Atmospheric carbonaceous aerosols from Indo-Gangetic Plain and Central Himalaya: impact of anthropogenic sources

Contrasting compositions of PM2.5 in Northern Thailand during La Niña (2017) and El Niño (2019) years

There have been a very limited number of systematic studies on PM2.5 compositions and their source contribution in Southeast Asia. This study aims to explore the characteristics of PM2.5 composition collected in Chiang Mai (Thailand) during La Niña and El Niño years and to apportion their sources during smoke haze and non-haze periods. The average PM2.5 concentration of smoke haze episode in 2019 (El Niño) was much higher than in 2017 (La Niña). The ratios of organic carbon (OC) to elemental carbon (EC), as well as K (biomass burning (BB) tracer) to PM2.5, were higher during smoke haze episodes in 2019 than in 2017 indicating a significant influence from BB. The ratios of secondary organic carbon (SOC) levels to primary organic carbon (POC) levels during smoke haze episodes were higher than those in non-haze period, which indicated greater SOC contributions or more photo-oxidation of precursors in haze episodes with high ambient temperatures. However, the ratios of soil markers (Ca and Mg) during non-haze period were high implying that soil source contributed more to PM2.5 concentrations when there less BB occurred. The positive Matrix Factorization (PMF) model revealed that the source of BB, characterized by high K fractions, was the largest contributor during smoke haze episodes accounting for 50% (2017) and 79% (2019). Climate conditions influence meteorological patterns, particularly during incidences of extreme weather such as droughts, which affect the scale and frequency of open burning and thus air pollution levels.

Water-soluble organic aerosols over South Asia - Seasonal changes and source characteristics

Water-soluble organic carbon (WSOC) has been identified as a key component in atmospheric aerosols due to its ability to act as cloud condensation nuclei (CCN) owing to their highly hygroscopic nature. This paper discusses about the spatio-temporal variability in WSOC mass concentration, sources (primary and secondary contributions), the role of long-range air-mass transport in modulating their abundance, at distinct sectors over South Asia. We found from our observations that, photochemical ageing of primary organic aerosols that are derived from biomass emissions, significantly contribute to the total WSOC budget over South Asia. The wide range of water-soluble compounds released by biomass burning can contribute directly to the WSOC fraction or undergo further atmospheric processing, such as oxidation or ageing, leading to the formation of additional WSOC. WSOC/OC (organic carbon) ratio and the correlation between the WSOC and secondary organic carbon (SOC) are used for assessing the contribution from secondary sources. The three different ratios are used to delineate different source processes; OC/EC (elemental carbon) for source identification, WSOC/OC for long-range atmospheric transport (ageing) and WSOC/SOC to understand the primary and secondary contribution of WSOC. The present investigation revealed that, the primary OC that have undergone significant chemical processing as a result of long-range transport have a substantial influence on WSOC formation over South Asia, especially in Indo Gangetic Plain outflow regions such as southern peninsular and adjacent marine regions. Overall, oxidation and ageing of primary organic aerosols emitted from biomass burning was found to serve as an important source of WSOC over South Asia.

Characteristics and sources of carbonaceous aerosols in a semi-arid city: Quantifying anthropogenic and meteorological impacts

Carbonaceous aerosols have great adverse impacts on air quality, human health, and climate. However, there is a limited understanding of carbonaceous aerosols in semi-arid areas. The correlation between carbonaceous aerosols and control measures is still unclear owing to the insufficient information regarding meteorological contribution. To reveal the complex relationship between control measures and carbonaceous aerosols, offline and online observations of carbonaceous aerosols were conducted from October 8, 2019 to October 7, 2020 in Hohhot, a semi-arid city. The characteristics and sources of carbonaceous aerosols and impacts of anthropogenic emissions and meteorological conditions were studied. The annual mean concentrations (± standard deviation) of fine particulate matter (PM_2.5), organic carbon (OC), and elemental carbon (EC) were 42.81 (±40.13), 7.57 (±6.43), and 2.25 (±1.39) μg m^-3, respectively. The highest PM_2.5 and carbonaceous aerosol concentrations were observed in winter, whereas the lowest was observed in summer. The result indicated that coal combustion for heating had a critical role in air quality degradation in Hohhot. A boost regression tree model was applied to quantify the impacts of anthropogenic emissions and meteorological conditions on carbonaceous aerosols. The results suggested that the anthropogenic contributions of PM_2.5, OC, and EC during the COVID-19 lockdown period were 53.0, 15.0, and 2.36 μg m^-3, respectively, while the meteorological contributions were 5.38, 2.49, and -0.62 μg m^-3, respectively. Secondary formation caused by unfavorable meteorological conditions offset the emission reduction during the COVID-19 lockdown period. Coal combustion (46.4% for OC and 35.4% for EC) and vehicular emissions (32.0% for OC and 50.4% for EC) were the predominant contributors of carbonaceous aerosols. The result indicated that Hohhot must regulate coal use and vehicle emissions to reduce carbonaceous aerosol pollution. This study provides new insights and a comprehensive understanding of the complex relationships between control strategies, meteorological conditions, and air quality.

Characteristics of fine carbonaceous aerosols in Wuhai, a resource-based city in Northern China:Insights from energy efficiency and population density

As one of the predominant compositions of PM_2.5 (particulate matter with aerodynamic diameter ≤2.5 μm), carbonaceous aerosols not only have adverse effects on air quality, but also can affect climate change. Although there are extensive recent studies on carbonaceous aerosols, comprehensive studies on their socioeconomic influencing factors in a resource-based city are relatively limited. In this study, the spatial-temporal variations of organic carbon (OC), elemental carbon (EC), and secondary organic carbon (SOC) were investigated in January, July, and October in 2015 and April in 2016 in Wuhai and its surrounding areas. The population distribution and industry layout have led to the uneven spatial-temporal distribution of carbonaceous aerosols. The concentrations of carbonaceous aerosols were higher in winter due to the unfavorable meteorology and the increased emissions from heating. The SOC is a significant contributor to OC in the cold season (52.0% for January). Primary carbonaceous aerosols pollution is higher in the industrial sites of resource-based cities, whereas the SOC makes a significant contribution in the residential sites. The results of backward-trajectory and concentration-weighted trajectory analysis suggest that the local emissions and short-range atmospheric transport from nearby areas have a significant impact on PM_2.5 and carbonaceous aerosols. A strong correlation between population density and OC/EC ratio was found, indicating that the megacities with high population density have a higher SOC contribution than the resource-based cities. Resource-based cities are characterized by high level of primary OC emissions, whereas cities with high energy efficiency have a more significant SOC contribution. These results provide a more comprehensive understanding of carbonaceous aerosols in a resource-based city.

Chemical Composition and Source Apportionment of Total Suspended Particulate in the Central Himalayan Region

The present study analyzes data from total suspended particulate (TSP) samples collected during 3 years (2005–2008) at Nainital, central Himalayas, India and analyzed for carbonaceous aerosols (organic carbon (OC) and elemental carbon (EC)) and inorganic species, focusing on the assessment of primary and secondary organic carbon contributions (POC, SOC, respectively) and on source apportionment by positive matrix factorization (PMF). An average TSP concentration of 69.6 ± 51.8 µg m−3 was found, exhibiting a pre-monsoon (March–May) maximum (92.9 ± 48.5 µg m−3) due to dust transport and forest fires and a monsoon (June–August) minimum due to atmospheric washout, while carbonaceous aerosols and inorganic species expressed a similar seasonality. The mean OC/EC ratio (8.0 ± 3.3) and the good correlations between OC, EC, and nss-K+ suggested that biomass burning (BB) was one of the major contributing factors to aerosols in Nainital. Using the EC tracer method, along with several approaches for the determination of the (OC/EC)pri ratio, the estimated SOC component accounted for ~25% (19.3–29.7%). Furthermore, TSP source apportionment via PMF allowed for a better understanding of the aerosol sources in the Central Himalayan region. The key aerosol sources over Nainital were BB (27%), secondary sulfate (20%), secondary nitrate (9%), mineral dust (34%), and long-range transported mixed marine aerosol (10%). The potential source contribution function (PSCF) and concentration weighted trajectory (CWT) analyses were also used to identify the probable regional source areas of resolved aerosol sources. The main source regions for aerosols in Nainital were the plains in northwest India and Pakistan, polluted cities like Delhi, the Thar Desert, and the Arabian Sea area. The outcomes of the present study are expected to elucidate the atmospheric chemistry, emission source origins, and transport pathways of aerosols over the central Himalayan region.

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.

Severe air pollution and characteristics of light-absorbing particles in a typical rural area of the Indo-Gangetic Plain

Total suspended particles (TSP) were collected in Lumbini from April 2013 to March 2016 to better understand the characteristics of carbonaceous aerosol (CA) concentrations, compositions and sources and their light absorption properties in rural region of severe polluted Indo-Gangetic Plain (IGP). Extremely high TSP (203.9 ± 109.6 μg m^-3), organic carbon (OC 32.1 ± 21.7 μg m^-3), elemental carbon (EC 6.44 ± 3.17 μg m^-3) concentrations were observed in Lumbini particularly during winter and post-monsoon seasons, reflecting the combined influences of emission sources and weather conditions. SO₄^2- (7.34 ± 4.39 μg m^-3) and Ca²⁺ (5.46 ± 5.20 μg m^-3) were the most dominant anion and cation in TSP. These components were comparable to those observed in urban areas in South and East Asia but significantly higher than those in remote regions over the Himalayas and Tibetan Plateau, suggesting severe air pollution in the study region. Various combustion activities including industry, vehicle emission, and biomass burning are the main reasons for high pollutant concentrations. The variation of OC/EC ratio further suggested that biomass such as agro-residue burning contributed a lot for CA, particularly during the non-monsoon season. The average mass absorption cross-section of EC (MAC_EC) and water-soluble organic carbon (MAC_WSOC) were 7.58 ± 3.39 and 1.52 ± 0.41 m² g^-1, respectively, indicating that CA in Lumbini was mainly affected by local emissions. Increased biomass burning decreased MAC_EC; whereas, it could result in high MAC_WSOC during the non-monsoon season. Furthermore, dust is one important factor causing higher MAC_WSOC during the pre-monsoon season.

Carbonaceous Aerosols in Contrasting Atmospheric Environments in Greek Cities: Evaluation of the EC-tracer Methods for Secondary Organic Carbon Estimation

This study examines the carbonaceous-aerosol characteristics at three contrasting urban environments in Greece (Ioannina, Athens, and Heraklion), on the basis of 12 h sampling during winter (January to February 2013), aiming to explore the inter-site differences in atmospheric composition and carbonaceous-aerosol characteristics and sources. The winter-average organic carbon (OC) and elemental carbon (EC) concentrations in Ioannina were found to be 28.50 and 4.33 µg m−3, respectively, much higher than those in Heraklion (3.86 µg m−3 for OC and 2.29 µg m−3 for EC) and Athens (7.63 µg m−3 for OC and 2.44 µg m−3 for EC). The winter OC/EC ratio in Ioannina (6.53) was found to be almost three times that in Heraklion (2.03), indicating a larger impact of wood combustion, especially during the night, whereas in Heraklion, emissions from biomass burning were found to be less intense. Estimations of primary and secondary organic carbon (POC and SOC) using the EC-tracer method, and specifically its minimum R-squared (MRS) variant, revealed large differences between the sites, with a prevalence of POC (67–80%) in Ioannina and Athens and with a larger SOC fraction (53%) in Heraklion. SOC estimates were also obtained using the 5% and 25% percentiles of the OC/EC data to determine the (OC/EC)pri, leading to results contrasting to the MRS approach in Ioannina (70–74% for SOC). Although the MRS method provides generally more robust results, it may significantly underestimate SOC levels in environments highly burdened by biomass burning, as the fast-oxidized semi-volatile OC associated with combustion sources is classified in POC. Further analysis in Athens revealed that the difference in SOC estimates between the 5% percentile and MRS methods coincided with the semi-volatile oxygenated organic aerosol as quantified by aerosol mass spectrometry. Finally, the OC/Kbb+ ratio was used as tracer for decomposition of the POC into fossil-fuel and biomass-burning components, indicating the prevalence of biomass-burning POC, especially in Ioannina (77%).

Seasonal changes in carbonaceous aerosols over a tropical coastal location in response to meteorological processes

Near-surface atmospheric aerosols (PM₁₀) collected from a tropical coastal location in south-west peninsular Indian region for a duration of 6 years (2012-18) (N = 461) were analysed for carbonaceous aerosol components, the less studied aerosol species. Organic carbon (OC), its water soluble-insoluble (WSOC and WIOC) components, primary-secondary (POC and SOC) fractions and elemental carbon (EC) were examined for understanding the annual, seasonal, day-night variations in abundance pattern along with associated physical and meteorological processes. Total carbonaceous aerosols accounting for 36% of the collected aerosol mass with 31.5% organic matter (OM) and 4.5% EC respectively, exhibited consistent seasonal pattern throughout the study period with high concentration during winter followed by post-monsoon, pre-monsoon and monsoon. Delineation of marine and continental components of carbonaceous species based on their relative dominance during different air-mass periods, shows that while marine aerosols were a combination of natural sources comprising of volatile, semi-volatile species and secondary organics (from marine VOC precursors); the continental aerosols were composed of anthropogenic combustion sources (fossil fuel, biomass emissions etc). Based on the measurements of OC and EC during 2005-09 and 2012-18, their long term trends (for more than a decade) were investigated. Although OC showed an increasing tendency, EC exhibited a decrease with the total carbonaceous aerosols exhibiting a gradual decreasing trend over the years, indicating that they do not strictly reverberate the reported increasing trend observed over north-central parts of India. This can be presumed to be due to the reduced anthropogenic inputs over the location owing to the control measures and policies. The strong convective activity and large scale monsoon phenomena also helps in the effective dispersion of pollutants. Making use of comprehensive measurement of carbonaceous aerosols and the previous measurements of other aerosol components, an improved chemical composition model is presented.

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.

Carbonaceous aerosols and pollutants over Delhi urban environment: Temporal evolution, source apportionment and radiative forcing

Particulate matter (PM2.5) samples were collected over Delhi, India during January to December 2012 and analysed for carbonaceous aerosols and inorganic ions (SO4(2-) and NO3(-)) in order to examine variations in atmospheric chemistry, combustion sources and influence of long-range transport. The PM2.5 samples are measured (offline) via medium volume air samplers and analysed gravimetrically for carbonaceous (organic carbon, OC; elemental carbon, EC) aerosols and inorganic ions (SO4(2-) and NO3(-)). Furthermore, continuous (online) measurements of PM2.5 (via Beta-attenuation analyser), black carbon (BC) mass concentration (via Magee scientific Aethalometer) and carbon monoxide (via CO-analyser) are carried out. PM2.5 (online) range from 18.2 to 500.6μgm(-3) (annual mean of 124.6±87.9μgm(-3)) exhibiting higher night-time (129.4μgm(-3)) than daytime (103.8μgm(-3)) concentrations. The online concentrations are 38% and 28% lower than the offline during night and day, respectively. In general, larger night-time concentrations are found for the BC, OC, NO3(-)and SO4(2-), which are seasonally dependent with larger differences during late post-monsoon and winter. The high correlation (R(2)=0.74) between OC and EC along with the OC/EC of 7.09 (day time) and 4.55 (night-time), suggest significant influence of biomass-burning emissions (burning of wood and agricultural waste) as well as secondary organic aerosol formation during daytime. Concentrated weighted trajectory (CWT) analysis reveals that the potential sources for the carbonaceous aerosols and pollutants are local emissions within the urban environment and transported smoke from agricultural burning in northwest India during post-monsoon. BC radiative forcing estimates result in very high atmospheric heating rates (~1.8-2.0Kday(-1)) due to agricultural burning effects during the 2012 post-monsoon season.