Background Aerosol Optical Depth Research Articles

Satellite-Based Background Aerosol Optical Depth Determination via Global Statistical Analysis of Multiple Lognormal Distribution

Determining background aerosol optical depth threshold value (BAOD) is critical to aerosol type identification and air pollution control. This study presents a statistical method to select the best BAOD threshold value using the VIIRS DB AOD products at 1 × 1 degree resolution from 2012 to 2019 as a major testbed. A series of multiple lognormal distributions with 1 to 5 peaks are firstly applied to fit the AOD histogram at each grid point, and the distribution with the highest correlation coefficient (R) gives preliminary estimations of BAOD, which is defined as either the intersection point of the first two normal distribution curves when having multiple peaks, or the midpoint between the peak AOD and the first AOD with non-zero probability when the mono peak is the best fit. Then, the lowest 1st to 100th percentile AOD distributions are compared with the preliminary BAOD distribution on a global scale. The final BAOD is obtained from the best cutoff percentile AOD distributions with the lowest bias compared with preliminary BAOD. Results show that the lowest 30th percentile AOD is the best estimation of BAOD for different AOD datasets and different seasons. Analysis of aerosol chemical information from MERRA-2 further supports this selection. Based on the BAOD, we updated the VIIRS aerosol type classification scheme, and the results show that the updated scheme is able to achieve reliable detection of aerosol type change in low aerosol loading conditions.

Background Optical Depth Correction over Urban Areas to Improve Land Aerosol Retrieval from Himawari-8

A conventional clear-sky minimum reflectance method, which has been widely used for AOD retrieval from geostationary satellites, usually has less accuracy over urban than other land areas. Urban areas usually have more complex surface properties and various aerosol types from different emission sources. When the surface reflectance is calculated from the clear-sky minimum reflectance, background aerosol optical depth (BOD) is assumed to be closed to zero. This assumption generates larger surface reflectance which leads to underestimation of AOD retrieved. This study proposed a correction for BOD value to be applied for AOD retrieval primary over urban areas where the pollution or natural source aerosol is persistent for long term. The study area covers Indonesia’s land region, while for evaluating the impact of specific treatment over urban areas we used the AERONET data from Bandung, Pontianak, and Makassar sites. The comparison of AOD retrieved from modified BOD and the AERONET ground-based data showed that the corrected surface reflectance improved the accuracy of AOD in the three sites, with a correlation coefficient increased from 0.23 to 0.37 and the fraction of ‘good retrieval’ changed from 35% to 51%.

Improving GOES Advanced Baseline Imager (ABI) aerosol optical depth (AOD) retrievals using an empirical bias correction algorithm

Abstract. The Advanced Baseline Imager (ABI) on board the Geostationary Operational Environmental Satellite-R (GOES-R) series enables retrieval of aerosol optical depth (AOD) from geostationary satellites using a multiband algorithm similar to those of polar-orbiting satellites' sensors, such as the Moderate Resolution Imaging Spectroradiometer (MODIS) and Visible Infrared Imaging Radiometer Suite (VIIRS). However, this work demonstrates that the current version of GOES-16 (GOES-East) ABI AOD has diurnally varying biases due to limitations in the land surface reflectance relationships between the 0.47 µm band and the 2.2 µm band and between the 0.64 µm band and 2.2 µm band used in the ABI AOD retrieval algorithm, which vary with the Sun–satellite geometry and NDVI (normalized difference vegetation index). To reduce these biases, an empirical bias correction algorithm has been developed based on the lowest observed ABI AOD of an adjacent 30 d period and the background AOD at each time step and at each pixel. The bias correction algorithm improves the performance of ABI AOD compared to AErosol RObotic NETwork (AERONET) AOD, especially for the high and medium (top 2) quality ABI AOD. AOD data for the period 6 August to 31 December 2018 are used to evaluate the bias correction algorithm. After bias correction, the correlation between the top 2 quality ABI AOD and AERONET AOD improves from 0.87 to 0.91, the mean bias improves from 0.04 to 0.00, and root-mean-square error (RMSE) improves from 0.09 to 0.05. These results for the bias-corrected top 2 qualities ABI AOD are comparable to those of the corrected high-quality ABI AOD. By using the top 2 qualities of ABI AOD in conjunction with the bias correction algorithm, the areal coverage of ABI AOD is increased by about 100 % without loss of data accuracy.

Exploring the Constraints on Simulated Aerosol Sources and Transport Across the North Atlantic With Island‐Based Sun Photometers

Atmospheric aerosol over the North Atlantic Ocean impacts regional clouds and climate. In this work, we use a set of sun photometer observations of aerosol optical depth (AOD) located on the Graciosa and Cape Verde islands, along with the GEOS‐Chem chemical transport model to investigate the sources of these aerosol and their transport over the North Atlantic Ocean. At both locations, the largest simulated contributor to aerosol extinction is the local source of sea‐salt aerosol. In addition to this large source, we find that signatures consistent with long‐range transport of anthropogenic, biomass burning, and dust emissions are apparent throughout the year at both locations. Model simulations suggest that this signal of long‐range transport in AOD is more apparent at higher elevation locations; the influence of anthropogenic and biomass burning aerosol extinction is particularly pronounced at the height of Pico Mountain, near the Graciosa Island site. Using a machine learning approach, we further show that simulated observations at these three sites (near Graciosa, Pico Mountain, and Cape Verde) can be used to predict the simulated background aerosol imported into cities on the European mainland, particularly during the local winter months, highlighting the utility of background AOD monitoring for understanding downwind air quality.

Assessment of Sun photometer Langley calibration at the high-elevation sites Mauna Loa and Izaña

Abstract. The aim of this paper is to analyze the suitability of the high-mountain stations Mauna Loa and Izaña for Langley plot calibration of Sun photometers. Thus the aerosol optical depth (AOD) characteristics and seasonality, as well as the cloudiness, have been investigated in order to provide a robust estimation of the calibration uncertainty as well as the number of days that are suitable for Langley calibrations. The data used for the investigations belong to the AERONET and GAW-PFR networks, which maintain reference Sun photometers at these stations with long measurement records: 22 years at Mauna Loa and 15 years at Izaña. In terms of clear-sky and stable aerosol conditions, Mauna Loa (3397 m a.s.l.) exhibits on average 377 Langley plots (243 morning and 134 afternoon) per year suitable for Langley plot calibration, whereas Izaña (2373 m a.s.l.) shows 343 Langley plots (187 morning and 155 afternoon) per year. The background AOD (500 nm) values, on days that are favorable for Langley calibrations, are in the range 0.01–0.02 throughout the year, with well-defined seasonality that exhibits a spring maximum at both stations plus a slight summer increase at Izaña. The statistical analysis of the long-term determination of extraterrestrial signals yields to a calibration uncertainty of ∼ 0.25–0.5 %, this uncertainty being smaller in the visible and near-infrared wavelengths and larger in the ultraviolet wavelengths. This is due to atmospheric variability produced by changes in several factors, mainly the AOD. The uncertainty cannot be reduced based only on quality criteria of individual Langley plots and averaging over several days is shown to reduce the uncertainty to the needed levels for reference Sun photometers.

Retrieval of Aerosol Optical Depth From INSAT‐3D Imager Over Asian Landmass and Adjoining Ocean: Retrieval Uncertainty and Validation

AbstractAn algorithm for aerosol optical depth (τ) retrieval from imager data onboard geostationary Indian National Satellite (INSAT‐3D) is described. The basic principles of the algorithm are adopted from Geostationary Observational Environmental Satellite Aerosol and Smoke Algorithm, where darkest observations over a time period and constant background aerosol optical depth (τb) value are used to derive surface reflectance. However, in INSAT‐3D algorithm spatially and temporally dynamic τb derived from Moderate Resolution Imaging Spectroradiometer (MODIS)‐aqua is used. The theoretical simulations suggest retrieval uncertainty of around ±30–45% depending on the certainty of aerosol optical properties and other parameters used in radiative transfer calculations. Retrieval uncertainty is less over dark regions and lower scattering angles. INSAT‐3D τ at 0.55 μm is derived operationally for the first time over Asia and adjoining ocean at 30‐min temporal resolution. The retrieval algorithm is validated against in situ Sun‐sky radiometer measurements at 26 Aerosol Robotic Network sites and Sun‐photometer measurements during Sagar‐Sampada cruise in the northern Arabian Sea. The validation study encompassing 3,803 and 844 data points over land and ocean stations show correlations ranging from 0.70 to 0.91 and 0.79 to 0.91, respectively. The uncertainty of retrievals is within 45% over land and within 30% over ocean. The diurnal variability of INSAT‐3D and Aerosol Robotic Network τ in India and southeast Asia is also presented. The comparison of INSAT‐3D and MODIS‐aqua monthly average τ shows correlation ranging from 0.74 to 0.89 and 0.65 to 0.97 over Indian landmass and adjoining ocean, respectively. The validation suggests that INSAT‐3D can be used for aerosol monitoring and can be merged with MODIS‐aqua/terra aerosol product to generate combined aerosol product at high temporal resolution.

Refined Use of Satellite Aerosol Optical Depth Snapshots to Constrain Biomass Burning Emissions in the GOCART Model

AbstractSimulations of biomass burning (BB) emissions in global chemistry and aerosol transport models depend on external inventories, which provide location and strength for BB aerosol sources. Our previous work shows that to first order, satellite snapshots of aerosol optical depth (AOD) near the emitted smoke plume can be used to constrain model‐simulated AOD, and effectively, the smoke source strength. We now refine the satellite‐snapshot method and investigate where applying simple multiplicative emission adjustment factors alone to the widely used Global Fire Emission Database version 3 emission inventory can achieve regional‐scale consistency between Moderate Resolution Imaging Spectroradiometer (MODIS) AOD snapshots and the Goddard Chemistry Aerosol Radiation and Transport model. The model and satellite AOD are compared globally, over a set of BB cases observed by the MODIS instrument during the 2004, and 2006–2008 biomass burning seasons. Regional discrepancies between the model and satellite are diverse around the globe yet quite consistent within most ecosystems. We refine our approach to address physically based limitations of our earlier work (1) by expanding the number of fire cases from 124 to almost 900, (2) by using scaled reanalysis‐model simulations to fill missing AOD retrievals in the MODIS observations, (3) by distinguishing the BB components of the total aerosol load from background aerosol in the near‐source regions, and (4) by including emissions from fires too small to be identified explicitly in the satellite observations. The small‐fire emission adjustment shows the complimentary nature of correcting for source strength and adding geographically distinct missing sources. Our analysis indicates that the method works best for fire cases where the BB fraction of total AOD is high, primarily evergreen or deciduous forests. In heavily polluted or agricultural burning regions, where smoke and background AOD values tend to be comparable, this approach encounters large uncertainties, and in some regions, other model‐ or measurement‐related factors might contribute significantly to model‐satellite discrepancies. This work sets the stage for a larger study within the Aerosol Comparison between Observations and Models (AeroCOM) multimodel biomass burning experiment. By comparing multiple model results using the refined technique presented here, we aim to separate BB inventory from model‐specific contributions to the remaining discrepancies.

Sensitivity of the WRF-Chem (V3.6.1) model to different dust emission parametrisation: assessment in the broader Mediterranean region

Abstract. In this study we aim to assess the WRF-Chem model capacity to reproduce dust transport over the eastern Mediterranean. For this reason, we compare the model aerosol optical depth (AOD) outputs to observations, focusing on three key regions: North Africa, the Arabian Peninsula and the eastern Mediterranean. Three sets of four simulations have been performed for the 6-month period of spring and summer 2011. Each simulation set uses a different dust emission parametrisation and for each parametrisation, the dust emissions are multiplied with various coefficients in order to tune the model performance. Our assessment approach is performed across different spatial and temporal scales using AOD observations from satellites and ground-based stations, as well as from airborne measurements of aerosol extinction coefficients over the Sahara. Assessment over the entire domain and simulation period shows that the model presents temporal and spatial variability similar to observed AODs, regardless of the applied dust emission parametrisation. On the other hand, when focusing on specific regions, the model skill varies significantly. Tuning the model performance by applying a coefficient to dust emissions may reduce the model AOD bias over a region, but may increase it in other regions. In particular, the model was shown to realistically reproduce the major dust transport events over the eastern Mediterranean, but failed to capture the regional background AOD. Further comparison of the model simulations to airborne measurements of vertical profiles of extinction coefficients over North Africa suggests that the model realistically reproduces the total atmospheric column AOD. Finally, we discuss the model results in two sensitivity tests, where we included finer dust particles (less than 1 µm) and changed accordingly the dust bins' mass fraction.

On the sensitivity of anthropogenic aerosol forcing to model‐internal variability and parameterizing a Twomey effect

AbstractDespite efforts to accurately quantify the effective radiative forcing (ERF) of anthropogenic aerosol, the historical evolution of ERF remains uncertain. As a further step toward a better understanding of ERF uncertainty, the present study systematically investigates the sensitivity of the shortwave ERF at the top of the atmosphere to model‐internal variability and spatial distributions of the monthly mean radiative effects of anthropogenic aerosol. For this, ensembles are generated with the atmospheric model ECHAM6.3 that uses monthly prescribed optical properties and changes in cloud‐droplet number concentrations designed to mimic that associated with the anthropogenic aerosol using the new parameterization MACv2‐SP. The results foremost highlight the small change in our best estimate of the global averaged all‐sky ERF associated with a substantially different pattern of anthropogenic aerosol radiative effects from the mid‐1970s (–0.51 Wm–2) and present day (–0.50 Wm–2). Such a small change in ERF is difficult to detect when model‐internal year‐to‐year variability (0.32 Wm–2 standard deviation) is considered. A stable estimate of all‐sky ERF requires ensemble simulations, the size of which depends on the targeted precision, confidence level, and the magnitude of model‐internal variability. A larger effect of the pattern of the anthropogenic aerosol radiative effects on the globally averaged all‐sky ERF (15%) occurs with a strong Twomey effect through lowering the background aerosol optical depth in regions downstream of major pollution sources. It suggests that models with strong aerosol‐cloud interactions could show a moderate difference in the global mean ERF associated with the mid‐1970s to present‐day change in the anthropogenic aerosol pattern.

Modulation in Direct Radiative Forcing Caused by Wind Generated Sea-Salt Aerosols

Sea-salt aerosols, prominent natural aerosols over the ocean, play a vital role in direct and indirect radiative forcing. Since surface winds are the prime cause of sea-salt generation, we have developed an empirical relationship between aerosol optical depth (AOD) and sea-surface winds over the study region (60-70 degrees E; 40 degrees S-20 degrees N). The latitudinal variation of background aerosol optical depth (tau(0)) and the wind index (b) are estimated as they are essential inputs for the estimation of the spatial variation of sea-salt aerosols and are used over the Arabian Sea (AS) to generate spatial heterogeneity of sea-salt aerosols. The latitudinal variation of tau(0) and b show a nearly exponential and linear increase, respectively, as we moved towards the north. We used an empirical-cum-model approach to construct an aerosol system to reproduce the observed AOD and aerosol optical properties. Utilizing this information as input to a radiative transfer model, we worked out direct radiative forcing (DRF) over the study region in the short wave (0.2-4 mu m) and long wave (8-14 mu m) region at the surface, top of the atmosphere (TOA) and in the atmosphere. Short wave cooling at the surface, TOA and heating in the atmosphere are estimated as 40 W m(-2), 32 W m(-2) and 8 W m(-2), respectively. Long wave heating due to the sea-salt aerosols estimated at the surface, TOA and in the atmosphere is about 9 W m-2, 6 W m-2 and 15 W m-2 respectively. Long wave forcing (LWF) partly counterbalances the effect of short wave forcing (SWF) and the cooling at the surface and at TOA. The highest value of such an offset at the surface was observed over the AS (similar to 23%) and that at TOA was similar to 19%, obviously at high wind conditions. This implies that over the AS sea-salt aerosols (in coarse mode) contribute significantly to LWF compared to other adjacent oceanic regions.

Factors affecting O3 and NO2photolysis frequencies measured in the eastern Mediterranean during the five‐year period 2002–2006

The photolysis frequencies of ozone (O3; to singlet oxygen, JO1D) and nitrogen dioxide (NO2; JNO2) were recorded at the remote coastal site Finokalia (35°20′N, 25°40′E), on the island of Crete, Greece, during the period 2002–2006. We present a study of their main climatological aspects and a quantification of the effect of aerosol and total ozone column on these frequencies. The 5‐yr mean Aerosol Optical Depth (AOD) at 380 nm in the area is equal to 0.27 ± 0.13 and reduces JNO2by 5%–14% at a solar zenith angle (sza) of 60°, compared to an aerosol‐free atmosphere. It also leads to a similar reduction of JO1D by 5%–15% at the 60° sza, for an average total ozone column (300–320 Dobson units (DU)). The effect of regional background AOD (∼0.1) is a reduction of JNO2 and JO1D by up to 6% for sza in the range 15° to75°, respectively. During high aerosol loads (AOD 0.5–0.7) the percentage reduction of Js was found to be as much as 30%–40% at high sza. The day‐to‐day variability of total ozone column over the area, of the order of 20 DU, was found to result in a 12% change in JO1D at 60° sza as compared with zero AOD conditions. A reduction of Js corresponding to a 24% decrease in the local noon JO1D value and a 5% decrease in local noon JNO2 were found to result in a 12% reduction in the 24 h mean net chemical production of O3, using a chemical box model.

An update on polar aerosol optical properties using POLAR-AOD and other measurements performed during the International Polar Year

An updated set of time series of derived aerosol optical depth (AOD) and Ångström’s exponent α from a number of Arctic and Antarctic stations was analyzed to determine the long-term variations of these two parameters. The Arctic measurements were performed at Ny-Ålesund (1991–2010), Barrow (1977–2010) and some Siberian sites (1981–1991). The data were integrated with Level 2.0 AERONET sun-photometer measurements recorded at Hornsund, Svalbard, and Barrow for recent years, and at Tiksi for the summer 2010. The Antarctic data-set comprises sun-photometer measurements performed at Mirny (1982–2009), Neumayer (1991–2004), and Terra Nova Bay (1987–2005), and at South Pole (1977–2010). Analyses of daily mean AOD were made in the Arctic by (i) adjusting values to eliminate volcanic effects due to the El Chichón, Pinatubo, Kasatochi and Sarychev eruptions, and (ii) selecting the summer background aerosol data from those affected by forest fire smoke. Nearly null values of the long-term variation of summer background AOD were obtained at Ny-Ålesund (1991–2010) and at Barrow (1977–2010). No evidence of important variations in AOD was found when comparing the monthly mean values of AOD measured at Tiksi in summer 2010 with those derived from multi-filter actinometer measurements performed in the late 1980s at some Siberian sites. The long-term variations of seasonal mean AOD for Arctic Haze (AH) conditions and AH episode seasonal frequency were also evaluated, finding that these parameters underwent large fluctuations over the 35-year period at Ny-Ålesund and Barrow, without presenting well-defined long-term variations. A characterization of chemical composition, complex refractive index and single scattering albedo of ground-level aerosol polydispersions in summer and winter–spring is also presented, based on results mainly found in the literature. The long-term variation in Antarctic AOD was estimated to be stable, within ±0.10% per year, at the three coastal sites, and nearly null at South Pole, where a weak increase was only recently observed, associated with an appreciable decrease in α, plausibly due to the formation of thin stratospheric layers of ageing volcanic particles. The main characteristics of chemical composition, complex refractive index and single scattering albedo of Antarctic aerosols are also presented for coastal particles sampled at Neumayer and Terra Nova Bay, and continental particles at South Pole.

Seasonal variations in aerosol optical properties over China

[1] Seasonal variations in background aerosol optical depth (AOD) and aerosol type are investigated over various ecosystems in China based upon three years' worth of meteorological data and data collected by the Chinese Sun Hazemeter Network. In most parts of China, AODs are at a maximum in spring or summer and at a minimum in autumn or winter. Minimum values (0.10∼0.20) of annual mean AOD at 500 nm are found in the Qinghai-Tibetan Plateau, the remote northeast corner of China, the northern forest ecosystems and Hainan Island. Annual mean AOD ranges from 0.25 to 0.30 over desert and oasis areas, as well as the desertification grasslands in northern China; the annual mean AOD over the Loess Plateau is moderately high at 0.36. Regions where the highest density of agricultural and industrial activities are located and where anthropogenic sulphate aerosol and soil aerosol emissions are consistently high throughout the whole year (e.g., the central-eastern, southern and eastern coastal regions of China) experience annual mean AODs ranging from 0.50∼0.80. Remarkable seasonal changes in the main types of aerosol over northern China (characterized by the Angstrom exponent, α) are seen. Due to biomass and fossil fuel burning from extensive agricultural practices in northern rural areas, concentrations of smoke and soot aerosols rise dramatically during autumn and winter (high α), while the main types of aerosol during spring and summer are dust and soil aerosols (low α). Over southeastern Asia, biomass burning during the spring leads to increases in smoke and soot emissions. Over the Tibetan Plateau and Hainan Island where the atmosphere is pristine, the main types of aerosol are dust and sea salt, respectively.

Estimating the maritime component of aerosol optical depth and its dependency on surface wind speed using satellite data

Abstract. Six years (2003–2008) of satellite measurements of aerosol parameters from the Moderate Resolution Imaging Spectroradiometer (MODIS) and surface wind speeds from Quick Scatterometer (QuikSCAT), the Advanced Microwave Scanning Radiometer (AMSR-E), and the Special Sensor Microwave Imager (SSM/I), are used to provide a comprehensive perspective on the link between surface wind speed and marine aerosol optical depth over tropical and subtropical oceanic regions. A systematic comparison between the satellite derived fields in these regions allows to: (i) separate the relative contribution of wind-induced marine aerosol to the aerosol optical depth; (ii) extract an empirical linear equation linking coarse marine aerosol optical depth and wind intensity; and (iii) identify a time scale for correlating marine aerosol optical depth and surface wind speed. The contribution of wind induced marine aerosol to aerosol optical depth is found to be dominated by the coarse mode elements. When wind intensity exceeds 4 m/s, coarse marine aerosol optical depth is linearly correlated with the surface wind speed, with a remarkably consistent slope of 0.009±0.002 s/m. A detailed time scale analysis shows that the linear correlation between the fields is well kept within a 12 h time frame, while sharply decreasing when the time lag between measurements is longer. The background aerosol optical depth, associated with aerosols that are not produced in-situ through wind driven processes, can be used for estimating the contributions of terrestrial and biogenic marine aerosol to over-ocean satellite retrievals of aerosol optical depth.

Optical properties of boreal region biomass burning aerosols in central Alaska and seasonal variation of aerosol optical depth at an Arctic coastal site

Long‐term monitoring of aerosol optical properties at a boreal forest AERONET site in interior Alaska was performed from 1994 through 2008 (excluding winter). Large interannual variability was observed, with some years showing near background aerosol optical depth (AOD) levels (<0.1 at 500 nm) while 2004 and 2005 had August monthly means similar in magnitude to peak months at major tropical biomass burning regions. Single scattering albedo (ω0; 440 nm) at the boreal forest site ranged from ∼0.91 to 0.99 with an average of ∼0.96 for observations in 2004 and 2005. This suggests a significant amount of smoldering combustion of woody fuels and peat/soil layers that would result in relatively low black carbon mass fractions for smoke particles. The fine mode particle volume median radius during the heavy burning years was quite large, averaging ∼0.17 μm at AOD(440 nm) = 0.1 and increasing to ∼0.25 μm at AOD(440 nm) = 3.0. This large particle size for biomass burning aerosols results in a greater relative scattering component of extinction and, therefore, also contributes to higher ω0. Additionally, monitoring at an Arctic Ocean coastal site (Barrow, Alaska) suggested transport of smoke to the Arctic in summer resulting in individual events with much higher AOD than that occurring during typical spring Arctic haze. However, the springtime mean AOD(500 nm) is higher during late March through late May (∼0.150) than during summer months (∼0.085) at Barrow partly due to very few days with low background AOD levels in spring compared with many days with clean background conditions in summer.

Multi-spectral Lidar characterization of the vertical structure of Saharan dust aerosol over southern Spain

Measurements during Saharan dust transport events have been performed at the Andalusian Centre for Environmental Studies (CEAMA) (37.16°N, 3.58°W, 680 m a.s.l.), located in the city of Granada (south-eastern Iberian Peninsula). During 2005, as many as 22 dust outbreaks took place over the study area in south-eastern Spain. The arrival of Saharan air masses induced sharp changes in the background aerosol optical depth (AOD) and the Angström exponent during the analysed events. This fact, in addition to a back-trajectory analysis, identified the Saharan dust events. In order to improve knowledge of Saharan dust over the southern Iberian Peninsula, vertical characterization of temporal evolutions has been performed in terms of the backscatter coefficient, aerosol depolarization ratio and colour ratio. Two severe events in July 2005 have been selected for detailed analysis. Both events lasted several days, allowing the characterization of the evolution of the dust layers over our station. The impact of Saharan dust outbreaks has been followed with the Lidar, and their effects at the surface have been determined in the air quality network of the regional government.

Aerosols in polar regions: A historical overview based on optical depth and in situ observations

Large sets of filtered actinometer, filtered pyrheliometer and Sun photometer measurements have been carried out over the past 30 years by various groups at different Arctic and Antarctic sites and for different time periods. They were examined to estimate ensemble average, long‐term trends of the summer background aerosol optical depth AOD(500 nm) in the polar regions (omitting the data influenced by Arctic haze and volcanic eruptions). The trend for the Arctic was estimated to be between −1.6% and −2.0% per year over 30 years, depending on location. No significant trend was observed for Antarctica. The time patterns of AOD(500 nm) and Ångström's parameters α and β measured with Sun photometers during the last 20 years at various Arctic and Antarctic sites are also presented. They give a measure of the large variations of these parameters due to El Chichon, Pinatubo, and Cerro Hudson volcanic particles, Arctic haze episodes most frequent in winter and spring, and the transport of Asian dust and boreal smokes to the Arctic region. Evidence is also shown of marked differences between the aerosol optical parameters measured at coastal and high‐altitude sites in Antarctica. In situ optical and chemical composition parameters of aerosol particles measured at Arctic and Antarctic sites are also examined to achieve more complete information on the multimodal size distribution shape parameters and their radiative properties. A characterization of aerosol radiative parameters is also defined by plotting the daily mean values of α as a function of AOD(500 nm), separately for the two polar regions, allowing the identification of different clusters related to fifteen aerosol classes, for which the spectral values of complex refractive index and single scattering albedo were evaluated.

Climatic effects of different aerosol types in China simulated by the UCLA general circulation model

The climatic effects of various types of aerosol in China have been investigated by using the atmospheric general circulation model (AGCM) developed at the University of California, Los Angeles (UCLA). The model includes an efficient and physically based radiation parameterization scheme specifically developed for application to clouds and aerosols. Simulation results show that inclusion of a background aerosol optical depth of 0.2 reduces the global mean net surface solar flux by about 5 W m−2 and produces a decrease in precipitation in the tropics as a result of decreased temperature contrast between this area and the middle to high latitudes, which suppresses tropical convection. These decreases have partially corrected the overestimates in the surface solar flux and precipitation in the UCLA AGCM simulations without the aerosol effect. The experiment with increased aerosol optical depths in China shows a noticeable increase in precipitation in the southern part of China in July due to the cooling in the midlatitudes that leads to the strengthening of the Hadley circulation. Aerosol types play an important role in the determination of the global mean radiation budget and regional climate. While sulfates mainly reflect solar radiation and induce negative forcing at the surface, black carbon and large dust particles absorb substantial solar radiation and have a positive solar forcing at the top of the atmosphere, but reduce the solar radiation reaching the surface. Large dust particles also have a significant effect on thermal IR radiation under clear conditions, but this effect is largely masked by clouds generated from the model in AGCM simulations. Black carbon and large dust particles in China would heat the air column in the middle to high latitudes and tend to move the simulated precipitation inland, i.e., toward the Himalayas. The inclusion of black carbon in our simulations has not produced the “north drought/south flooding” precipitation pattern that has frequently occurred in China during the past 50 years.

Retrievals of thin cloud optical depth from a multifilter rotating shadowband radiometer

A method is developed for accurate retrieval of thin cloud optical depth from measurements of a multifilter rotating shadowband radiometer (MFRSR). A key feature of this technique is correction of strong forward scattering of solar radiation into the instrument's field of view, which causes underestimation of retrieved cloud optical depth. To develop the correction method, measurements from the MFRSR under various atmospheric conditions are simulated by using a modified DISORT code that can accurately compute radiative intensity for strong forward scattering by thin clouds. These simulations are used to develop a polynomial fitting technique that corrects forward scattering into direct beam component. The correction is then applied to real MFRSR measurements. First, temporal and spectral variations in direct beam observations are used to discriminate between aerosol and thin cloud conditions. The true thin cloud optical depth, then, is inferred by removing background aerosol optical depth and by applying the polynomial fitting technique to correct the forward scattering contribution. An analysis of uncertainties associated with this method is also conducted and the results show that the retrievals are accurate to better than 5% or 0.05 when optical depth is less than 1.

Contribution of the different aerosol species to the aerosol mass load and optical depth over the northeastern tropical Atlantic

Simultaneous ground‐based measurements of chemical composition, size distribution, and column optical thickness at 670 nm of atmospheric aerosols have been performed at Sal Island (Cape Verde) during the winter season when African dust is transported in the lower troposphere. Mineral dust and, occasionally, sea salt dominate the aerosol mass load, whereas the excess sulfates plus the carbonaceous aerosol (particulate organic matter and black carbon) contributions to the mass load remain lower than 5% on average. We compute the total aerosol optical depth (AOD) by combining optical properties derived from measured size distributions and vertical concentration profile of each aerosol type estimated from surface elemental concentrations and meteorological observations. Results are very consistent with direct Sun photometer measurements. This allows us to derive the chemical apportionment of AOD in this region: mineral dust from Africa controls the total AOD and generally dominates AOD even in the absence of dust outbreak; on average, sea salt, excess sulfate, and carbonaceous aerosols all together only contribute to an averaged background AOD of 0.04 at 670 nm.