TOMS Aerosol Index Research Articles

Possible combined influences of absorbing aerosols and anomalous atmospheric circulation on summertime diurnal temperature range variation over the middle and lower reaches of the Yangtze River

Based on the temperature data from the China Meteorological Administration, NCEP–NCAR reanalysis data, and the TOMS Aerosol Index (AI), we analyze the variations in the summertime diurnal temperature range (DTR) and temperature maxima in the middle and lower reaches of the Yangtze River (MLRYR) in China. The possible relationships between the direct warming effect of the absorbing aerosol and temperature variations are further investigated, although with some uncertainties. It is found that the summertime DTR exhibits a decreasing trend over the most recent 50 years, along with a slight increasing tendency since the 1980s. The trend of the maximum temperature is in agreement with those of the DTR and the absorbing aerosols. To investigate the causes of the large anomalies in the temperature maxima, composite analyses of the circulation anomalies are performed. When anomalous AI and anomalous maximum temperature over the MLRYR have the same sign, an anomalous circulation with a quasi-barotropic structure occurs there. This anomalous circulation is modulated by the Rossby wave energy propagations from the regions northwest of the MLRYR and influences the northwestern Pacific subtropical high over the MLRYR. In combination with aerosols, the anomalous circulation may increase the maximum temperature in this region. Conversely, when the anomalous AI and anomalous maximum temperature in the MLRYR have opposite signs, the anomalous circulation is not equivalently barotropic, which possibly offsets the warming effect of aerosols on the maximum temperature changes in this region. These results are helpful for a better understanding of the DTR changes and the occurrences of temperature extremes in the MLRYR region during boreal summer.

The Synoptic Patterns Associated with Spring Widespread Dusty Days in Central and Eastern Saudi Arabia

Four synoptic regimes were identified as accompanying the widespread dust in central and eastern Saudi Arabia. The widespread cases of dust were classified based on the value and spread of the aerosol index data from the TOMS aerosol index (TOMS AI) satellite over the area of interest. The synoptic regimes of these dust cases were recognized using the Empirical Orthogonal Function (EOF) analysis of their mean sea level pressure (SLP), which was obtained from the National Centers for Environmental Prediction and the National Center for Atmospheric Research (NCEP/NCAR) Reanalysis Project dataset. The variations of the analyzed SLP of these four regimes appeared as meridional distributions for the first two regimes and zonal distributions for the second two regimes. A surface synoptic study of the first two regimes showed that the most significant features were either a strong low-pressure system over the eastern region or a strong high-pressure system over the western region. The synoptic features for the less significant regimes (the second two regimes) were characterized by the interaction between the northern high-pressure belt, which shifted northward because of the significant regime decrease, and the southern low-pressure belt. In addition, the upper synoptic study showed that the upper synoptic systems support the surface systems. Moreover, the study showed that the surface northerly wind over the eastern Arabian Peninsula is the dominant wind during strong dust activity, whereas the surface southerly wind is dominant during weak dust activity.

Diagnostic study of spring dusty days over the southwest region of the Kingdom of Saudi Arabia

Three classes of dust over the southwest region of the Kingdom of Saudi Arabia were classified based on both the values and distribution of the TOMS Aerosol Index. These classes include a narrow spread class (i.e., a weak class), a moderate spread class, and a wide spread class (i.e., a strong class). A diagnostic study of these classes indicated that there are common atmospheric features for dust cases, and the intensity of these features increases with the characteristic strength of the dust cases. These features are the horizontal circulation of anticyclonic wind flow associated with lower layer upward motion through the zonal stripe between 17°N and 25°N through the meridional cross-section in addition to a vertical circulation, especially for the layers above 850 hPa. These two circulation patterns merge to form an unstable atmosphere and a deep cyclone. Furthermore, there are areas of convergence and divergence at the points of deepening upward motion and shallow upward motion, respectively. Finally, all these features collectively force the dust from the Arabian Desert to move into the southwest region of the Kingdom of Saudi Arabia.

Impact of elevated aerosol layer on the cloud macrophysical properties prior to monsoon onset

Atmospheric aerosols alter the radiation balance by absorption/scattering of solar radiation, and indirectly by modifying the cloud microphysical properties. Observations during the Cloud Aerosol Interaction and Precipitation Enhancement Experiment (CAIPEEX) provide a unique opportunity to investigate the aerosol–cloud interaction in a dry to wet transition phase prior to the onset of southwest monsoon. It is observed that aerosol loading increased over the central Indian region in spite of the increase in surface rainfall. This aerosol loading was observed mainly in the 2–5 km level above surface. The origin and influence of elevated aerosol layer have been investigated with the help of WRF-Chem simulations by conducting sensitivity experiments for dust emissions, modified based on the satellite observations. To enhance the dust emissions, the erodible fraction over the Thar Desert region is enhanced to an average factor of 1.7 based on TOMS aerosol index (AI) and USGS land use category, which contributed to enhanced dust emissions by a factor of 1.25 over the study region. This enhancement of dust emission from Thar Desert can result in an increased radiative heating due to elevated aerosol layers, which leads to an increase in the ice mixing ratio and ice water content in the regions of dry to wet transition. It is shown that even natural dust emissions (without changes in anthropogenic emissions) may also influence the spatial and temporal distribution of cloud and precipitation and the hydrological cycle.

Numerical Simulations of Asian Dust-Aerosols and Regional Impact on Weather and Climate- Part II: PRCM-Dust Model Simulation

ABSTRACTThe Purdue Regional Climate Model (PRCM)-Dust, an on-line coupled regional climate-dust model, has been developed to study the dust life cycle and radiative effect of dust on regional weather and climate in Asia in April 1998. The dust model is built on the PRCM discussed in Part I. The dust module includes the major phases of the mineral dust cycle, such as emission, advection, diffusion, and deposition. Furthermore, the optical properties of the dust aerosols are included in the radiation calculation of the PRCM, so that the feedback of radiative forcing of dust-aerosols can be considered in the PRCM modeling. The PRCM-Dust successfully simulated the uplifted dusts to reached around 500–800 hPa over the source regions and remained at 3–5 km or higher to be transported further eastward. The spatial and temporal distributions of the dust aerosols were consistent with the satellite images, the TOMS Aerosol Index maps, lidar observations, and the surface network reports. The regional climatic impacts due to radiative effect of dust aerosols include warming over the high concentration regions in North Asia and cooling in dust-free South Asia. Since there is no nudging, restart, or other artificial forcing in the PRCM-Dust model, the simulated dust concentrations, size distribution, as well as meteorological and soil environments follow the conservation laws of physics. They are suitable to study the interactions among dusts, soil, and regional weather/climate.

Analysis of Saharan dust intrusions into the Carpathian Basin (Central Europe) over the period of 1979–2011

Aeolian dust particles and dust storms play substantial role in climatic and other environmental processes of the Earth system. The largest and most important dust source areas are situated in the Sahara, from where several hundred thousand tons of mineral dust is emitted each year and transported towards the European continent. Here we show that 130 Saharan dust events (SDEs) reached the atmosphere of the Carpathian Basin from 1979 to 2011 by using the NASA's daily TOMS Aerosol Index data, satellite images and backward trajectory calculations of NOAA HYSPLIT model. Monthly trends of dust events demonstrate that the main period of dust transportation is in the spring, with a secondary maximum in the summer (in July and August). This seasonal distribution match well the seasonality of Saharan dust emissions. However synoptic meteorological conditions govern primarily the occurrence of long-range dust transport towards Central Europe. Based on their different meteorological backgrounds (geopotential field, wind vector and meridional flow), SDEs were classified into three main types. By using composite mean maps of synoptic situations and backward trajectories, the possible source areas have also been identified for the different types of events. Finally, we provide a short discussion on how the African mineral dust could contribute to the local aeolian sedimentation of the Carpathian Basin during the Plio-Pleistocene.

Influence of anomalous dry conditions on aerosols over India: Transport, distribution and properties

A synergy of satellite and ground‐based radiometric observations, along with chemical transport modeling, was used for the assessment of the influence of drought monsoon conditions of 2002 and prolonged dry pre‐monsoon period of 2003 on aerosol properties over south Asia, with emphasis over northern India. Reanalysis data are also examined for studying the dry anomalous period from the climatological mean, that show prevalence of westerlies under anticyclonic circulation and subsidence favoring the accumulation of aerosols. TRMM observations over south Asia indicate significant rainfall deficit over northwestern India in July 2002 and May–June 2003. Subsequently, the anomalous and prolonged dry conditions favored heavy aerosol buildup as indicated by strong positive anomalies (20–80%) of MODIS aerosol optical depth (AOD) as well as significant increase in TOMS aerosol index (AI) during July 2002 and May–June 2003 compared to the long‐term monthly means. The largest increase in aerosol loading is observed over northern India, encompassing the Indo‐Gangetic Plains (IGP) that is in the downwind region of dust outflow from the Thar Desert and long‐range transport from Arabia and Middle East. Ground‐based sunphotometer observations at Delhi and Kanpur also show enhanced presence of desert‐dust aerosols during July 2002 and May–June 2003, characterized by large AOD and significantly low Angstrom Exponent. In addition, modifications in columnar aerosol size distribution toward larger coarse‐mode fraction and higher single scattering albedo at longer wavelengths were observed, thus supporting the observation of enhanced dust influx. SPRINTARS model simulations also show the enhanced dust loading over northern India during the studied months, which is in general agreement with the satellite and ground‐based observations.

Evaluating WRF-Chem multi-scale model in simulating aerosol radiative properties over the tropics — A case study over India

We evaluated the performance of WRF-Chem multi-scale model over the tropics, to simulate the regional distribution and optical properties of aerosols, and its effect on radiation over India for a winter month. The model is evaluated using measurements obtained from upper-air soundings, AERONET sun photometers, various satellite instruments, and pyranometers. The simulated downward shortwave flux was overestimated when the effect of aerosols and clouds, on radiation, was -2 neglected. The simulated downward shortwave radiation was 1 to 20 Wm closer to the observations when we included aerosol-cloud-radiation interaction in the simulation. The model usually underestimated particulate concentration for the few observations available. This is likely due to turbulent mixing, transport errors and the lack of dust emission/scheme and the secondary organic aerosol treatment in the model. The model efficiently captured the broad regional hotspots such as, higher aerosol optical depth over the northern parts of India, especially over the Indo-Gangetic basin and lower aerosol optical depth over southern parts of India. The regional distribution of aerosol optical depth agreed well with the AVHRR aerosol optical depth and the TOMS aerosol index pattern. The magnitude and wavelength-dependence of simulated aerosol optical depth was also similar to the AERONET observations across India. The difference in surface shortwave radiation between two simulations that included and neglected aerosol-radiation (aerosol-radiation-cloud) interactions -2 were as high as -25 (-30) Wm . The spatial variations of these differences were also compared with the AVHRR observation. This study suggests that the model is able to qualitatively simulate the distribution of particulates and its impact on radiation over India; however, additional measurements of particulate mass and composition are needed to fully evaluate the model performance.

The Linkage Between Solar Insolation and Dust in the Major World Deserts

The annual link between the solar insolation, the surface temperature, and the dust load in the atmosphere as estimated by the TOMS Aerosol Index was compared in five major sandy deserts around the globe. A high correlation was found between the insolation and the dust in all deserts, while the correlation of dust with temperature was lower. The peaks of the insolation and the dust occurred almost simultaneously on the same date in three out of the five deserts i.e. Sahara, S.W. North American, Atacama. In the other two deserts i.e., Australian, Taklimakan, the peak of the dust occurs earlier in the year. The temperature peaks occur on a later date in all the deserts. Our conclusion is, that in the deserts of the world the radiation is the major cause of the total dust loading.

Stratospheric impact of the Chisholm pyrocumulonimbus eruption: 1. Earth‐viewing satellite perspective

The pyrocumulonimbus storm near Chisholm, Alberta, on 28 May 2001 has been studied in depth. However, the impact of this eruption on the lower stratosphere has not been characterized. Here and in a companion paper we explore this topic. This paper focuses on the “young” Chisholm smoke plume, from the age of ∼3 h to 1 week, as observed by Earth‐viewing satellite instruments. (The companion paper presents strictly profile data.) GOES visible and infrared image loops reveal the pyroconvective life cycle and initial transport of the smoke cloud. MISR stereographic heights are the first of their kind for a stratospheric cloud, showing smoke up to 5 km above the tropopause on 29 May. MODIS IR and visible images are analyzed to give constraints on plume height, thickness, and particle size. Infrared brightness temperature analyses reveal unique aspects of the “day‐after” Chisholm plume. Particle sizes are 1/3 to 1/2 compared to normal cirrus crystals. The daytime 29 May plume is optically thick at tropopause temperatures yet smoky brown. A transition from deep anvil blow off to “dry” smoke is still occurring after ∼1.5 d. TOMS aerosol index is used as a proxy for areas of particularly high smoke plume altitude. The Chisholm smoke in the upper troposphere and lower stratosphere is traced with AI for 1 week as the plume blows across North America to western Europe. First estimates are made of stratospheric smoke mass in relation to emissions during pyroconvection. The 29 May stratospheric Chisholm pyroCb plume contains a mass between ∼1.39 × 104 and 1.09 × 105 t. This represents between ∼10% and 121% of total particle mass emitted from the fire on 28 May, calling into question some frequently assumed values for smoke single scatter albedo and/or emission estimates. Strictly in terms of mass, the stratospheric Chisholm plume amounted to ∼15% of background Northern Hemispheric stratospheric sulfate aerosol. Overall, the young pyroCb plume is seen to be a peculiar mixture of smoke aerosols and water‐ice that confounds operational cloud/aerosol detection routines and exhibits extreme, and still mysterious, composition and life cycle features.

Asian dust initiated early spring bloom in the northern East/Japan Sea

Data for the TOMS aerosol index and SeaWiFS chlorophyll a clearly indicate that an early spring bloom in the northern East/Japan Sea can be initiated during the Asian dust event in association with precipitation. Spring bloom is normally initiated in this area as the surface mixed layer becomes shallower than critical depth, in accordance with Sverdrup's critical depth hypothesis. However, after the passage of Asian dust accompanied by precipitation, spring bloom was initiated about one month earlier than the bloom during non‐dust event years. We found that deepening of critical depth coincided with the early spring bloom observed during wet‐dust events. This early initiation of spring bloom indicates that the supply of bio‐available nutrients such as iron through wet deposition induces deepening of the critical depth, which results in such an early initiation of the bloom in the area.

Comparison of simulating mineral dust aerosols in east asia by two emission schemes

Two common surface-dust emission schemes using critical wind speed and friction velocity were compared with the regional climate model RegCM3 in East Asia. In the comparison, transport of mineral dust and its distribution were simulated from March to April, 2001. Simulation results were also compared with TOMS aerosol index, showing that obvious differences exist in dust emission quantity and its column burden simulated by the dust emission schemes of friction velocity and wind speed criteria. The results obtained by the wind speed criterion are higher than that by friction velocity, bringing forth the problem whether or not the dust emission scheme matches the model. The obvious difference in the two schemes also explains the uncertainty of simulating mineral dust aerosol by modeling.

Assessing Goddard Institute for Space Studies ModelE aerosol climatology using satellite and ground‐based measurements: A comparison study

A physically based aerosol climatology is important to address questions of global climate change. We evaluate the aerosol climatology used in the GISS ModelE (Schmidt et al., 2006), by characterizing and comparing the geographic distribution and seasonal variability of aerosol optical depth (AOD) and particle size via Ångström exponent (A) against available satellite and ground‐based measurements, i.e., MODIS, MISR, POLDER, AVHRR, and AERONET data. There are a number of model parameters, particularly those related to aerosol size specification, that can be better constrained by comparison to satellite data. Our comparison shows that there are large differences in the satellite and ground‐based global distributions of AOD. The differences between the observations increase for the Ångström exponent. Given the uncertainties associated with satellite retrieval results, the agreement in the distributions of global optical depth between GCM aerosols and satellite data is qualitatively reasonable. However, the Ångström exponent of the GCM aerosol is clearly biased low compared to satellite data, implying that the GCM aerosol sizes are overestimated. There is qualitative agreement of the ModelE aerosol single scattering albedo ϖ with TOMS Aerosol Index (AI) and AERONET data. The comparisons show insufficient aerosol absorption at most locations, suggesting a possible underestimation of black carbon distributions in the GCM. However, a more quantitative comparison first requires a readjustment of the GCM aerosol size specification.

Astronomical extinction over the ELT Moroccan sites from aerosol satellite data

AbstractTwo Moroccan sites have been selected to be characterized for the ELT telescopes. Those sites are in the Atlas, between Oukaïmeden (where the national observatory is situated) and the Canary Islands. For this preliminary study, we have used the Nimbus-7 TOMS aerosol index to derive the astronomical extinction. This work builds on the findings of an earlier study which established the link between these two parameters over the Canary Islands.

Study of desert dust events over the southwestern Iberian Peninsula in year 2000: two case studies

Abstract. Strong desert dust events occurring in 2000 over the southwestern Atlantic coast of the Iberian Peninsula are detected and evaluated by means of the TOMS Aerosol Index (A.I.) at three different sites, Funchal (Madeira Island, Portugal), Lisboa (Portugal) and El Arenosillo (Huelva, Spain). At the El Arenosillo station, measurements from an AERONET Cimel sunphotometer allow a more precise retrieval of the spectral AOD and the derived alpha Ångström coefficient. After using different threshold values of these parameters, we conclude that it is difficult to establish reliable and robust criteria for an automatic estimation of the number of dust episodes and the total number of dusty days per year. As a result, additional information, such as airmass trajectories, were used to improve the estimation, from which reasonable results were obtained (although some manual editting was still needed). A detailed characterization of two selected desert dust episodes, a strong event in winter and another of less intensity in summer, was carried out using AOD derived from Brewer spectrometer measurements. Size distribution parameters and radiative properties, such as arefractive index and the aerosol single scattering albedo derived from Cimel data, were analyzed in detail for one of these two case studies. Although specific to this dust episode, the retrieved range of values of these parameters clearly reflect the characteristics of desert aerosols. Back-trajectory analysis, synoptic weather maps and satellite images were also considered together, as supporting data to assess the aerosol desert characterization in this region of study.

North African soil dust and European pollution transport to America during the warm season: Hidden links shown by a passive tracer simulation

A mesoscale atmospheric model coupled with a dispersion model is used to establish source‐receptor relationships between the European‐North African aerosol episodes observed over the Caribbean every year during summer and their main sources. The simulation period extends from 15 July to 16 August 1991, concurrent with the flight campaign of the European Project RECAPMA (Millán et al., 1997; Gangoiti et al., 2001). We now extend the simulations to reach the tropical Atlantic and the Caribbean, and we make use of the TOMS aerosol index data (Nimbus 7 satellite) to substantiate the simulations at the regional‐to‐continental scale. Our simulations, based on passive tracer release from selected source regions, show that emissions from southern Europe can either take a full‐Atlantic shortcut to the American coasts or enter the Mediterranean basin eastward to turn back to America following a longer pathway: The emissions cross the southern Mediterranean coast to northern Africa, and then they are vented to the middle troposphere (2000–6000 m MSL), mixed within the Saharan air layer (SAL) and transported westward to the tropical Atlantic (TA) and America. Recirculations around the Atlas Mountains and perturbed conditions over the area modulate the export of the European tracer (and soil dust) into America. We found transit times of 11–12 days for the full Atlantic shortcut and 20–26 days for the longer pathway across northern Africa. Under the simulated conditions, southern Europe can contribute with more than the 50% of its emissions to America, while northern Europe shows longer transit times and less transport efficiency.

A potential climatic index for total Saharan dust: the Sun insolation

It is shown that the daily course of the total amount of dust in the atmosphere, based on the TOMS aerosol index (AI), is highly correlated (0.98) with the integrated daily solar insolation over the Sahara region for spatially and temporally averaged data. Moreover, the annual maximum is even reached nearly at the same day, i.e., the 17th (18th) June for the surface (top of the atmosphere) solar insolation, as compared to 21st June for the date of maximum atmospheric dust loading. Other factors like cloudiness, rainfall soil moisture, and wind probably play a more dominant role in smaller areas. The fact that on a very large scale, like the whole of the Sahara, the solar insolation becomes the one single forcing directly correlated with the total amount of dust in the atmosphere is suggesting a simple climatic index for the total dust loading in the atmosphere. This index is shown for the Saharan region only, while more studies are needed for other regions. The 3–4 day time‐lag between the maximum insolation and the maximum dust loading in the atmosphere is compared to about a one‐month delay with the maximum ground or air temperatures and may serve as an estimate to the surface response to the major forcing of the Sun. The maximum ground temperature, delayed to the 17th July, can be explained by the delay in the lagged‐heating of the deeper soil layers. The 3–4 day delay only with the Sun insolation is probably linked to the period in which the dust is maintained in the atmosphere until it is deposited. One application to the present finding may be for climate models, since such a high correlation for spatially and temporarily averaged data may be used as an integrated index for the total amount of Saharan dust and for potentially validating the complex model parameterizations in climate models.

Analysis of TOMS Aerosol Index and Single Scattering Albedo over the Central Indian Region

Aerosol effects are significant in processes related to the local, regional and global analysis of air pollution. Large‐scale biomass burning and forest fire events produce smoke plumes that transport smoke thousands of kilometers causing air quality and health related problems. Satellite observations provide efficient means to determine aerosol physical properties on the temporal and spatial scales needed to understand and monitor their effects on the earth‐atmosphere system. In the present study Earth Probe/Total Ozone Mapping Spectrometer (EPTOMS) data and ground‐based observations were used to study the aerosol index and single scattering albedo variation. Monthly variations in AI suggest that the concentration of UV‐absorbing aerosols gradually increases from January to May coinciding with the biomass burning activities in the region. Day average fraction of black carbon (BC) to total aerosol mass concentration is observed to be 7‐10%. The trends in satellite‐derived single scattering albedo correlate well with ground measured values.

Inverse modeling of biomass burning emissions using Total Ozone Mapping Spectrometer aerosol index for 1997

We present results from an inverse model study to determine biomass smoke emissions for the year 1997 by comparison of modeled aerosol index (AI) with that measured by the EP TOMS instrument. The IMPACT model with Data Assimilation Office (DAO) meteorology data in 1997 is utilized to obtain aerosol spatial and temporal distributions. Then a radiative transfer model is applied to generate the modeled AI. A Bayesian inverse technique is applied to optimize the difference between the modeled AI and the EP TOMS AI in the same period by regulating monthly a priori biomass smoke emissions in seven predefined regions. The modeled AI with a posteriori emissions is generally in better agreement with the EP TOMS AI. The a posteriori emissions from Indonesia increase by a factor of 8–10 over the a priori emissions due to the Indonesian fires in 1997. The annual total a posteriori source increases by about 13% for the year 1997 (6.31 Tg/yr black carbon and 67.27 Tg/yr smoke) in the base scenario, with a larger adjustment of monthly emissions. The sensitivity of this result to the a priori uncertainties, the height of the smoke layer, the cloud screening criteria, the inclusion of an adjustment of emissions outside the main biomass burning regions, and the inclusion of the covariances between observations in different locations is discussed in a set of sensitivity scenarios. The sensitivity scenarios suggest that the inverse model results are most sensitive to the assumed uncertainty for a priori emissions and the altitude of aerosol layer in the model and are less sensitive to other factors. In the scenario where the uncertainty of a priori emissions is increased to 100% (300% in Indonesia), the total annual black carbon emission is increased to 6.87 Tg/yr, and the smoke emission increases to 73.39 Tg/yr. The a posteriori emissions in Indonesia in the scenario with increased uncertainty are in better agreement with both the TOMS AI and with previous estimates for the Indonesian fires in 1997. In the scenario where biomass smoke from large fires are elevated by 1 km in altitude, the annual total black carbon emissions are 5.68 Tg/yr, and the smoke emissions are 60.44 Tg/yr, almost unchanged from the a priori emissions.

Saharan dust outbreak over southeastern Spain as detected by sun photometer

Characterization of mineral aerosol properties in southeastern Spain is important due to its proximity to the African continent that permit studying the Saharan dust just entering European continent before its possible mixing with anthropogenic emissions in Europe. Two Saharan dust events that occurred during 30 August–3 September and on 10 September 1998 were observed at an inland location on the outskirts of Granada in southeastern Spain. Detailed analyses including back trajectories and TOMS aerosol index maps showed transport of Saharan dust from the Sahara desert and North Africa to the study area. The dust episodes were associated with marked increases in aerosol optical depth ( δ a) at all wavelengths examined in this study. Thus δ a (500 nm) increased from a value of ∼0.2 corresponding to normal conditions up to 0.6 in the first event and up to 0.55 in the second. At the end of these events, the arrival of air masses from the Atlantic caused a sharp decrease in δ a (500 nm) that reaches values close to those obtained before the event (∼0.2). The Angström exponent α reached a minimum of 0.36 in the first event and 0.37 in the second. The Angström exponent α and aerosol optical depth values during dust events agree well with those obtained under the same kind of events in AERONET and SKYNET sites, as well as with the results obtained in ACE-2 and ACE-Asia experiments. The aerosol size distributions, retrieved from aerosol optical depth using King's method, demonstrated how the large size fraction of aerosol associated with Saharan dust dominated during these events. When Saharan dust was present, the retrieved aerosol size distributions were bimodal with a well-defined mode centred at a radius of 0.6 μm, and showed an evident increase in the large particles mode with radii in the range 0.4–2 μm. The small particle concentration during the two events did not present any marked change, and was similar to those observed on days without Saharan dust. Large particle concentration was higher by a factor of 8 than during normal conditions.