GOES-8 and NOAA-14 AVHRR retrieval of smoke aerosol optical thickness during SCAR-B

Using the NOAA-14 1-km Advanced Very High Resolution Radiometer (AVHRR) and the Geostationary Operational Environmental Satellite (GOES-8) imager data, smoke aerosol optical thickness ( ) is retrieved over land during the Smoke, Clouds and Radiation-Brazil (SCAR-B) experiment in Brazil during August-September 1995. The satellite-retrieved values are then compared against ground-based sunphotometer derived values from the Aerosol Robotic Network (AERONET) program. Both the AVHRR and GOES-8 retrieved values are in excellent agreement with the AERONET derived values with linear correlation coefficients of 0.93. A single scattering albedo of 0.90 (at 0.67 w m) provides the best fit between the GOES-8 and AERONET values. The sensitivity of the retrieved to assumed surface albedo and aerosol single scattering albedo are also examined. A simple multi-spectral thresholding algorithm is used to separate smoke from other features from GOES-8 satellite imagery and regional maps of are provided. Our results show that the aerosol properties used in this paper are adequate to characterize biomass burning aerosols and can be used in studies that model the role of biomass burning on regional climate.     

Aerosol phase function derived from CIMEL measurements

An approach to simulate the signal at the top of the atmosphere (TOA) in the visible and near-infrared regions is based on the exploitation of CIMEL sky radiances in three spectral bands (i.e. 440 nm, 675 nm and 870 nm). An iterative method, developed at the Laboratoire Interdisciplinaire des Sciences de l'Environnement (LISE, Wimereux, France), allows the aerosol phase function to be extracted from these ground-based measurements. Sky radiances are corrected for the multiple scattering based on the use of a radiative transfer tool and the aerosol phase function is derived from the primary scattering approximation. In order to cover the largest range of scattering angles, only the sky radiances acquired at low solar elevations are employed in this retrieval. These extreme geometrical conditions impose to adapt the radiative transfer code and to check its performances. Limits, performances and accuracy of this inverse method are discussed and illustrated both from the radiative transfer computations and from the CIMEL measurements. Moreover, thanks to the fact that the Aerosol Robotic Network (AERONET) also proposes CIMEL derived aerosol phase functions, the latter have been compared with our results. The substantial discrepancies that appear between the two sets are explained by the different objectives used in the two retrieval algorithms.     

Validation of MODIS derived aerosol optical depth over the Yangtze River Delta in China

MODIS derived aerosol optical depths (AODs) at 550 nm are compared with sunphotometer CE318 measurements at 7 sites located at Yangtze River Delta (YRD) in China from July to October, 2007. The evaluation result indicates that MODIS AODs (Collection 5, C005) are in good agreement with those from CE318 in dense vegetation regions, but show more differences in those regions with complex underlying surface (such as at lake water and urban surface sites). Reasons for these differences are discussed after removing cases with significant errors caused by validation scheme. The final validation result shows that MODIS AODs are in good agreement with CE318 with a correlation coefficient of 0.85 and RMS of 0.15. 90% of MODIS cases fall in the range of Δτ = ± 0.05 ± 0.20τ, indicating MODIS aerosol retrieval algorithm, aerosol models and surface reflectance estimate are generally suitably reasonable for aerosol retrieval in YRD. However, MODIS AODs show a systemic errors with fitted line of y = 0.75x + 0.13, indicating underestimation of AOD when aerosol loadings are high. Aerosol models and surface reflectance estimations are dominant sources of MODIS aerosol retrieval errors.     

双通道算法与MODIS算法反演中国近海气溶胶光学特性的对比研究

利用双通道和IMAPP气溶胶反演算法处理TERRA/MODIS L1B数据得出中国近海气溶胶的光学厚度,与AERONET太阳光度计的反演结果作对比分析,验证了反演方法的可行性。同时,对各海域的反演结果及表征粒子谱宽度的Angstrom指数(α)的变化情况进行了分析,结果表明：在东海和日本以南等广阔海域,两种反演算法的结果同AERONET太阳光度计的观测结果基本一致,相关性较好;在渤海和黄海近海岸一带两者气溶胶光学厚度的反演值均偏高,其原因主要是由这些海域的二类水体的影响导致的。探讨分析了这些海域的水域特征及光学特性,为研究发展适合中国近海气溶胶特性的反演算法提供了依据。     

Physical and chemical properties of surface and column aerosols at a rural New England site during MODIS overpass

Surface-based measurements of aerosol optical depth at a rural site in southern New Hampshire (43.11°N, 70.95°W) are compared to retrievals of the same parameter by the Moderate Resolution Imaging Spectrometer (MODIS) during April-August, 2001. Hourly averages of aerosol optical depth (AOD) were derived using a multi-filter rotating shadowband radiometer (MFRSR) at the time of NASA's Terra satellite overpass. The MODIS Level 2 aerosol product at a wavelength of 550 nm was directly compared to the MFRSR interpolated AOD at 550 nm. We were able to compare the two AOD measurement platforms on 46 days (out of a possible 128 days) and observed a good agreement between the two methods (R=0.81; slope=0.95±0.10). However, there were 11 days during this study period when MODIS measured AOD at the site, but the MFRSR did not due to excessive cloud cover. There were also 7 days when clear skies prevailed at the site during the time of MODIS overpass, but there was no AOD retrieved by MODIS. Surface measurements of fine particle (PM_2.5) mass, chemical composition, and optical properties were also performed during summer 2001. A good correlation (R=0.87) between fine particle mass and AOD measured by the MFRSR was observed. A comparison between fine particle light extinction at the surface and MFRSR AOD (at the same wavelength) also showed good agreement (R=0.80). Aerosol chemical analysis revealed that ammonium sulfate was the main aerosol component during times of very high turbidity, while organic carbon dominated during times of below-average turbidity.     

Ground-, satellite- and simulation-based analysis of a strong dust event over Abastumani,Georgia, during May 2009

A strong dust event over Abastumani, Georgia, during May 2009 was studied using light detection and ranging (lidar), satellite and sun photometric measurements. High aerosol optical depth (AOD) values (0.45–0.57) at 500 nm were measured over the closest Aerosol Robotic Network (AERONET) site (Erdemli, Turkey), whereas over Georgia, the AOD measured by the Moderate Resolution Imaging Spectroradiometer (MODIS) was about 0.9 at 550 nm. The AERONET data analysis showed a mean aerosol effective radius of about 2.5 μm, whereas the mean value of the Ångström exponent (α) (wavelength pair 440/870 nm) was smaller than 1, indicating the dominance of large aerosols. The aerosol lidar over Abastumani showed the existence of a strong particle load from the near ground up to a height of 3.5 km. The BSC-DREAM8b forecast model showed that the dust aerosols travelled from the Saharan and the Arabic deserts to the studied area, even reaching southern Russia, covering a total distance of about 5500 km, in the height region from about 2 to 11.5 km.     

Comparing MODIS and AERONET aerosol optical depth over China

The newest daily and monthly Moderate Resolution Imaging Spectroradiometer (MODIS) aerosol optical depths (AOD or τ) dataset over land, C005, retrieved using the second-generation operational algorithm, were evaluated using a ground-based Aerosol Robotic Network (AERONET) dataset from 13 sites over China. The dataset covers the period 2003–2006. Daily MODIS C005 AODs over China were found to have a positive bias with a relationship of τ_MODIS?=?0.135?+?1.022τ_AERONET, for which the offset is larger than reported global validation results. However, the relationship τ_MODIS?=?0.021?+?0.929τ_AERONET showed that monthly MODIS C005 AODs were an overestimation for small AOD and underestimation for high AOD. Both daily and monthly MODIS AOD retrievals showed poor performance in extreme aerosol conditions, e.g. under dust events or heavy urban/industrial haze. Nevertheless, both daily and monthly MODIS C005 AOD datasets can be used for investigation of aerosol spatial distribution and temporal variation over China.     

Aerosol characteristics during the coolest June month over New Delhi,northern India

June 2008, which is also the transition month between two major seasons for Indo-Gangetic Basin (IGB), has been identified the coolest June over New Delhi during the past century, showing mean temperature of 31.6 ± 1.7°C, which was found to be ～2°C less than its climatological mean (33.9°C). Aerosol optical properties for this month and thus obtained physical parameters have been studied using data from the CIMEL sun/sky radiometer, installed in New Delhi under the Aerosol Robotic Network (AERONET) programme. Results reveal bimodal aerosol volume size distribution. The monthly mean values for aerosol optical depth (AOD) at 500 nm (0.96 ± 0.31) and Ångström exponent at the wavelength pair of 440–870 nm (0.79 ± 0.42) show significant lower values whereas single scattering albedo at 675 nm shows a significantly larger value (0.94 ± 0.04) compared with previous measurements over the station. Results suggest dominance of scattering-type particles such as water-soluble aerosols from anthropogenic sources and dust aerosols from natural sources with higher relative humidity over the station. Radiative forcing caused due to the aerosols for the month of June 2008, which have been computed using the radiative-transfer model, informs low forcing at the top of atmosphere (TOA,?+14 W m^?2) as well as at surface (?33 W m^?2). The resultant atmospheric forcing (+47 W m^?2) indicates warming effect that caused heating of lower atmosphere at the rate of 0.89 K day^?1.     

Aerosol optical properties from sun photometric measurements in Hangzhou district,China

Along with fast economic development, the Yangtze Delta region is experiencing rapid environmental changes. A high concentration of aerosols with diverse properties is emitted in this region, providing a unique opportunity for understanding the impact of environmental change on climate systems, especially in urban regions. Aerosol optical properties including aerosol optical depth (AOD), the Ångström exponent (α), and water vapour and their relationship with aerosol particle concentrations were examined using ground-based measurements at three sites in the Hangzhou district: Hangzhou (120.01° E, 30.26° N), Lin'an (119.73° E, 30.26° N) and Qiandaohu (119.05° E, 29.55° N). The results from these observations show that there exists strong temporal and spatial variation in aerosol optical properties and particle matter (PM) concentrations. Monthly averaged AODs at the Hangzhou and Lin'an sites show a similar trend. AODs in Qiandaohu have the highest values in winter and spring and the lowest values in summer and autumn. In 2008, the minimum monthly mean α for every site was lowest in December due to the weather conditions. From spring to summer, these three locations experienced a drop in AOD and an increase in α due to the humidity-swelling effect. There is a positive correlation between AOD and aerosol particle concentrations; the correlation coefficients were obtained as 0.40, 0.55 and 0.69 in Hangzhou, Lin'an and Qiandaohu, respectively. The association between daily PM₁₀ concentration and AOD is marginally statistically significant at the three sites, which suggests that AOD is a useful tool for mapping PM distribution over large spatial domains.     

Intercomparison of CALIOP,MODIS, and AERONET aerosol optical depth over China during the past decade

ABSTRACT

Aerosol optical depth (AOD) data from Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) were intercompared and validated against ground-based measurements from Aerosol Robotic Network (AERONET) as well as space-based Moderate Resolution Imaging Spectroradiometer (MODIS) over China during June 2006 to December 2015. This article aims to evaluate CALIOP daytime AOD using MODIS and AERONET AODs. Comparing the AOD between CALIOP and AERONET in different regions over China using quality control flags to screen the AOD data, we find that CALIOP AOD is generally lower than AERONET AOD especially at optical depths over 0.4 likely due to differences in the cloud screening algorithms and general retrieval uncertainty. Comparison between CALIOP AOD and MODIS AOD results show that the overall spatio-temporal distribution of CALIOP AOD and MODIS AOD is basically consistent. As for the spatial distribution, both data sets show several high-value regions and low-value regions in China. CALIOP is systematically lower than MODIS over China, especially over high AOD value regions for all seasons. As for the temporal variation, both data sets show a significant seasonal variation: AOD is largest in spring, then less in summer, and smallest in winter and autumn. A long-term linear trend analysis based on the domain averaged monthly mean CALIOP and MODIS AOD shows agreement among CALIOP and MODIS for the trends over the 10-year period in four regions examined. The trends in AOD derived from CALIOP and MODIS indicate a decline in aerosol loading in China since 2006. It is found from frequency comparison that CALIOP and MODIS AOD generally exhibit a degree of correlation over China. Statistical frequency analysis shows that CALIOP AOD frequency distribution shows a higher peak than MODIS AOD when AOD < 0.4. For the most part, mean MODIS AOD is higher than mean CALIOP AOD. Evaluation of CALIOP AOD retrievals provides the prospect for application of CALIOP data. The intercomparison suggests that CALIOP has systematically underestimated daytime AOD retrievals, especially deteriorating with increasing AOD, and therefore, CALIOP daytime AOD retrievals should be treated with some degree of caution when the AOD is over 0.4.     

A path radiance estimation algorithm using reflectance measurements in radiometric control areas

A new algorithm of path radiance estimation based on measurements of surface reflectance at radiometric control areas (RCAs) is proposed. Path radiance estimates of the proposed RCA-based method were compared against estimates of other methods including the dark object subtraction (DOS) method, the multi-band regression (MBR) technique and the covariance matrix method (CMM). The RCA-based method is superior to other methods based on three qualitative assessment criteria and a quantitative assessment based on measurements of molecule and aerosol optical depths (AODs) available from the Aerosol Robotic Network (AERONET) and Moderate Resolution Imaging Spectroradiometer (MODIS) data archive. Asphalt-paved surface, which could be easily identified in most images, was also found to be a good choice for RCAs. The DOS method and the CMM tend to overestimate path radiances. Although in our study the MBR technique and the RCA-based method seem to perform equally well, estimates of the MBR technique may be less reliable.     

A new method for improving the retrieved aerosol fine-mode fraction from MODIS over ocean

A new method for improving the retrieved aerosol fine-mode fraction (₅₅₀) based on the current Moderate Resolution Imaging Spectroradiometer (MODIS) ocean algorithm is proposed. In the current MODIS ocean algorithm, the top of the atmosphere (TOA) apparent reflectance needs calculation from lookup tables (LUTs). The weighting parameters used in the calculation show an obvious spectral dependence, which is not taken into account in the current algorithm. The main measure taken in this study is to consider the spectral dependence of the weighting parameters. The MODIS aerosol products and the Aerosol Robotic Network (AERONET) data of Hong Kong Hok Tsui, Midway Island, Martha’s Vineyard Coastal Observatory (MVCO) and COVE, Virginia, where aerosols exhibit different loading and size distribution, are used to test the new method. The results show that the new method improves the retrieved fine-mode fraction, which is underestimated in anthropogenic-dominated aerosol conditions and overestimated in the sea salt-dominated aerosol conditions by the current algorithm. The correlation of the retrieved fine-mode fraction between the new method and AERONET is much higher (correlation coefficient, r?=?0.92) than that between the current MODIS and AERONET (r?=?0.80). The retrieved aerosol optical depth (AOD) is also improved. More AODs retrieved from the new method lie within the expected error bars.     

Optical–microphysical properties of smoke haze from Siberian forest fires in summer 2012

A comprehensive experiment on the study of dynamics of optical and microphysical characteristics of submicron aerosols in the near-ground air layer was carried out in June–August of 2012 under conditions of extremely strong smoke haze originating from vast forest fires in Siberia. Variations of the spectral angular scattering coefficients and the degree of linear polarization in the visible wavelength range are studied, as well as the mass concentrations of dry fractions of aerosol and black carbon, relative content of black carbon (BC fraction), single scattering albedo, particle size distribution function, complex refractive index of the particulate matter, and parameter of condensation activity of aerosol. Peculiarities of differences between optical and microphysical characteristics of near-ground aerosol in smoke haze and under smokeless conditions are revealed. This study is important for validation of results of remote sensing of the composition of smoke haze and background atmosphere obtained from satellite data and sun photometric measurements at the Aerosol Robotic Network (AERONET).     

Validating a new algorithm for estimating aerosol optical depths over land from MODIS imagery

Aerosols greatly affect the signals of satellite sensor imagery for remote sensing of land surfaces and play a dual role in global climate change and the hydrological cycle. However, there has not been a reliable method for estimating aerosol properties over land directly from multispectral remotely sensed imagery. In a recent study, a new algorithm to estimate aerosol optical depths (AODs) from Moderate‐Resolution Imaging Spectroradiometer (MODIS) imagery suitable for all land surfaces was proposed. It is based on a sequence of imagery over a period of time with the assumption that the surface property is relatively stable and atmospheric conditions vary much more dramatically. Although this algorithm was validated over several sites, more validation was necessary. In this study, this algorithm was validated using 3‐month measurements at 25 AErosol RObotic NETwork (AERONET) sites in North America. The validation results show that this algorithm can estimate AODs with close agreement with the AERONET measurements [R ² = 0.69, root mean square error (RMSE) 0.06].     

Toward aerosol optical depth retrievals over land from GOES visible radiances: determining surface reflectance

Frequent observations of aerosol over land are desirable for aviation, air pollution and health applications. Thus, a method is proposed here to correct surface effects and retrieve aerosol optical depth using visible reflectance measurements from the Geostationary Operational Environmental Satellite (GOES). The surface contribution is determined from temporal compositing of visible imagery, where darker pixels correspond to less atmospheric attenuation and surface reflectance is deduced from the composite using radiative transfer. The method is applied to GOES‐8 imagery over the eastern US. Retrieved surface reflectance is compared with separate retrievals using a priori ground‐based observations of aerosol optical depth. The results suggest that surface reflectances can be determined to within ±0.04. The composite‐derived surface reflectance is further analysed by retrieving aerosol optical depth and validating retrievals with Aerosol Robotic Network (AERONET) observations. This analysis indicates that the retrieved optical depth is least biased, hence the surface reflectance is most accurate, when the composite time period varies seasonally. Aerosol optical depth retrievals from this validation are within ±0.13 of AERONET observations and have a correlation coefficient of 0.72. While aerosol optical depth retrieval noise at low optical depths may be limiting, the retrieval accuracy is adequate for monitoring large outbreaks of aerosol events.     

Ozone Monitoring Instrument aerosol products: a comparison study with ground-based airborne sun photometer measurements over Europe

Airborne sun photometer measurements are used to evaluate retrievals of extinction aerosol optical depth (AOD). These data are extracted from spatially coincident and temporally near-coincident measurements by the Ozone Monitoring Instrument (OMI) aboard the Aura satellite taken during 2005. OMI-measured top of atmosphere (TOA) reflectances are routinely inverted to yield aerosol products such as AOD using two different retrieval techniques: the Aura OMI Near-Ultraviolet Aerosol Data Product, OMAERUV, and the multi-wavelength Aura OMI Aerosol Data Product, OMAERO. In this work, we propose a study that specifically compares the instantaneous aerosol optical thicknesses retrieved from OMI at several locations containing sites and those of the Aerosol Robotic Network (AERONET). The result of the comparison shows that, just over Europe, OMI aerosol optical thicknesses are better retrieved in the multi-wavelength retrieval than in the near-ultraviolet. Correlations have been improved by applying a simple criterion to avoid scenes probably contaminated by thin clouds, and surface scattering. The ultraviolet irradiance positive bias in the OMI data is corrected using a procedure based on global climatological fields of aerosol absorption optical depth. The results generally show a bias significantly reduced by 5–20%, a lower variability and an unchanged, high correlation coefficient.     

利用PARASOL数据反演陆地气溶胶光学厚度

研究了利用PARASOL多角度偏振数据反演我国陆地气溶胶,假设单次散射在扣除大气分子和地表的偏振辐亮度后,根据气溶胶光学性质查找表,采用最佳匹配的方法,选择最适合的气溶胶模式,得到气溶胶光学厚度。并用AERONET的北京站和香河站的地基观测数据对结果进行了验证。结果表明,多角度偏振方法反演陆地气溶胶精度稳定,受季节和地表类型的影响很小,但精度较低还需作进一步的改进。     

AERONET observations of direct and indirect aerosol effects over a South European conurbation

Sun photometer measurements over the Thessaloniki AErosol RObotic NETwork (AERONET) site were conducted during 2005–2007 and provide a method for determining the temporal variability of aerosol optical properties. Monthly mean aerosol optical thickness (AOT) showed higher values during the warmer months and lower values during the colder months. The single scattering albedo (SSA) and also the imaginary part of the refractive index (k) indicate more absorbing particles during autumn/winter than summer/spring. Aerosol direct radiative forcing measurements are also presented. The monthly change in aerosol radiative forcing was examined at the top of the atmosphere (TOA; from??8.0 to??28.0 W m^?2), at the bottom of the atmosphere (BOA; from??19.0 to??55.0 W m^?2) and within the atmosphere (ATM; from 11.0 to 35.0 W m^?2); the ATM is affected by both the SSA and k. We also investigated the interaction between aerosol optical properties over Thessaloniki and cloud parameters over a region of the Northern Aegean. For the autumn and spring mean monthly values, high correlation coefficients were found between AOT and cloud fraction, the SSA and cloud fraction, AOT and cloud condensation nuclei per square centimetre (CCN/cm²), and cloud optical depth and AOT, and finally a negative correlation was found between AOT and cloud effective radius.     

Estimation of variable atmospheric parameters for the atmospheric correction of satellite images

The information about variable components of the atmosphere (aerosol, water vapour, and ozone) during acquisition is required for the atmospheric correction of spectral images acquired by shortwave sensors of the Earth observing remote-sensing satellites. The procedure to estimate aerosol optical depth and columnar water vapour by the inversion of the atmospheric radiative transfer model 6S using moderate-resolution spectra of incident solar radiation is proposed. Comparison to the results obtained by the Aerosol Robotic Network AERONET at an AERONET site at the distance of 50 km on days when both sensors were in the same air mass shows systematic overestimation both of aerosol optical depth and of columnar water vapour if aerosol optical depth is estimated in the wavelength range of 365–425 nm and columnar water in the range of 895–985 nm using spectra of total irradiance. If more wavelengths and diffuse-to-total spectral irradiance ratio are implemented in the inversion, the bias of estimated water vapour decreases, but aerosol optical depth is underestimated. The estimates at 50 km distance are well correlated. The modelled spectral irradiance using estimated atmospheric parameters matches the measured spectra with high accuracy. In the spectral bands of the Sentinel-2 MultiSpectral Instrument (MSI), the differences do not exceed 2%.     

Observation of an agricultural biomass burning in central and east China using merged aerosol optical depth data from multiple satellite missions

Agricultural biomass burning (ABB) in central and east China occurs every year from May to October and peaks in June. During the period from 26 May to 16 June 2007, one strong ABB procedure happened mainly in Anhui, Henan, Jiangsu and Shandong provinces. This article focuses on analysis of this ABB procedure using a comprehensive set of aerosol optical depth (AOD) data merged by using the optimal interpolation method from the Moderate Resolution Imaging Spectroradiometer, the Multi-angle Imaging Spectroradiometer (MIRS) as well as Sea-viewing Wide Field-of-view Sensor (SeaWiFS)-derived AOD products. In addition, the following additional data are used: fire data from the National Satellite Meteorological Centre of China Meteorological Administration, the mass trajectory analyses from hybrid single-particle Lagrangian integrated trajectory (HYSPLIT) model and ground-based AOD and Ångström data derived from the Aerosol Robotic Network and China Aerosol Remote Sensing Network. The results show that merged satellite AOD values can expand the spatial coverage of agricultural biomass aerosol distributions with good accuracy (R = 0.93, root mean square error = 0.37). Based on the merged AOD images, the highest AOD values were found concentrated in central China as well as in eastern China before 6 June and further extended to northeast China after 12 June. AODs from ground measurement show that eastern China always keeps high AOD values (>1.0), with a maximum exceeding 3.0 and extending as high as nearly 5.0 during this ABB event. With the help of the HYSPLIT model, we analysed the ABB sources and examined how transport paths affect the concentrations of air pollutants in some sites. The results show that Henan, Jiangsu and Anhui provinces are the three main sources in this ABB.