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1.
There is widespread use of passive remote sensing techniques to quantify trace gas column densities in volcanic plumes utilizing scattered sunlight as a light source. Examples include passive DOAS, COSPEC, and the SO2 camera. In order to calculate trace gas concentrations or volcanic emission fluxes, knowledge about the optical path through the plume is necessary. In the past, a straight photon path through the plume has always been assumed although it was known that this is not always true. Here we present the results of model studies conducted specifically to quantify the effects of realistic radiative transfer in and around volcanic plumes on ground-based remote sensing measurements of SO2. The results show that measurements conducted without additional information on average photon paths can be inaccurate under certain conditions, with possible errors spanning more than an order of magnitude. Both over and underestimation of the true column density can occur. Actual errors depend on parameters such as distance between instrument and plume, plume SO2 concentration, plume aerosol load, as well as aerosol conditions in the ambient atmosphere. As an example, a measurement conducted with an SO2 camera is discussed, the results of which can only be correctly interpreted if realistic radiative transfer is considered. Finally, a method is presented which for the first time allows the retrieval of actual average photon paths in spectroscopic (i.e. DOAS) measurements of adequate resolution. By allowing for a wavelength dependent column density during the evaluation of DOAS measurements, we show how radiative transfer effects can be corrected using information inherently available in the measured spectra, thus greatly enhancing the accuracy of DOAS measurements of volcanic emissions.  相似文献   

2.
We simulated geostationary satellite observations to assess the potential for high spatial-and temporal-resolution monitoring of air pollution in China with a focus on tropospheric ozone(O_3), nitrogen dioxide(NO_2), sulfur dioxide(SO_2), and formaldehyde(HCHO). Based on the capabilities and parameters of the payloads onboard sun-synchronous satellites, we simulated the observed spectrum based on a radiative transfer model using a geostationary satellite model. According to optimal estimation theory, we analyzed the sensitivities and retrieval uncertainties of the main parameters of the instrument for the target trace gases. Considering the retrieval error requirements of each trace gas, we determined the major instrument parameter values(e.g., observation channel, spectral resolution, and signal-to-noise ratio). To evaluate these values, retrieval simulation was performed based on the three-dimensional distribution of the atmospheric components over China using an atmospheric chemical transportation model. As many as 90% of the experiments met the retrieval requirements for all target gases. The retrieval precision of total-column and stratospheric O_3 was 2%. In addition, effective retrieval of all trace gases could be achieved at solar zenith angles larger than 70°. Therefore, the geostationary satellite observation and instrument parameters provided herein can be used in air pollution monitoring in China. This study offers a theoretical basis and simulation tool for improving the design of instruments onboard geostationary satellites.  相似文献   

3.
A high-accuracy surface modeling (HASM) method based on the fundamental theorem of surfaces, is developed to simulate XCO2 surfaces using the GOSAT retrieval XCO2 data. Two tests are designed to investigate the simulation accuracy. The first test divides the existing satellite retrieval XCO2 data into training points and testing points, and simulates the XCO2 surface using the training points while computing the simulation error using the testing points. The absolute mean error (MAE) of the testing points is 1.189 ppmv, and the corresponding values of the comparison methods, Ordinary Kriging, IDW, and Spline are 1.203, 1.301, and 1.355 ppmv, respectively. The second test simulates the XCO2 surface using all the satellite retrieval points and uses the TCCON (Total Carbon Column Observing Network) site observation values as the ture values. For the six typical TCCON sites, the HASM simulation MAE is 1.688 ppmv, and the satellite retrieval MAE at the same sites is 2.147 ppmv. These results indicate that HASM can successfully simulate XCO2 surfaces based on satellite retrieval data.  相似文献   

4.
We propose an algorithm that combines a pre-processing step applied to the a priori state vector prior to retrievals, with the modified damped Newton method (MDNM), to improve convergence. The initial constraint vector pre-processing step updates the initial state vector prior to the retrievals if the algorithm detects that the initial state vector is far from the true state vector in extreme cases where there are CO2 emissions. The MDNM uses the Levenberg-Marquardt parameter γ, which ensures a positive Hessian matrix, and a scale factor α, which adjusts the step size to optimize the stability of the convergence. While the algorithm iteratively searches for an optimized solution using observed spectral radiances, MDNM adjusts parameters γ and α to achieve stable convergence. We present simulated retrieval samples to evaluate the performance of our algorithm and comparing it to existing methods. The standard deviation of our retrievals adding random noise was less than 3.8 ppmv. After pre-processing the initial estimate when it was far from the true value, the CO2 retrieval errors in the boundary layers were within 1.2 ppmv. We tested the MDNM algorithm’s performance using GOSAT L1b data with cloud screening. Our preliminary validations comparing the results to TCCON FTS measurements showed that the average bias was less than 1.8 ppm and the correlation coefficient was approximately 0.88, which was larger than for the GOSAT L2 product.  相似文献   

5.
Active Long Path Differential Optical Absorption Spectroscopy (LP-DOAS) measurements of halogen oxides were conducted at Masaya Volcano, in Nicaragua from April 14 to 26, 2007. The active LP-DOAS system allowed night-time halogen measurements and reduced the ClO detection limit by an order of magnitude when compared to previous passive DOAS measurements, as wavelengths below 300 nm could be used for the DOAS retrievals. BrO was detected with an average BrO/SO2 molecular ratio of approximately 3 × 10−5 during the day. However, BrO values were below the detection limit of the instrument for all night-time measurements, a strong indication that BrO is not directly emitted, but rather the result of photochemical formation in the plume itself according to the autocatalytic “bromine explosion” mechanism. Despite the increased sensitivity, both ClO and OClO could not be detected. The achieved upper limits for the X/SO2 ratios were 5 × 10−3 and 7 × 10−6, respectively. A rough calculation suggests that ClO and OClO should be present at similar abundances in volcanic plumes. Since the DOAS technique is orders of magnitude more sensitive for OClO than for ClO, this indicates that OClO should always be detectable in plumes in which ClO is found. However, further LP-DOAS studies are needed to conclusively clarify the role of chlorine oxides in volcanic plumes.  相似文献   

6.
The Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) is a useful tool for detecting low quantities of sulfur dioxide at passively degassing volcanoes such as Lascar volcano, Chile. Two mini-UV spectrometers (MUSes) were used to make transects of Lascar volcano’s sulfur dioxide plume on December 7, 2004, during a coordinated overpass of ASTER. SO2 burdens were retrieved using the thermal infrared channels of the acquired ASTER image. This allowed for a direct comparison between the two methods in order to validate the ASTER SO2 retrieval. The results were extremely encouraging with ASTER deriving SO2 fluxes within the range of fluxes obtained by the MUSe.  相似文献   

7.
The seasonal cycle of the main lunar tidal constituent M 2 is studied globally by an analysis of a high-resolution ocean circulation and tide model (STORMTIDE) simulation, of 19 years of satellite altimeter data, and of multiyear tide-gauge records. The barotropic seasonal tidal variability is dominant in coastal and polar regions with relative changes of the tidal amplitude of 5–10 %. A comparison with the observations shows that the ocean circulation and tide model captures the seasonal pattern of the M 2 tide reasonably well. There are two main processes leading to the seasonal variability in the barotropic tide: First, seasonal changes in stratification on the continental shelf affect the vertical profile of eddy viscosity and, in turn, the vertical current profile. Second, the frictional effect between sea-ice and the surface ocean layer leads to seasonally varying tidal transport. We estimate from the model simulation that the M 2 tidal energy dissipation at the sea surface varies seasonally in the Arctic (ocean regions north of 60°N) between 2 and 34 GW, whereas in the Southern Ocean, it varies between 0.5 and 2 GW. The M 2 internal tide is mainly affected by stratification, and the induced modified phase speed of the internal waves leads to amplitude differences in the surface tide signal of 0.005–0.0150 m. The seasonal signals of the M 2 surface tide are large compared to the accuracy demands of satellite altimetry and gravity observations and emphasize the importance to consider seasonal tidal variability in the correction processes of satellite data.  相似文献   

8.
A refined empirical model of the Dynamics Explorer-1 far-ultraviolet (FUV) imaging photometer’s response to Earth’s quiet time FUV dayglow has been developed for thermospheric studies. The mean photometer response is based upon FUV observations in 156 images obtained during the first five months of imager operations (September 1981–January 1982) and is determined as a function of solar and satellite zenith angles, observational azimuth and solar clock angles, and solar radio flux. Variations with each parameter are characterized and, where possible, fitted with an appropriate function. The fitted response, based on the n-th power of the cosine of the solar zenith angle, is within 10% of actual mean values at all observed solar and satellite zenith angles and is consistent with the results of a first-principles calculation. Subtraction of the model background from other DE-1 images indirectly reveals the enhancement or diminution of thermospheric O/N2 column density ratios due to transport and Joule heating effects. An analysis of summer storm-time images from the Southern Hemisphere demonstrates the use of the model in revealing these effects. The technique developed here is readily applicable to other FUV data sets.  相似文献   

9.
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10.
The correlation spectrometer (COSPEC), the principal tool for remote measurements of volcanic SO2, is rapidly being replaced by low-cost, miniature, ultraviolet (UV) spectrometers. We compared two of these new systems with a COSPEC by measuring SO2 column amounts at Kīlauea Volcano, Hawaii. The two systems, one calibrated using in-situ SO2 cells, and the other using a calibrated laboratory reference spectrum, employ similar spectrometer hardware, but different foreoptics and spectral retrieval algorithms. Accuracy, signal-to-noise, retrieval parameters, and precision were investigated for the two configurations of new miniature spectrometer. Measurements included traverses beneath the plumes from the summit and east rift zone of Kīlauea, and testing with calibration cells of known SO2 concentration. The results obtained from the different methods were consistent with each other, with <8% difference in estimated SO2 column amounts up to 800 ppm m. A further comparison between the COSPEC and one of the miniature spectrometer configurations, the ‘FLYSPEC’, spans an eight month period and showed agreement of measured emission rates to within 10% for SO2 column amounts up to 1,600 ppm m. The topic of measuring high SO2 burdens accurately is addressed for the Kīlauea measurements. In comparing the foreoptics, retrieval methods, and resultant implications for data quality, we aim to consolidate the various experiences to date, and improve the application and development of miniature spectrometer systems.  相似文献   

11.
The characteristics of spatial and temporal distribution of tropospheric NO2 column density concentration over China are presented, on the basis of measurements from the satellite instruments GOME and SCIAMACHY. From these observations, monthly averaged tropospheric NO2 variations are determined for the period of 1997 to 2006. The trend and seasonal cycle are also investigated. The possible source of tropospheric NO2 over megacity area is discussed in this paper. The results show a large growth of tropospheric NO2 over eastern China, especially above the industrial areas with a fast economical growth, such as, Yangtze Rive Delta region and Pearl River Delta region because of the prominent anthropogenic activity. There is a rapid increase of tropospheric NO2 over megacities in China. For instance, Shanghai had a linear significant increase in NO2 columns of ～20% per year (reference year 1997) in the period of 1997-2006, which is the rapidest increase among all the selected cities. The seasonal pattern of the NO2 concentration shows a difference between the east and west in China. In the eastern part of China, an expected winter maximum in seasonal cycle is found because of the prominent anthropogenic activity and meteorological conditions. In the western part this cycle shows a NO2 maximum in summer time, which is attributed to natural emissions, especially soil emissions and lightning. A quickly increasing vehicle population may contribute to the increase of tropospheric NO2 over megacities in China for the remarkable correlation for vehicle population with tropospheric NO2.  相似文献   

12.
The simultaneous quantitative determination of two-dimensional bromine monoxide (BrO) and sulphur dioxide (SO2) distributions in volcanic gas plumes is described. Measurements at the fumarolic field on the island Vulcano (autumn 2004) and in the plume of Mt. Etna volcano (spring 2005) were carried out with an Imaging DOAS instrument. The SO2 fluxes of several fumaroles were estimated from two-dimensional distributions of SO2. Additionally, the first two-dimensional distributions of BrO within a volcanic plume were successfully retrieved. Slant column densities of up to 2.6 × 1014 molecules per square centimetre were detected in the plume of Mt. Etna. The investigation of the BrO/SO2 ratio, calculated from the two-dimensional distributions of SO2 and BrO, shows an increase from the centre to the edge of the volcanic plume. These results have significance for the involvement of ozone during BrO formation processes in volcanic emissions.  相似文献   

13.
Soil moisture (SM) can be retrieved from active microwave (AM), passive microwave (PM) and thermal infrared (TIR) observations, each having unique spatial and temporal coverages. A limitation of TIR‐based retrievals is a dependence on cloud‐free conditions, whereas microwave retrievals are almost all weather proof. A downside of SM retrievals from PM is the coarse spatial resolution. Although SM retrievals at coarse spatial resolution proved to be valuable for global‐scale and continental‐scale studies, their value for regional‐scale studies remains limited. To increase the use of SM retrievals from PM observations, an existing method to enhance their spatial resolution was applied. We present an intercomparison study over the Iberian Peninsula for three SM products on two different spatial sampling grids. The remotely sensed SM products were also compared with in situ observations from the Remedhus network. Variations between ground data and satellite‐based SM are observed; all three remotely sensed SM products show good agreement to the ground observations. The comparison shows that these ground observations and satellite data are consistent, based on the correlation coefficient (R) and root mean square error (RMSE). The remotely sensed products were intercompared after sampling at 25 × 25 km2 and after applying the smoothing filter‐based intensity modulation (SFIM) downscaling technique at 10 × 10 km2 grids. After the application of the SFIM technique, the SM retrievals from PM observations show better agreement with the other remotely sensed SM products for approximately 40% of the study area. For another 40% of the study area, we found a similar agreement between these product combinations, whereas in extreme environments, both arid and densely vegetated regions, the agreement decreases after the application of the SFIM technique. Agreement between retrievals of absolute SM content from PM and TIR observations is generally high (R = 0.77 for semi‐arid areas). This study enhances our understanding of the remotely sensed SM products for improvements of SM retrieval and merging strategies. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

14.
Satellite observations of atmospheric CO2 are able to truly capture the variation of global and regional CO2 concentration.The model simulations based on atmospheric transport models can also assess variations of atmospheric CO2 concentrations in a continuous space and time,which is one of approaches for qualitatively and quantitatively studying the atmospheric transport mechanism and spatio-temporal variation of atmospheric CO2 in a global scale.Satellite observations and model simulations of CO2 offer us two different approaches to understand the atmospheric CO2.However,the difference between them has not been comprehensively compared and assessed for revealing the global and regional features of atmospheric CO2.In this study,we compared and assessed the spatio-temporal variation of atmospheric CO2 using two datasets of the column-averaged dry air mole fractions of atmospheric CO2(XCO2)in a year from June 2009 to May 2010,respectively from GOSAT retrievals(V02.xx)and from Goddard Earth Observing System-Chemistry(GEOS-Chem),which is a global 3-D chemistry transport model.In addition to the global comparison,we further compared and analyzed the difference of CO2 between the China land region and the United States(US)land region from two datasets,and demonstrated the reasonability and uncertainty of satellite observations and model simulations.The results show that the XCO2 retrieved from GOSAT is globally lower than GEOS-Chem model simulation by 2 ppm on average,which is close to the validation conclusion for GOSAT by ground measures.This difference of XCO2 between the two datasets,however,changes with the different regions.In China land region,the difference is large,from 0.6 to 5.6 ppm,whereas it is 1.6 to 3.7 ppm in the global land region and 1.4 to 2.7 ppm in the US land region.The goodness of fit test between the two datasets is 0.81 in the US land region,which is higher than that in the global land region(0.67)and China land region(0.68).The analysis results further indicate that the inconsistency of CO2concentration between satellite observations and model simulations in China is larger than that in the US and the globe.This inconsistency is related to the GOSAT retrieval error of CO2 caused by the interference among input parameters of satellite retrieval algorithm,and the uncertainty of driving parameters in GEOS-Chem model.  相似文献   

15.
Based on the optimal estimation method, a satellite XCO2 retrieval algorithm was constructed by combining LBLRTM with VLIDORT. One-year GOSAT/TANSO observations over four TCCON stations were tested by our algorithm, and retrieval results were compared with GOSAT L2 B products and ground-based FTS measurements. Meanwhile, the influence of CO2 line mixing effect on retrieval was estimated, and the research showed that neglecting CO2 line mixing effect could result in approximately 0.25% XCO2 underestimation. The accuracy of XCO2 retrievals was similar to GOSAT L2 B products at cloud-free footprints with aerosol optical depth less than 0.3, and 1% accuracy of XCO2 retrievals can be reached based on the validation result with TCCON measurements.  相似文献   

16.
Automated detection of fog and low stratus in nighttime satellite data has been implemented on the basis of numerous satellite systems in past decades. Commonly, differences in small-droplet emissivities at 11μm and 3.9μm are utilized. With Meteosat SEVIRI, however, this method cannot be applied with a fixed threshold due to instrument design: The 3.9μm band is exceptionally wide and overlaps with the 4μm CO2 absorption band. Therefore, the emissivity difference varies with the length of the slant atmospheric column between sensor and object. To account for this effect, the new technique presented in this paper is based on the dynamical extraction of emissivity difference thresholds for different satellite viewing zenith angles. In this way, varying concentrations of CO2 and column depths are accounted for. The new scheme is exemplified in a plausibility study and shown to provide reliable results.  相似文献   

17.
On 27 February 2007, a new eruption occurred on Stromboli which lasted until 2 April. It was characterized by effusive activity on the Sciara del Fuoco and by a paroxysmal event (15 March). This crisis represented an opportunity for us to refine the model that had been developed previously (2002–2003 eruption) and to improve our understanding of the relationship between the magmatic dynamics of the volcano and the geochemical variations in the fluids. In particular, the evaluation of the dynamic equilibrium between the volatiles (CO2 and SO2) released from the magma and the corresponding fluids discharged from the summit area allowed us to evaluate the level of criticality of the volcanic activity. One of the major accomplishments of this study is a 4-year database of summit soil CO2 flux on the basis of which we define the thresholds (low–medium–high) for this parameter that are empirically based on the natural volcanological evolution of Stromboli. The SO2 fluxes of the degassing plume and the CO2 fluxes emitted from the soil at Pizzo Sopra la Fossa are also presented. It is noteworthy that geochemical signals of volcanic unrest have been clearly identified before, during and after the effusive activity. These signals were found almost simultaneously in the degassing plume (SO2 flux) and in soil degassing (CO2 flux) at the summit, although the two degassing processes are shown to be clearly different. The interpretation of the results will be useful for future volcanic surveillance at Stromboli.  相似文献   

18.
We present a study on the retrieval sensitivity of the column-averaged dry-air mole fraction of CO2 (XCO2) for the Chinese carbon dioxide observation satellite (TanSat) with a full physical forward model and the optimal estimation technique. The forward model is based on the vector linearized discrete ordinate radiative transfer model (VLIDORT) and considers surface reflectance, gas absorption, and the scattering of air molecules, aerosol particles, and cloud particles. XCO2 retrieval errors from synthetic TanSat measurements show solar zenith angle (SZA), albedo dependence with values varying from 0.3 to 1 ppm for bright land surface in nadir mode and 2 to 8 ppm for dark surfaces like snow. The use of glint mode over dark oceans significantly improves the CO2 information retrieved. The aerosol type and profile are more important than the aerosol optical depth, and underestimation of aerosol plume height will introduce a bias of 1.5 ppm in XCO2. The systematic errors due to radiometric calibration are also estimated using a forward model simulation approach.  相似文献   

19.
This study aims at evaluating the uncertainty in the prediction of soil moisture (1D, vertical column) from an offline land surface model (LSM) forced by hydro-meteorological and radiation data. We focus on two types of uncertainty: an input error due to satellite rainfall retrieval uncertainty, and, LSM soil-parametric error. The study is facilitated by in situ and remotely sensed data-driven (precipitation, radiation, soil moisture) simulation experiments comprising a LSM and stochastic models for error characterization. The parametric uncertainty is represented by the generalized likelihood uncertainty estimation (GLUE) technique, which models the parameter non-uniqueness against direct observations. Half-hourly infra-red (IR) sensor retrievals were used as satellite rainfall estimates. The IR rain retrieval uncertainty is characterized on the basis of a satellite rainfall error model (SREM). The combined uncertainty (i.e., SREM + GLUE) is compared with the partial assessment of uncertainty. It is found that precipitation (IR) error alone may explain moderate to low proportion of the soil moisture simulation uncertainty, depending on the level of model accuracy—50–60% for high model accuracy, and 20–30% for low model accuracy. Comparisons on the basis of two different sites also yielded an increase (50–100%) in soil moisture prediction uncertainty for the more vegetated site. This study exemplified the need for detailed investigations of the rainfall retrieval-modeling parameter error interaction within a comprehensive space-time stochastic framework for achieving optimal integration of satellite rain retrievals in land data assimilation systems.  相似文献   

20.
The global rate of fossil fuel combustion continues to rise, but the amount of CO2 accumulating in the atmosphere has not increased accordingly. The causes for this discrepancy are widely debated. Particularly, the location and drivers for the interannual variability of atmospheric CO2 are highly uncertain. Here we examine links between global atmospheric CO2 growth rate (CGR) and the climate anomalies of biomes based on (1986–1995) global climate data of ten years and accompanying satellite data sets. Our results show that four biomes, the tropical rainforest, tropical savanna, C4 grassland and boreal forest, and their responses to climate anomalies, are the major climate-sensitive CO2 sinks/sources that control the CGR. The nature and magnitude by which these biomes respond to climate anomalies are generally not the same. However, one common influence did emerge from our analysis; the extremely high CGR observed for the one extreme El Niño year was caused by the response of the tropical biomes (rainforest, savanna and C4 grassland) to temperature.  相似文献   

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