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Classifying aerosol types using spectral aerosol optical properties plays a key role in promoting the accuracy of remote sensing observations and expanding our knowledge of the internal relations between aerosol and climate. However, studies on verifying the correctness for aerosol type classification methods are particularly scarce. In this study, data from three month optical and chemical measurements of atmospheric aerosols in Harbin, Northeast China, were used to verify the optical classification methods by comparing their results with the actual aerosol types. Aerosol optical properties like aerosol optical depth (AOD), Angstrom Exponent (AE), and single scattering albedo (SSA) were obtained through a CE318 sun-photometer, and aerosol type was then identified based on four previously published aerosol classification schemes. To acquire the actual aerosol states, satellite observations from MODIS and CALIPSO was combined and then the morphology, chemical composition, and molecular structure of sample particles were examined by energy-dispersive X-ray spectroscopy (SEM-DES), X-Ray fluorescence spectrophotometer (XRF), and Fourier transform infrared spectroscopy (FTIR) measurements. Results show that aerosol types over urban Harbin varied a lot during the studying period and mixed aerosol was the dominant type. The optical classification methods mentioned in this study could identify aerosol types correctly in most cases, but sometimes made improper estimations. Besides, they are able to descript the distinct variation of aerosol type on seasonal and daily scale, but at present the result is still too rough to discriminate the detailed changes of actual aerosol state accurately. The combination of different classification methods or chemical measurements would help reducing the misjudgment of aerosol types. Our findings demonstrate that a more comprehensive way to identify changes in aerosol state is urgently needed to enhance our understanding of the impacts of aerosols on climate and remote sensing.

As one of the largest metropolitan areas in the world, Beijing has a high aerosol loading. Although the government has taken various measures to reduce emissions, the large-scale and intermittent occurrence of pollution is still a major issue. To understand the mechanisms and characteristics of pollution, Raman-Mie lidar and CE-318 sun photometer were used to study the aerosol vertical distribution and optical properties during dust, haze and clean periods in Beijing that occurred in September to October 2017. Combined with meteorological data, the sources and transmission paths of the pollution events were analyzed. The results are as follows. For the dust event, irregular particles with a large linear particle depolarization ratio played a leading role. The aerosol boundary layer height reached 3.5 km. The volume concentration of coarse particles was obviously larger in the volume size distribution. Strong northwest winds at high altitudes cause high-altitude long-distance transmission of pollutants. And this process can exacerbate air quality of Beijing. For the haze event, high-spherical water-soluble particles were dominant. The extinction coefficient exceeded 1 km−1 while the linear particle depolarization ratio was relatively small. The aerosol boundary layer height was

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), Angstrom exponent (α 440–870 ), Single Scattering Albedo (SSA), and aerosol particle-size distribution were derived from the observational data. The overall mean AOD 500 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 α 440–870 value for the period studied was 1.50 ± 0.16. The monthly average of α 440–870 reached a minimal value of 1.32 in August and a maximal value of 1.61 in May. Average SSA at 440 nm 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 0.2 μm. 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.

In this study, the Simplified Aerosol Retrieval Algorithm (SARA) was applied to both MODIS and Visible Infrared Imaging Radiometer Suits (VIIRS) images to retrieve aerosol optical depth (AOD) over water for blue, green and red wavelengths at 500 m (MODISSARA) and 750 m (VIIRSSARA) resolutions, respectively. Retrievals were compared with the Terra–MODIS Dark Target (DT) AOD at 3 km resolution (MODIS3K) and the VIIRS Environmental Data Record (VIIRSEDR) AOD at 6 km resolution. Validation was conducted using 86 Microtops II Sun photometer AOD measurements collected over different classes of water quality (low to high sediment levels) for seven days (6, 8, 9, 13, and 15, 16 and 17 October 2014) between 10:00 to 15:00 local time. Thirty-eight to fifty percent and 44–54% of the MODIS3K and the VIIRSEDR AOD retrievals respectively, fall within the expected error (EE) with root mean square error from 0.12 to 0.14 and mean absolute error from 0.10 to 0.11. The MODISSARA and the VIIRSSARA AOD retrievals are well correlated with the ground–based measurements (R: MODISSARA = 0.86–0.89 and VIIRSSARA = 0.85–0.94), with a larger number of retrievals falling within the EE MODISSARA = 73–77% and VIIRSSARA = 71–82%) than MODIS3K and VIIRSEDR. The results indicate that the SARA algorithm is more robust than MODIS3K and VIIRSEDR global AOD products, and can retrieve accurate AOD over low to highly sedimented water surfaces, similar to good AOD retrievals over land.

The optical properties of dust aerosols were measured using narrow-band data from a portable sun photometer at four desert and semi-desert stations in northwestern China from 2004 to 2007. Ground-based and satellite observations indicated absorbing dust aerosol loading over the region surrounded by eight large-scale deserts. Radiation forcing was identified by using the Santa Barbara DISORT Atmospheric Radiative Transfer (SBDART) model. The ranges of annual mean aerosol optical depth (AOD), Angstrom exponents, and single-scattering albedo (SSA) were from 0.25 to 0.35, from − 0.73 to 1.18, and from 0.77 to 0.86, respectively. The ranges of annual mean aerosol direct radiative forcing values at the top of the atmosphere (TOA), mid-atmosphere, and on the surface were from 3.9 to 12.0, from 50.0 to 53.1, and from − 39.1 to − 48.1 W/m 2 , respectively. The aerosols' optical properties and radiative characteristics showed strong seasonal variations in both the desert and semi-desert regions. Strong winds and relatively low humidity will lead dust aerosols in the atmosphere to an increase, which played greatly affected these optical properties during spring and winter in northwestern China. Based on long-term observations and retrieved data, aerosol direct radiative forcing was confirmed to heat the atmosphere (50–53 W/m 2 ) and cool the surface (− 39 to − 48 W/m 2 ) above the analyzed desert. Radiative forcing in the atmosphere in spring and winter was 18 to 21 W/m 2 higher than other two seasons. Based on the dust sources around the sites, the greater the AOD, the more negative the forcing. The annual averaged heating rates for aerosols close to the ground (1 km) were approximately 0.80–0.85 K/day.

Columnar aerosol properties retrieved from MICROTOPS II Sun Photometer measurements during 2010–2013 over Pune (18°32′N; 73°49′E, 559 m amsl), a tropical urban station in India, are analyzed to identify aerosol types in the atmospheric column. Identification/classification is carried out on the basis of dominant airflow patterns, and the method of discrimination of aerosol types on the basis of relation between aerosol optical depth (AOD500 nm) and Angstrom exponent (AE, α). Five potential advection pathways viz., NW/N, SW/S, N, SE/E and L have been identified over the observing site by employing the NOAA-HYSPLIT air mass back trajectory analysis. Based on AE against AOD500 nm scatter plot and advection pathways followed five major aerosol types viz., continental average (CA), marine continental average (MCA), urban/industrial and biomass burning (UB), desert dust (DD) and indeterminate or mixed type (MT) have been identified. In winter, sector SE/E, a representative of air masses traversed over Bay of Bengal and Eastern continental Indian region has relatively small AOD (τpλ = 0.43 ± 0.13) and high AE (α = 1.19 ± 0.15). These values imply the presence of accumulation/sub-micron size anthropogenic aerosols. During pre-monsoon, aerosols from the NW/N sector have high AOD (τpλ = 0.61 ± 0.21), and low AE (α = 0.54 ± 0.14) indicating an increase in the loading of coarse-mode particles over Pune. Dominance of UB type in winter season for all the years (i.e. 2010–2013) may be attributed to both local/transported aerosols. During pre-monsoon seasons, MT is the dominant aerosol type followed by UB and DD, while the background aerosols are insignificant.

Ground and satellite remote sensing measurements have revealed heavy aerosol loading in China; however, aerosol optical properties and direct radiative forcing in Northeast China – important in climate modeling and remote sensing – have not been widely studied. We studied four years of continuous sun photometer measurements at Tongyu, a typical semi-arid rural site in Northeast China, to better understand column-integrated aerosol optical properties and direct radiative forcing. The annual average aerosol optical depth (AOD) at 500 nm was 0.20 ± 0.26; the Angstrom exponent (AE) between 440 and 870 nm was 1.37 ± 0.64; and the single scattering albedo (SSA) at 440 nm was 0.91 ± 0.05. The AOD at this rural site was a quarter of that observed in the polluted North China Plain and Yangtze River Delta regions. Anthropogenic fine-mode particles were the dominant contributor to AOD. The AOD and AE showed generally opposite seasonal variation patterns. Relatively higher AOD values in summer (0.26 ± 0.27) and spring (0.24 ± 0.30) were likely related to long-range transportation of anthropogenic aerosols from southern industrial regions in summer, and the increased contribution of dust events in spring. The minimum AOD (0.16 ± 0.22) was concurrent with the maximum AE (1.75 ± 0.76), observed in winter. On average, the absorption AOD (AAOD) at 440 nm and its absorption Angstrom exponent (AAE) between 440 and 870 nm were 0.06 ± 0.03 and 1.04 ± 0.43, respectively. The mean AAE was considerably higher than 1 in autumn and winter, indicating that brown carbon from biomass burning contributed greatly to aerosol absorption. The AAE was lower than 1 in summer and spring, related to the coating of black carbon particles. Large negative aerosol direct radiative forcing was estimated at the bottom of the atmosphere, with relatively lower values estimated at the top of the atmosphere; the means were − 26.28 and − 9.42 W m − 2 , respectively. This resulted in a strong cooling effect on the surface, but warming in the atmosphere, potentially impacting the regional climate.

In this work we propose a technique to retrieve optical depths for optically thin clouds. This method complements the current performance of AERONET for optically thick cloud measurements using data eliminated by the cloud-screening algorithm that are not useful to derive aerosol properties, therefore inexpensively increasing the capabilities of the sun-photometers. The proposed method is based on the computation of the apparent cloud optical depths and on a forward scattering correction method that exploits state-of-the-art ice cloud scattering models. This complementary procedure is applied to Cimel sun-photometer measurements performed at the Evora Geophysics Centre (Portugal, 38.6°N, 7.9°W, 293 m asl) included in the AERONET network in order to obtain a climatology of optical depths for optically thin clouds over middle-latitude continental regions of the southwestern Iberian Peninsula. Main features regarding annual and seasonal variability and the relative changes in the infrared radiative forcing from 2007 to 2010 are also reported and analyzed.

Trans‐boundary biomass burning smoke episodes have increased dramatically during the past 20–30 years and have become an annual phenomenon in the South‐East‐Asia region. On 15th October 2010, elevated levels of fire activity were detected by remote sensing satellites (e.g. MODIS). On the same date, measurements of fine particulate matter (PM2.5) at Singapore and Malaysia found high levels of fine mode particles in the local environment. All these observations were indicative of the initial onset of a smoke episode that lasted for several days. In this work, we investigate the temporal evolution of this smoke episode by analyzing the physical and optical properties of smoke particles with the aid of an AERONET Sun photometer, an MPLNet micropulse lidar, and surface PM2.5 measurements. Elevated levels of fire activity coupled with high aerosol optical depth and PM2.5 were observed over a period of nine days. Increased variability of parameters such as aerosol optical depth, Angstrom exponent number and its fine mode equivalents all indicated high levels of fine particulate presence in the atmosphere. Smoke particle growth due to aging, coagulation and condensation mechanisms was detected during the afternoons and over several days. Retrieved lidar ratios were compatible with the presence of fine particulate within the boundary/aerosol layer. Moreover, retrieved particle size distribution as well as single scattering albedo indicated the prevalence of the fine mode particulate regime as well as particles showing enhanced levels of absorption respectively.

Aerosol concentration and aerosol type, retrieved from observations with CIMEL sun-photometers at three sub-arctic locations at the Scandinavian Peninsula are presented. The observations were made at ALOMAR-Andenes in Norway, Abisko in Sweden and Sodankyla in Finland. This field campaign took place in late spring and summer 2007 as part of the activities of the International Polar Year (IPY) within the POLARCAT project at ALOMAR and Abisko. Aerosol properties were characterized using the relationship between the aerosol optical depth and the Angstrom Exponent. The characteristics of the predominant aerosol type and microphysics are largely determined by the location of the site (continental or coastal). During summer the fine mode particles dominate, as indicated by the fine mode volume fraction and the Angstrom Exponent. The aerosol concentration was on average very low, except during an event in which long-range transported aerosols (dust and pollution) were detected. © 2011 Elsevier B.V.

To reduce uncertainties in the assessment of aerosol effects on regional climate and validate the satellite products, aerosol optical measurements were made at a remote site, Nam Co Station (AERONET) in central Tibetan Plateau, since August 2006. Very low aerosol optical values were observed with annual mean Aerosol Optical Depth (AOD) of 0.05 at 500 nm during August 2006 to July 2007. Angstrom parameters were also low with an average of 0.42 ± 0.27. Angstrom parameter varies greatly over a narrow range of AOD, indicating the occurrence of different types of aerosol particles. The seasonal variation of the monthly average AOD shows maximum values in the spring (April, May) and remains high values in summer monsoon season (June, July, August, and September). The former corresponds to the prevalence of soil dust particle in spring and the later reflects the enhanced human activities in summer. The seasonal variation of columnar size distribution is also consistent with this result. Distinct seasonal variation of water vapor content was observed, with high that occurred in summer monsoon season. Nam Co appears to represent a clean continental background site for atmospheric composition investigation. This work could fill the gap which exists in our knowledge on the global aerosol distribution over the huge high elevation area (i.e. Tibetan Plateau).

We evaluate first the mesoscale Transport of Atmospheric Pollutants Model (TAPOM) performance in calculated aerosol surface concentrations and aerosol optical depth (AOD) values for the greater Milan area in Italy during June 2001. A comparison of the aerosol concentrations and AOD calculations is made with several data sets. Satellite data from both the MISR and MODIS instruments on board the Terra platform are used, as well as sun photometer data from both the AERONET and the Swiss CHARM networks and EMEP measurement data. Calculated monthly mean of SO4=, and NO3− at the EMEP Ispra station are overestimated by a factor 1.5 and 2 respectively. EC corresponds well with measurements when compared to the year 2000. OC is underestimated by a factor 1.5. Calculated AOD values are in general in good agreement with sun photometer and satellite data. Differences between model AOD and observations for dry clear sky days are smaller than for other days when, cirrus and Saharan dust are observed. We find for some clear sky days a good agreement between calculated and observed AOD spatial distribution. AOD retrievals by remote sensing instruments are not always consistent with each other. Significant differences are found between satellite AOD retrieval for the same area and time frame. Therefore it makes it difficult to use remote sensing data for model comparison. Secondly, we used the model to study scale issues in aerosol modelling at three horizontal resolutions (5 × 5 km, 10 × 10 km and 20 × 20 km) through calculations of AOD and other aerosol properties for the same area and period. A finer model resolution shows a more detailed AOD distribution pattern following the domain's orography than the coarser resolution, and AOD values of the finer resolutions correspond better with the observations than the coarser resolution. Thirdly, we study the role of the boundary conditions on aerosol concentrations and AOD calculations for Ispra. A sensitivity analysis showed that when the boundary conditions are set to zero, SO4=, NH4+ and NO3− are underestimated by a factor 2.5, 2 and 8 when compared to measurements, leading to a strong underestimation of the AOD values at all times when compared to AERONET AOD. This indicates that the boundary conditions are very important for urban scale modelling.

Daily averaged retrievals of AERONET sun photometer measurements from March 2003 to March 2004 are used to provide preliminary results on the characterization of aerosol properties and changes over south–east Italy (40°20′N, 18°6′E). It is shown that aerosol optical and microphysical properties and the dominating aerosol types depend on seasons. Aerosol-parameter frequency distributions reveal the presence of individual modes that lead to the assumption that moderately absorbing urban–industrial and marine-polluted aerosols dominate in spring–summer and autumn–winter, respectively. It is shown that aerosol optical depths (AODs), single scattering albedos (SSAs), and Angstrom coefficients (A) of urban–industrial (spring–summer) aerosols are characterized by lognormal distributions with peak values of 0.20±0.03, 0.94±0.01, and 1.58±0.03, respectively. On the contrary AOD, SSA and A values of maritime-polluted (autumn–winter) aerosols are characterized by lognormal distributions with peak values of 0.049±0.008, 0.974±0.003, and 0.7±0.1, respectively. It is also shown that the frequency distribution of real n and imaginary k refractive indices permits inference of the dominant aerosol constituents: sea-salt, water soluble, soot, and mineral particles. Finally, it is shown that dust outbreaks do not significantly affect the seasonal evolution of aerosol parameters, and that sunphotometry retrievals along dust events are in satisfactory accord with experimental findings indicating that moderately-absorbing (0.005≤ k ≤0.05) dust particles with a high content of illite are mainly advected over the Mediterranean basin during Sahara dust storms.

Aerosol optical parameters, polarized phase function and single-scattering albdeo, have been retrieved from ground-based sun photometer measurements in Beijing 2003. The measured aerosol optical thickness varies from 0.12 to 0.77 with an average value of 0.39. The measured Angstrom coefficient ranges from 0.75 to 1.47 with an average value of 1.21. The retrieved single-scattering albedo at 870 nm is within the 0.76–0.94 range and the average value is 0.85, suggests there are considerable aerosol absorptions in Beijing. The maximum value of retrieved polarized phase function at 870 nm ranges from 0.068 to 0.225 with an average value of 0.16, and it illustrates good correlations with the Angstrom coefficient, i.e. the relative size of aerosol particles. Analyses of measurements and theoretical calculations show the polarized phase function is sensitive to aerosol size distribution and complex refractive index, especially the imaginary part of the refractive index which denotes aerosol light absorbing effects. These results suggest that the polarized phase function is an effective and unique aerosol optical parameter and is able to improve the retrieval of aerosol physical properties.

Characterization of aerosol optical properties, such as aerosol optical depth, Angstrom exponent, and volume size distribution at the semiarid site of Tombstone Arizona (31823VN, 110805VW, 1408 m) will be presented for one annual cycle. In this region, extensive observations of selected optical parameters such as aerosol optical depth (AOD) have been made in the past and reported on in the literature. Less is known about other optical characteristics that are important in climate modeling and remote sensing. New observational techniques and inversion methods allow for the expansion of the earlier information. Observations have been taken with a state of the art sun photometer for a 1-year period and their analysis will be presented here. Monthly mean AODs at 500 nm were found to be in the range of 0.03–0.12; the monthly mean Angstrom exponent ranged from 0.9 to 1.6, being higher in spring and summer and lower in late fall and winter. Volume size distributions exhibit clear dominance of smaller particles, with a gradual increase in size from winter to spring and into summer. Annual variation of the radii of the smaller and the larger particles ranged between 0.05–0.4 and 4–8 Am, respectively. Radiance measurements at 940 nm were used to estimate precipitable water. The retrieved values compared within limits of uncertainty with independently derived estimates from the National Center for Environmental Prediction (NCEP) regional weather forecast model. An interesting outcome from

The aerosol optical thickness (AOT) τa computed from the spectral sun photometer in Thala (Tunisia) exhibited variability ranging from approximately 0.03 to greater than 2.0 at 870 nm for March–October 2001. These measurements are compared to the aerosol optical thickness computed in Ouagadougou (Burkina-Faso), Banizoumbou (Niger), IMC Oristano (Sardinia) and Rome Tor Vergata (Italy). Analysis of τa data from this observation network suggests that there is a high temporal and spatial variability of τa in the different sites. The Angstrom wavelength exponent α was found to vary with the magnitude of the aerosol optical thickness, with values as high as 1.5 for very low τa, and values of −0.1 for high τa situations. The relationship between the two parameters τa and α is investigated. Values of the turbidity coefficient β have been determined in Thala (Tunisia) for 8 months in 2001 based on a direct fitting method of the Angstrom power law expression using sun photometer data. The monthly averaged values of the turbidity coefficient β vary between 0.15 and 0.33. The months of July and October experienced the highest turbidity, while April experienced the lowest aerosol loading on average. The turbidity shows a maximum and minimum values for the Southwest and the Northwest wind directions, respectively. The single scattering albedo ωo for the 870 nm wavelength obtained from solar aureole data in Thala is analysed according to the particles' origin.

The aerosol index (AI) derived from backscattered UV radiances measured by the total ozone mapping spectrometer (TOMS) provides global coverage spanning nearly 20 years. Published maps of significant global sources of UV-absorbing aerosol produced from AI do not include northern Australia, despite the extensive grass fires that persist throughout the dry season, a result that could be interpreted to mean that radiative heating by smoke in northern Australia is not significant. In this paper, we show that such is not the case. The radiative heating is intense, persists throughout most of the dry season, and is likely to moderate the local climate. Numerical simulations to determine confidence limits for radiative heating based solely on AI data show that large uncertainties remain, because AI does not characterize either the vertical distribution or optical properties of aerosol uniquely. However, when AI is combined with sun-photometer observations of aerosol optical depth (AOD) at Jabiru in Kakadu National Park, the uncertainty is reduced markedly. Analysis of the combined data sets shows that moderate amounts of aerosol (with AOD≈0.5) persist throughout September and October. The associated radiative heating cannot be neglected, even in comparison with the heating produced by water vapour. The dominant sources of uncertainty, namely, the single-scattering albedo and the vertical distribution of the aerosol, are quantified. Simulations show that the uncertainty may be reduced significantly if sun-photometer and AI data are analyzed in conjunction with LIDAR data to determine the vertical distribution of aerosol.

A 2-year feasibility study was conducted at the USGS EROS Data Center, South Dakota (43.733°N, 96.6167°W) to assess whether a four-band, ground-based, sun photometer could be used to discriminate sky types. The results indicate that unique spectral signatures do exist between sunny skies (including clear and hazy skies) and cirrus, and cirrostratus, altocumulus or fair-weather cumulus, and thin stratocumulus or altostratus, and fog/fractostratus skies. There were insufficient data points to represent other cloud types at a statistically significant level.