new instrument is equipped with a specially developed corona discharge
ion source. According to laboratory measurements, the overall accuracy and detection limits are estimated to be, respectively,
25% and 2 × 10 5 molecule cm -3 for OH and 30% and 1 × 10 5 molecule cm -3 for HO 2 at 10 min integration times. The detection limit for measurements of OH radicals under polluted conditions is 5 × 10 5 molecules cm -3 at 10 min integration times. Examples of ambient air measurements during a field campaign near Paris in July 2007 are presented
demonstrating the capability of the new instrument, although with reduced performance due to the employment of non isotopic
SO 2. 相似文献
7.
Size-segregated aerosol particles were collected using a high volume MOUDI sampler at a coastal urban site in Xiamen Bay, China, from March 2018 to June 2020 to examine the seasonal characteristics of aerosol and water-soluble inorganic ions (WSIIs) and the dry deposition of nitrogen species. During the study period, the annual average concentrations of PM1, PM2.5, PM10, and TSP were 14.8?±?5.6, 21.1?±?9.0, 35.4?±?14.2 μg m?3, and 45.2?±?21.3 μg m?3, respectively. The seasonal variations of aerosol concentrations were impacted by the monsoon with the lowest value in summer and the higher values in other seasons. For WSIIs, the annual average concentrations were 6.3?±?3.3, 2.1?±?1.2, 3.3?±?1.5, and 1.6?±?0.8 μg m?3 in PM1, PM1-2.5, PM2.5–10, and PM>10, respectively. In addition, pronounced seasonal variations of WSIIs in PM1 and PM1-2.5 were observed, with the highest concentration in spring-winter and the lowest in summer. The size distribution showed that SO42?, NH4+ and K+ were consistently present in the submicron particles while Ca2+, Mg2+, Na+ and Cl? mainly accumulated in the size range of 2.5–10 μm, reflecting their different dominant sources. In spring, fall and winter, a bimodal distribution of NO3? was observed with one peak at 2.5–10 μm and another peak at 0.44–1 μm. In summer, however, the fine mode peak disappeared, likely due to the unfavorable conditions for the formation of NH4NO3. For NH4+ and SO42?, their dominant peak at 0.25–0.44 μm in summer and fall shifted to 0.44–1 μm in spring and winter. Although the concentration of NO3–N was lower than NH4–N, the dry deposition flux of NO3–N (35.77?±?24.49 μmol N m?2 d?1) was much higher than that of NH4–N (10.95?±?11.89 μmol N m?2 d?1), mainly due to the larger deposition velocities of NO3–N. The contribution of sea-salt particles to the total particulate inorganic N deposition was estimated to be 23.9—52.8%. Dry deposition of particulate inorganic N accounted for 0.95% of other terrestrial N influxes. The annual total N deposition can create a new productivity of 3.55 mgC m?2 d?1, accounting for 1.3–4.7% of the primary productivity in Xiamen Bay. In light of these results, atmospheric N deposition could have a significant influence on biogeochemistry cycle of nutrients with respect to projected increase of anthropogenic emissions from mobile sources in coastal region. 相似文献
8.
The present study comprehensively reports the simultaneous measurement of wet deposition of total inorganic nitrogen (TIN; which is the sum of the NH4+-N and NO3?-N) at three different sites in Nr emission hotspot of Indo-Gangetic plain (IGP) over a year-long temporal scale from October 2017 to September 2018. At rural Meetli (MTL) site, urban Baraut (BRT) site and industrial Loni (LNI) site, the annual wet deposition of NH4+-N was estimated as 21.87, 19.48 and 7.43 kg N ha?1 yr?1, respectively; the annual wet deposition NO3?-N was estimated as 12.96, 12.17 and 4.44 kg N ha?1 yr?1, respectively; and the annual wet deposition of TIN was estimated as 34.83, 31.64 and 11.87 kg N ha?1 yr?1, respectively. NH4+-N was dominantly contributing species in annual, monsoon and non-monsoon-time wet deposition of TIN at all sites. The spatial gradient (variability) in percent contribution of NH4+ to total annual volume-weighted mean (VWM) concentration of all analyte ions was observed as MTL (43.23%)?>?BRT (37.90%)?>?LNI (30%). On the other hand, the spatial gradient in percent contribution of NO3? to total annual VWM concentration of all analyte ions was observed as MTL (7.45%)?>?BRT (6.89%)?>?LNI (5.32%). The extremely narrow range of NH4+-N/NO3?-N ratios (ranging from 1.60 at BRT site to 1.69 at LNI site) showed the approximately equal relative abundance of oxidized and reduced nitrogen (N) deposition across all sites. Inferences from enrichment factor analysis, principal component analysis and Pearson’s correlation coefficient analysis suggested that across all sites, virtually all NH4+-N and NO3?-N depositions were originated anthropogenically. The annual wet deposition of TIN measured in this study showed?≥?6865%,?≥?6228% and?≥?2274% increment than the natural N deposition rate at MTL, BRT and LNI site, respectively. These empirically measured annual wet depositions of TIN also emanated theoretical transgression of critical N load threshold across all sites therefore signifying probable undermining of long-term elastic stability and resilience of ecosystems against stressor in the study domain. 相似文献
9.
Summary Nocturnal eddy-covariance carbon dioxide fluxes have uncertainties arising from non-stationary atmospheric processes. Low-level
jets (LLJ) are one of the prominent nocturnal boundary-layer phenomena observed over non-mountainous terrain, and are capable
of generating shear and turbulence close to the ground. The influence of intermittent LLJ activity on nocturnal carbon dioxide
exchange measurements is investigated using wind profile observations and eddy-covariance flux measurements over a tall forest
canopy. Results suggest that the buildup and venting of CO 2 are closely associated with LLJ activity during the night. Of significance in quantifying nocturnal fluxes, this paper demonstrates
how low-level jet activity introduces sporadic coupling between the canopy and the atmosphere. 相似文献
10.
Single-tower eddy-covariance measurements represent the complete surface flux of a scalar only under idealized conditions.
Therefore, we often find an underestimation of energy fluxes expressed as a lack of energy balance closure at many sites.
In this study, a multi-tower approach to measure atmospheric energy fluxes based on spatial averaging is evaluated and possible
mechanisms causing a lack of energy balance closure are analysed, focussing on daytime data only. It is shown that the multi-tower
technique is also unable to measure the entire flux for our site, likely because the assumption of horizontal homogeneity
is violated. Heterogeneity-induced and buoyancy-driven quasi-stationary circulations are probably the dominant processes causing
the underestimation of energy fluxes. A dependence of the energy balance residual on stability is found, with residuals close
to zero for stable stratification, a maximum under unstable to near-neutral conditions and still relatively large residuals
for stronger instability. Assuming the processes transporting energy and CO 2 are similar, the implications on long-term CO 2 flux measurements are analysed. Accordingly, the resulting selective systematic error of cumulative net ecosystem exchange
estimates for agricultural regions such as ours can be of the order of more than 100%, since mainly the fluxes during periods
of net CO 2 uptake are underestimated while periods of net CO 2 release are much less affected by this bias. Further investigations about this issue are highly warranted. 相似文献
11.
We measured the methane flux of a forest canopy throughout a year using a relaxed eddy accumulation (REA) method. This sampling system was carefully validated against heat and CO 2 fluxes measured by the eddy covariance method. Although the sampling system was robust, there were large uncertainties in the measured methane fluxes because of the limited precision of the methane gas analyzer. Based on the spectral characteristics of signals from the methane analyzer and the diurnal variations in the standard deviation of the vertical wind velocity, we found the daytime and nighttime precision of half-hourly methane flux measurements to be approximately 1.2 and 0.7?μg?CH 4?m ?2?s ?1, respectively. Additional uncertainties caused by the dilution effect were estimated to affect the accuracy by as much as 0.21?μg?CH 4?m ?2?s ?1 on a half-hourly basis. Diurnal and seasonal variations were observed in the measured fluxes. The biological emission from plant leaves was not observed in our studies, and thus could be negligible at the canopy-scale exchange. The annual methane sink was 835?±?175?mg?CH 4?m ?2?year ?1 (8.35?kg?CH 4?ha ?1?year ?1), which was comparable to the flux range of 379–2,478?mg?CH 4?m ?2?year ?1 previously measured in other Japanese forest soils. This study indicated that the REA method could be a promising technique to measure canopy scale methane fluxes over forests, but further improvement of precision of the analyzer will be required. 相似文献
12.
Aerosol and rain samples were collected between 48°N and 55°S during the KH-08-2 and MR08-06 cruises conducted over the North
and South Pacific Ocean in 2008 and 2009, to estimate dry and wet deposition fluxes of atmospheric inorganic nitrogen (N).
Inorganic N in aerosols was composed of ~68% NH 4+ and ~32% NO 3– (median values for all data), with ~81% and ~45% of each species being present on fine mode aerosol, respectively. Concentrations
of NH 4+ and NO 3− in rainwater ranged from 1.7–55 μmol L −1 and 0.16–18 μmol L −1, respectively, accounting for ~87% by NH 4+ and ~13% by NO 3− of total inorganic N (median values for all data). A significant correlation ( r = 0.74, p < 0.05, n = 10) between NH 4+ and methanesulfonic acid (MSA) was found in rainwater samples collected over the South Pacific, whereas no significant correlations
were found between NH 4+ and MSA in rainwater collected over the subarctic ( r = 0.42, p > 0.1, n = 6) and subtropical ( r = 0.33, p > 0.5, n = 6) western North Pacific, suggesting that emissions of ammonia (NH 3) by marine biological activity from the ocean could become a significant source of NH 4+ over the South Pacific. While NO 3− was the dominant inorganic N species in dry deposition, inorganic N supplied to surface waters by wet deposition was predominantly
by NH 4+ (42–99% of the wet deposition fluxes for total inorganic N). We estimated mean total (dry + wet) deposition fluxes of atmospheric
total inorganic N in the Pacific Ocean to be 32–64 μmol m −2 d −1, with 66–99% of this by wet deposition, indicating that wet deposition plays a more important role in the supply of atmospheric
inorganic N than dry deposition. 相似文献
13.
A mesoscale meteorological model (FOOT3DK) is coupled with a gas exchange model to simulate surface fluxes of CO 2 and H 2O under field conditions. The gas exchange model consists of a C3 single leaf photosynthesis sub-model and an extended big
leaf (sun/shade) sub-model that divides the canopy into sunlit and shaded fractions. Simulated CO 2 fluxes of the stand-alone version of the gas exchange model correspond well to eddy-covariance measurements at a test site
in a rural area in the west of Germany. The coupled FOOT3DK/gas exchange model is validated for the diurnal cycle at singular
grid points, and delivers realistic fluxes with respect to their order of magnitude and to the general daily course. Compared
to the Jarvis-based big leaf scheme, simulations of latent heat fluxes with a photosynthesis-based scheme for stomatal conductance
are more realistic. As expected, flux averages are strongly influenced by the underlying land cover. While the simulated net
ecosystem exchange is highly correlated with leaf area index, this correlation is much weaker for the latent heat flux. Photosynthetic
CO 2 uptake is associated with transpirational water loss via the stomata, and the resulting opposing surface fluxes of CO 2 and H 2O are reproduced with the model approach. Over vegetated surfaces it is shown that the coupling of a photosynthesis-based
gas exchange model with the land-surface scheme of a mesoscale model results in more realistic simulated latent heat fluxes. 相似文献
14.
We measured the surface energy budget of an Alpine grassland in highly complex terrain to explore possibilities and limitations
for application of the eddy-covariance technique, also for CO 2 flux measurements, at such non-ideal locations. This paper focuses on the influence of complex terrain on the turbulent energy
measurements of a characteristic high Alpine grassland on Crap Alv (Alp Weissenstein) in the Swiss Alps during the growing
season 2006. Measurements were carried out on a topographic terrace with a slope of 25 ◦ inclination. Flux data quality is assessed via the closure of the energy budget and the quality flag method used within the
CarboEurope project. During 93% of the time the wind direction was along the main valley axis (43% upvalley and 50% downvalley
directions). During the transition times of the typical twice daily wind direction changes in a mountain valley the fraction
of high and good quality flux data reached a minimum of ≈50%, whereas during the early afternoon ≈70% of all records yielded
good to highest quality (CarboEurope flags 0 and 1). The overall energy budget closure was 74 ± 2%. An angular correction
for the shortwave energy input to the slope improved the energy budget closure slightly to 82 ± 2% for afternoon conditions.
In the daily total, the measured turbulent energy fluxes are only underestimated by around 8% of net radiation. In summary,
our results suggest that it is possible to yield realistic energy flux measurements under such conditions. We thus argue that
the Crap Alv site and similar topographically complex locations with short-statured vegetation should be well suited also
for CO 2 flux measurements. 相似文献
15.
Chemical compositions of precipitation samples collected from a remote and high elevation site (Nam Co Station, 30°46.44??N, 90°59.31??E, 4730?m?a.s.l.) in central Tibetan Plateau (TP, hereafter) from August 2005 to August 2009 are investigated. During the study period, Ca 2+ and HCO 3 - have the highest concentrations among ions and are the dominant cation and anion in precipitation, taking 27.46?% and 30.84?% to the total ions respectively. Empirical Orthogonal Functions (EOFs) analyses reveal that crustal aerosol inputs significantly contributed to the loading of Ca 2+, Mg 2+, SO 4 2- and HCO 3 - in precipitation, while lake salt plays a major source of K + and Cl -. Seasonal variations of ionic wet deposition fluxes show high values during monsoon seasons due to large precipitation amount. Among the cations, annual Ca 2+ flux is the largest (86.26?eq hm ?2), Na + and NH 4 + fluxes are following. Among anions, HCO 3 - has the highest flux (98.66?eq hm ?2) while that of NO 3 - is the lowest. Annual wet deposition of nitrogen has varied considerably with the average value of 0.70?kg?ha ?1 a ?1 at Nam Co Station. About 80?% of total nitrogen flux occurs during the monsoon seasons when precipitation is concentrated, in which NH 4 + and NO 3 - contributed to 61?% and 39?% of the total nitrogen deposition. Thus, our ionic concentrations and wet deposition fluxes in precipitation can provide a useful dataset to assess atmospheric environment and its impacts on ecosystem in the inland TP. 相似文献
16.
In the present study, the wet and dry depositions of particulate NO 3−, SO 42−, Cl − and NH 4+ were measured using a wet/dry sampler as a surrogate surface. Gas phase compounds of nitrogen, sulfur and chloride (HNO 3, NH 3, SO 2 and HCl) were measured by an annular denuder system (ADS) equipped with a back up filter for the collection of particles with diameter ≤ 5 μm. Ambient concentrations of NO, NO 2 and SO 2 were also taken into consideration. Sampling was conducted at an urban site in the center of the city of Thessaloniki, northern Greece. The presence of the aerosol species was examined by cold/warm period and the possible compounds in dry deposits were also considered. Dry deposition fluxes were found to be well correlated with ambient particle concentrations in order to be used for the calculation of particle deposition velocity. Average particulate deposition velocities calculated were 0.36, 0.20, 0.20 and 0.10 cm s − 1 for Cl −, NO 3−, SO 42− and NH 4+, respectively. Total dry deposition fluxes (gas and particles) were estimated at 3.24 kg ha − 1 year − 1 for chloride (HCl + p-Cl), 9.97 kg ha − 1 year − 1 for nitrogen oxidized (NO + NO 2 + HNO 3 + p-NO 3), 5.32 kg ha − 1 year − 1 for nitrogen reduced (NH 3 + p-NH 4) and 15.77 kg ha − 1 year − 1 for sulfur (SO 2 + p-SO 4). 70–90% total dry deposition was due to gaseous species deposition. The contribution of dry deposition to the total (wet + dry) was at the level of 60–70% for sulfur and nitrogen (oxidized and reduced), whereas dry chloride deposition contributed 35% to the total. The dry-to-wet deposition ratio of all the studied species was found to be significantly associated with the precipitation amount, with nitrogen species being better and higher correlated. Wet, dry and total depositions measured in Thessaloniki, were compared with other countries of Europe, US and Asia. 相似文献
17.
In this study, 24-h PM 2.5 samples were collected using Harvard Honeycomb denuder/filter-pack system during different seasons in 2006 and 2007 at an urban site in Guangzhou, China. The particles collected in this study were generally acidic (average strong acidity ([H +]) ~ 70 nmol m ? 3). Interestingly, aerosol sulfate was not fully neutralized in the ammonia-rich atmosphere (NH 3 ~ 30 ppb) and even when NH 4+]/[SO 42?] was larger than 2. Consequently, strong acidity ([H +]) as high as 170 nmol m ? 3 was observed in these samples. The kinetic rate of neutralization of acidity (acidic sulfate) by ambient ammonia was significantly higher than the rate of formation of ammonium nitrate involving HNO 3 and NH 3 for [NH 4+]/[SO 42?] ≤ 1.5 and much lower for NH 4+]/[SO 42?] > 1.5. Therefore, higher nitrate principally formed via homogeneous gas phase reactions involving ammonia and nitric acid were observed for [NH 4+]/[SO 42?] > 1.5. However, little nitrate, probably formed via heterogeneous processes e.g. reaction of HNO 3 with sea salt or crustal species, was observed for [NH 4+]/[SO 42?] ≤ 1.5. These demonstrate a clear transition in the pathways of ambient ammonia to form aerosol ammonium at [NH 4+]/[SO 42?] = 1.5 and evidently explain the observed high acidity due to the unneutralized sulfate in the ammonium-rich aerosol (NH 4+]/[SO 42?] > 1.5). In fact, the measured acidity was almost similar to the excess acid defined as the acid that remains at [NH 4+]/[SO 42?] = 1.5 due to the un-neutralized fraction of sulfate ([H +] = 0.5[SO 42?]). The presence of high excess acid and ammonium nitrate significantly lowered the deliquescence relative humidity of ammonium sulfate (from 80% to 40%) in the ammonium-rich samples. 相似文献
18.
Space-borne measurements of atmospheric greenhouse gas concentrations provide global observation constraints for top-down estimates of surface carbon flux.Here,the first estimates of the global distribution of carbon surface fluxes inferred from dry-air CO_2 column (XCO_2) measurements by the Chinese Global Carbon Dioxide Monitoring Scientific Experimental Satellite (Tan Sat) are presented.An ensemble transform Kalman filter (ETKF) data assimilation system coupled with the GEOS-Chem global chemistry transport model is used to optimally fit model simulations with the Tan Sat XCO_2 observations,which were retrieved using the Institute of Atmospheric Physics Carbon dioxide retrieval Algorithm for Satellite remote sensing (IAPCAS).High posterior error reduction (30%–50%) compared with a priori fluxes indicates that assimilating satellite XCO_2 measurements provides highly effective constraints on global carbon flux estimation.Their impacts are also highlighted by significant spatiotemporal shifts in flux patterns over regions critical to the global carbon budget,such as tropical South America and China.An integrated global land carbon net flux of 6.71±0.76 Gt C yr~(-1) over12 months (May 2017–April 2018) is estimated from the Tan Sat XCO_2 data,which is generally consistent with other inversions based on satellite data,such as the JAXA GOSAT and NASA OCO-2 XCO_2 retrievals.However,discrepancies were found in some regional flux estimates,particularly over the Southern Hemisphere,where there may still be uncorrected bias between satellite measurements due to the lack of independent reference observations.The results of this study provide the groundwork for further studies using current or future Tan Sat XCO_2 data together with other surfacebased and space-borne measurements to quantify biosphere–atmosphere carbon exchange. 相似文献
19.
A 1-year set of measurements of CO 2 and energy turbulent fluxes above and within a 25-m pine forest in southern Brazil is analyzed. The study focuses on the coupling state between two levels and its impact on flux determination by the eddy-covariance method. The turbulent series are split in their typical temporal scales using the multiresolution decomposition, a method that allows proper identification of the time scales of the turbulent events. Initially, four case studies are presented: a continually turbulent, a continually calm, a calm then turbulent, and an intermittent night. During transitions from calm to turbulent, large scalar fluxes of opposing signs occur at both levels, suggesting the transference of air accumulated in the canopy during the stagnant period both upwards and downwards. Average fluxes are shown for the entire period as a function of turbulence intensity and a canopy Richardson number, used as an indicator of the canopy coupling state. Above the canopy, CO 2 and sensible heat fluxes decrease in magnitude both at the neutral and at the very stable limit, while below the canopy they increase monotonically with the canopy Richardson number. Latent heat fluxes decrease at both levels as the canopy air becomes more stable. The average temporal scales of the turbulent fluxes at both levels approach each other in neutral conditions, indicating that the levels are coupled in that case. Average CO 2 fluxes during turbulent periods that succeed very calm ones are appreciably larger than the overall average above the canopy and smaller than the average or negative within the canopy, indicating that the transfer of air accumulated during calm portions at later turbulent intervals affects the flux average. The implications of this process for mean flux determination are discussed. 相似文献
20.
Cloud/fog samples were collected during spring of 2007 in the highly polluted North China Plain in order to examine the impact of pollution and dust particles on cloud water chemistry. The volume weighted mean pH of cloud water was 3.68. The cloud acidity was shown to be associated with air mass origins. Cloud water with its air mass trajectories originating from the southern part of China was more acidic than those from northern China. Anthropogenic source and dust had obvious impact on cloud water composition as indicated by the very high mean concentrations of SO 42? (1331.65 μeq L ? 1), NO 3? (772.44 μeq L ? 1), NH 4+ (1375.92 μeq L ? 1) and Ca 2+ (625.81 μeq L ? 1) in the observation periods. During sandstorm days, cloud pH values were relatively high, and the concentrations of all the ions in cloud water reached unusual high levels. Significant decreases in the mass concentrations of PM 2.5 and PM 10 were observed during cloud events. The average scavenging ratio for PM 2.5 and PM 10 was 52.0% and 55.7%, respectively. Among the soluble ions in fine particles, NO 3?, K + and NH 4+ tend to be more easily scavenged than Ca 2+ and Na +. 相似文献
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