Identification of aerosol types over Indo-Gangetic Basin: implications to optical properties and associated radiative forcing

Aerosols and black carbon variability using OMI and MERRA-2 and their relationship to near-surface air temperature

An extinction of incoming solar radiation is taking place by absorption and scattering by dust, water droplets, and gaseous molecules. Such phenomena are responsible for altering meteorological variables. In the present study, temporal analysis of the aerosol optical thickness (AOT) and black carbon (BC) surface mass concentration was undertaken using an ozone monitoring instrument (OMI) and modern-era retrospective analysis for research and applications, version 2 (MERRA-2) satellite from the year 2018 to 2022. The study was mainly focused on the western states of India which are Rajasthan, Gujarat, and Maharashtra. The correlation of AOT and BC surface mass concentration with near-surface temperature (2m above ground level) was analyzed. BC and temperature shows strong negative correlation as BC is known for its absorption of radiation. It accumulates in the atmosphere and contributes to atmospheric warming while simultaneously bringing down the near-surface air temperature due to the reduced sunlight reaching the ground. Also, seasonal analysis was conducted for winter, summer, monsoon, and post-monsoon, which shows the higher values of AOT in monsoon; however, seasonal average BC surface mass concentration was found high in winter in each year for all three states. AERONET data from Jaipur, Rajasthan, and Pune, Maharashtra for the year 2021 was used to further evaluate the AOT generated from OMI. The results demonstrated a significant connection, with R2 values of 0.62 and 0.69, respectively. The temperature retrieved from MERRA-2 was also validated with ground truth data of the Continuous Ambient Air Quality Monitoring Station (CAAQMS) at both stations showing high agreement with R2 > 0.70.

Radiative effects of absorbing aerosol types over South Asia

A comprehensive study on classifying the aerosol types and absorbing aerosol types, and quantifying the effect of absorbing aerosols on aerosol optical and radiative properties using four years (2015-2016, 2018-2019) of high-quality Aerosol Robotic Network (AERONET) datasets over Kanpur (urban) and Gandhi College (rural) in the Indo-Gangetic Plain (IGP) region is conducted on a seasonal scale, for the first time. Biomass burning (BB), urban-industrial, and mixed aerosol types are always present, whereas dust aerosol and mostly dust absorbing aerosol types are only present in pre-monsoon and monsoon seasons. During winter and post-monsoon seasons, BB aerosols and mostly black carbon (MBC) absorbing aerosols dominate, and the contribution of aerosol optical depth (AOD) and single scattering albedo (SSA) corresponding to MBC to total AOD and SSA are higher. SSA for MBC varies over a broader range due to mixing of BC with water-soluble aerosols. During pre-monsoon and monsoon seasons, mixing of dust with anthropogenic aerosols increases the amount of mixed aerosol type. Surface cooling and atmospheric heating efficiency for mixed aerosols are higher than MBC and dust aerosols due to enhancement in aerosol absorption over both locations. Seasonal analysis of aerosol radiative properties showed that during winter and post-monsoon, MBC absorbing aerosols are the major contributor in controlling/influencing the total aerosol radiative forcing (ARF) and heating rate (HR). During the other seasons, each absorbing aerosol type significantly influences ARF depending on their AOD and SSA values. In addition to Kanpur and Gandhi College, data from seven other AERONET sites located at Karachi, Lahore, Jaipur, Lumbini, Pokhara, Bhola, and Dhaka in South Asia are analysed to conduct a regional-scale examination of aerosol optical parameters and radiative effects due to different absorbing aerosol types. As the aerosol characteristics and trends are similar over these sites, the findings from such a regional-scale analysis can be an appropriate representative for the South Asian region. The regional analysis revealed that the annual mean atmospheric ARF (ARF_ATM) and ARF efficiency (ARFE_ATM), and HR are higher for MBC, followed by mixed and MD aerosols over South Asia due to higher AOD, and higher absorbing efficiency of MBC aerosols. In comparison, mixed aerosols exhibit higher ARF_ATM over East Asia. This quantification of absorbing aerosol types over a global aerosol hotspot will be useful for an accurate quantification of climate impacts of aerosols.

Characterization of columnar aerosol over a background site in Central Asia

Ground-based observational characterization of atmosphere aerosols over Central Asia is very limited. This study investigated the columnar aerosol characteristics over Issyk-Kul, Kyrgyzstan, a background site in Central Asia using the long-term (∼14 years: August 2007-November 2021) data acquired with the Cimel sunphotometer. The mean aerosol optical depth (AOD) and Ångström exponent (AE) during the observation period were 0.14 ± 0.10 and 1.19 ± 0.41, respectively. Both AOD and AE varied across seasons, with highest AOD in spring (0.17 ± 0.17). Regarding the aerosol types, clean continental aerosols were dominant type (65%), followed by mixed aerosols (∼19%), clean marine aerosols (∼14%), dust (0.8%), and urban/industrial and biomass burning aerosol (0.7%). The aerosol volume size distribution was bimodal indicating the influence of both anthropogenic and natural aerosols with clear dominance of coarse mode during the spring season. Mainly dust and mixed aerosols were present during high aerosol episodes while the coarse mode aerosol volume concentration was 7.5 (strong episodes) and ∼19 (extreme episodes) times higher than the whole period average. Aerosol over this background sites were from local and regional sources with some contribution of long-range transport.

Investigation of June 2020 giant Saharan dust storm using remote sensing observations and model reanalysis

This paper investigates the characteristics and impact of a major Saharan dust storm during June 14th–19th 2020 on atmospheric radiative and thermodynamics properties over the Atlantic Ocean. The event witnessed the highest ever aerosol optical depth for June since 2002. The satellites and high-resolution model reanalysis products well captured the origin and spread of the dust storm. The Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) measured total attenuated backscatter and aerosol subtype profiles, lower angstrom exponent values (~ 0.12) from Modern-Era Retrospective Analysis for Research and Application—version 2 (MERRA-2) and higher aerosol index value from Ozone monitoring instrument (> 4) tracked the presence of elevated dust. It was found that the dust AOD was as much as 250–300% higher than their climatology resulting in an atmospheric radiative forcing ~ 200% larger. As a result, elevated warming (8–16%) was observed, followed by a drop in relative humidity (2–4%) in the atmospheric column, as evidenced by both in-situ and satellite measurements. Quantifications such as these for extreme dust events provide significant insights that may help in understanding their climate effects, including improvements to dust simulations using chemistry-climate models.

Identification of key aerosol types and mixing states in the central Indian Himalayas during the GVAX campaign: the role of particle size in aerosol classification

Studies in aerosol properties, types and sources in the Himalayas are important for atmospheric and climatic issues due to high aerosol loading in the neighboring plains. This study uses in situ measurements of aerosol optical and microphysical properties obtained during the Ganges Valley Aerosol eXperiment (GVAX) at Nainital, India over the period June 2011-March 2012, aiming to identify key aerosol types and mixing states for two particle sizes (PM₁ and PM₁₀). Using a classification matrix based on SAE vs. AAE thresholds (scattering vs. absorption Ångström exponents, respectively), seven aerosol types are identified, which are highly dependent on particle size. An aerosol type named "large/BC mix" dominates in both PM₁ (45.4%) and PM₁₀ (46.9%) mass, characterized by aged BC mixed with other aerosols, indicating a wide range of particle sizes and mixing states. Small particles with low spectral dependence of the absorption (AAE < 1) account for 31.6% and BC-dominated aerosols for 14.8% in PM₁, while in PM₁₀, a large fraction (39%) corresponds to "large/low-absorbing" aerosols and only 3.9% is characterized as "BC-dominated". The remaining types consist of mixtures of dust and local emissions from biofuel burning and display very small fractions. The main optical properties e.g. spectral scattering, absorption, single scattering albedo, activation ratio, as well as seasonality and dependence on wind speed and direction of identified types are examined, revealing a large influence of air masses originating from the Indo-Gangetic Plains. This indicates that aerosols over the central Himalayas are mostly composed by mixtures of processed and transported polluted plumes from the plains. This is the first study that identifies key aerosol populations in the central Indian Himalayas based on in situ measurements and the results are highly important for aerosol-type inventories, chemical transport models and reducing the uncertainty in aerosol radiative forcing over the third pole.

Spatio-temporal assessment of ambient air quality, their health effects and improvement during COVID-19 lockdown in one of the most polluted cities of India

The present work aims to investigate seasonal variations in air pollution levels in Lucknow and assess the ambient air quality of the city together with highlighting the health impacts of major pollutants like PM₁₀, PM_2.5, SO₂, NO₂, Pb, Ni and aerosols from 2010 to 2019. The maximum and minimum values of PM₁₀, PM_2.5, SO₂, NO₂, Pb and Ni were found to be 270.75 and 122.45 μg/m³, 124.95 and 95.52 μg/m³, 25.60 and 8.05 μg/m³, 75.65 and 23.85 μg/m³, 0.66 and 0.03 μg/m³ and 0.07 and 0.01 ng/m³, respectively. Health impact of particulate matter has also been assessed with AirQ+, and it was estimated that long-term exposure of PM₁₀ was attributed to between 37 and 48% for post-neonatal (age 1-12 months) mortality rate due to all causes, whereas long-term attributable proportions in mortality due to exposure of PM_2.5 were to about 19 to 28% from all causes. Further, an attempt has also been made to evaluate the impact of lockdown amid COVID-19 on the ambient air quality of Lucknow. During the lockdown, PM_2.5 levels reduced by 65% (at Gomti Nagar), 23% (at central school), 79% (at Lalbagh) and 35% (at Talkatora), due to which, air quality index of Gomti Nagar came down to 43, well below 50 which falls in the healthy range. NO₂ levels also came down. However, levels of SO₂ did not show significant reduction. Correlating the data between aerosol optical depth and Angstrom exponent by Pearson correlation analysis revealed a significant positive correlation (r = 0.65, P < 0.001).

Light absorption enhancement of particulate matters and their source apportionment over the Asian continental outflow site and South Yellow Sea

Light absorption enhancement of black carbon due to the aerosol mixing states is an important parameterization for climate modeling, while emission source contributions to the enhancement factor are unclear. An intensive campaign was conducted simultaneously at a China coastal site (Qingdao city) and maritime sites (South Yellow Sea, SYS) in August and Nov to Dec 2018. The absorption enhancement (E_MAC) of the black carbon was calculated using a two-step solvent dissolution protocol and found 1.96 ± 0.68, 1.64 ± 0.38, and 2.40 ± 0.76 for Qingdao summer (QS), Qingdao autumn (QA), and SYS, respectively. Positive matrix factorization (PMF) model identified six sources of PM_2.5 and E_MAC, which were secondary aerosol (with contribution 27.9% and 29.2%), coal combustion (24.9% and 20.2%), industrial emissions (15.2% and 25.4%), sea salt (6.9% and 9.6%), vehicle emissions (12.1% and 10.9%), and soil dust (13.0% and 4.7%), respectively. These sources increased the absorption of black carbon by a factor of 1.25 ± 0.11 (secondary aerosol), 1.21 ± 0.20 (industrial emissions), 1.17 ± 0.08 (coal combustion), 1.09 ± 0.07 (vehicle emissions), 1.08 ± 0.17 (sea salt), and 1.04 ± 0.10 (soil dust). Based on the correlation between PM and E_MAC source contributions, we estimated that secondary aerosols, industrial emissions, and coal combustion contributed to 74.8% of absorption enhancement at a regional scale in China. The source apportionment for E_MAC offers a new diagnosis for each source regarding aerosol forcing simulation which inputs from the individual emission sector.

Environmental Association of Burning Agricultural Biomass in the Indus River Basin

Intensification of smog episodes, following harvesting of paddy crops in agricultural plains of the Indus basin in the Indian subcontinent, are often attributed to farming practice of burning standing stubble during late autumn (October, November) months. Biomass burning (paddy stubble residual) is a preferred technique to clear farmlands for centuries by farmers in that basin. However, despite stable agricultural landholding and yield, smog is being increasingly associated with burning agricultural biomass, thus creating a paradox. Here, we show that the concentration of smog (NOx, PM2.5, SO2) in the ambient air exceeds the safe threshold limits throughout the entire year in the region. This study argues that agricultural biomass burning is an ephemeral event in the basin that may act as a catalyst to a deteriorated air quality in the entire region. Results further demonstrate that simultaneous saturation of air pollutants along with high ambient moisture content and low wind speeds following the monsoon season are strongly related to aggravated smog events. Findings from this study should help make holistic mitigation and intervention policies to monitor air quality for sustainability of public health in agricultural regions where farming activities are a dominant economic driver for society.

Columnar aerosol properties and radiative effects over Dushanbe, Tajikistan in Central Asia

This paper presents the results of the study on columnar aerosol optical and physical properties and radiative effects directly observed over Dushanbe, the capital city of Tajikistan, a NASA AERONET site (equipped with a CIMEL sunphotometer) in Central Asia. The average aerosol optical depth (AOD) and Ångström exponent (AE) during the observation period from July 2010 to April 2018 were found to be 0.28 ± 0.20 and 0.82 ± 0.40, respectively. The highest seasonal AOD (0.32 ± 0.24), accompanied by the lowest average AE (0.61 ± 0.25) and fine-mode fraction in AOD (0.39), was observed during summer due to the influence of coarse particles like dust from arid regions. Fine particles were found in significant amounts during winter. The 'mixed aerosol' was identified as the dominant aerosol type with presence of 'dust aerosol' during summer and autumn seasons. Aerosol properties like volume size distribution, single scattering albedo, asymmetry parameter and refractive index suggested the influence of coarse particles (during summer and autumn). Most of the air masses reaching this site transported local and regional emissions, including from beyond Central Asia, explaining the presence of various aerosol types in Dushanbe's atmosphere. The seasonal aerosol radiative forcing efficiency (ARFE) in the atmosphere was found high (>100 Wm^-2) and consistent throughout the year. Consequently, this resulted in similar seasonally coherent high atmospheric solar heating rate (HR) of 1.5 K day^-1 during summer-autumn-winter, and ca. 0.9 K day^-1 during spring season. High ARFE and HR values indicate that atmospheric aerosols could exert significant implications to regional air quality, climate and cryosphere over the central Asian region and downwind Tianshan and Himalaya-Tibetan Plateau mountain regions with sensitive ecosystems.

Long-term (2008-2018) aerosol properties and radiative effect at high-altitude sites over western trans-Himalayas

Analysis of the climatology of aerosol properties is performed over Hanle (4500 m) and Merak (4310 m), two remote-background sites in the western trans-Himalayas, based on eleven years (2008-2018) of sun/sky radiometer (POM-01, Prede) measurements. The two sites present very similar atmospheric conditions and aerosol properties allowing us to examine them as continuous single-data series. The annual average aerosol optical depth at 500 nm (AOD₅₀₀) is 0.04 ± 0.03, associated with an Ångström exponent (AE_440-870) of 0.58 ± 0.35 and a single scattering albedo (SSA₅₀₀) of 0.95 ± 0.05. AOD₅₀₀ exhibits higher values in May (~0.07) and lower in winter (~0.03), while AE_400-870 minimizes in spring, indicating influence by coarse-mode dust aerosols, either emitted regionally or long-range transported. The de-convolution of AOD₅₀₀ into fine and coarse modes justifies the aerosol seasonality and sources, while the marginal diurnal variation in all aerosol properties reveals a weak influence from local sources, except for some few aerosol episodes. The aerosol-volume size distribution presents a mode value at ~10 μm with secondary peaks at accumulation (~ 2 μm) and fine modes (~0.03 μm) and low variability between the seasons. A classification of the aerosol types based on the fine-mode fraction (FMF) vs. SSA₅₀₀ relationship reveals the dominance of aerosols in the FMF range of 0.4-0.6, characterized as mixed (39%), followed by fine aerosols with high scattering efficiency (26%), while particles related to dust contribute ~21%, with low fractions of fine-absorbing aerosols (~13%). The aerosol radiative forcing (ARF) estimates reveal a small cooling effect at the top of the atmosphere (-1.3 Wm^-2), while at the surface, the ARF ranges from -2 Wm^-2 to -6 Wm^-2 on monthly basis. The monthly-mean atmospheric radiative forcing (~1 to 4 Wm^-2) leads to heating rates of 0.04 to 0.13 K day^-1. These ARF values are higher than the global averages and may cause climate implications over the trans-Himalayan region.

Spatial variability of sedimentary carbon in South Yellow Sea, China: impact of anthropogenic emission and long-range transportation

During the last few decades, sedimentary carbons gain great concerns of research interest among the scientific committee worldwide due to their adverse impact on aquatic chemistry, ecology, and hence human health along with global climate change. In the present study, we investigated the spatial distribution of mass concentration of sedimentary carbon (viz. black carbon: BC, and its components, char and soot) along with their burial fluxes in the surface sediments of the South Yellow Sea (SYS). The concentration of sedimentary carbon is measured by using an emerging method of thermal/optical reflectance. The observed BC concentration is found in the range of 0.02-1.02 mg g^-1 with a mean value of 0.49 ± 0.26 mg g^-1. The mean burial fluxes of BC, char, and soot also have a similar spatial variation to their concentration with the mean value along with relative standard deviation (in bracket) 22.43 ± 12.49 (~ 56%), 5.90 ± 3.99 (~ 68%), and 16.53 ± 10.67 (65%), respectively. Relatively lower value of char/soot ratio, i.e., 0.48 ± 0.22, indicates the dominance of soot in surface sediments that could be mainly derived from the fossil fuel combustion which is further confirmed from emission inventory data suggesting maximum contribution, i.e., ~ 66-80%, of the total BC emission emitted from residential and industrial emission sources. The back trajectories analysis revealed a significant impact of long-range transportation on BC concentration in the surface sediments of SYS. Further study of BC concentrations in sea sediments and their interaction with other organic/inorganic compounds in continental shelves is highly needed for a better understanding of the global carbon cycle.

Spatiotemporal Trends of Aerosols over Urban Regions in Pakistan and Their Possible Links to Meteorological Parameters

Aerosol optical depth (AOD) has become one of the most crucial parameters for climate change assessment on regional and global scales. The present study investigates trends in AOD using long-term data derived from moderate resolution imaging spectro-radiometer (MODIS) over twelve regions in Pakistan. Different statistical tests are used to assess the annual and seasonal trends in AOD. Results reveal increasing AOD trends over most of the selected regions with an obvious increase over the north and northeastern parts of the study area. Annually, increasing trends (0.0002–0.0047 year−1) were observed over seven regions, with three being statistically significant. All the selected regions experience increasing AOD trends during the winter season with six being statistically significant while during the summer season seven regions experience increasing AOD trends and the remaining five exhibit the converse with two being statistically significant. The changes in the sign and magnitude of AOD trends have been attributed to prevailing meteorological conditions. The decreasing rainfall and increasing temperature trends mostly support the increasing AOD trend over the selected regions. The high/low AOD phases during the study period may be ascribed to the anomalies in mid-tropospheric relative humidity and wind fields. The summer season is generally characterized by high AOD with peak values observed over the regions located in central plains, which can be attributed to the dense population and enhanced concentration of industrial and vehicular emissions over this part of the study area. The results derived from the present study give an insight into aerosol trends and could form the basis for aerosol-induced climate change assessment over the study area.

Aerosol Parameters During Winter and Summer Seasons and Meteorological Implications

The MICROTOPS II aerosol optical depth (AOD) and Moderate Resolution Imaging Spectroradiometer (MODIS) Aqua AOD and angstrom exponent (AE) were collected during December 2016 to May 2017. Higher AOD values were recorded during winter (December - February) and summer (March - May) Months. These values were observed by MICROTOPS II (0.35 - 1.279) and MODIS (0.222 - 1.904) during winter season. During summer AOD values were recorded by MICROTOPS II (0.272 – 1.744) and MODIS (0.227 – 1.33). Whereas MODIS AE (0.218 – 1.799) values were found high during winter season indicates about the dominance of fine particulates. During summer months MODIS AE (.001 – 1.648) values are indicating about the mixing of the aerosol particles. The relative humidity values during December and January months were found to be high, while its values were found decreasing during late February due to transition phase. Relative humidity values were recorded low during summer (36 – 86%) months in compare to the winter (50 - 100%) months.

The climatology of aerosol optical thickness and radiative effects in Southeast Asia from 18-years of ground-based observations

The present study utilizes 18 years of long-term (2001-2018) data collected from six active AERONET sites over the Indo-Gangetic Plain (IGP) and the North China Plain (NCP) areas in Southeast Asia. The annual mean (±SD) aerosol optical thickness at 440 nm (AOT₄₄₀) was found high at XiangHe (0.92 ± 0.69) and Taihu (0.90 ± 0.51) followed by Beijing (0.81 ± 0.69), Lahore (0.81 ± 0.43), and Kanpur (0.73 ± 0.35) and low at Karachi (0.52 ± 0.23). Seasonally, high AOT₄₄₀ with corresponding high Ångström exponent (ANG_440-870) noticed during JJA for all sites, except Kanpur, suggesting the dominance of fine-mode particles, generally associated with large anthropogenic emissions. Climatologically, an increasing (decreasing) trend was observed over IGP (NCP) sites, with the highest (lowest) percentage of departures in AOT₄₄₀ found over Beijing (Karachi). We further identified major aerosol types which showed the dominance of biomass burning, urban-industrial followed by the mixed type of aerosols. In addition, single scattering albedo (SSA), asymmetry parameter (ASP), volume size distribution (VSD), and complex aerosol refractive index (RI) showed significant temporal and spectral changes, illustrating the complexity of aerosol types. At last, the annual mean direct aerosol radiative forcing at the top, bottom, and within the atmosphere for all sites were found in the range from -17.36 ± 3.75 to -45.17 ± 4.87 W m^-2, -64.6 ± 4.86 to -93.7 ± 10.27 W m^-2, and 40.5 ± 6.43 to 68.25 ± 7.26 W m^-2, respectively, with an averaged atmospheric heating rate of 0.9-2.3 K day^-1. A large amount of anthropogenic aerosols showed a significant effect of heating (cooling) on the atmosphere (surface) results obviously, due to an increased rate of atmospheric heating. Therefore, the thermodynamic effects of anthropogenic aerosols on the atmospheric circulation and its structure should be taken into consideration for future study over the experimental sites.

Integrated assessment of health risk and climate effects of black carbon in the Pearl River Delta region, China

BACKGROUND

Black carbon (BC) caused by incomplete combustion of fossil and bio-fuel has a dual effect on health and climate. There is a need for systematic approaches to evaluation of health outcomes and climate impacts relevant to BC exposure.

OBJECTIVES

We propose and illustrate for the first time, to our knowledge, an integrated analysis of a region-specific health model with climate change valuation module to quantify the health and climate consequences of BC exposure.

METHODS

Based on the data from regional air pollution monitoring stations from 2013 to 2014 in the Pearl River Delta region (PRD), China, we analyzed the carcinogenic and non-carcinogenic effects and the relative risk of cause-specific mortality due to BC exposure in three typical cities of the PRD (i.e. Guangzhou, Jiangmen and Huizhou). The radiative forcing (RF) and heating rate (HR) were calculated by the Fu-Liou-Gu (FLG) plane-parallel radiation model and the conversion of empirical formula. We further connected the health and climate impacts by calculating the excess mortalities attributed to climate warming due to BC.

RESULTS

Between 2013 and 2014, carcinogenic risks of adults and children due to BC exposure in the PRD were higher than the recommended limits (1 × 10^-6 to 1 × 10^-4), resulting in an excess of 4.82 cancer cases per 10,000 adults (4.82 × 10^-4) and an excess of 1.97 cancer cases per 10,000 children (1.97 × 10^-4). Non-carcinogenic risk caused by BC was not found. The relative risks of BC exposure on mortality were higher in winter and dry season. The atmospheric RFs of BC were 26.31 W m^-2, 26.41 W m^-2, and 22.45 W m^-2 for Guangzhou, Jiangmen and Huizhou, leading to a warming of the atmosphere in the PRD. The estimated annual excess mortalities of climate warming due to BC were 5052 (95% CI: 1983, 8139), 5121 (95% CI: 2010, 8249) and 4363 (95% CI: 1712, 7032) for Guangzhou, Jiangmen and Huizhou, respectively.

CONCLUSION

Our estimates suggest that current levels of BC exposure in the PRD region posed a considerable risk to human health and the climate. Reduction of BC emission could lead to substantial health and climate co-benefits.

The Concentrations and Removal Effects of PM10 and PM2.5 on a Wetland in Beijing

Particulate matter (PM) is an essential source of atmospheric pollution in metropolitan areas since it has adverse effects on human health. However, previous research suggested wetlands can remove particulate matter from the atmosphere to land surfaces. This study was conducted in the Hanshiqiao Wetland National Nature Reserve in Beijing during 2016. The concentrations of PM10 and PM2.5 on a wetland and bare land in the park, as well as metrological data, were collected during the whole year. Based on the observed data, removal efficiency of each land use type was calculated by empirical models and the relationships between concentrations and metrological factors were also analyzed. The results indicated that: (1) In general, the PM10 and PM2.5 concentrations on the bare land surface were higher than those on the wetland surface, in both of which the highest value appeared at night and evening, while the lowest value appeared near noon. In terms of season, the average concentration of PM10 was higher in winter (wetland: 137.48 μg·m−3; bare land: 164.75 μg·m−3) and spring (wetland: 205.18 μg·m−3; bare land: 244.85 μg·m−3) in general. The concentration of PM2.5 on the wetland surface showed the same pattern, while that on the bare land surface was higher in spring and summer. (2) Concentrations of PM10 and PM2.5 were significantly correlated with the relative humidity (p < 0.01) and inversely correlated with wind speed (p < 0.05). The relationship between PM10 and PM2.5 concentrations and temperature was more complicated—it showed a significantly negative correlation (p < 0.01) between them in winter and spring, however, the correlation was insignificant in autumn. In summer, only the correlation between PM10 concentration and temperature on the wetland surface was significant (p < 0.01). (3) The dry removal efficiency of PM10 was greater than that of PM2.5. The dry removal efficiencies of PM10 and PM2.5 followed the order of spring > winter > autumn > summer on the wetland. This study seeks to provide practical measures to improve air quality and facilitate sustainable development in Beijing.

Aerosol columnar characteristics and their heterogeneous nature over Varanasi, in the central Ganges valley

The Indo-Gangetic Basin (IGB) experiences one of the highest aerosol loading over the globe with pronounced inter-/intra-seasonal variability. Four-year (January 2011-December 2014) continuous MICROTOPS-II sun-photometer measurements at Varanasi, central Ganges valley, provide an opportunity to investigate the aerosol physical and optical properties and their variability. A large variation in aerosol optical depth (AOD: from 0.23 to 1.89, mean of 0.82 ± 0.31) and Ångström exponent (AE: from 0.19 to 1.44, mean of 0.96 ± 0.27) is observed, indicating a highly turbid atmospheric environment with significant heterogeneity in aerosol sources, types and optical properties. The highest seasonal means of both AOD and AE are observed in the post-monsoon (October-November) season (0.95 ± 0.31 for AOD and 1.16 ± 0.14 for AE) followed by winter (December, January, February; 0.97 ± 0.34 for AOD and 1.09 ± 0.20 for AE) and are mainly attributed to the accumulation of aerosols from urban and biomass/crop residue burning emissions within a shallow boundary layer. In contrast, during the pre-monsoon and monsoon seasons, the aerosols are mostly coming from natural origin (desert and mineral dust) mixed with pollution in several cases. The spectral dependence of AE, the aerosol "curvature" effect and other graphical techniques are used for the identification of the aerosol types and their mixing processes in the atmosphere. Furthermore, the aerosol source-apportionment assessment using the weighted potential source contribution function (WPSCF) analysis reveals the different aerosol types, emission sources and transport pathways.

Observation of optical properties and sources of aerosols at Buddha's birthplace, Lumbini, Nepal: environmental implications

For the first time, aerosol optical properties are measured over Lumbini, Nepal, with CIMEL sunphotometer of the Aerosol Robotic Network (AERONET) program. Lumbini is a sacred place as the birthplace of Lord Buddha, and thus a UNESCO world heritage site, located near the northern edge of the central Indo-Gangetic Plains (IGP) and before the Himalayan foothills (and Himalayas) to its north. Average aerosol optical depth (AOD) is found to be 0.64 ± 0.38 (0.06-3.28) over the sampling period (January 2013-December 2014), with the highest seasonal AOD during the post-monsoon season (0.72 ± 0.44). More than 80% of the daily averaged AOD values, during the monitoring period, are above 0.3, indicating polluted conditions in the region. The levels of aerosol load observed over Lumbini are comparable to those observed at several heavily polluted sites in the IGP. Based on the relationship between AOD and Ångstrom exponent (α), anthropogenic, biomass burning, and mixed aerosols are found to be the most prevalent aerosol types. The aerosol volume-size distribution is bi-modal during all four seasons with modes centered at 0.1-0.3 and 3-4 μm. For both fine and coarse modes, the highest volumetric concentration of ~ 0.08 μm^-3 μm^-2 is observed during the post-monsoon and pre-monsoon seasons. As revealed by the single-scattering albedo (SSA), asymmetry parameter (AP), and refractive index (RI) analyses, aerosol loading over Lumbini is dominated by absorbing, urban-industrial, and biomass burning aerosols.

Temporal variability in aerosol characteristics and its radiative properties over Patiala, northwestern part of India: Impact of agricultural biomass burning emissions

A comprehensive measurements of aerosol optical depth (AOD), particulate matter (PM) and black carbon (BC) mass concentrations have been carried out over Patiala, a semi-urban site in northwest India during October 2008 to September 2010. The measured aerosol data was incorporated in an aerosol optical model to estimate various aerosol optical parameters, which were subsequently used for radiative forcing estimation. The measured AOD at 500 nm (AOD₅₀₀) shows a significant seasonal variability, with maximum value of 0.81 during post-monsoon (PoM) and minimum of 0.56 during winter season. The Ångström exponent (α) has higher values (i.e. more fine-mode fraction) during the PoM/winter periods, and lower (i.e. more coarse-mode fraction) during pre-monsoon (PrM). In contrast, turbidity coefficient (β) exhibits an opposite trend to α during the study period. BC mass concentration varies from 2.8 to 13.9 μg m^-3 (mean: 6.5 ± 3.2 μg m^-3) during the entire study period, with higher concentrations during PoM/winter and lower during PrM/monsoon seasons. The average single scattering albedo (SSA at 500 nm) values are 0.70, 0.72, 0.82 and 0.75 during PoM, winter, PrM and monsoon seasons, respectively. However, inter-seasonal and inter-annual variability in measured aerosol parameters are statistically insignificant at Patiala. These results suggest strong changes in emission sources, aerosol composition, meteorological parameters as well as transport of aerosols over the station. Higher values of AOD, α and BC, along with lower SSA during PoM season are attributed to agriculture biomass burning emissions over and around the station. The estimated aerosol radiative forcing within the atmosphere is positive (i.e. warming) during all the seasons with higher values (∼60 Wm^-2) during PoM-08/PoM-09 and lower (∼40 Wm^-2) during winter-09/PrM-10. The present study highlights the role of BC aerosols from agricultural biomass burning emissions during post-monsoon season for atmospheric warming at Patiala.

Aerosol characterization and radiative properties over Kavaratti, a remote island in southern Arabian Sea from the period of observations

Long-term measurements of spectral aerosol optical depth (AOD) using sun/sky radiometer for a period of five years (2009-2014) from the remote island location at Kavaratti (KVT; 10.56°N, 72.64°E) in the southern Arabian sea have been analysed. Climatologically, AODs decrease from October to reach maximum of ~0.6 (at 500nm) in March, followed by a sudden fall towards May. Significant modulations of intra-seasonal timescales over this general pattern are noticed due to the changes in the relative strength of distinctively different sources. The corresponding changes in aerosol inversion parameters reveal the presence of coarse-mode aerosols during spring and fine-mode absorbing aerosols in autumn and winter months. An overall dominance of a mixed type of aerosols (~41%) with maximum in winter (~53%) was found via the AOD₅₀₀ vs. Ångström exponent (α_440-870) relationship, while biomass-burning aerosols or thick urban/industrial plumes contribute to ~19%. Spectral dependence of Ångström exponent and aerosol absorbing properties were used to identify the aerosol types and its modification processes. Based on air mass back trajectory analysis, we revealed that the advection of aerosols from Indian subcontinent and western regions plays a major role in modifying the optical properties of aerosols over the observational site. The shortwave aerosol direct radiative forcing estimated via SBDART model ranges from -11.00Wm^-2 to -7.38Wm^-2, -21.51Wm^-2 to -14.33Wm^-2 and 3.17Wm^-2 and 10.0Wm^-2 at top of atmosphere, surface and within the atmosphere, respectively. This atmospheric forcing translates into heating rate of 0.62-1.04Kday^-1. Furthermore, the vertical profiles of aerosols and heating rate exhibit significant increase in lower (during winter and autumn) and mid troposphere (during spring). This may cause serious climate implications over Kavaratti with further consequences on cloud microphysics and monsoon rainfall.

Declining pre-monsoon dust loading over South Asia: Signature of a changing regional climate

Desert dust over the Indian region during pre-monsoon season is known to strengthen monsoon circulation, by modulating rainfall through the elevated heat pump (EHP) mechanism. In this context, an insight into long term trends of dust loading over this region is of significant importance in understanding monsoon variability. In this study, using long term (2000 to 2015) aerosol measurements from multiple satellites, ground stations and model based reanalysis, we show that dust loading in the atmosphere has decreased by 10 to 20% during the pre-monsoon season with respect to start of this century. Our analysis reveals that this decrease is a result of increasing pre-monsoon rainfall that in turn increases (decreases) wet scavenging (dust emissions) and slowing circulation pattern over the Northwestern part of the sub-continent.

Column-integrated aerosol optical properties and direct radiative forcing over the urban-industrial megacity Nanjing in the Yangtze River Delta, China

Aerosol optical properties were measured and analyzed through the ground-based remote sensing Aerosol Robotic Network (AERONET) over an urban-industrial site, Nanjing (32.21° N, 118.72° E, and 62 m above sea level), in the Yangtze River Delta, China, during September 2007-August 2008. The annual averaged values of aerosol optical depth (AOD500) and the Ångström exponent (AE440-870) were measured to be 0.94 ± 0.52 and 1.10 ± 0.21, respectively. The seasonal averaged values of AOD500 (AE440-870) were noticed to be high in summer (autumn) and low in autumn (spring). The characterization of aerosol types showed the dominance of mixed type followed by the biomass burning and urban-industrial type of aerosol at Nanjing. Subsequently, the curvature (a 2) obtained from the second-order polynomial fit and the second derivative of AE (α') were also analyzed to understand the dominant aerosol type. The single scattering albedo at 440 nm (SSA440) varied from 0.88 to 0.93 with relatively lower (higher) values during the summer (spring), suggesting an increase in black carbon and mineral dust (desert dust) aerosols of absorbing (scattering) nature. The averaged monthly and seasonal evolutions of shortwave (0.3-4.0 μm) direct aerosol radiative forcing (DARF) values were computed from the Santa Barbara DISORT Atmospheric Radiative Transfer (SBDART) model both at the top of atmosphere (TOA) and bottom of atmosphere (SUR) during the study period. Further, the aerosol forcing efficiency (AFE) and the corresponding atmospheric heating rates (AHR) were also estimated from the forcing within the atmosphere (ATM). The derived DARF values, therefore, produced a warming effect within the atmosphere due to strong absorption of solar radiation.