Climatology Of Aerosol Optical Depth Research Articles

Aerosol climatology, variability, and trends over the Indo-Gangetic Plain in CMIP6 models

We evaluate the performance of Coupled Model Intercomparison Project Phase 6 (CMIP6) models in simulating aerosol optical depth (AOD) climatology, variability, and trends over the Indo-Gangetic Plain (IGP) considered for the period of space-borne MODIS satellite observations during 2003–2014. The multi-model ensemble (MME) mean of CMIP6 models, is considered when better correlations (≥0.5) of models against the MODIS-derived AOD observations using standard statistical skill metrics. Analysis suggests that the CMIP6 MME successfully reproduces the spatial distribution of AOD climatology and its seasonal variability, albeit with some discrepancies. In particular, CMIP6 MME underestimates AOD spatial distributions across the IGP region (∼ bais varies between 0.2 and 0.4 during different seasons). CMIP6 MME captures coarse-mode aerosol distribution similar to AERONET over the IGP, while it has limited skill in capturing the fine-mode aerosol distribution, which could be due to uncertainties pertinent to anthropogenic aerosol emissions. Furthermore, the CMIP6 MME captures the seasonal evolution of aerosols over the IGP and its sub-regions (i.e., eastern and western IGP regions). Notably, the CMIP6 MME successfully captures the dominance among various aerosol species and their contributions to total AOD over the IGP. The CMIP6 MME mimic the observed AOD trends (varies between 0.01 and 0.03 year−1) well across the study regions, except the western IGP, where the MME is unable to replicate declining AOD trends during the pre-monsoon and monsoon seasons compared to MODIS. By conducting a principal component analysis on AOD data, it is apparent that the CMIP6 MME captures AOD variances that are comparable to MODIS observations, albeit with some discrepancies. This finding of the present research will help in policymaking, and mitigation strategies in the region.

Aerosol optical depth climatology from the high-resolution MAIAC product over Europe: differences between major European cities and their surrounding environments

Abstract. The aerosol optical depth (AOD) is a derived measurement useful to investigate the aerosol load and its distribution at different spatio-temporal scales. In this work we use long-term (2000–2021) MAIAC (Multi-Angle Implementation of Atmospheric Correction) retrievals with 1 km resolution to investigate the climatological AOD variability and trends at different scales in Europe: a continental (30–60∘ N, 20∘ W–40∘ E), a regional (100 × 100 km2) and an urban–local scale (3 × 3 km2). The AOD climatology at the continental scale shows the highest values during summer (JJA) and the lowest during winter (DJF) seasons. Regional and urban–local scales are investigated for 21 cities in Europe, including capitals and large urban agglomerations. Analyses show AOD average (550 nm) values between 0.06 and 0.16 at the urban–local scale while also displaying a strong north–south gradient. This gradient corresponds to a similar one in the European background, with higher AOD being located over the Po Valley, the Mediterranean Basin and eastern Europe. Average enhancements of the local with respect to regional AOD of 57 %, 55 %, 39 % and 32 % are found for large metropolitan centers such as Barcelona, Lisbon, Paris and Athens, respectively, suggesting a non-negligible enhancement of the aerosol burden through local emissions. Negative average deviations are observed for other cities, such as Amsterdam (−17 %) and Brussels (−6 %), indicating higher regional background signal and suggesting a heterogeneous aerosol spatial distribution that conceals the urban–local signal. Finally, negative statistically significant AOD trends for the entire European continent are observed. A stronger decrease rate at the regional scale with respect to the local scale occurs for most of the cities under investigation.

Climatology and interannual variability of type-dependent aerosol optical depth and vertical distribution over southwest China and northern India from multiple satellite and aerosol reanalysis datasets

Aerosol loading and properties have an important influence on the radiative effect of aerosols and regional climate change. Based on the long-term datasets of aerosol optical products retrieved from multiple satellite observations (MODIS/Terra, MODIS/Aqua, MISR, and CALIOP/CALIPSO) and a reanalysis product (CAMS) from 2007 to 2019, the climatology and interannual variability in total aerosol optical depth (AOD) and type-dependent AODs related to aerosol particle properties and components over southwest China (SWC) and northern India (NI) are investigated, and the vertical distribution of type-dependent aerosol extinctions are also examined. Overall, consistent spatial distribution characteristics of the AOD climatology are captured across the different datasets, albeit with slight differences in magnitude. Multi-year mean high AODs are mainly concentrated in areas such as the Sichuan Basin (SCB) (∼0.5) and NI (∼0.6), dominated by anthropogenic aerosols, and the Tarim Basin (∼0.5), dominated by mineral dust aerosols. In contrast, the central Tibetan Plateau (CTP) area, with a relatively clean background, has a lower AOD (∼0.1) on a multi-year average basis. When the total AOD is separated by chemical components, the CAMS reanalysis demonstrates that sulfate and carbonaceous aerosols (CAs) contribute nearly equally to AOD in SCB, while CAs is the largest contributor to the high aerosol loading in NI. An enhanced vertical aerosol extinction coefficient (AEC) is mainly located below 1 km in both SCB and NI, reaching 0.35 km−1 and 0.55 km−1. In CTP, the AEC has two peaks (∼0.02 km−1) at 2.5 km and 3.5 km. By partitioning the column AOD by different altitude ranges, we find that about 73.8% (80.6%) of the AOD in SCB (NI) is located in the lower troposphere (0–2 km). The resulting trend analyses show that a significant decreasing (increasing) trend (P < 0.05) in AOD in SCB (NI) is accurately captured by both the satellite products and the CAMS reanalysis. In contrast, a significant decline (P < 0.1) in the CTP region is observed in CAMS only, while a non-significant decline is detected in the satellite products. The results of this study have potential implications for understanding the vertical distribution and interannual variability of regional aerosol loading.

15-Year Analysis of Direct Effects of Total and Dust Aerosols in Solar Radiation/Energy over the Mediterranean Basin

The direct radiative effects of atmospheric aerosols are essential for climate, as well as for other societal areas, such as the energy sector. The goal of the present study is to exploit the newly developed ModIs Dust AeroSol (MIDAS) dataset for quantifying the direct effects on the downwelling surface solar irradiance (DSSI), induced by the total and dust aerosol amounts, under clear-sky conditions and the associated impacts on solar energy for the broader Mediterranean Basin, over the period 2003–2017. Aerosol optical depth (AOD) and dust optical depth (DOD) derived by the MIDAS dataset, along with additional aerosol and dust optical properties and atmospheric variables, were used as inputs to radiative transfer modeling to simulate DSSI components. A 15-year climatology of AOD, DOD and clear-sky global horizontal irradiation (GHI) and direct normal irradiation (DNI) was derived. The spatial and temporal variability of the aerosol and dust effects on the different DSSI components was assessed. Aerosol attenuation of annual GHI and DNI were 1–13% and 5–47%, respectively. Over North Africa and the Middle East, attenuation by dust was found to contribute 45–90% to the overall attenuation by aerosols. The GHI and DNI attenuation during extreme dust episodes reached 12% and 44%, respectively, over particular areas. After 2008, attenuation of DSSI by aerosols became weaker mainly because of changes in the amount of dust. Sensitivity analysis using different AOD/DOD inputs from Copernicus Atmosphere Monitoring Service (CAMS) reanalysis dataset revealed that using CAMS products leads to underestimation of the aerosol and dust radiative effects compared to MIDAS, mainly because the former underestimates DOD.

Seasonal simulations of summer aerosol optical depth over the Arabian Peninsula using WRF‐Chem: Validation, climatology, and variability

AbstractThis study investigates the climatology and variability of summer aerosol optical depth (AOD) over the Arabian Peninsula (AP) using a long‐term high‐resolution Weather Research and Forecasting model coupled with the chemistry module (WRF‐Chem) simulation, available ground‐based and satellite observations, and reanalysis products from 2008 to 2018. The simulated spatial distribution of the summer AOD agrees well with the satellite observations and reanalysis over the AP, with spatial correlation coefficients of 0.81/0.83/0.89 with MODIS‐A/MODIS‐T/MERRA‐2, respectively. Higher values of summertime AOD are broadly found over the eastern AP regions and the southern Red Sea and minima over the northern Red Sea and northwest AP, consistent with observational datasets. The WRF‐Chem simulation suggests that the two regions of high AOD are associated with dust advected from the Tigris–Euphrates by the northwesterly summer Shamal wind in the eastern AP and from the African Sahara via Sudan by westerly winds through the Tokar Gap for the southern AP. The high AOD over the south‐central east AP is due to locally generated dust by the action of northerly winds, modulated by variations in relative humidity, vertical motion, soil moisture, and soil temperature over the desert regions. The vertical extent of this dust is primarily driven by upward motion triggered by thermal convection over the local source region. In terms of interannual variability, summer AOD exhibits significant year‐to‐year variations over the AP region. In particular, enhanced (reduced) AOD over the southern AP (Persian Gulf) is observed during La Niña conditions, favoured by stronger (weaker) Tokar westerly (northwesterly summer Shamal) winds.HighlightsThis study evaluated the long‐term simulations of AOD using high resolution WRF‐Chem model over the AP.WRF‐Chem successfully simulated the summer AOD climatology and its variability.Potential driving factors of summer AOD loading over the AP is explored.Summer AOD variability in the AP is mainly driven by northern Shamal winds and westerly Tokar winds.Enhanced (reduced) AOD over the southern AP (Arabian Gulf) is observed during La Niña conditions.

Climatology of aerosol optical depth over Eastern Europe based on 19 years (2000–2018) MODIS TERRA data

AbstractThis study investigates the spatial–temporal evolution of aerosols, their optical properties (aerosol optical depth [AOD] and ångström exponent [AE]) and the trend over a period of 19 years in the Eastern European countries. The data used for the study are from moderate‐resolution imaging spectroradiometer (MODIS) Terra Collection 6.1 aerosol products with the use of Dark Target algorithm for the period from 2000 to 2018. The satellite‐based AOD products were validated against the AERONET AOD measurements in four stations, located in different regions. The results demonstrate high coherence in the setting of high values of correlation (R = 0.8–0.9) and low values of root‐mean‐square error (RMSE [0.066–0.130]) and mean absolute error (MAE [0.048–0.067]). However, of the total amount of data from AERONET stations, only 68.9% of available data fall within expected error (EE) over land. Generally, all countries recorded a low AOD (83.2% less than 0.2) and high AE (82.5% higher than 1). Mean AOD and AE values varied from 0.17 ± 0.12 and 1.31 ± 0.27 (Russia) to 0.24 ± 0.14 and 1.31 ± 0.33 (Czech Republic). There was a gradual decrease in aerosol load in all countries, the highest tendencies of AOD reduction are observed in Czech Republic, Bulgaria, Slovakia and Hungary (by −0.0028, −0.0027, −0.0026 and −0.0025 per year), while the lowest tendencies of reduction are observed in Russia and Moldova (by −0.001 and −0.0006 per year). Seasonal averaged AOD values have maximum values in summer and minimum values in winter. We studied predominant aerosol types, which are based on AOD and AE interrelation, the results showed the predominance of clean continental and mixed aerosols over all countries of the region.

Aerosol optical depth climatology over Central Asian countries based on Aqua-MODIS Collection 6.1 data: Aerosol variations and sources

The aim of this study is to explore the aerosol optical depth (AOD) – an indicator of air quality, based on satellite data over Central Asia. Due to limited studies and lack of knowledge regarding AOD variations over this region, five countries (Kazakhstan, Kyrgyzstan, Tajikistan, Turkmenistan, and Uzbekistan) and fifteen sites including the capital cities were selected for the analysis. Long-term Aqua-MODIS Collection 6.1 Deep Blue (DB) AOD observations at 10 km spatial resolution were obtained for 2002–2017 and the Ångström exponent (AE) was calculated using spectral AOD observations. AOD and AE were explored based on seasonal and inter-annual scale. The relationship between aerosol loading and economic indicator (gross domestic product) was also analyzed. Highest frequencies of low AOD and high AE were observed in all countries. Average AOD (AE) values during the study period were 0.17 ± 0.09 (1.29 ± 0.17) for Kazakhstan, 0.11 ± 0.13 (1.41 ± 0.13) for Kyrgyzstan, 0.17 ± 0.14 (1.36 ± 0.18) for Tajikistan, 0.20 ± 0.10 (1.10 ± 0.28) for Turkmenistan, and 0.18 ± 0.09 (1.18 ± 0.24) for Uzbekistan. Summer season experienced highest aerosol loading over Kazakhstan, Tajikistan, and Uzbekistan, whereas spring season experienced highest aerosol loading over Kyrgyzstan and Turkmenistan possibly due to the influence of less precipitation and dust from the arid regions. All fifteen cities selected in this study exhibited increasing aerosol load. Aerosol types based on the relationship between AOD and AE were investigated, results suggested the clean continental aerosol as a major aerosol type over the region, whereas the contribution of mixed aerosols was observed during spring and summer seasons. This study suggests for establishment of ground-based aerosol monitoring stations network and inclusions of modeling works to understand the complex nature of aerosol over this vast landmass.

How Long should the MISR Record Be when Evaluating Aerosol Optical Depth Climatology in Climate Models?

This study used the nearly continuous 17-year observation record from the Multi- angle Imaging SpectroRadiometer (MISR) instrument on the National Aeronautics and Space Administration (NASA) Terra Earth Observing System satellite to determine which temporal subsets are long enough to define statistically stable speciated aerosol optical depth (AOD) climatologies (i.e., AOD by particle types) for purposes of climate model evaluation. A random subsampling of seasonally averaged total and speciated AOD retrievals was performed to quantitatively assess the statistical stability in the climatology, represented by the minimum record length required for the standard deviation of the subsampled mean AODs to be less than a certain threshold. Our results indicate that the multi-year mean speciated AOD from MISR is stable on a global scale; however, there is substantial regional variability in the assessed stability. This implies that in some regions, even 17 years may not provide a long enough sample to define regional mean total and speciated AOD climatologies. We further investigated the agreement between the statistical stability of total AOD retrievals from MISR and the Moderate Resolution Imaging Spectroradiometer (MODIS), also on the NASA Terra satellite. The difference in the minimum record lengths between MISR and MODIS climatologies of total AOD is less than three years for most of the globe, with the exception of certain regions. Finally, we compared the seasonal cycles in the MISR total and speciated AODs with those simulated by three global chemistry transport models in the regions of climatologically stable speciated AODs. We found that only one model reproduced the observed seasonal cycles of the total and non-absorbing AODs over East China, but the seasonal cycles in total and dust AODs in all models are similar to those from MISR in Western Africa. This work provides a new method for considering the statistical stability of satellite-derived climatologies and illustrates the value of MISR’s speciated AOD data record for evaluating aerosols in global models.

Aerosol climatology over the Mexico City basin: Characterization of optical properties

Climatology of Aerosol Optical Depth (AOD), Single Scattering Albedo (SSA), and aerosol particle-size distribution were analyzed using a 15-year (1999–2014) dataset from AErosol RObotic NETwork (AERONET) observations over the Mexico City (MC) basin. The atmosphere over this site is dominated by two main aerosol types, represented by urban/industrial pollution and biomass-burning particles. Due to the specific meteorological conditions within the basin, seasons are usually classified into three as follows: Dry Winter (DW) (November–February); Dry Spring (DS) (March–April), and the RAiny season (RA) (May–October), which are mentioned throughout this article. Using a CIMEL sun photometer, we conducted continuous observations over the MC urban area from January 1999 to December 2014. Aerosol Optical Depth (AOD), Ångström exponent (α440–870), Single Scattering Albedo (SSA), and aerosol particle-size distribution were derived from the observational data. The overall mean AOD500 during the 1999–2014 period was 0.34±0.07. The monthly mean AOD reached a maximal value of 0.49 in May and a minimal value of 0.27 in February and March. The average α440–870 value for the period studied was 1.50±0.16. The monthly average of α440–870 reached a minimal value of 1.32 in August and a maximal value of 1.61 in May. Average SSA at 440nm was 0.89 throughout the observation period, indicating that aerosols over Mexico City are composed mainly of absorptive particles. Concentrations of fine- and coarse-mode aerosols over MC were highest in DS season compared with other seasons, especially for particles with radii measuring between 0.1 and 0.2μm. Results from the Spectral De-convolution Algorithm (SDA) show that fine-mode aerosols dominated AOD variability in MC. In the final part of this article, we present a classification of aerosols in MC by using the graphical method proposed by Gobbi et al. (2007), which is based on the combined analysis of α and its spectral curvature δα.

Retrieval and Validation of Atmospheric Aerosol Optical Depth From AVHRR Over China

As Advanced Very High Resolution Radiometer (AVHRR) lacks a 2.1- $\mu\text{m}$ band, the widely used “dark target” algorithm cannot be used to retrieve aerosol optical depth (AOD) from AVHRR over land. Instead, a multiple regression algorithm has been developed to process a time series of AVHRR Level_1b measurements over China (15°–45° N, 75°–135° E) for AOD retrieval. As the apparent reflectance of AVHRR is closely related to AOD, which can be provided by Moderate Resolution Imaging Spectroradiometer (MODIS), spatially and temporally collocated Aqua/MODIS AOD and AVHRR Level_1b measurements from four years (January 2008–December 2011) were used to generate the regression coefficients. Angle information, normalized difference vegetation index, water vapor, and surface elevation are chosen in addition to the apparent reflectance as predictors for different surface types. By applying the regression coefficients to AVHRR, the AOD product from independent AVHRR Level_1b measurements (May 2003–December 2007) was generated. Validation with AErosol RObotic NETwork (AERONET) AOD and comparison with MODIS AOD products have been conducted to evaluate the uncertainty of the AVHRR AOD from May 2003 to December 2007. The distribution pattern of the seasonal mean AOD from AVHRR is consistent with that of the MODIS AOD. Taking regions with rapid economic development, for example, the regional monthly mean AOD for these two data sets agrees well, with a consistent tendency and high correlation coefficients. When compared with AERONET from four sites in China, the results are also encouraging. Results show that the multiple regression method offers the potential to generate an AOD climatology data record from a long-term AVHRR Level_1b data set over land.

WRF-Solar: Description and Clear-Sky Assessment of an Augmented NWP Model for Solar Power Prediction

AbstractWRF-Solar is a specific configuration and augmentation of the Weather Research and Forecasting (WRF) Model designed for solar energy applications. Recent upgrades to the WRF Model contribute to making the model appropriate for solar power forecasting and comprise 1) developments to diagnose internally relevant atmospheric parameters required by the solar industry, 2) improved representation of aerosol–radiation feedback, 3) incorporation of cloud–aerosol interactions, and 4) improved cloud–radiation feedback. The WRF-Solar developments are presented together with a comprehensive characterization of the model performance for forecasting during clear skies. Performance is evaluated with numerical experiments using a range of different external and internal treatment of the atmospheric aerosols, including both a model-derived climatology of aerosol optical depth and temporally evolving aerosol optical properties from reanalysis products. The necessity of incorporating the influence of atmospheric aerosols to obtain accurate estimations of the surface shortwave irradiance components in clear-sky conditions is evident. Improvements of up to 58%, 76%, and 83% are found in global horizontal irradiance, direct normal irradiance, and diffuse irradiance, respectively, compared to a standard version of the WRF Model. Results demonstrate that the WRF-Solar model is an improved numerical tool for research and applications in support of solar energy.

An Operational Radiometric Landsat Preprocessing Framework for Large-Area Time Series Applications

We developed a large-area preprocessing framework for multisensor Landsat data, capable of processing large data volumes. Cloud and cloud shadow detection is performed by a modified Fmask code. Surface reflectance is inferred from Tanre's formulation of the radiative transfer, including adjacency effect correction. A precompiled MODIS water vapor database provides daily or climatological fallback estimates. Aerosol optical depth (AOD) is estimated over dark objects (DOs) that are identified in a combined database and image-based approach, where information on their temporal persistency is utilized. AOD is inferred with consideration of the actual target reflectance and background contamination effect. In case of absent DOs in bright scenes, a fallback approach with a modeled AOD climatology is used instead. Topographic normalization is performed by a modified C-correction. The data are projected into a single coordinate system and are organized in a gridded data structure for simplified pixel-based access. We based the assessment of the produced data set on an exhaustive analysis of overlapping pixels: 98.8% of the redundant overlaps are in the range of the expected ±2.5% overall radiometric algorithm accuracy. AOD is in very good agreement with Aerosol Robotic Network sunphotometer data ( $R^{2}$ : 0.72 to 0.79, low intercepts, and slopes near unity). The uncertainty in using the water vapor fallback climatology is approximately ±2.8% for the TM SWIR1 band in the wet season. The topographic correction was considered successful by an investigation of the nonrelationship between the illumination angle and the corrected radiance.

An assessment of the quality of aerosol retrievals over the Red Sea and evaluation of the climatological cloud-free dust direct radiative effect in the region

Ground-based and satellite observations are used in conjunction with the Rapid Radiative Transfer Model (RRTM) to assess climatological aerosol loading and the associated cloud-free aerosol direct radiative effect (DRE) over the Red Sea. Aerosol optical depth (AOD) retrievals from the Moderate Resolution Imaging Spectroradiometer and Spinning Enhanced Visible and InfraRed Imager (SEVIRI) instruments are first evaluated via comparison with ship-based observations. Correlations are typically better than 0.9 with very small root-mean-square and bias differences. Calculations of the DRE along the ship cruises using RRTM also show good agreement with colocated estimates from the Geostationary Earth Radiation Budget instrument if the aerosol asymmetry parameter is adjusted to account for the presence of large particles. A monthly climatology of AOD over the Red Sea is then created from 5 years of SEVIRI retrievals. This shows enhanced aerosol loading and a distinct north to south gradient across the basin in the summer relative to the winter months. The climatology is used with RRTM to estimate the DRE at the top and bottom of the atmosphere and the atmospheric absorption due to dust aerosol. These climatological estimates indicate that although longwave effects can reach tens of W m−2, shortwave cooling typically dominates the net radiative effect over the Sea, being particularly pronounced in the summer, reaching 120 W m−2 at the surface. The spatial gradient in summertime AOD is reflected in the radiative effect at the surface and in associated differential heating by aerosol within the atmosphere above the Sea. This asymmetric effect is expected to exert a significant influence on the regional atmospheric and oceanic circulation.

Development and basic evaluation of a prognostic aerosol scheme (v1) in the CNRM Climate Model CNRM-CM6

Abstract. We have implemented a prognostic aerosol scheme (v1) in CNRM-CM6, the climate model of CNRM-GAME and CERFACS, based upon the GEMS/MACC aerosol module of the ECMWF operational forecast model. This scheme describes the physical evolution of the five main types of aerosols, namely black carbon, organic matter, sulfate, desert dust and sea salt. In this work, we describe the characteristics of our implementation, for instance, taking into consideration a different dust scheme or boosting biomass burning emissions by a factor of 2, as well as the evaluation performed on simulation output. The simulations consist of time slice simulations for 2004 conditions and transient runs over the 1993–2012 period, and are either free-running or nudged towards the ERA-Interim Reanalysis. Evaluation data sets include several satellite instrument AOD (aerosol optical depth) products (i.e., MODIS Aqua classic and Deep-Blue products, MISR and CALIOP products), as well as ground-based AERONET data and the derived AERONET climatology, MAC-v1. The uncertainty of aerosol-type seasonal AOD due to model internal variability is low over large parts of the globe, and the characteristics of a nudged simulation reflect those of a free-running simulation. In contrast, the impact of the new dust scheme is large, with modelled dust AODs from simulations with the new dust scheme close to observations. Overall patterns and seasonal cycles of the total AOD are well depicted with, however, a systematic low bias over oceans. The comparison to the fractional MAC-v1 AOD climatology shows disagreements mostly over continents, while that to AERONET sites outlines the capability of the model to reproduce monthly climatologies under very diverse dominant aerosol types. Here again, underestimation of the total AOD appears in several cases, sometimes linked to insufficient efficiency of the aerosol transport away from the aerosol sources. Analysis of monthly time series at 166 AERONET sites shows, in general, correlation coefficients higher than 0.5 and lower model variance than observed. A large interannual variability can also be seen in the CALIOP vertical profiles over certain regions of the world. Overall, this prognostic aerosol scheme appears promising for aerosol-climate studies. There is room, however, for implementing more complex parameterisations in relation to aerosols.

Radiative signature of absorbing aerosol over the eastern Mediterranean basin

Abstract. The effects of absorbing aerosols on the atmospheric radiation budget and dynamics over the eastern Mediterranean region are studied using satellites and ground-based observations, and radiative transfer model calculations, under summer conditions. Climatology of aerosol optical depth (AOD), single scattering albedo (SSA) and size parameters were analyzed using multi-year (1999–2012) observations from Moderate Resolution Imaging Spectroradiometer (MODIS), Multi-angle Imaging SpectroRadiometer (MISR) and AErosol RObotic NETwork (AERONET). Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP)-derived aerosol vertical distributions and their classifications are used to calculate the AOD of four dominant aerosol types: dust, polluted dust, polluted continental, and marine aerosol over the region. The seasonal mean (June–August 2010) AODs are 0.22 ± 0.02, 0.11 ± 0.04, 0.10 ± 0.04 and 0.06 ± 0.01 for polluted dust, polluted continental, dust and marine aerosol, respectively. Changes in the atmospheric temperature profile as a function of absorbing aerosol loading were derived for the same period using observations from the AIRS satellite. We inferred heating rates in the aerosol layer of ~1.7 ± 0.8 K day−1 between 925 and 850 hPa, which is attributed to aerosol absorption of incoming solar radiation. Radiative transfer model (RTM) calculations show significant atmospheric warming for dominant absorbing aerosol over the region. A maximum atmospheric forcing of +16.7 ± 7.9 Wm−2 is calculated in the case of polluted dust, followed by dust (+9.4 ± 4.9 Wm−2) and polluted continental (+6.4 ± 4.5 Wm−2). RTM-derived heating rate profiles for dominant absorbing aerosol show warming of 0.1–0.9 K day−1 in the aerosol layer (&lt; 3.0 km altitudes), which primarily depend on AODs of the different aerosol types. Diabatic heating due to absorbing aerosol stabilizes the lower atmosphere, which could significantly reduce the atmospheric ventilation. These conditions can enhance the "pollution pool" over the eastern Mediterranean.

Micropulse lidar‐derived aerosol optical depth climatology at ARM sites worldwide

This paper focuses on climatology of the vertical distribution of aerosol optical depth (AOD (z)) from micropulse lidar (MPL) observations for climatically different locations worldwide. For this, a large data set obtained by MPL systems operating at 532 nm during the 4 year period 2007–2010 was used to derive vertical profiles of AOD (z) by combining the corresponding AOD data as an input from an independent measurement using nearly colocated multifilter rotating shadowband radiometer (MFRSR) systems at five different U.S. Department of Energy Atmospheric Radiation Measurement (ARM) Program sites—three permanent sites (SGP in north‐central Oklahoma, at 36.6°N, 97.5°W, 320 m; TWP‐Darwin in the tropical western Pacific, at 12.4°S, 130.9°E, 30 m; and NSA at Barrow on the North Slope of Alaska, at 71.3°N, 156.6°W, 8 m) and two mobile facility sites (GRW at Graciosa Island in the Azores, at 39°N, 28°W, 15 m; and FKB in the Black Forest of Germany, at 48.5°N, 8.4°E, 511 m). Therefore, amount of data used in this study is constrained by the availability of the MFRSR data. The MPL raw data were averaged for 30 s in time and 30 m in altitude. The diurnally averaged AOD (z) profiles from 4 years were combined to obtain a multiyear vertical profile of AOD (z) climatology at various ARM sites, including diurnal, day‐to‐day, and seasonal variabilities. Most aerosols were found to be confined to 0–2 km (approximately the planetary boundary layer region) at all sites; however, all sites exhibited measurable aerosols well above the mixed layer, with different height maxima. The entire data set demonstrates large day‐to‐day variability at all sites. However, there is no significant diurnal variation in AOD (z) at all sites. Significant interannual variability was observed at the SGP site. Clear seasonal variations in AOD (z) profiles exist for all five sites, but seasonal behavior was distinct. Moreover, the different seasonal variability for the lower level (0 to ~2 km) versus the level above indicates a contribution of different types of air masses from different sources. The lower annual mean AOD (z) values (0.093 ± 0.033 for daytime and 0.093 ± 0.05 for nighttime) observed near the surface at GRW are not unexpected for maritime aerosols (mostly sea salt), and the corresponding higher values at SGP (0.118 ± 0.038 for daytime and 0.11 ± 0.05 for nighttime), FKB (0.124 ± 0.042 for daytime and 0.127 ± 0.047 for nighttime), and TWP (0.13 ± 0.078 for daytime and 0.14 ± 0.073 for nighttime) are usual for continental aerosols. The annual mean AOD (z) values observed near the surface during daytime and nighttime for NSA were 0.1 ± 0.042 and 0.09 ± 0.037, respectively. These results will aid the scientific community in understanding aerosol properties and boundary layer dynamics and in improving the incorporation of aerosol radiative effects into global climate models.

Estimating Aerosol Optical Depth Over the Broader Greek Area from MODIS Satellite

A first-to-date comprehensive climatological aspect of the regime of aerosol loading over the entire broader Greek region (19°E–30°E and 34°N–43°N) is obtained using pixel-level (50 km × 50 km) monthly aerosol optical depth (AOD) products derived from raw Level-2 (10 km × 10 km) MODIS-Terra AOD data at 550 nm. The AOD climatology (for the 8-year period from March 2000 to February 2008) indicates that the study region is significantly loaded by aerosols (mean regional AOD equal to 0.196 ± 0.030 on an annual basis, with values ranging from 0.126 up to 0.382 at pixel level). It is also found that our high-resolution AOD data can reveal spatial patterns that are not evident in studies based on lower resolution data. There is a distinct longitudinal gradient of AODs, with higher values in the eastern than western part of the Greek peninsula. There is also a strong latitudinal gradient with decreasing values from south to north attributed to the presence of the Sahara Desert in northern Africa. The annual AOD cycle presents double maximum values, in spring and summer, and minimum ones in winter. There is also a remarkable year-by-year variability of AOD levels, especially as to their maxima, influenced by varying transport of desert dust, from the south, or biomass burning aerosols, from the north, to the region under prevailing favorable synoptic conditions. In this work, the processing, consisting in averaging over space and time, is done applying/testing five different criteria varying in their flexibility/severity in both spatial and temporal data availability. The criteria selection affects the magnitude of computed regional mean AOD value modifying it by up to 19 %, although the patterns of geographical distribution of AOD and its intra-annual variability do not change drastically.

A climatology of aerosol optical depth over China from recent 10 years of MODIS remote sensing data

ABSTRACTUsing the recent 10‐year (March 2000 to February 2010) MODIS data of aerosol optical depth (AOD), the distributions of annual and seasonal mean AOD over China are presented, and the trends and seasonal variations in AOD over 10 regions in China are analysed. The spatial pattern of annual mean AOD is characterized generally with two low centres and two high centres over China. Two low AOD centres are located in the areas with a high vegetation cover and a sparse population in (1) the high‐latitude region in Northeast China with AOD of about 0.2 and (2) the high‐altitude region in Southwest China with AOD from 0.1 to 0.2. These two low AOD centres are connected by a low AOD zone (0.2–0.3) in a northeast–southwest direction across China. Demarcated by this low AOD zone, two high centres with AOD of about 0.8 are situated in (1) the most densely populated and industrialized regions in China with high anthropogenic aerosols from North China Plain, Yangtze River areas covering Sichuan Basin, Central China and Yangtze River Delta to South China with Pearl River Delta region and (2) Tarim Basin in Northwest China with high natural aerosols dominated with desert dust. The spatial AOD patterns over China keep seasonally unchanged, but the strengths of the AOD‐centres vary from season to season. The wintertime AOD is lowest in China. The monthly AOD variations from March to September in Southern China correspond with high AOD before, after the rain periods and low AOD during the rain periods of Asian summer monsoon. Asian summer monsoons also make a notable impact on the seasonal cycle of aerosols in China. The AOD in Northern China changes monthly with a single peak between April and June and a low in the winter months. The positive trends in AOD occur mostly in the aerosol source regions with higher annual mean AOD (&gt;0.25), while the negative trends are found in the regions with lower annual mean AOD (&lt;0.25) over China.

A 4-D climatology (1979–2009) of the monthly tropospheric aerosol optical depth distribution over the Mediterranean region from a comparative evaluation and blending of remote sensing and model products

Abstract. Since the 1980s several spaceborne sensors have been used to retrieve the aerosol optical depth (AOD) over the Mediterranean region. In parallel, AOD climatologies coming from different numerical model simulations are now also available, permitting to distinguish the contribution of several aerosol types to the total AOD. In this work, we perform a comparative analysis of this unique multi-year database in terms of total AOD and of its apportionment by the five main aerosol types (soil dust, sea-salt, sulfate, black and organic carbon). We use 9 different satellite-derived monthly AOD products: NOAA/AVHRR, SeaWiFS (2 products), TERRA/MISR, TERRA/MODIS, AQUA/MODIS, ENVISAT/MERIS, PARASOL/POLDER and MSG/SEVIRI, as well as 3 more historical datasets: NIMBUS7/CZCS, TOMS (onboard NIMBUS7 and Earth-Probe) and METEOSAT/MVIRI. Monthly model datasets include the aerosol climatology from Tegen et al. (1997), the climate-chemistry models LMDz-OR-INCA and RegCM-4, the multi-model mean coming from the ACCMIP exercise, and the reanalyses GEMS and MACC. Ground-based Level-2 AERONET AOD observations from 47 stations around the basin are used here to evaluate the model and satellite data. The sensor MODIS (on AQUA and TERRA) has the best average AOD scores over this region, showing a relevant spatio-temporal variability and highlighting high dust loads over Northern Africa and the sea (spring and summer), and sulfate aerosols over continental Europe (summer). The comparison also shows limitations of certain datasets (especially MERIS and SeaWiFS standard products). Models reproduce the main patterns of the AOD variability over the basin. The MACC reanalysis is the closest to AERONET data, but appears to underestimate dust over Northern Africa, where RegCM-4 is found closer to MODIS thanks to its interactive scheme for dust emissions. The vertical dimension is also investigated using the CALIOP instrument. This study confirms differences of vertical distribution between dust aerosols showing a large vertical spread, and other continental and marine aerosols which are confined in the boundary layer. From this compilation, we propose a 4-D blended product from model and satellite data, consisting in monthly time series of 3-D aerosol distribution at a 50 km horizontal resolution over the Euro-Mediterranean marine and continental region for the 2003–2009 period. The product is based on the total AOD from AQUA/MODIS, apportioned into sulfates, black and organic carbon from the MACC reanalysis, and into dust and sea-salt aerosols from RegCM-4 simulations, which are distributed vertically based on CALIOP climatology. We extend the 2003–2009 reconstruction to the past up to 1979 using the 2003–2009 average and applying the decreasing trend in sulfate aerosols from LMDz-OR-INCA, whose AOD trends over Europe and the Mediterranean are median among the ACCMIP models. Finally optical properties of the different aerosol types in this region are proposed from Mie calculations so that this reconstruction can be included in regional climate models for aerosol radiative forcing and aerosol-climate studies.

Validation of an hourly resolved global aerosol model in answer to solar electricity generation information needs

Abstract. Solar energy applications need global aerosol optical depth (AOD) information to derive historic surface solar irradiance databases from geostationary meteorological satellites reaching back to the 1980's. This paper validates the MATCH/DLR model originating in the climate community against AERONET ground measurements. Hourly or daily mean AOD model output is evaluated individually for all stations in Europe, Africa and the Middle East – an area highly interesting for solar energy applications being partly dominated by high aerosol loads. Overall, a bias of 0.02 and a root-mean-square error (RMSE) of 0.23 are found for daily mean AOD values, while the RMSE increases to 0.28 for hourly mean AOD values. Large differences between various regions and stations are found providing a feedback loop for the aerosol modelling community. The difference in using daily means versus hourly resolved modelling with respect to hourly resolved observations is evaluated. Nowadays state-of-the-art in solar resource assessment relies on monthly turbidity or AOD climatologies while at least hourly resolved irradiance time series are needed by the solar sector. Therefore, the contribution of higher temporally modelled AOD is evaluated.