Fine Mode Aerosols Research Articles

Water vapor retrieval using the Airborne Multiangle SpectroPolarimetric Imager

The Airborne Multiangle SpectroPolarimetric Imager (AirMSPI) acquires radiance measurements in eight spectral bands centered at 355, 380, 445, 470*, 555, 660*, 865*, and 935 nm, three of which also measure linear polarization (denoted by asterisks). A two-step retrieval approach is developed to retrieve total precipitable water vapor (PWV) abundance: a) the first step is a fast estimate of water vapor obtained by comparing the ratio of observed radiance in the near-infrared (NIR) bands L935nm/L865nm at multiple view angles against pre-calculated lookup table values as a function of PWV abundance, on the assumption of no aerosols and a moderately bright land surface; (b) the second step refines the estimated solution by accounting for aerosol loading, particle properties, and surface reflection, which are retrieved from AirMSPI measurements in its ultraviolet to NIR aerosol bands. Our retrieval is tested using 27 AirMSPI datasets with low to moderately high aerosol loadings and PWV up to about 3.5 cm, acquired during four NASA field campaigns plus one AirMSPI engineering test flight. Comparison of the retrieved total PWV amount against AERONET reference data shows a mean percentage difference 8.6%. The quantities that inform the second step of the PWV retrieval—namely, aerosol optical depth (AOD), aerosol single scattering albedo (SSA), and aerosol effective radius—are also compared to the AERONET reference data. The mean absolute differences (MADs) are less than 0.0253 and less than 0.036 for AOD and SSA in the visible, respectively. The MADs are 0.036 μm and 1.232 μm for the effective radius of fine and coarse mode aerosols, respectively. Comparison to data from the Cloud Physics Lidar places the MAD of aerosol layer height at about 360 m.

New global aerosol fine-mode fraction data over land derived from MODIS satellite retrievals.

The space-borne measured fine-mode aerosol optical depth (fAOD) is a gross index of column-integrated anthropogenic particulate pollutants, especially over the populated land. The fAOD is the product of the AOD and the fine-mode fraction (FMF). While there exist numerous global AOD products derived from many different satellite sensors, there have been much fewer, if any, global FMF products with a quality good enough to understand their spatiotemporal variations. This is key to understanding the global distribution and spatiotemporal variations of air pollutants, as well as their impacts on global environmental and climate changes. Modifying our newly developed retrieval algorithm to the latest global-scale Moderate Resolution Imaging Spectroradiometer (MODIS) aerosol product (Collection 6.1), a global 10-year FMF product is generated and analyzed here. We first validate the product through comparisons with the FMF derived from Aerosol Robotic Network (AERONET) measurements. Among our 169,313 samples, the satellite-derived FMFs agreed with the AERONET spectral deconvolution algorithm (SDA)-retrieved FMFs with a root-mean-square error (RMSE) of 0.22. Analyzed using this new product are the global patterns and interannual and seasonal variations of the FMF over land. In general, the FMF is large (>0.80) over Mexico, Myanmar, Laos, southern China, and Africa and less than 0.5 in the Sahelian and Sudanian zones of northern Africa. Seasonally, higher FMF values occur in summer and autumn. The linear trend in the satellite-derived and AERONET FMFs for different countries was explored. The upward trend in the FMFs was particularly strong over Australia since 2008. This study provides a new global view of changes in FMFs using a new satellite product that could help improve our understanding of air pollution around the world.

Impacts of Atmospheric Boundary Layer Vertical Structure on Haze Pollution Observed by Tethered Balloon and Lidar

In this paper, the characteristics of the atmospheric boundary layer (ABL) vertical structure over the North China Plain (NCP) during a comprehensive observation experiment conducted during 15–21 December 2018 were investigated. Observational data were obtained with a large tethered balloon, Doppler wind lidar, and ground-level instruments. The maximum concentration of PM2.5 exceeded 200 µg m−3, and the ratio of PM2.5/PM10 was approximately 0.4 (its maxi-mum was approximately 0.8) during the whole observation period, indicating the explosive growth of dominant fine-mode aerosols in the winter heating season. Elevated concentrations of pollutants decreased the solar irradiance received by the ground, resulting in lower temperature at ground level. Our results illustrate three distinct types of vertical profiles: Type 1 (convective state)—the concentration of PM2.5 decreased nearly linearly with increase of the height below approximately 600 m; Type 2 (stable state)—the PM2.5 concentration sharply decreased from the ground to approximately 200 m; and Type 3 (multilayer structure)—some pollutants were suspended aloft in the upper air layer. Diurnal evolution of the vertical profiles of PM2.5 and their relationship with the changes in meteorological factors were identified. From daytime to nighttime, the vertical profiles evolved from Type 1 to Type 2 or Type 3. All the 33 vertical PM2.5 profiles that we obtained showed a strong relationship with elements of the ABL structure, such as the distributions of winds, the inversion layer, and turbulence activities. A light-wind layer and weak turbulence activity, especially within the inversion layer, contributed greatly to the accumulation of pollutants. Vertical PM2.5 concentration patterns were also greatly affected by local ground-level emission sources and regional transport processes.

Understanding global changes in fine-mode aerosols during 2008–2017 using statistical methods and deep learning approach

Despite their extremely small size, fine-mode aerosols have significant impacts on the environment, climate, and human health. However, current understandings of global changes in fine-mode aerosols are limited. In this study, we employed newly developed satellite retrieval data and an attentive interpretable deep learning model to explore the status, changes, and association factors of the global fine-mode aerosol optical depth (fAOD) and aerosol fine-mode fraction (FMF) from 2008 to 2017. At the global scale, the results show a significant increasing trend in land FMF (2.34 × 10−3/year); however, the FMF over the ocean and the fAOD over land and ocean did not reveal significant trends. Between 2008 and 2017, high levels of both fAOD (>0.30) and FMF (>0.75) were identified over China, southeastern Asia, India, and Africa. Seasonally, global land FMF showed high values in summer (>0.70) and low values in spring (<0.65), while land fAOD was high in summer (>0.15) but low in winter (<0.13). Importantly, Australia and Mexico experienced significant increasing trends in FMF during all four seasons. At the regional scale, a significant decline in fAOD was identified in China, which indicates that government emission controls and reductions have been effective in recent decades. The deep learning model was used to interpret the result and showed that O3 was significantly associated with changes in both the FMF and fAOD. This finding suggests the importance of synergizing the regulations for both O3 and fine particles. Our work comprehensively examined global spatial and seasonal fAOD and FMF changes and provides a holistic understanding of global anthropogenic impacts.

New in situ aerosol hyperspectral optical measurements over 300–700 nm – Part 1: Spectral Aerosol Extinction (SpEx) instrument field validation during the KORUS-OC cruise

Abstract. In situ observations of spectrally resolved aerosol extinction coefficients (300–700 nm at ∼ 0.8 nm resolution) from the May–June 2016 Korea–United States Ocean Color (KORUS-OC) oceanographic field campaign are reported. Measurements were made with the custom-built Spectral Aerosol Extinction (SpEx) instrument that previously has been characterized only using laboratory-generated aerosols of known size and composition. Here, the performance of SpEx under realistic operating conditions in the field was assessed by comparison to extinction coefficients derived from commercial instruments that measured scattering and filter-based absorption coefficients at three discrete visible wavelengths. Good agreement was found between these two sets of extinction coefficients with slopes near unity for all three wavelengths within the SpEx measurement error (± 5 Mm−1). The meteorological conditions encountered during the cruise fostered diverse ambient aerosol populations with varying sizes and composition at concentrations spanning 2 orders of magnitude. The sampling inlet had a 50 % size cut of 1.3 µm diameter particles such that the in situ aerosol sampling suite deployed aboard ship measured fine-mode aerosols only. The extensive hyperspectral extinction data set acquired revealed that nearly all measured spectra exhibited curvature in logarithmic space, such that Ångström exponent (α) power law fits could lead to large errors compared to measured values. This problem was particularly acute for α values calculated over only visible wavelengths and then extrapolated to the UV, highlighting the need for measurements in this wavelength range. Second-order polynomial fits to the logarithmically transformed data provided a much better fit to the measured spectra than the linear fits of power laws. Building on previous studies that used total column aerosol optical depth observations to examine the information content of spectral curvature, the relationship between α and the second-order polynomial fit coefficients (a1 and a2) was found to depend on the wavelength range of the spectral measurement such that any given α maps into a line in (a1, a2) coefficient space with a slope of −2LN(λch), where λch is defined as the single wavelength that characterizes the wavelength range of the measured spectrum (i.e., the “characteristic wavelength”). Since the curvature coefficient values depend on λch, it must be taken into account when comparing values from spectra obtained from measurement techniques with different λch. Previously published work has shown that different bimodal size distributions of aerosols can exhibit the same α yet have differing spectral curvature with different (a1, a2). This implies that (a1, a2) contain more information about size distributions than α alone. Aerosol size distributions were not measured during KORUS-OC, and the data reported here were limited to the fine fraction, but the (a1, a2) maps obtained from the SpEx data set are consistent with the expectation that (a1, a2) may contain more information than α – a result that will be explored further with future SpEx and size distribution data sets.

Simultaneous measurements of PM1 and PM10 aerosol scattering properties and their relationships in urban Beijing: A two-year observation

The aerosol scattering properties of submicron (PM1) and sub-10 μm particles (PM10) under dry conditions (RH <30%) were investigated in Beijing from 2018 to 2019. Using the simultaneous measurement of PM1 and PM10, the scattering properties of super micron (PM10–1) particles were also calculated. At 550 nm, the average of scattering coefficient (σsp) and asymmetry parameter (g) were 208.7 ± 204.9 Mm−1 and 0.61 ± 0.04 for PM10, 140.6 ± 130.2 Mm−1 and 0.60 ± 0.04 for PM1, and 69.8 ± 82.2 Mm−1 and 0.62 ± 0.04 for PM10–1, respectively, while the backscattering ratio (b) values were 0.13 ± 0.02 for PM10 and PM1, and 0.12 ± 0.02 for PM10–1. The mass scattering efficiencies (MSE) for PM10, PM1 and PM10–1 were 2.43 ± 2.37, 3.67 ± 0.96, and 1.73 ± 1.82 m2 g−1, respectively. In 2019, σsp decreased by approximately 18.4% for PM10, and 16.7% for PM1 compared with those in 2018, which was quite similar to the decrease of 17% and 19% for PM10 and PM2.5 mass concentrations during the same time period. The scattering Ångström exponent (SAE450/700), which was 1.88 ± 0.29 for PM1 and 1.50 ± 0.27 for PM10 indicated size distributions dominated by fine mode aerosols. This is also evidenced by the high submicron scattering ratio (Rsp) (71.1% ± 7.9%). The high SAE, Rsp, and high PM1 σsp in the study suggest that control of fine particle pollution is important to reduce overall PM pollution in urban Beijing. In addition, with an increase in σsp, b, Rsp, and SAE gradually decreased, while g and MSE increased. The clearly scattering coefficient-dependent MSE suggests that high aerosol loading and high MSE both play an important role in degraded visibility during heavy pollution periods.

Analysis of Aerosol Type and Fine- and Coarse-mode Aerosol Direct Radiative Forcing over Regions in East and Southeast Asia Based on AERONET Version 3 Data

ABSTRACT Based on the particle linear depolarization ratio (PLDR) and single-scattering albedo (SSA) values obtained for 1020 nm from Aerosol Robotic Network (AERONET) Version 3 Level 2.0 data collected in five regions, namely, northern China, Northwest Asia, the Tibetan Plateau, southern China and Southeast Asia, we classified seven types of aerosol (viz., purely dust, dust-dominated, pollution-dominated, non-absorbing [NA], weakly absorbing [WA], moderately absorbing [MA] and strongly absorbing [SA]) in order to assess the spatial and temporal distributions of their constituents and the radiative effects of their fine- and coarse-mode particles. The fine fraction dominated in northern China and also played a crucial role on the Tibetan Plateau and in southern China and Southeast Asia, whereas the coarse fraction prevailed in Northwest Asia. Furthermore, the fine-mode aerosol on the Tibetan Plateau exhibited its maximum radiative forcing efficiency (110.3 W m–2) during high concentrations of SA aerosol. Also, the lowest values for both the radiative forcing and the radiative forcing efficiency in southern China occurred during summer. Finally, the various aerosol constituents displayed distinct spatial and temporal distributions in Southeast Asia, with the SA aerosol contributing approximately 20% of the total aerosol on the Indochinese Peninsula and the NA and WA aerosol forming the largest percentages on the Malay Peninsula.

Analysis of Aerosol Optical Depth and Angstrom Exponents over an AERONET site at Pokhara, Nepal

The monthly variability of Aerosol Optical Depth at 0.50 μm (AOD0.50) and Ångström exponents (AE) based on spectral AODs over an Aerosol Robotic Network (AERONET) site Pokhara, are analyzed by using aerosol data of the year 2017. The AOD0.50 are characterized by low average values (0.21± 0.12) in monsoon, and highest values in pre-monsoon (0.67± 0.14) followed by winter (0.46± 0.28) and post-monsoon (0.33±.02) with an overall mean of 0.43 ± 0.02. The average AE obtained by using AODs at 0.44 μm and 0.87 μm is 1.20± 0.04 in pre-monsoon, 1.37± 0.05 in monsoon, 1.41±.01 in post-monsoon, and 1.37± 0.07 in winter with an annual average value of 1.35 ± 0.08. These overall variations of AE indicate that the majority of aerosol loading during the study period was mixture of fine and coarse mode aerosols and the influence of anthropogenic aerosols. The monthly average AOD suggests low aerosol loading in the months of the monsoon season (June to September) than other months of pre-monsoon season (March to May) and post-monsoon season (October and November).&#x0D; BIBECHANA 18 (2021) 118-127

Spatiotemporal relationship between Himawari-8 hourly columnar aerosol optical depth (AOD) and ground-level PM2.5 mass concentration in mainland China

Himawari-8 aerosol products have been widely used to estimate the near-surface hourly PM2.5 concentrations due to the high temporal resolution. However, most studies focus on the evaluation model. As the foundation of the estimation, the relationship between near-surface PM2.5 and columnar aerosol optical depth (AOD) has not been comprehensively investigated. In this study, we investigate the relationship between PM2.5 and advanced Himawari imager (AHI) AOD for 2016–2018 across mainland China on different spatial and temporal scales and the factors affecting the association. We calculated the Pearson correlation coefficients and the PM2.5/AOD ratio as the analysis indicators in 345 cities and 14 urban agglomerations based on the collocations of PM2.5 and AHI AOD. From 9:00 to 17:00 local time, the PM2.5-AOD correlation become significantly stronger while The PM2.5/AOD ratio markedly decrease in Beijing-Tianjin-Hebei, Yangtze River Delta, and Chengyu regions. The strongest correlation is between 12:00 and 14:00 LT (at noon) and between 13:00 and 17:00 LT (afternoon), respectively. The ratio in a day shows an obvious unimodal mode, and the peak occurred at around 10:00 or 11:00 LT, especially in autumn and winter. There is a pronounced variation of the PM2.5-AOD relationship in a week during the winter. Moreover, there are the strongest correlation and the largest ratio for most urban agglomerations during the winter. We also find that PM2.5 and AOD are not always correlated under different meteorological conditions and precursor concentrations. Furthermore, for the scattering-dominated fine-mode aerosol, there is a high correlation and a low ratio between PM2.5 and AOD. The correlation between PM2.5 and AHI AOD significantly increases with increasing the number of AOD retrievals on a day. The findings will provide meaningful information and important implications for satellite retrieval of hourly PM2.5 concentration and its exposure estimation in China, especially in some urban agglomerations.

Sensitive Research of Scattering and Absorbing Aerosols on Sky Polarization Pattern

Sky polarization patterns are a relatively new and interesting field of polarized remote sensing. However, most current research mainly focuses on Rayleigh scattering or different conditions of aerosol optical depth. In this study, the sky downward polarization patterns are calculated for both degree of linear polarization and angle of polarization with scattering and absorbing aerosol situations. When coarse-mode aerosol changes from scattering to absorbing, the decreasing trend in the sky downward degree-of-linear-polarization largely slows down when aerosol optical depth increases. For fine-mode aerosol, on the other hand, the change of pattern is not sensitive to the absorbing property of aerosol. Sky downward angle-of-polarization patterns for different levels of aerosol optical depth and aerosol modes are similar, with little change. The results suggest that in order to accurately use sky polarization for remote sensing or bionic navigation, it is necessary to characterize aerosol microphysical properties first, especially when coarse absorbing aerosol exists.

Estimation of Surface Concentrations of Black Carbon from Long-Term Measurements at Aeronet Sites over Korea

We estimated fine-mode black carbon (BC) concentrations at the surface using AERONET data from five AERONET sites in Korea, representing urban, rural, and background. We first obtained the columnar BC concentrations by separating the refractive index (RI) for fine-mode aerosols from AERONET data and minimizing the difference between separated RIs and calculated RIs using a mixing rule that can represent a real aerosol mixture (Maxwell Garnett for water-insoluble components and volume average for water-soluble components). Next, we acquired the surface BC concentrations by establishing a multiple linear regression (MLR) between in-situ BC concentrations from co-located or adjacent measurement sites, and columnar BC concentrations, by linearly adding meteorological parameters, month, and land-use type as the independent variables. The columnar BC concentrations estimated from AERONET data using a mixing rule well reproduced site-specific monthly variations of the in-situ measurement data, such as increases due to heating and/or biomass burning and long-range transport associated with prevailing westerlies in the spring and winter, and decreases due to wet scavenging in the summer. The MLR model exhibited a better correlation between measured and predicted BC concentrations than those based on columnar concentrations only, with a correlation coefficient of 0.64. The performance of our MLR model for BC was comparable to that reported in previous studies on the relationship between aerosol optical depth and particulate matter concentration in Korea. This study suggests that the MLR model with properly selected parameters is useful for estimating the surface BC concentration from AERONET data during the daytime, at sites where BC monitoring is not available.

A synergy of linear model and wavelet analysis towards space-time characterization of aerosol optical depth (AOD) during pre-monsoon season (2007–2016) over Indian sub-continent

The pre-monsoon aerosol concentration plays a significant role in modifying precipitation amount over the Indian sub-continent. A large variety of aerosol from different sources produces a complex radiative and climate response through the interaction with the hydrometeorological parameters. In this study, we analyzed the space-time dynamics of aerosol optical depth (AOD) in relation to the meteorological and surface parameters over Indian sub-continent during pre-monsoon season from 2007 to 2016. The level-3 daily aerosol products from Moderate Resolution Imaging Spectroradiometer (MODIS) and Multi-angle Imaging Spectroradiometer (MISR) were used. The IMD gridded rainfall and temperature, ECMWF derived RH and wind velocity, and GLADS derived soil moisture data were also used at daily time scale. For the space-time model, the Mann-Kendall trend test and a pixel-based multiple linear regression were used, while, wavelet transformation was used on these daily observations for analyzing periodicities of AOD. The time-series average shows moderate to high AOD (0.4–0.8), including a consistent positive anomaly in the Indo-Gangetic basin (IGP) in north India. A significant inter-annual variation is also observed both in MODIS and MISR datasets. The trend statistics shows an increasing trend of aerosol concentration in the eastern and southern India. The linear regression shows a variable response of AOD with changing magnitude of meteorological factors. However, a substantial spatial coverage of significantly decreased AOD is observed with increasing soil moisture content (β < −0.04). The wavelet analysis manifests the abundances of 32–128 days of cycle with a periodic interjection of 8–32 days of cycle suggesting occurrence of fine and coarse mode aerosols events, respectively. The coherency of time series AOD and other covariates shows varying leading and lagging dynamics in these two principal periodicities. The findings, however, evidenced a notable difference in the space-time patterns of AOD in MODIS and MISR datasets. The analyzed AOD cycles are coincided with the Madden-Julian-Oscillation (MJO) that recurs every 30–60 days interval. The findings also support the theoretical proposition of elevated heat pump theory (EHP) driven by fine mode aerosols for occurring pre-monsoon and monsoon precipitation over the Indian sub-continent. The analyzed periodicity of AOD can provide useful insights in studying the short/long term variability of precipitation over polluted environments during the pre-monsoon season.

The interaction of ozone and aerosol in a semi-arid region in the Middle East: Ozone formation and radiative forcing implications

Investigations on the variations in ozone concentration, aerosol properties, and ozone-aerosol interactions can help to better understand the atmospheric radiation budget and their effects on climate. In this study, the interaction of ozone and aerosol was analyzed over Zanjan, Iran. The seasonal cycle of total column ozone demonstrated that high ozone concentrations often occur in early spring and winter. However, the seasonal cycle of tropospheric ozone showed that high concentrations of tropospheric ozone often happen in the spring and summer. The high values of Aerosol Optical Depth (AOD), Particulate Matter (PM10) and Absorption Aerosol Optical Depth (AAOD) were observed in the spring and summer, whereas low values in the fall and winter. The results showed that tropospheric ozone and AOD were negatively correlated during the spring and summer seasons. Especially on dusty days, the aerosol has reduced the concentration of ozone in the atmosphere. The correlation coefficients between PM10 and ozone were negative during dusty days/months. Moreover, the relation of ozone with a fine mode fraction of AOD depicted negative correlations in the spring and summer and positive correlation in the fall and winter. A comprehensive study on the effects of the aerosol types on ozone concentration showed that anthropogenic fine mode aerosols such as urban/industrial had a positive impact on ozone production, contrary to the effect of dust, which reduced ozone. This study showed that coarse mode aerosols such as dust with higher AOD, reduced the sunlight to reach the earth's surface. As a result, aerosol radiative forcing (ARF) values were found to be more negative at the surface; thus, the photolysis rate decreased considerably, and the concentration of surface ozone was reduced significantly.

Geochemical characteristics of trace elements in size-resolved coastal urban aerosols associated with distinct air masses over tropical peninsular India: Size distributions and source apportionment

Trace elements in atmospheric particulate matter play a significant role in air quality, health and biogeochemical cycles. The present study reports on geochemical characteristics of size-resolved trace elements in PM10 aerosols collected under different air masses over a coastal urban location in peninsular India. A contrast in elemental distribution was observed for the particle size above 7.0 μm and below 1.1 μm under the influence of northeasterly air masses as characterized by Al > Fe > Zn and Fe > Al > Zn, respectively. The concentrations of the crustal elements (Al, Fe, Ti, P, Ba, Co) were high and illustrated by a unimodal size distribution with a peak in coarse mode (>2.0 μm) during northwesterly air masses. On the other hand, combustion-derived metals (Cu, Zn, Cd, Sb, and Pb) were maximized under northeasterly air masses, characterized by unimodal size distribution with a peak in fine mode (<2.0 μm). The enrichment factor (EF) analysis reveals the contribution of anthropogenic emissions to Cd, Sb, Pb, Zn, Cu, Cr, Ni, As, and Sn metals, particularly to the high enrichment of trace metals in fine mode. These results suggest that crustal emissions are major sources of trace metals in coarse mode aerosols; whereas combustion derived anthropogenic emissions contribute to the fine mode aerosols. The positive matrix factorization (PMF) analysis revealed that crustal sources (52–90%) were most abundant for particles >7.0 μm, whereas combustion related emissions such as vehicular and traffic sources are predominant for particles <1.1 μm. The present study demonstrates that trace metals in coastal urban aerosols are affected by changes in emission sources/strengths and regional transport of air masses originated from the northeasterly and northwesterly parts of the tropical Indian subcontinent.

Interannual and seasonal variations in the aerosol optical depth of the atmosphere in two regions of Spitsbergen (2002–2018)

Abstract. In this work, hourly averaged sun photometer data from Barentsburg and Ny-Ålesund, both located on Spitsbergen in the European Arctic, are compared. Our data set comprises the years from 2002 to 2018 with overlapping measurements from both sites during the period from 2011 to 2018. For more turbid periods (aerosol optical depth, AOD, τ0.5&gt;0.1), we found that Barentsburg is typically more polluted than Ny-Ålesund, especially in the shortwave spectrum. However, the diurnal variation in the AOD is highly correlated. Next, τ was divided into a fine and coarse mode. It was found that the fine-mode aerosol optical depth generally dominates and also shows a larger interannual than seasonal variation. The fine-mode optical depth is in fact largest in spring during the Arctic haze period. Overall the aerosol optical depth seems to decrease (at 500 nm the fine-mode optical depth decreased by 0.016 over 10 years), although this is hardly statistically significant.

Spatio-temporal distribution of aerosol optical depth and cloud properties over Lake Tana Basin, Upper Blue Nile River Basin, Ethiopia

The spatio temporal variations (with respect to space and time) of some of atmospheric parameters over Lake Tana Basin, Ethiopia have been studied from 2001 to 2019 to understand their relationships among aerosol optical depth AOD) and rainfall. The origin and movement of air masses which bring aerosol particles have been studied by using the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model. Aerosol optical depth (AOD) values were found to be maximum during the summer season (June–August), but minimum during the winter season (December–February). The annual average of AOD over the Lake Tana Basin varies from 0.21 to 0.34 during 2001–2019 while monthly and seasonal AOD values vary between 0.05 and 0.32, and 0.07 and 0.25 respectively. The HYSPLIT air mass back trajectories model indicate that air masses were traced from Eritrean Plateau, Red sea, Arabian Peninsula, Gulf of Aden and Afar depression. The negative relationship between AOD and Angstrom exponent indicates that the fine mode aerosol particles are dominate over the Tana Lake Basin. The stronger positive correlation coefficient between AOD and cloud fraction (CF) over the Gonder region (north of the Lake Tana Basin) as compared to that for Dangila region (south-west of the Lake Tana Basin; r = 0.61 and r = 0. 58 respectively), indicates the source of moisture from the Red Sea region. Nevertheless, the south-easterly increase in the spatial correlation between AOD and water vapor (WV) over the Lake Tana Basin suggests the prime source of moisture form the Western Indian Ocean. Thus, satellite derived AOD data are useful for the study of rainfall patterns.

Investigation of the relationship between the fine mode fraction and Ångström exponent: Cases in Korea

Based on long-term AERONET observations in the Korean peninsula, we evaluate how well the Ångström exponent (AE) can describe the regional aerosol size well in comparison to the fine mode fraction (FMF). Despite close relationships between AE and FMF previously reported, we found that AE is not always linearly correlated to the FMF in the Korean peninsula, particularly during the summertime. Seasonal patterns of volume size distribution indicate that the volume median and peak radius of the fine mode are obviously enhanced in the summer. Since the ambient conditions in Korean summer are highly humid, cloudy, and stagnant, the size of regional fine mode aerosol can increase significantly due to hygroscopic growth and/or cloud processing in this highly polluted atmosphere. In contrast to the AE, the AE ratio (AE at longer wavelengths divided by AE at shorter wavelengths; AER) correlates better with the FMF variation in summer, but a little worse in the spring. This suggests that the combined consideration of AE and AER may provide a better characterization relative to FMF. Thus, we try to estimate the FMF based on the multiple linear regression method using both AE and AER. As expected, the estimated FMF shows remarkably high linear correlations with the AERONET FMF without any seasonal bias. These findings imply that the FMF in South Korea can be simply obtained if just the AOT measurement at multiple wavelengths (in the range from ultraviolet to near-infrared channels) are available.

Impact assessment of continental and marine air-mass on size-resolved aerosol chemical composition over coastal atmosphere: Significant organic contribution in coarse mode fraction

Size-segregated aerosols (five stages from <1.1 to >7 μm in diameter) collected from a coastal location in south-west peninsular India were analysed for water-soluble ionic species, carbonaceous aerosols and metallic species on a seasonal basis for two years (2017–19). The annual mean concentration of the components Na+, Ca2+, Mg2+, Cl−, NO3−, Al, Fe exhibited coarse mode (> 2 μm) enhancement; while NH4+, K+, SO42−, MSA−, oxalate, Organic carbon (OC), Elemental carbon (EC) peaked in fine mode (< 2 μm). The contribution of fine mode aerosols dominated under the influence of continental air-mass and a relative shift towards coarse mode size regime with respect to marine air-mass was observed. The relative contribution of analysed species showed the dominance of sea-salt fraction in coarse mode during monsoon, anthropogenic ions in fine mode during post-monsoon, crustal species in coarse mode during pre-monsoon and carbonaceous species in fine mode during winter. While nss-SO42− was found to be a major component of fine mode aerosol fraction, OC contributed significantly for both coarse and fine mode. Distinct seasonal variation in OC size-distribution was observed. Fine mode OC dominated during post-monsoon and winter while coarse mode OC peaked during pre-monsoon and monsoon, indicating its multiple sources and formation pathways. The water-soluble fraction of OC predominantly existed in fine mode and was modulated by changing synoptic air-masses. The water insoluble OC fraction was found to be modified by sea-land breeze activities and exhibited coarse mode nature. The significant contribution of OC in coarse mode size regime over this coastal atmosphere has been attributed to; the condensation of gaseous precursors on pre-existing coarse particles leading to the formation of coarse mode WSOC and the primary emission from natural terrestrial and marine organic matter resulting in the emission of coarse mode WIOC.

Climatology of Fine and Coarse Mode Aerosol Optical Thickness Over East and South Asia Derived From POLDER/PARASOL Satellite

AbstractThe climatological characteristics of total aerosol optical thickness (AOT) together with fine and coarse mode AOT (fAOT and cAOT) and fine mode fraction (FMF = fAOT/AOT) over East and South Asia are presented here for the period March 2005 to October 2013. The characteristics are retrieved from POLDER/PARASOL measurements using recently developed aerosol component module of GRASP algorithm. The satellite retrievals were validated and presented a good agreement with the AERONET products (e.g., R of ~0.8–0.9 for AOT, fAOT, Ångström exponent, and FMF). The results show a seasonal cycle and strong relation to the emission sources. For example, FMF has a peak of nearly 1.0 in March in the Indo‐China Peninsula region, reaching a minimum of about 0.4 in April in Northwestern China region. These patterns can be explained by dominance of anthropogenic pollution or desert dust emissions. Maps of fine, coarse, and total AOT, as well as of FMF for the East and South Asia region, are presented and analyzed in scope of seasonal and interannual variabilities.

Investigation of Aerosol Climatology and Long-Range Transport of Aerosols over Pokhara, Nepal

This study presents the spectral monthly and seasonal variation of aerosol optical depth (τAOD), single scattering albedo (SSA), and aerosol absorption optical depth (AAOD) between 2010 and 2018 obtained from the Aerosol Robotic Network (AERONET) over Pokhara, Nepal. The analysis of these column-integrated aerosol optical data suggests significant monthly and seasonal variability of aerosol physical and optical properties. The pre-monsoon season (March to May) has the highest observed τAOD(0.75 ± 0.15), followed by winter (December to February, 0.47 ± 0.12), post-monsoon (October and November, 0.39 ± 0.08), and monsoon seasons (June to September, 0.27 ± 0.13), indicating seasonal aerosol loading over Pokhara. The variability of Ångström parameters, α, and β, were computed from the linear fit line in the logarithmic scale of spectral τAOD, and used to analyze the aerosol physical characteristics such as particle size and aerosol loading. The curvature of spectral τAOD, α’, computed from the second-order polynomial fit, reveals the domination by fine mode aerosol particles in the post-monsoon and winter seasons, with coarse mode dominating in monsoon, and both modes contributing in the pre-monsoon. Analysis of air mass back trajectories and observation of fire spots along with aerosol optical data and aerosol size spectra suggest the presence of mixed types of transboundary aerosols, such as biomass, urban-industrial, and dust aerosols in the atmospheric column over Pokhara.