Daily Mean Aerosol Optical Depth Research Articles

Sustainable data-driven insights: Statistical analysis and artificial intelligence-driven modelling of aerosol concentrations in Hyderabad district, India

Air pollution stands as a pressing issue in contemporary times, leading to the loss of millions of lives and exerting detrimental effects on the economy. The aerosols especially particulate matter, which are dispersions of matter in air medium play an important role in manipulating the climatological variables in an area. The current study was developed in response to the need to study aerosols and particulates on annual levels using 20-year (2002–2021) daily mean Aerosol Optical Depth (AOD) product released by Moderate Resolution Imaging Spectrometer (MODIS) sensors, and to generate prediction models for AOD using artificial intelligence (AI) techniques for Hyderabad district in India. The results of daily mean analysis revealed a rising trend in the number of days with severe AOD (> 1). Yearly mean AOD distribution showed a percentage increase of 45.31 % from 2002 to 2021. Furthermore, factor analysis was carried out to check for correlations of AOD and PM2.5 with various meteorological and pollutant variables. It was observed that both PM2.5 and AOD had significant weak to moderate (p < 0.05; r < 0.5) correlations with both pollutants and meteorological variables. The hybrid deep learning-based CNN-LSTM was identified as the best-fit model to predict AOD, outperforming MLP – ARIMA and MLP models. CNN – LSTM showed an R2 of 0.70, MAE of 0.08, MSE of 0.02 and RMSE of 0.14.

Case study of biomass-burning aerosol events over Sofia, Bulgaria, based on AERONET sun photometer data

In the present work, the peculiarities were analyzed of aerosol events over Sofia, Bulgaria, characterized by a daily-mean aerosol optical depth (AOD) exceeding 0.3 and Ångström exponent (AE) above unity. Such aerosol situations arise most frequently in summer and early autumn, and sometimes in late autumn and winter. The analysis was based on AERONET sun photometer data and showed that the corresponding aerosol ensembles consisted mainly of fine-fraction biomass-burning (BB) smoke particles due to summer wildfires or domestic heating in winter, with a high sphericity factor (SF) that reached unity and a low linear particle depolarization ratio (LDR) of the order of thousandths. In the cases of aged BB smoke particles, the values of AE decrease without essential changes in SF and LDR. The presence or the periodic appearance of desert dust and/or marine aerosols during the day will also lead to a lower AE, accompanied however by a decreasing SF and an increasing LDR.

Application of Optimal Interpolation to Spatially and Temporally Sparse Observations of Aerosol Optical Depth

Aerosol optical depth (AOD) is one of the basic characteristics of atmospheric aerosol. A global ground-based network of sun and sky photometers, the Aerosol Robotic Network (AERONET) provides AOD data with low uncertainty. However, AERONET observations are sparse in space and time. To improve data density, we merged AERONET observations with a GEOS-Chem chemical transport model prediction using an optimal interpolation (OI) method. According to OI, we estimated AOD as a linear combination of observational data and a model forecast, with weighting coefficients chosen to minimize a mean-square error in the calculation, assuming a negligible error of AERONET AOD observations. To obtain weight coefficients, we used correlations between model errors in different grid points. In contrast with classical OI, where only spatial correlations are considered, we developed the spatial-temporal optimal interpolation (STOI) technique for atmospheric applications with the use of spatial and temporal correlation functions. Using STOI, we obtained estimates of the daily mean AOD distribution over Europe. To validate the results, we compared daily mean AOD estimated by STOI with independent AERONET observations for two months and three sites. Compared with the GEOS-Chem model results, the averaged reduction of the root-mean-square error of the AOD estimate based on the STOI method is about 25%. The study shows that STOI provides a significant improvement in AOD estimates.

Aerosol physico–optical–radiative characterization and classification during summer over Ny-Ålesund, Arctic

ABSTRACT With an aim to study the year-to-year changes in physical, optical, and radiative properties of polar aerosols, special observation campaigns were conducted during three consecutive summers of 2010, 2011 and 2012 over Ny-Ålesund (78.9°N, 11.9°E, 42 m AMSL), Arctic, employing a ground-based Multi-channel Solar-radiometer (MICROTOPS II Sun- photometer). In the present study, the columnar Aerosol Optical Depth (AOD) at λ = 500 nm and the Angstrom exponent (α) data sets are used to analyse the daily mean AOD and α by using a liner regression (best-fit) method. The results highlight that the columnar aerosol optical depth (mean AOD@500 nm for the study period) is 0.144 ± 0.05. Signature of secondary aerosol formation through marine sources based on variation in two wavelength AODs is also studied. These columnar AOD measurements over the study region are found to be in good agreement within experimental limitations with the synchronous MODIS satellite derived values. The 3 years’ α mean value is found to be 0.797 ± 0.328 within the range of 0.01–1.4. The frequency of occurrence and relative frequency histograms of α exhibited two modes centred at around 1.2 and 0.8. The primary and secondary modes reveal that the marine aerosols in the study region are mixed type, and they contribute about 44%, with the accumulation particles alone contributing about 31%. We found a close correspondence between aerosol optical properties and local meteorological conditions over the experimental location. The aerosol radiative forcing estimations revealed higher negative aerosol radiative forcing (indicating cooling) at Bottom-of-the-Atmosphere (BOA) and positive radiative forcing (indicating warming) in the Atmosphere (ATM) and Top-of-the-Atmosphere (TOA) during 2012 as compared to those obtained during 2010 and 2011, which is consistent with the observed AOD and α. The airmass back-trajectory model analysis also showed intrusion of anthropogenic aerosols from neighbouring regions into the study region, particularly during the year 2012.

Optical and physical properties, time-period, and severity of dust activities as a function of source for the main dust sources of the Middle East

To achieve long-time and comparable aerosol properties with the maximum geographical coverage possible in the Middle East, four AERONET sites (Kuwait university, Solar village, IASBS, and Mezaira) are selected. Aerosol optical depth (AOD) at 870 nm, Ångström exponent (α) for two wavelengths, 440 and 870 nm, real and imaginary parts of aerosol refractive index at 870 nm (mr, mi), and aerosol effective radius are studied for these sites during 2010–2017. Daily-mean values of aerosol properties (mr≥ 1.51, mi≤ 0.013, and α ≤ 0.88), show that dust particles are the dominant aerosol type in the atmosphere of the Middle East area. It has been seen that dusty days (daily-mean AOD(870) ≥ 0.50, and α ≤ 0.50), assign a notable percentage (14%–19%) of all days of data for all sites with the exception of the IASBS site. Based on the monthly-averaged MODIS-Terra Deep Blue AOD at 550 nm, it is seen four main dust sources in the Middle East: Mesopotamian area in Iraq and East Syria, northern part of the Arabian Peninsula, Elrob Elkhali desert at southern part of the Arabian Peninsula, and sources of East Arabian Peninsula located in Oman country. All sources of dust are active during dry months in spring and summer seasons. The monthly-averaged wind field data of ERA-interim ECMWF model reveals that dust events in northern part of the Arabian Peninsula are mainly originated from North Arabian Peninsula (Tigris-Euphrates basin) sources in spring (summer). In East Arabian Peninsula (Mezaira site), dusty conditions have continued until late summer and it is related to dust activities of sources, located at southern (Elrob Elkhali desert) and eastern part (Oman country) of the Arabian Peninsula. Dust recorded in Iran plateau (IASBS site), can be originated from sources of Tigris-Euphrates basin via westerly wind at higher altitudes. Also, it seems that dust particles originated from Tigris-Euphrates basin, have smaller sizes and higher values of complex refractive index and therefore it is essential to consider source functionality of dust particles optical and physical properties for each sub-region of the Middle East.

Trends of Aerosol Optical Properties over the Heavy Industrial Zone of Northeastern Asia in the Past Decade (2004–15)

The heavy industrial zone of northeastern Asia is dominated by year-round industrial scattering aerosols that undergo hygroscopic growth in summer. With the rapid economic development over the past decade, aerosol optical depth (AOD) has increased (6.35% yr−1) with an annual-mean AOD of 0.61 ± 0.13. Simultaneously, the aerosol particle size and aerosol scattering have increased, with an annual-mean scattering aerosol optical depth (SAOD) reaching 0.58 ± 0.15. However, considering that the annual AOD/gross domestic product (GDP) per capita decreased, the environmental degradation caused by aerosol emission is expected to reach a turning point based on the environmental Kuznets curve (EKC) hypothesis. In addition, annual-mean radiative forcing at the top, bottom, and interior of the atmospheric column reached −2.35 ± 2.33, −54.16 ± 7.26, and 51.81 ± 7.93 W m−2, respectively. The increase in unit SAOD contributes to the growth in net negative top-of-atmosphere (TOA) forcing and surface (SFC) forcing, and unit absorption aerosol optical depth (AAOD) increases together with atmosphere (ATM) forcing. Moreover, the cooling effect of aerosols on the Earth–atmosphere system showed an increase over the most recent 10 years related to the increase in scattering aerosol from development in the old industrial base. Except for local sources, under the western air masses, the circum–Bohai Sea economic zone was the potential source area of anthropogenic aerosols throughout the year with annual daily mean AOD, single-scattering albedo (SSA), TOA forcing, and SFC forcing values of 0.88, 0.93, −8.08, and −63.05 W m−2, respectively. The Mongolian Plateau was the potential natural dust source area under the northeastern air masses.

AEROCAN, the Canadian sub-network of AERONET: Aerosol monitoring and air quality applications

Previous studies have demonstrated the utility of AERONET (Aerosol Robotic Network) aerosol optical depth (AOD) data for monitoring the spatial variability of particulate matter (PM) in relatively polluted regions of the globe. AEROCAN, a Canadian sub-network of AERONET, was established 20 years ago and currently consists of twenty sites across the country. In this study, we examine whether the AEROCAN sunphotometer data provide evidence of anthropogenic contributions to ambient particulate matter concentrations in relatively clean Canadian locations. The similar weekly cycle of AOD and PM2.5 over Toronto provides insight into the impact of local pollution on observed AODs. High temporal correlations (up to r = 0.78) between daily mean AOD (or its fine-mode component) and PM2.5 are found at southern Ontario AEROCAN sites during May–August, implying that the variability in the aerosol load resides primarily in the boundary layer and that sunphotometers capture day-to-day PM2.5 variations at moderately polluted sites. The sensitivity of AEROCAN AOD data to anthropogenic surface-level aerosol enhancements is demonstrated using boundary-layer wind information for sites near sources of aerosol or its precursors. An advantage of AEROCAN relative to the Canadian in-situ National Air Pollution Surveillance (NAPS) network is the ability to detect free tropospheric aerosol enhancements, which can be large in the case of lofted forest fire smoke or desert dust. These aerosol plumes eventually descend to the surface, sometimes in populated areas, exacerbating air quality. In cases of large AOD (≥0.4), AEROCAN data are also useful in characterizing the aerosol type. The AEROCAN network includes three sites in the high Arctic, a region not sampled by the NAPS PM2.5 monitoring network. These polar sites show the importance of long-range transport and meteorology in the Arctic haze phenomenon. Also, AEROCAN sunphotometers are, by design and due to regular maintenance, the most valuable monitors available for long term aerosol trends. Using a variety of data analysis techniques and timescales, the usefulness of this ground-based remote-sensing sub-network for providing information relevant to air quality is demonstrated.

Investigation on seasonal variations of aerosol properties and its influence on radiative effect over an urban location in central India

Aerosol plays an important role in modulating solar radiation, which are of great concern in perspective of regional climate change. The study analysed the physical and optical properties of aerosols over an urban area and estimated radiative effect using three years in-situ data from sunphotometer, aethalometer and nephelometer as input to radiative transfer model. Aerosols properties indicate the dominance of fine mode aerosols over the study area. However presence of coarse mode aerosols is also found during pre-monsoon [March-April-May]. Daily mean aerosol optical depth showed a minimum during winter [Dec-Jan-Feb] (0.45–0.52) and a maximum during pre-monsoon (0.6–0.7), while single scattering albedo (ω) attains its maximum (0.78 ± 0.05) in winter and minimum (0.67 ± 0.06) during pre-monsoon and asymmetry factor varied in the range between 0.48 ± 0.02 to 0.53 ± 0.04. Episodic events of dust storm and biomass burning are identified by analyzing intrinsic aerosol optical properties like scattering Ångström exponent (SAE) and absorption Ångström exponent (AAE) during the study periods and it has been observed that during dust storm events ω is lower (∼0.77) than that of during biomass burning (∼0.81). The aerosol direct radiative effect at top of the atmosphere during winter is −11.72 ± 3.5 Wm−2, while during pre-monsoon; it is −5.5 ± 2.5 Wm−2, which can be due to observed lower values of ω during pre-monsoon. A large positive enhancement of atmospheric effect of ∼50.53 Wm−2 is observed during pre-monsoon compared to winter. Due to high aerosol loading in pre-monsoon, a twofold negative surface forcing is also observed in comparison to winter.

Measurement of aerosol optical depth and sub-visual cloud detection using the optical depth sensor (ODS)

Abstract. A small and sophisticated optical depth sensor (ODS) has been designed to work in the atmosphere of Mars. The instrument measures alternatively the diffuse radiation from the sky and the attenuated direct radiation from the Sun on the surface. The principal goals of ODS are to retrieve the daily mean aerosol optical depth (AOD) and to detect very high and optically thin clouds, crucial parameters in understanding the Martian meteorology and climatology. The detection of clouds is undertaken at twilight, allowing the detection and characterization of clouds with opacities below 0.03 (sub-visual clouds). In addition, ODS is capable to retrieve the aerosol optical depth during nighttime from moonlight measurements. Recently, ODS has been selected at the METEO meteorological station on board the ExoMars 2018 Lander. In order to study the performance of ODS under Mars-like conditions as well as to evaluate the retrieval algorithms for terrestrial measurements, ODS was deployed in Ouagadougou (Africa) between November 2004 and October 2005, a Sahelian region characterized by its high dust aerosol load and the frequent occurrence of Saharan dust storms. The daily average AOD values retrieved by ODS were compared with those provided by a CIMEL sunphotometer of the AERONET (Aerosol Robotic NETwork) network localized at the same location. Results represent a good agreement between both ground-based instruments, with a correlation coefficient of 0.77 for the whole data set and 0.94 considering only the cloud-free days. From the whole data set, a total of 71 sub-visual cirrus (SVC) were detected at twilight with opacities as thin as 1.10−3 and with a maximum of occurrence at altitudes between 14 and 20 km. Although further optimizations and comparisons of ODS terrestrial measurements are required, results indicate the potential of these measurements to retrieve the AOD and detect sub-visual clouds.

Changes in column aerosol optical properties during extreme haze-fog episodes in January 2013 over urban Beijing

Several dense haze-fog (HF) episodes were occurred in the North China Plain (NCP), especially over Beijing in January 2013 characterized by a long duration, a large influential region, and an extremely high PM2.5 values (>500 μg m−3). In this study, we present the characteristics of aerosol optical properties and radiative forcing using Cimel sun-sky radiometer measurements during HF and no haze-fog (NHF) episodes occurred over Beijing during 1–31 January, 2013. The respective maximum values of daily mean aerosol optical depth at 440 nm (AOD440) were observed to be 1.21, 1.43, 1.52, and 2.21 occurred on 12, 14 19, and 28 January. It was found that the Ångström exponent (AE) values were almost higher than 1.0 during all the days with its maximum on 26 January (1.53), suggests the dominance of fine-mode particles. The maximum (minimum) aerosol volume size distributions occurred during dense HF (NHF) days with larger particle volumes of fine-mode. The single scattering albedo, asymmetry parameter, and complex refractive index values during HF events suggest the abundance of fine-mode particles from anthropogenic (absorbing) activities mixed with scattering dust particles. The average shortwave direct aerosol radiative forcing (DARF) values at the bottom-of-atmosphere (BOA) during HF and NHF days were estimated to be 112.29 ± 42.18 W m−2 and −58.61 ± 13.09 W m−2, while at the top-of-atmosphere (TOA) the forcing values were −45.78 ± 22.17 W m−2 and −18.64 ± 5.84 W m−2, with the corresponding heating rate of 1.61 ± 0.48 K day−1 and 1.12 ± 0.31 K day−1, respectively. The DARF values retrieved from the AERONET were in good agreement with the SBDART computed both at the TOA (r = 0.95) and the BOA (r = 0.97) over Beijing in January 2013.

Assessment of aerosol optical and micro-physical features retrieved from direct and diffuse solar irradiance measurements from Skyradiometer at a high altitude station at Merak: Assessment of aerosol optical features from Merak.

Optical and micro-physical features of aerosol are reported using Skyradiometer (POM-01L, Prede, Japan) observations taken from a high-altitude station Merak, located in north-eastern Ladakh of the western trans-Himalayas region during January 2011 to December 2013. The observed daily mean aerosol optical depth (AOD, at 500 nm) at the site varied from 0.01 to 0.14. However, 75 % of the observed AOD lies below 0.05 during the study period. Seasonal peaks of AOD occurred in spring as 0.06 and minimum in winter as 0.03 which represents the aged background aerosols at the site. Yearly mean AOD at 500 nm is found to be around 0.04 and inter-annual variations of AOD is very small (nearly ±0.01). Angstrom exponent (a) varied seasonally from 0.73 in spring to 1.5 in autumn. About 30 % of the observed a lies below 0.8 which are the indicative for the presence of coarse-mode aerosols at the site. The station exhibits absorbing aerosol features which prominently occurred during spring and that may be attributed by the transported anthropogenic aerosol from Indo-Gangatic Plain (IGP). Results were well substantiated with the air mass back-trajectory analysis. Furthermore, seasonal mean of single scattering albedo (SSA at 500 nm) varied from of 0.94 to 0.98 and a general increasing trend is noticed from 400 to 870 nm wavelengths. These features are apparently regional characteristics of the site. Aerosol asymmetry factor (AS) decreases gradually from 400 to 870 nm and varied from 0.66 to 0.69 at 500 nm across the seasons. Dominance of desert-dust aerosols, associated by coarse mode, is indicated by tri-modal features of aerosol volume size distribution over the station during the entire seasons.

Use of MODIS data to assess atmospheric aerosol before, during, and after community evacuations related to wildfire smoke

Wildfires can pose direct and indirect threats to public health via fire and smoke, respectively. Evacuations are used to manage both types of threat given situational information about fires and their behaviour. However, there has been limited documented use of objective remote sensing data to inform the decision-making process. We conduct a retrospective assessment of 41 smoke-related evacuations in Canada between 2000 and 2007 using smoke and cloud data from the Terra MODIS. Daily mean aerosol optical depth (AOD) measurements were used to represent smoke levels in the area surrounding each evacuation site. Time series of these data were plotted for the four days prior to evacuation, the evacuation date, and the four days following the evacuation. Daily AOD values were scaled using the daily cloud mask to crudely account for the fact that heavy aerosol can be classified as cloud. Time series plots and the percentage differences between pre- and post-evacuation AOD were used to contextualize each evacuation. Out of 41 cases, approximately half met each of the criteria used to assess public health protection: (1) post-evacuation AOD values over the risk threshold for sensitive populations; (2) peak AOD exposures occurring post-evacuation; and (3) mean AOD higher post-evacuation than pre-evacuation. Some evacuations did not meet any of these criteria. Although evacuation decisions would never be made in the absence of situational intelligence, we conclude that systematic use of objective and readily available remote sensing data can help to retrospectively evaluate smoke-related evacuations, and may be useful for prospective, real-time application with further development.

Aerosol properties and radiative forcing over Kanpur during severe aerosol loading conditions

The present work analyzes the aerosol episode (AE) days and examines the modification in aerosol properties and radiative forcing during the period 2001–2010 based on Kanpur-AERONET data. AEs are defined as the days having daily-mean aerosol optical depth (AOD) above the decadal mean + 1 STD (standard deviation); the threshold value is defined at 0.93. The analysis identifies 277 out of 2095 days (13.2%) of AEs over Kanpur, which are most frequently observed during post-monsoon (78 cases, 18.6%) and monsoon (76, 14.7%) seasons due to biomass-burning episodes and dust influence, respectively. On the other hand, the AEs during winter and pre-monsoon are lesser in both absolute and percentage values (65, 12.5% and 58, 9.1%, respectively). The modification in aerosol properties on the AE days is strongly dependent on season; during post-monsoon and winter, the AEs are associated with enhanced presence of fine-mode aerosols from anthropogenic emissions and/or biomass burning, while during pre-monsoon and monsoon seasons, they are mostly associated with dust. Aerosol radiative forcing (ARF) calculated using SBDART shows much more surface (∼−69 to −97 Wm−2) and Top of Atmosphere cooling (−20 to −30 Wm−2) as well as atmospheric heating (∼43 to 71 Wm−2) during the AE days as compared to seasonal means. These forcing values are mainly controlled by the higher AODs and the modified aerosol characteristics (Angstrom Exponent α, single scattering albedo SSA) during the AE days in each season. Furthermore, the vertical profiles of aerosols and atmospheric radiative heating exhibit significant increase in lower and mid troposphere during the AE days. This may cause serious climate implications over Ganges Basin and surrounding regions with further consequences on cloud microphysics, monsoon rainfall and melting of Himalayan glaciers.

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

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

Ground-based in situ measurements of near-surface aerosol mass concentration over Anantapur: Heterogeneity in source impacts

Surface measurements of aerosol physical properties were made at Anantapur (14.62°N, 77.65°E, 331 m a.s.l), a semiarid rural site in India, during August 2008–July 2009. Measurements included the segregated sizes of aerosolsas as well as total mass concentration and size distributions of aerosols measured at low relative humidity (RH 68% to the total aerosol mass concentration. Accumulation aerosol mass fraction, A f (= M a/M t) was highest during winter (mean value of A f ∼ 0.80) and lowest (A f ∼ 0.64) during the monsoon season. The regression analysis shows that both R eff and R m are dependent on coarse-mode aerosols. The relationship between the simultaneous measurements of daily mean aerosol optical depth at 500 nm (AOD500) and PM2.5 mass concentration ([PM2.5]) shows that surface-level aerosol mass concentration increases with the increase in columnar aerosol optical depth over the observation period.

Aerosol climatology in an Alpine valley

Aerosol optical depth (AOD) is measured with a Precision Filter Radiometer in Innsbruck, Austria, from 2007 to 2011. The 5‐year time series is analyzed with respect to the Ångström parameterα and quadratic coefficient γ. Information on the aerosol size distribution is obtained by adopting a graphical framework showing the particle fine mode fraction as well as the effective fine mode radius. The aerosol conditions in Innsbruck reveal a typical midlatitudinal pattern for continental urban sites with low pollution. Small particles originating from traffic pollution as well as natural sources dominate leading to an overall mean α of 1.53 +/− 0.28 and γ of −0.54 +/− 0.25. Low AOD at 500 nm is observed in winter with multiannual monthly means as low as 0.08 and higher AOD in spring and summer with multiannual monthly means up to 0.18. The maximum daily mean AOD at 500 nm always remains below 0.5. Special events such as Saharan dust events or presence of volcanic ash are detected and discussed.

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.

Polar aerosol characterization, sources and impacts

Aerosols are known to cause important effects on weather and climate of Polar Regions and their radiation balance of the polar surface-atmosphere system, especially in the regions characterized by high surface-reflectance conditions, which also prevails the heterogeneous chemistry of aerosols. Therefore, the knowledge of the aerosol physical and optical properties needs to be improved on both spatial and temporal scales. To characterize these physico-chemical and optical properties, studies have been carried out over both the polar regions [Antarctica (‘Maitri’ (70.76oS, 11.74oE) and Arctic “Himadri” (79°N, 11°E) during the summer period of 24th (2004-05), 26th (2006-07) Indian Antarctica Expedition, and during 14th Indian Arctic Expedition in 2010. Total column aerosol optical depth (AOD), ozone (TCO), precipitable water content (PWC), and direct radiative forcing using a multi-channel solar-radiometer (Microtops II); and short-wave global radiative flux using a wide-band pyranometer for their characteristics. In the Arctic, an Andersen Sampler, Black Carbon Aethalometer was also operated to determine the chemical properties of aerosols.&#x0D; The aerosol optical, physical and radiative properties, and their interface with simultaneously measured gases and their chemical composition have been investigated. The results showed that the daily mean AOD at a characteristic wavelength of 500 nm was found to be 0.042 with an average Angstrom coefficient of 0.24, revealing abundance of coarse-mode particles in Antarctica, and Arctic average AOD was observed 0.11 with an average Angstrom coefficient of 2.84, suggesting fine-mode particles. The TCO measured by the surface-based ozone monitor matched reasonably within 5% with that of the Total Ozone Mapping Spectrometer (TOMS) satellite sensor. Variability in ozone on daily scale, during the study period, was less than 4% over the Antarctica region and more or less same for Arctic.&#x0D; The January 2005 fluxes were found to be less by about 20% as compared to those in February 2005. The average short-wave direct radiative forcing due to aerosols showed cooling at the surface with an average value of -0.47 W/m2 during the study period. In this paper, we briefly describe the equipment deployed, data archival, their analysis techniques and salient results obtained over the Indian polar stations, ‘Maitri’ and ‘Himadri’.

Characterization of aerosols and pre-cursor gases over Maitri during 24th Indian Antarctica Expedition

Within the framework of the 24th Indian Antarctica Expedition (IAE), observations of total column aerosol optical depth (AOD), ozone (TCO) and precipitable water content (TCW) using a multi-channel solar-radiometer (MICROTOPS-II: Microprocessor-controlled Total Ozone Portable Spectrometer-II), and observations of short-wave global radiative flux using a wide-band pyranometer have been carried out over the Indian Antarctica station Maitri (70.76° S, 11.74° E) and the southern Indian Ocean during December 2004–February 2005. These extensive datasets have been utilized to investigate the aerosol optical, physical and radiative properties, and their interface with simultaneously measured gases. Data over the Oceanic region have been collected from the ship front deck. The daily mean AOD at a characteristic wavelength of 500 nm was found to be 0.042 with an average Angstrom coefficient of 0.24, revealing an abundance of coarse-mode particles. Interestingly, the January fluxes were found to be less by about 20% compared with those in February. The average short-wave direct radiative forcing due to aerosols showed cooling at the surface with an average value of −0.47 Wm−2. The TCO increased from about 252 DU around 38° S to about 312 DU at 70° S, showing a gradual increase in ozone with increasing latitude. The TCO measured by the surface-based ozone monitor matched reasonably well with that observed by the Total Ozone Mapping Spectrometer (TOMS) satellite sensor within 5%. Variability in ozone on a daily scale during the study period was less than 4% over the Antarctica region.

Study of the correlation between the near-ground PM 10 mass concentration and the aerosol optical depth

This work aims to analyze the relationship between in situ measurements of PM 10 mass concentrations near the ground and daily mean Aerosol Optical Depth (AOD) values. Measurements were performed with sun-photometers part of the Aerosol Robotic Network (AERONET) for AOD, and low-volume samplers near the ground for in situ PM 10 mass concentrations. The analysis was applied for some periods of 2007 and 2009. It is known that several factors like aerosol vertical distribution or hygroscopic growth factor could affect the link between PM 10 ground measurements and AOD. Using a new filtering condition, our linear regression analysis results have shown a significant correlation coefficient, around 0.70. It may be concluded that the columnar optical observation can be transferred to the near-surface conditions with a rather high degree of confidence. This study showed that it is possible to use sun photometric measurements to improve existent air quality surveillance or to extend their spatial coverage.