Column Aerosol Optical Thickness Research Articles

Estimation of surface-level PM2.5 concentration using aerosol optical thickness through aerosol type analysis method

Surface-level particulate matter is closely related to column aerosol optical thickness (AOT). Previous researches have successfully used column AOT and different meteorological parameters to estimate surface-level PM concentration. In this study, the performance of a selected linear model that estimates surface-level PM2.5 concentration was evaluated following the aerosol type analysis method (ATAM) for the first time. We utilized 443 daily average data for Xuzhou, Jiangsu province, collected using Aerosol Robotic Network (AERONET) during the period October 2013 to April 2016. Several parameters including atmospheric boundary layer height (BLH), relative humidity (RH), and effective radius of the aerosol size distribution (Ref) were used to assess the relationship between the column AOT and PM2.5 concentration. By including the BLH, ambient RH, and effective radius, the correlation (R2) increased from 0.084 to 0.250 at Xuzhou, and with the use of ATAM, the correlation increased further to 0.335. To compare the results, 450 daily average data for Beijing, pertaining to the same period, were utilized. The study found that model correlations improved by varying degrees in different seasons and at different sites following ATAM. The average urban industry (UI) aerosol ratios at Xuzhou and Beijing were 0.792 and 0.451, respectively, demonstrating poorer air conditions at Xuzhou. PM2.5 estimation at Xuzhou showed lower correlation (R2 = 0.335) compared to Beijing (R2 = 0.407), and the increase of R2 at Xuzhou and Beijing site following use of ATAM were 33.8% and 12.4%, respectively.

Evaluation of the Multi-Angle Implementation of Atmospheric Correction (MAIAC) Aerosol Algorithm through Intercomparison with VIIRS Aerosol Products and AERONET.

The Multi-Angle Implementation of Atmospheric Correction (MAIAC) algorithm is under evaluation for use in conjunction with the Geostationary Coastal and Air Pollution Events (GEO-CAPE) mission. Column aerosol optical thickness (AOT) data from MAIAC are compared against corresponding data from the Visible Infrared Imaging Radiometer Suite (VIIRS) instrument over North America during 2013. Product coverage and retrieval strategy, along with regional variations in AOT through comparison of both matched and un-matched seasonally gridded data are reviewed. MAIAC shows extended coverage over parts of the continent when compared to VIIRS, owing to its pixel selection process and ability to retrieve aerosol information over brighter surfaces. To estimate data accuracy, both products are compared with AERONET Level 2 measurements to determine the amount of error present and discover if there is any dependency on viewing geometry and/or surface characteristics. Results suggest that MAIAC performs well over this region with a relatively small bias of -0.01; however there is a tendency for greater negative biases over bright surfaces and at larger scattering angles. Additional analysis over an expanded area and longer time period are likely needed to determine a comprehensive assessment of the products capability over the Western Hemisphere.

Evaluation of the representativeness of ground-based visibility for analysing the spatial and temporal variability of aerosol optical thickness in China

Although visibility is a widely-used indicator to quantify the aerosol loadings, only a few studies have been analyzed the representativeness of visibility in deriving Aerosol Optical Thickness (AOT). In this paper, ground-based visibility, MODerate-resolution Imaging Spectroradiometer (MODIS) and Multi-angle Imaging SpectroRadiometer (MISR) monthly AOT products between July 2002 and December 2014 were analyzed in order to extract the dominant modes of variability using the Singular Value Decomposition (SVD) method. The method has significant merit to reduce data dimension and examine both spatial and temporal variability simultaneously. Results indicated that the satellite retrieved AOTs agreed well with ground-based visibility in terms of inter-annual variability. The correlation coefficients in the first deseasonalized mode are greater than 0.65 between visibility and satellite AOT products. However, large differences were observed in the seasonal variability between ground-based visibility and AOT. In addition, Aerosol vertical distribution from LIdar climatology of Vertical Aerosol Structure for space-based lidar simulation studies (LIVAS) and cloud data from ground-based meteorological station were used to investigate the seasonal variability disagreement. The AOT values derived from LIVAS extinction coefficients between 0 and 500 m above surface have a stronger relationship with visibility, than total column AOT with visibility. It also indicates that seasonal variation of aerosol vertical distribution is the main cause of the disagreement between two parameters, and the uncertainties of satellite products also contribute to the disagreement. Results in this study highlighted that the visibility observation could only be used to depict the inter-annual AOT and more ancillary information could be used for studying seasonal AOT variation.

Temporal variability of aerosol optical thickness vertical distribution observed from CALIOP

AbstractTemporal variability in the vertical distribution of aerosol optical thickness (AOT) derived from the 0.532 µm aerosol extinction coefficient is described using Cloud‐Aerosol Lidar with Orthogonal Polarization (CALIOP) observations over 8.5 years (June 2006 to December 2014). Temporal variability of CALIOP column‐integrated AOT is largely consistent with total column AOT trends from several passive satellite sensors, such as the Moderate Resolution Imaging Spectroradiometer, Multiangle Imaging Spectroradiometer, and the Sea‐viewing Wide Field‐of‐view Sensor. Globally, a 0.0002 AOT per year positive trend in deseasonalized CALIOP total column AOT for daytime conditions is attributed to corresponding changes in near‐surface (i.e., 0.0–0.5 km or 0.5–1.0 km above ground level (agl)) aerosol particle loading, while a −0.0006 AOT per year trend during nighttime is attributed to elevated (i.e., 1.0–2.0 km or >2.0 km agl) aerosols. Regionally, increasing daytime CALIOP AOTs are found over Southern Africa and India, mostly due to changes in aerosol loading at the 1.0–2.0 km and 0.0–0.5 km agl layers, respectively. Decreasing daytime CALIOP AOTs are observed over Northern Africa, Eastern U.S., and South America (due mostly to elevated aerosol loading), while the negative CALIOP AOT trends found over Eastern China, Europe, and Western U.S. are due mostly to aerosol layers nearer the surface. To our knowledge, this study is the first to provide both a globally comprehensive estimation of the temporal variation in aerosol vertical distribution and an insight into passive sensor column AOT trends in the vertical domain.

Use of the CALIOP vertical feature mask for evaluating global aerosol models

Abstract. We use observations from the space-based Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) to evaluate global aerosol distributions simulated in the NASA Modern Era Retrospective Analysis for Research and Applications aerosol reanalysis (MERRAero). We focus particularly on an evaluation of aerosol types, using the CALIOP vertical feature mask (VFM) algorithm, and look especially at Saharan dust distributions during July 2009. MERRAero consists of an aerosol simulation produced in the Goddard Earth Observing System version 5 (GEOS-5) Earth system model and incorporates assimilation of MODIS-derived aerosol optical thickness (AOT) to constrain column aerosol loadings. For comparison to the CALIOP VFM we construct two synthetic VFMs using the MERRAero aerosol distributions: a CALIOP-like VFM in which we simulate the total attenuated backscatter and particle depolarization ratio from the MERRAero output and pass those into the CALIOP VFM typing algorithm (MERRAero-CALIOP), and an extinction-based VFM in which we use the MERRAero-simulated species-resolved extinction to map the MERRAero species to the CALIOP VFM types (MERRAero-Extinction). By comparing the MERRAero-CALIOP VFM to CALIOP VFM, we can diagnose the aerosol transport and speciation in MERRAero. By comparing the MERRAero-CALIOP and MERRAero-Extinction-simulated VFM, we perform a simple observing system experiment (OSE), which is useful for identifying limitations of the CALIOP VFM algorithm itself. We find that, despite having our column AOT constrained by MODIS, comparison to the CALIOP VFM reveals a greater occurrence of dusty aerosol layers in our MERRAero-CALIOP VFM due to errors in MERRAero aerosol speciation. Additionally, we find that the CALIOP VFM algorithm is challenged when classifying aerosol features when multiple aerosol types are present, as our application of the CALIOP VFM algorithm to MERRAero aerosol distributions classified marine-dominated aerosol layers with low aerosol loadings as polluted dust when the contribution of dust to the total extinction was low.

Tracking the Saharan Air Layer with shipborne lidar across the tropical Atlantic

Saharan dust was observed with shipborne lidar from 60° to 20°W along 14.5°N during a 1‐month transatlantic cruise of the research vessel Meteor. About 4500 km off the coast of Africa, mean extinction and backscatter‐related Ångström exponent of 0.1, wavelength‐independent extinction‐to‐backscatter ratios (lidar ratios) of around 45 sr, and particle linear depolarization ratio of 20% were found for aged dust (transport time >10 days). In contrast, dust with a shorter atmospheric residence time of 2–3 days showed Ångström exponents of −0.5 (backscatter coefficient) and 0.1 (extinction coefficient), mean lidar ratios of 64 and 50 sr, and particle linear depolarization ratios of 22 and 26% at 355 and 532 nm wavelength, respectively. Traces of fire smoke were also detected in the observed dust layers. The lidar observations were complemented by Aerosol Robotic Network handheld Sun photometer measurements, which revealed a mean total atmospheric column aerosol optical thickness of 0.05 for pure marine conditions (in the absence of lofted aerosol layers) and roughly 0.9 during a strong Saharan dust outbreak. The achieved data set was compared with first Consortium for Small Scale Modeling‐Multi‐Scale Chemistry Aerosol Transport simulations. The simulated vertical aerosol distribution showed good agreement with the lidar observations.

The inter-comparison of MODIS, MISR and GOCART aerosol products against AERONET data over China

Aerosols in East Asia have significant direct and indirect effects on the Earth's climate change. Several aerosol products have been generated to provide long-term monitoring of aerosol properties, but the discrepancies among them make the reliability of the products in doubt which leads to the uncertainty in the understanding of the long-time series of aerosols in East Asia, and especially in China. In this study, we investigate the applicability of Aerosol Optical Thickness (AOT) products derived from the measurements or model results of MODIS, MISR and GOCART through a statistical comparison, with Aerosol Robotic Network (AERONET) measurements over four sites located in China during a long-term period (2001–2011). Analysis shows that (1) only 43.06% of MODIS retrieved AOT values fall within ±(0.15τ+0.05) of the paired validation data from the AERONET for most sites, lower than the global averaged level (>66%) announced by Levy and Remer, in addition, MODIS/Terra shows a decreasing trend (−0.009/yr.) while MODIS/Aqua shows an increasing trend (+0.0012/yr.) and provides more stable AOT retrievals than MODIS/Terra during 2001–2011; (2) about 66.82% of MISR retrieved AOTs fall within the error envelop and MISR also shows a high correlation around 0.88 with AERONET data; (3) only 26.82% of GOCART model calculated AOTs fall within ±(0.15τ+0.05) of the paired validation data from the AERONET. GOCART also shows a low correlation (R=0.22–0.60) with AERONET data and thus cannot be well applied to total column AOT. Comparison of MODIS, MISR and GOCART products over the four sites in China and four sites located separately in Europe, North Africa and USA is carried out, which shows lower retrieval accuracy and larger discrepancy of products in China region. This is attributed to the fewer match-ups, larger aerosol load, moderate level of aerosol absorption and the complex nature of the aerosol mixtures. The analysis indicates the need for multiple-viewing-angle measurements of both intensity and polarization to provide the relevant aerosol parameters with sufficient accuracy for climate research.

Aerosol characterization: comparison between measured and modelled surface radiative forcing over Bay of Bengal

Under the aegis of Indian Space Research Organisation's ongoing climate change research programme, Integrated Campaign for Aerosols, gases and Radiation Budget (ICARB) was undertaken from 27 December 2008 to 30 January 2009 over the Bay of Bengal (BoB). Since measurements of this sort have not been made near the eastern-most coastline of BoB, a cruise was planned to have observations along the boundaries of the BoB. Here we report the columnar aerosol optical thickness (AOT), total water vapour (TWV), total ozone column (TOC) and aerosol direct surface forcing calculations by using a pair of handheld Microtops II (Ozonometer and Sunphotometer) and a gimbal-mounted short-wave (SW) pyranometer, with high temporal resolution (5 minutes). The presence of high aerosol loading and absorbing aerosols over the coastal and Andaman regions resulted in a diurnal mean aerosol surface radiative forcings of −24 and −35 W m−2, respectively. These results are in good agreement with radiative transfer model estimations.

Use of hourly Geostationary Operational Environmental Satellite (GOES) fire emissions in a Community Multiscale Air Quality (CMAQ) model for improving surface particulate matter predictions

[1] Large changes in surface-level PM2.5 concentrations and columnar aerosol optical thickness (AOT) were observed downwind of fires that originated in Georgia and Florida during the April–May 2007 period. In order to quantify the impacts of these wildfires on particulate matter air quality, Community Multiscale Air Quality (CMAQ) simulations were conducted by adding hourly fire emissions derived from the Geostationary Operational Environmental Satellite (GOES) imagers. The simulations include ambient aerosols by accounting for background emissions using the Sparse Matrix Operator Kernel Emissions (SMOKE) model. The impacts of fire emissions are obtained by comparing the CMAQ simulations with and without fire emissions. Overall, the CMAQ-derived PM2.5 reproduces the major smoke transport features of the Moderate Resolution Imaging Spectrometer (MODIS) AOT, but is systematically lower than the ground-based observations of PM2.5 mass concentrations during the fires. An increase of the satellite-derived fire emissions improves the simulated magnitude of PM2.5 concentrations. We also show that the disagreement between the CMAQ predictions and ground-based observations during the high PM2.5 episodes occurs when the predicted position of fire plume is not accurately located. The smoke position error grows more rapidly due to drift behavior of model wind error, so the position error dominates the accuracy of site-specific CMAQ PM2.5 predictions.

Using airborne high spectral resolution lidar data to evaluate combined active plus passive retrievals of aerosol extinction profiles

We derive aerosol extinction profiles from airborne and space‐based lidar backscatter signals by constraining the retrieval with column aerosol optical thickness (AOT), with no need to rely on assumptions about aerosol type or lidar ratio. The backscatter data were acquired by the NASA Langley Research Center airborne High Spectral Resolution Lidar (HSRL) and by the Cloud‐Aerosol Lidar with Orthogonal Polarization (CALIOP) instrument on the Cloud‐Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellite. The HSRL also simultaneously measures aerosol extinction coefficients independently using the high spectral resolution lidar technique, thereby providing an ideal data set for evaluating the retrieval. We retrieve aerosol extinction profiles from both HSRL and CALIOP attenuated backscatter data constrained with HSRL, Moderate‐Resolution Imaging Spectroradiometer (MODIS), and Multiangle Imaging Spectroradiometer column AOT. The resulting profiles are compared with the aerosol extinction measured by HSRL. Retrievals are limited to cases where the column aerosol thickness is greater than 0.2 over land and 0.15 over water. In the case of large AOT, the results using the Aqua MODIS constraint over water are poorer than Aqua MODIS over land or Terra MODIS. The poorer results relate to an apparent bias in Aqua MODIS AOT over water observed in August 2007. This apparent bias is still under investigation. Finally, aerosol extinction coefficients are derived from CALIPSO backscatter data using AOT from Aqua MODIS for 28 profiles over land and 9 over water. They agree with coincident measurements by the airborne HSRL to within ±0.016 km−1 ± 20% for at least two‐thirds of land points and within ±0.028 km−1 ± 20% for at least two‐thirds of ocean points.

Suspended particulate matter sampling at an urban AERONET site in Japan, part 2: relationship between column aerosol optical thickness and PM<sub>2.5</sub> concentration

The concentration of suspended particulate matter smaller than 2.5 µm (PM2.5) was linearly correlated with the column aerosol optical thickness (AOT) based on simultaneous measurements at a NASA/AERONET station at Kinki University Campus, Higashi-Osaka, Japan, between March 2004 and June 2006. The correlation coefficient differed with the aerosol type, being maximal when PM2.5 values were measured 120 minutes after AOT data for a dust episode, but almost independent of the time difference between measurements for anthropogenic aerosols. The obtained results were validated using data obtained at the Higashi-Osaka and Noto sites. Our results suggest that the PM2.5 mass concentration can be estimated from the AOT, and vice versa, and hence a distribution map of PM2.5 can be produced from the satellite-derived AOT map determined from the Aqua/MODIS sensor.

Errata: Suspended particulate matter sampling at an urban AERONET site in Japan, part 2: relationship between column aerosol optical thickness and PM<sub>2.5</sub>concentration

The <i>Journal of Applied Remote Sensing</i> (JARS) is an online journal that optimizes the communication of concepts, information, and progress within the remote sensing community to improve the societal benefit for monitoring and management of natural disasters, weather forecasting, agricultural and urban land-use planning, environmental quality monitoring, ecological restoration, and numerous other commercial and scientific applications.

Satellite Remote Sensing and Mesoscale Modeling of the 2007 Georgia/Florida Fires

During April and May 2007, several hundred fires burned uncontrollably in Georgia and Florida. The smoke from these fire events were visible throughout the Southeastern United States and had a major impact on particulate matter (PM) air quality near the surface. In this study, we show the strength of polar orbiting and geostationary satellite data in capturing the spatial distribution and diurnal variability of columnar smoke aerosol optical depth from these fires. We quantitatively evaluate PM air quality from satellites and ground-based monitors, near and far away (> 300 km) from fire source regions. We also show the changes in organic carbon concentrations (a tracer for smoke aerosols) before, during and after these fire events. Finally, we use fire locations and emissions retrieved and estimated from satellite observations as input to a regional mesoscale transport model to forecast the spatial distribution of aerosols and their impact on PM air quality. During the fire events, near the source regions, total column 550 nm aerosol optical thickness (AOT) exceeded 1.0 on several days and ground-based PM2.5 mass (particles less than 2.5 mum in aerodynamic diameter) reached unhealthy levels ( > 65.5 mug m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> ). Since the aerosols were reasonably well mixed in the first 1-2 km (as estimated from meteorology), the column AOT values derived from both geostationary and polar orbiting satellites and the surface PM2.5 were well correlated (linear correlation coefficient, r > 0.7). Several hundred miles away from the fire sources, in Birmingham, AL, the impact of the fires were also seen through the high AOT's and PM2.5 values. Correspondingly, PM2.5 mass due to organic carbon obtained from ground-based monitors showed a three fold increase during fire events when compared to background values. Satellite data were especially useful in capturing PM2.5 air quality in areas where there were no ground-based monitors. Although the mesoscale transport model captured the timing and location of aerosols, when compared to observations, the simulated mass concentrations are underestimated by nearly 70% due to various reasons including uncertainties in fire emission estimates, lack of chemistry in the model, and assumptions on vertical distribution of aerosols. Satellite products such as AOT, fire locations, and emissions from space-borne sensors are becoming a vital tool for assessing extreme events such as fires, smoke, and particulate matter air quality.

Impact of the mixing boundary layer on the relationship between PM2.5 and aerosol optical thickness

The purpose of this paper is to study the relationship between columnar aerosol optical thickness and ground-level aerosol mass. A set of Sun photometer, elastic backscattering lidar and TEOM measurements were acquired during April 2007 in Lille, France. The PM2.5 in the mixed boundary layer is estimated using the lidar signal, aerosol optical thickness, or columnar integrated Sun photometer size distribution and compared to the ground-level station measurements. The lidar signal recorded in the lowest level (240 m) is well correlated to the PM2.5 ( R 2 = 0.84). We also show that the correlation between AOT-derived and measured PM2.5 is significantly improved when considering the mixed boundary layer height derived from the lidar. The use of the Sun photometer aerosol fine fraction volume does not improve the correlation.

Estimating PM 2.5 over southern Sweden using space-borne optical measurements

In the present study Bremen aerosol retrieval (BAER) columnar aerosol optical thickness (AOT) data, according to moderate resolution imaging spectroradiometer (MODIS) and medium resolution imaging sensor (MERIS) level 1 calibrated satellite data, have been compared with AOT data obtained with the MODIS and MERIS retrieval algorithms (NASA and ESA, respectively) and by AErosol RObotic NETwork (AERONET). Relatively good agreement is found between these different instruments and algorithms. The R 2 and relative RMSD were 0.86 and 31% for MODIS when comparing with AERONET and 0.92 and 21% for MERIS. The aerosols investigated were influenced by low relative humidity. During this period, a relatively large range of aerosol loadings were detected; from continental background aerosol to particles emitted from agricultural fires. In this study, empirical relationships between BAER columnar AOT and ground-measured PM 2.5 have been estimated. Linear relationships, with R 2 values of 0.58 and 0.59, were obtained according to MERIS and MODIS data, respectively. The slopes of the regression of AOT versus PM 2.5 are lower than previous studies, but this could easily be explained by considering the effect of hygroscopic growth. The present AOT–PM 2.5 relationship has been applied on MERIS full resolution data over the urban area of Stockholm and the results have been compared with particle mass concentrations from dispersion model calculations. It seems that the satellite data with the 300 m resolution can resolve the expected increased concentrations due to emissions along the main highways close to the city. Significant uncertainties in the spatial distribution of PM 2.5 across land/ocean boundaries were particularly evident when analyzing the high resolution satellite data.

High-spectral resolution simulation of polarization of skylight: Sensitivity to aerosol vertical profile

[1] A vector radiative transfer model was used in conjunction with the line-by-line radiative transfer model and the database of high-resolution transmission (HITRAN) molecular absorption to simulate the degree of linear polarization of skylight in cloud-free conditions. Differences between simulated and measured polarization data in high-spectral resolution are found to be within 1% after aerosol scattering and gas absorptions are carefully considered. Limiting experiments are conducted at wavelengths around 0.760–0.765 μm O2-A absorption band for the same columnar aerosol optical thickness but different aerosol profiles. Results showed that the degree of linear polarization of skylight at surface varies strongly and is sensitive to the vertical change of tropospheric aerosol mass (or extinction) as the wavelengths approach to the edge of O2-A absorption band. However, such sensitivity is minimal at all wavelengths when the aerosol composition or single scattering properties are vertically homogeneous. This study suggests that the polarization data can be used together with radiance data to constrain the simulation of vertical distribution of aerosol composition in chemistry transport models.

Aerosol source plume physical characteristics from space‐based multiangle imaging

Models that assess aerosol effects on regional air quality and global climate parameterize aerosol sources in terms of amount, type, and injection height. The multiangle imaging spectroradiometer (MISR) aboard NASA’s Terra satellite retrieves total column aerosol optical thickness (AOT), and aerosol type over cloud‐free land and water. A stereo‐matching algorithm automatically retrieves reflecting‐layer altitude wherever clouds or aerosol plumes have discernable spatial contrast, with about 500‐m accuracy, at 1.1‐km horizontal resolution. Near‐source biomass burning smoke, volcanic effluent, and desert dust plumes are observed routinely, providing information about aerosol amount, particle type, and injection height useful for modeling applications. Compared to background aerosols, the plumes sampled have higher AOT, contain particles having expected differences in Angstrom exponent, size, single‐scattering albedo, and for volcanic plume and dust cloud cases, particle shape. As basic thermodynamics predicts, thin aerosol plumes lifted only by regional winds or less intense heat sources are confined to the boundary layer. However, when sources have sufficient buoyancy, the representative plumes studied tend to concentrate within discrete, high‐elevation layers of local stability; the aerosol is not uniformly distributed up to a peak altitude, as is sometimes assumed in modeling. MISR‐derived plume heights, along with meteorological profile data from other sources, make it possible to relate radiant energy flux observed by the moderate resolution imaging spectroradiometer (MODIS), also aboard the Terra spacecraft, to convective heat flux that plays a major role in buoyant plume dynamics. A MISR climatology of plume behavior based on these results is being developed.

Suspended particulate matter sampling at an urban AERONET site in Japan, part 1: clustering analysis of aerosols

The aerosol properties of urban atmospheric particles have been analyzed for radiometry data obtained using a multi-spectral photometer located at a NASA/AERONET station at Kinki University Campus, Higashi-Osaka, Japan, since 2002. The suspended particulate matter has been simultaneously measured at the same AERONET site since 2004. Our measurements and a clustering analysis reveal the aerosol types over the industrial city of Higashi-Osaka. It is shown that aerosols at Higashi-Osaka can be classified into three categories: (1) ordinary, which represents the background clear atmosphere of Higashi-Osaka, (2) anthropogenic aerosol events, and (2) dust episodes called Kosa, when large amounts of soil dust are transported to Higashi-Osaka from the Chinese mainland on westerly winds, especially in spring. We also found that the linear correlation exists between column aerosol optical thickness and PM2.5 concentration, and the correlation is better within each aerosol cluster than overall.

Is the top of atmosphere dust net radiative effect different between Terra and Aqua?

We assess the difference in Top of Atmosphere (TOA) cloud‐free Net Radiative Effect (NRE) of dust aerosols between the Terra and Aqua satellites using three years of collocated Moderate Resolution Imaging SpectroRadiometer (MODIS) and the Clouds and the Earth's Radiant Energy System (CERES) data over the Atlantic Ocean [0–30°N, 10–60°W]. The dust aerosol optical thickness at 0.55 μm (τdust) was first separated from the total aerosol column aerosol optical thickness (τ) and our results indicate that the Terra minus Aqua difference for both τ and τdust is approximately 10%, with Terra values generally being slightly higher. The resulting difference in TOA NRE from dust aerosols is less than 1 Wm−2. The difference between Terra and Aqua NRE lies well within previously reported uncertainties indicating that data from either satellite can be used interchangeably if independent adjustments for diurnal effects and clear‐sky sample biases are made.

Characterization of aerosol pollution events in France using ground-based and POLDER-2 satellite data

Abstract. We analyze the relationship between daily fine particle mass concentration (PM2.5) and columnar aerosol optical thickness derived from the Polarization and Directionality of Earth's Reflectances (POLDER) satellite sensor. The study is focused over France during the POLDER-2 lifetime between April and October 2003. We have first compared the POLDER derived aerosol optical thickness (AOT) with integrated volume size distribution derived from ground-based Sun Photometer observations. The good correlation (R=0.72) with sub-micron volume fraction indicates that POLDER derived AOT is sensitive to the fine aerosol mass concentration. Considering 1974 match-up data points over 28 fine particle monitoring sites, the POLDER-2 derived AOT is fairly well correlated with collocated PM2.5 measurements, with a correlation coefficient of 0.55. The correlation coefficient reaches a maximum of 0.80 for particular sites. We have analyzed the probability to find an appropriate air quality category (AQC) as defined by U.S. Environmental Protection Agency (EPA) from POLDER-2 AOT measurements. The probability can be up to 88.8% (±3.7%) for the "Good" AQC and 89.1% (±3.6%) for the "Moderate" AQC.