Source Quantification of South Asian Black Carbon Aerosols with Isotopes and Modeling

Roles of water-soluble aerosol coatings for the enhanced radiative absorption of black carbon over south asia and the northern indian ocean

Black Carbon (BC), formed by incomplete combustion, absorbs solar radiation and heats the atmosphere. We investigated the enhancement in optical absorption of BC due to coatings of water-soluble (WS) species in the polluted South Asian atmosphere. The BC Mass Absorption Cross-section (MAC; 678 nm) was estimated before and after removal of the WS components. Wintertime samples were collected from three South Asian receptor observatories intercepting large-footprint outflow: Bangladesh Climate Observatory Bhola (BCOB; integrating outflow of the Indo-Gangetic Plain), Maldives Climate Observatories at Hanimaadhoo (MCOH) and at Gan (MCOG), both reflecting outflow from the South Asian region. The ambient MAC observed at BCOB, MCOH and MCOG were 4.2 ± 1.4, 7.9 ± 1.9 and 7.1 ± 1.5 m2 g-1, respectively. The average enhancement of the BC MAC due to WS coatings (i.e., ws-EMAC) was identical at all three sites (1.6 ± 0.5) indicating that the anthropogenic aerosols had already evolved to a fully coated morphology at BCOB and/or that subsequent aging involved two compensating evolution processes of the coating. Inspecting the key coating component sulfate; the sulfate-to-BC ratio increased threefold when transitioning from BCOB to MCOH and by about 1.5 times from BCOB to MCOG. Conversely, both WS organic carbon (WSOC)/BC and water-insoluble OC (WIOC)/BC ratios declined with distance: WSOC/BC diminished by 84 % from BCOB to MCOH and by 80 % from BCOB to MCOG, while WIOC/BC dropped by about 63 % and 59 %, respectively. Such declines in WSOC and WIOC reflect a combination of photochemical oxidation and more efficient washout of OC compared to BC. The observed changes in the SO42-/BC and WSOC/BC ratios across South Asia highlight the significant impact of aerosol composition on the optical properties of Black Carbon (BC). These findings emphasize the need for detailed studies on aerosol composition to improve climate models and develop effective strategies for reducing the impact of anthropogenic aerosols on the climate.

Leakage of old carbon dioxide from a major river system in the Canadian Arctic

The Canadian Arctic is warming at an unprecedented rate. Warming-induced permafrost thaw can lead to mobilization of aged carbon from stores in soils and rocks. Tracking the carbon pools supplied to surrounding river networks provides insight on pathways and processes of greenhouse gas release. Here, we investigated the dual-carbon isotopic characteristics of the dissolved inorganic carbon (DIC) pool in the main stem and tributaries of the Mackenzie River system. The radiocarbon (14C) activity of DIC shows export of "old" carbon (2,380 ± 1,040 14C years BP on average) occurred during summer in sampling years. The stable isotope composition of river DIC implicates degassing of aged carbon as CO2 from riverine tributaries during transport to the delta; however, information on potential drivers and fluxes are still lacking. Accounting for stable isotope fractionation during CO2 loss, we show that a large proportion of this aged carbon (60 ± 10%) may have been sourced from biospheric organic carbon oxidation, with other inputs from carbonate weathering pathways and atmospheric exchange. The findings highlight hydrologically connected waters as viable pathways for mobilization of aged carbon pools from Arctic permafrost soils.

Atmospheric aerosol chemistry and source apportionment of PM10 using stable carbon isotopes and PMF modelling during fireworks over Hyderabad, southern India

This study examined the influence of fireworks on atmospheric aerosols over the Southern Indian city of Hyderabad during festival of Diwali using mass closure, stable carbon isotopes and the EPA-PMF model. Identification of chemical species in day and night time aerosol samples for 2019 and 2020 Diwali weeks showed increased concentrations of NH4+, NO3-, SO42-, K+, organic carbon (OC), Ba, Pb and Li, which were considered as tracers for fireworks. PM10 source apportionment was done using inorganic (trace elements, major ions) and carbonaceous (organic and elemental carbon; OC & EC) constituents, along with stable isotopic compositions of TC and EC. K+/Na+ ∼1 and K+nss/OC > 0.5 indicated contribution from fireworks. High NO3-, NH4+, Na+, Cl- and SO42- suggested the presence of deliquescent salts NaCl, NH4NO3 and (NH4)2SO4. TAE/TCE >1 suggested H+ exclusion, indicating possible presence of H2SO4 and NH4HSO4 in the aerosols. Ba, Pb, Sb, Sr and Fe increased by 305 (87), 12 (11), 12 (3), 3 (2) and 3 (4) times on Diwali nights, compared to pre-Diwali of 2019 (2020), and are considered as metallic tracers of fireworks. δ13CTC and δ13CEC in aerosols closely resembled that of diesel and C3 plant burning emissions, with meagre contribution from firecrackers during Diwali period. The δ13CEC was relatively depleted than δ13CTC and δ13COC. For both years, δ13COC-EC (δ13COC - δ13CEC) were positive, suggesting photochemical aging of aerosols during long-range transport, while for pre-Diwali 2019 and post-Diwali 2020, δ13COC-EC were negative with high OC/EC ratio, implying secondary organic aerosols formation. High toluene during Diwali week contributed to fresh SOA formation, which reacted with precursor 12C, leading to 13C depletions. Eight-factored EPA-PMF source apportionment indicated highest contribution from residue/waste burning, followed by marine/dust soil and fireworks, while least was contributed from solid fuel/coal combustion.

Dual carbon isotope-based brown carbon aerosol characteristics at a high-altitude site in the northeastern Himalayas: Role of biomass burning

PM2.5 samples (n = 34) were collected from January to April 2017 over Shillong (25.7°N, 91.9°E; 1064 m amsl), a high-altitude site situated in the northeastern Himalaya. The main aim was to understand the sources, characteristics, and optical properties of local vs long-range transported carbonaceous aerosols (CA) using chemical species and dual carbon isotopes (13C and 14C). Percentage biomass burning (BB)/biogenic fraction (fbio, calculated from 14C) varied from 67 to 92 % (78 ± 7) and correlated well with primary BB tracers like f60, and K+, suggesting BB as a considerable source. Rain events are shown to reduce the fbio fraction, indicating majority of BB-derived CA are transported. Further, δ13C (-26.6 ± 0.4) variability was very low over Shillong, suggesting it's limitations in source apportionment over the study region, if used alone. Average ratio of absorption coefficient of methanol-soluble BrC (BrCMS) to water-soluble BrC (BrCWS) at 365 nm was 1.8, indicating a significant part of BrC was water-insoluble. A good positive correlation between fbio and mass absorption efficiency of BrCWS and BrCMS at 365 nm with the higher slope for BrCMS suggests BB derived water-insoluble BrC was more absorbing. Relative radiative forcing (RRF, 300 to 2500 nm) of BrCWS and BrCMS with respect to EC were 11 ± 5 % and 23 ± 16 %, respectively. Further, the RRF of BrCMS was up to 60 %, and that of BrCWS was up to 22 % with respect to EC for the samples with fbio ≥ 0.85 (i.e., dominated by BB), reflecting the importance of BB in BrC RRF estimation.

Heterogenous distribution and burial flux of black carbon in Chinese lakes and its global implication

Black carbon (BC) plays a crucial role in global carbon cycle and climate change. However, its source and burial flux in environments are not well constrained. Here, we investigated surface sediments from 22 Chinese lakes across wide geographical areas and different socioeconomic status. The BC content accounts for 0.09-10.5 % of total organic carbon (TOC), and its average 14C age is older than that of TOC by 1640 years. The application of δ13C-based MixSIAR model shows that the contribution of fossil fuel combustion is highest in the most developed Eastern China (85.7 %) and lowest in the rural Qinghai-Tibetan Plateau (51.4 %), which is corroborated by the results from 14C-based two endmember mixing model. The BC data from this study and literatures suggest that the current BC burial flux is 126.4 ± 15.8 Gg year-1 in Chinese lakes, and approximately 2987 ± 1022 Gg year-1 in global lakes. Globally, lakes accumulate 1.2 %-6.4 % of the total BC production and thus are an important and heterogenous BC sink.

Provenance of Aerosol Black Carbon over Northeast Indian Ocean and South China Sea and Implications for Oceanic Black Carbon Cycling

Aerosol black carbon (BC) is a short-lived climate pollutant. The poorly constrained provenance of tropical marine aerosol BC hinders the mechanistic understanding of extreme climate events and oceanic carbon cycling. Here, we collected PM2.5 samples during research cruise NORC2016-10 through South China Sea (SCS) and Northeast Indian Ocean (NEIO) and measured the dual-carbon isotope compositions (δ13C-Δ14C) of BC using hydrogen pyrolysis technique. Aerosol BC exhibits six different δ13C-Δ14C isotopic spaces (i.e., isotope provinces). Liquid fossil fuel combustion, from shipping emissions and adjacent land, is the predominant source of BC over isotope provinces "SCS close to Chinese Mainland" (53.5%), "Malacca Strait" (53.4%), and "Open NEIO" (40.7%). C3 biomass burning is the major contributor to BC over isotope provinces "NEIO close to Southeast Asia" (55.8%), "Open NEIO" (41.3%), and "Open SCS" (40.0%). Coal combustion and C4 biomass burning show higher contributions to BC over "Sunda Strait" and "Open SCS" than the others. Overall, NEIO near the Bay of Bengal, Malacca Strait, and north SCS are three hot spots of fossil fuel-derived BC; the first two areas are also hot spots of biomass-derived BC. The comparable δ13C-Δ14C between BC in aerosol and dissolved BC in surface seawater may suggest atmospheric BC deposition as a potential source of oceanic dissolved BC.

Characterization of paddy-residue burning derived carbonaceous aerosols using dual carbon isotopes

A large scale paddy-residue burning (PRB) happens every year in the northwest Indo-Gangetic Plain (IGP) during the post-monsoon season, and winds transport pollutants from the source region up to the northern Indian Ocean affecting air quality of the IGP and marine region. In this study, day-night pairs of fine aerosol samples (n = 69) were collected during October-November over Patiala (30.2°N, 76.3°E, 250 m amsl), a site located in the source region of PRB. Carbonaceous aerosols (CA) were characterised using chemical species and dual carbon isotopes (¹³C and ¹⁴C) to estimate bio vs non-bio contributions and understand their characteristics. Percentage of bio fraction (f_bio, estimated using ¹⁴C) in CA varied from 74 % to 87 % (avg: 80 ± 3) during days and 71 % to 96 % (avg: 85 ± 7 %) during nights. Further, the f_bio was found to be better correlated with aerosol mass spectrometer derived f60 compare to levoglucosan (LG) or nssK⁺, suggesting f60 a useful proxy for PRB. The δ¹³C varied from -27.7 ‰ to -26.0 ‰ (avg: -27.0 ± 0.4 ‰) and - 28.7 ‰ to -26.4 ‰ (avg: -27.5 ± 0.7 ‰) during day and night, respectively. Measured δ¹³C of the samples was found to be more enriched than expected by 0.3 to 2.0 ‰, indicating the presence of aged CA also in Patiala even during PRB period. From f_bio versus δ¹³C correlation, and from Miller-Trans plot, δ¹³C of PRB is found to be -28.9 ± 1.1 ‰, which also infers that Miller-Trans plot can be used to understand source isotopic signature in the absence of radiocarbon measurements in aerosols. Further, the characteristics ratios of organic carbon (OC) to elemental carbon (EC) (11.9 ± 4.1), LG to potassium (K⁺) (0.84 ± 0.15), OC/LG (19.7 ± 2.0) and K⁺/EC (0.75 ± 0.27) were calculated by considering samples with f_bio higher than 0.90, which can be used for source apportionment studies. Such studies are crucial in assessing the effects of PRB on regional air quality and climate.

Black carbon aerosols over Indian Ocean have unique source fingerprint and optical characteristics during monsoon season

Effects of aerosols such as black carbon (BC) on climate and buildup of the monsoon over the Indian Ocean are insufficiently quantified. Uncertain contributions from various natural and anthropogenic sources impede our understanding. Here, we use observations over 5 y of BC and its isotopes at a remote island observatory in northern Indian Ocean to constrain loadings and sources during little-studied monsoon season. Carbon-14 data show a highly variable yet largely fossil (65 ± 15%) source mixture. Combining carbon-14 with carbon-13 reveals the impact of African savanna burning, which occasionally approach 50% (48 ± 9%) of the total BC loadings. The BC mass-absorption cross-section for this regime is 7.6 ± 2.6 m2/g, with higher values during savanna fire input. Taken together, the combustion sources, longevity, and optical properties of BC aerosols over summertime Indian Ocean are different than the more-studied winter aerosol, with implications for chemical transport and climate model simulations of the Indian monsoon.

Spatial distribution of fossil fuel derived CO2 over India using radiocarbon measurements in crop plants

Examining the contribution of fossil fuel CO₂ to the total CO₂ changes in the atmosphere is of primary concern due to its alarming levels of fossil fuel emissions over the globe, specifically developing countries. Atmospheric radiocarbon represents an important observational constraint and utilized to trace fossil fuel derived CO₂ (CO_2ff) in the atmosphere. For the first time, we have presented a detailed analysis on the spatial distribution of fossil fuel derived CO₂ (CO_2ff) over India using radiocarbon (Δ¹⁴C) measurements during three-year period. Analysis shows that the Δ¹⁴C values are varying between 29.33‰ to -34.06‰ across India in the year 2017, where highest value belongs to a location from Gujarat while lowest value belongs to a location from Chhattisgarh. Based on the Δ¹⁴C patterns, spatial distributions of CO_2ff mole fractions have been determined over India and the calculated values of CO_2ff mole fractions are varying between 4.85 ppm to 26.59 ppm across India. It is also noticed that the highest CO_2ff mole fraction is observed as 26.59 ppm from a site in Chhattisgarh. CO_2ff mole fraction values from four high altitude sites are found to be varied between 4.85 ppm to 14.87 ppm. Effect of sampling different crop plants from the same growing season and different crop plant organs (grains, leaves, stems) on the Δ¹⁴C and CO_2ff have been studied. Annual and intra seasonal variations in the Δ¹⁴C and CO_2ff mole fractions have also been analyzed from a rural location (Dholpur, Rajasthan).

Saccharides in atmospheric PM2.5 in tropical forest region of southwest China: Insights into impacts of biomass burning on organic carbon aerosols

Biomass burning (BB) in South and Southeast Asia has a strong impact on regional air quality, yet its impact on atmospheric PM_2.5 of tropical rainforest regions, a background region occupying a large area in South Asia, has rarely been investigated. In this work, we performed one-year PM_2.5 sampling during December 2018 to October 2019 at a tropical rainforest site in southwest China. PM_2.5 mass concentration, major chemical components, and ten saccharides were determined to study seasonal variations of PM_2.5 chemical composition, and further to understand possible impacts of BB to organic carbon (OC) aerosols at this background region. The concentration levels of PM_2.5, major PM_2.5 components, and total saccharides were significantly higher in dry season than in wet season. Besides, PM_2.5, OC, and total saccharides were highly correlated (R² > 0.64, p < 0.001) during the sampling period, suggesting they might share common sources. Source apportionment of saccharides revealed that BB was the main source in both seasons. Furthermore, the contributions of BB to OC (BB/OC) were estimated using levoglucosan as a molecular tracer while levoglucosan's chemical degradation was considered. It was found that over 80% of LG was degraded in both seasons, suggesting BB sources were largely from the transport of external air mass. The estimated BB/OC were over 50%, indicating BB was an important source of OC and likely of PM_2.5 in both seasons. The air-mass backward trajectory analysis and active fire spots data indicate intense BB emission sources were from South and Southeast Asia in dry season and the BB emissions in southern region of China could impact on the studied area in wet season.

Atmospheric Black Carbon Loadings and Sources over Eastern Sub-Saharan Africa Are Governed by the Regional Savanna Fires

Vast black carbon (BC) emissions from sub-Saharan Africa are perceived to warm the regional climate, impact rainfall patterns, and impair human respiratory health. However, the magnitudes of these perturbations are ill-constrained, largely due to limited ground-based observations and uncertainties in emissions from different sources. This paper reports multiyear concentrations of BC and other key PM_2.5 aerosol constituents from the Rwanda Climate Observatory, serving as a regional receptor site. We find a strong seasonal cycle for all investigated chemical species, where the maxima coincide with large-scale upwind savanna fires. BC concentrations show notable interannual variability, with no clear long-term trend. The Δ¹⁴C and δ¹³C signatures of BC unambiguously show highly elevated biomass burning contributions, up to 93 ± 3%, with a clear and strong savanna burning imprint. We further observe a near-equal contribution from C3 and C4 plants, irrespective of air mass source region or season. In addition, the study provides improved relative emission factors of key aerosol components, organic carbon (OC), K⁺, and NO₃^-, in savanna-fires-influenced background atmosphere. Altogether, we report quantitative source constraints on Eastern Africa BC emissions, with implications for parameterization of satellite fire and bottom-up emission inventories as well as regional climate and chemical transport modeling.

Stable carbon isotopes trace the effect of fossil fuels on fractions of particulate black carbon in a large urban lake in China

Black carbon (BC), the highly recalcitrant aromatic carbonaceous from the incomplete combustion of fossil fuel and biomass, is an important carbon sink in carbon cycle. Char and soot, the main components of BC, have significantly different origin and physicochemical characteristics (particle sizes and resultant transportability). The limited understanding of char and soot sources leads to poor insight into the effect of BC on carbon cycle. Sources of char and soot were investigated in this study using stable carbon isotopes to study the effect of BC on the organic carbon pool in a lake, thereby improving the knowledge of lacustrine carbon cycling. The concentration of BC in Taihu Lake ranged from 0.0 to 0.7 mg·L^-1and accounted for 10.9 ± 4.7% of the particulate organic carbon. The spatial-mean δ¹³C values of BC, char, and soot were -23.2 ± 2.0‰, -23.5 ± 2.2‰, and -22.9 ± 1.6‰, respectively. The BC in water was primarily derived from fossil fuels (66.0 ± 9.3%), with liquid fossil fuel accounting for 48.2 ± 13.2% of the BC. The contribution of liquid fossil fuel to soot (49.3%) was much higher than that to char (36.1%); correspondingly, the contributions of biomass and coal to soot (29.2% and 21.5%) were lower than those to char (38.1% and 25.8%). The contribution of liquid fossil fuel combustion to organic carbon (OC), char, and soot gradually increased from 31.9% to 49.3%. Biomass and coal combustion primarily contributed to char (38.1% and 25.8%) and OC (37.5% and 30.6%). The source apportionment of BC, char, and soot revealed the influence of anthropogenically driven BC, char, and soot on the lake and, by extension, to the global carbon cycle.

14C characteristics of organic carbon in the atmosphere and at glacier region of the Tibetan Plateau

As an important component of carbonaceous aerosols (CA), organic carbon (OC) exerts a strong, yet insufficiently constrained perturbation of the climate. In this study, we reported sources of OC based on its natural abundance radiocarbon (¹⁴C) fingerprinting in aerosols and water-insoluble organic carbon (WIOC) in snowpits across the Tibetan Plateau (TP) - one of the remote regions in the world and a freshwater reservoir for billions of people. Overall, the proportions from ¹⁴C-based non-fossil fuel contribution (f_non-fossil) for OC in aerosols was 74 ± 10%, while for WIOC in snowpits was 81 ± 10%, both of which were significantly higher than that of elemental carbon (EC). These indicated sources of OC (WIOC) and EC were different at remote TP. Spatially, high f_non-fossil of WIOC of snowpit samples appeared at the inner part of the TP, indicating the important contribution of local non-fossil sources. Therefore, local non-fossil sources rather than long-range transportation OC dominants its total amount of the TP. In addition, the contribution of local non-fossil sourced WIOC increased during the monsoon period because heavy precipitation removed a high ratio of long-range transportation WIOC. The results of this study showed that not only OC and EC but also their different fuel sources should be treated separately in models to investigate their sources and atmospheric transportation.

Characteristics, source apportionment and long-range transport of black carbon at a high-altitude urban centre in the Kashmir valley, North-western Himalaya

Six years of data (2012-2017) at an urban site-Srinagar in the Northwest Himalaya were used to investigate temporal variability, meteorological influences, source apportionment and potential source regions of BC. The daily BC concentration varies from 0.56 to 40.16 μg/m³ with an inter-annual variation of 4.20-7.04 μg/m³ and is higher than majority of the Himalayan urban locations. High mean annual BC concentration (6.06 μg/m³) is attributed to the high BC observations during winter (8.60 μg/m³) and autumn (8.31 μg/m³) with a major contribution from Nov (13.88 μg/m³) to Dec (13.4 μg/m³). A considerable inter-month and inter-seasonal BC variability was observed owing to the large changes in synoptic meteorology. Low BC concentrations were observed in spring and summer (3.14 μg/m³ and 3.21 μg/m³), corresponding to high minimum temperatures (6.6 °C and 15.7 °C), wind speed (2.4 and 1.6 m/s), ventilation coefficient (2262 and 2616 m²/s), precipitation (316.7 mm and 173.3 mm) and low relative humidity (68% and 62%). However, during late autumn and winter, frequent temperature inversions, shallow PBL (173-1042 m), stagnant and dry weather conditions cause BC to accumulate in the valley. Through the observation period, two predominant diurnal BC peaks were observed at ⁓9:00 h (7.75 μg/m³) and ⁓21:00 h (6.67 μg/m³). Morning peak concentration in autumn (11.28 μg/m³) is ⁓2-2.5 times greater than spring (4.32 μg/m³) and summer (5.23 μg/m³), owing to the emission source peaks and diurnal boundary layer height. Diurnal BC concentration during autumn and winter is 65% and 60% higher than spring and summer respectively. During autumn and winter, biomass burning contributes approximately 50% of the BC concentration compared to only 10% during the summer. Air masses transport considerable BC from the Middle East and northern portions of South Asia, especially the Indo-Gangetic Plains, to Srinagar, with serious consequences for climate, human health, and the environment.

Dual-carbon isotope constraints on source apportionment of black carbon in the megacity Guangzhou of the Pearl River Delta region, China for 2018 autumn season

Black carbon (BC) aerosol negatively affects air quality and contributes to climate warming globally. However, little is known about the relative contributions of different source control measures to BC reduction owing to the lack of powerful source-diagnostic tools. We combine the fingerprints of dual-carbon isotope using an optimized Bayesian Markov chain Monte Carlo (MCMC) scheme and for the first time to study the key sources of BC in megacity Guangzhou of the Pearl River Delta (PRD) region, China in 2018 autumn season. The MCMC model-derived source apportionment of BC shows that the dominant contributor is petroleum combustion (39%), followed by coal combustion (34%) and biomass burning (27%). It should be noted that the BC source pattern is highly sensitive to the variations of air masses transported with an enhanced contribution of fossil source from the eastern area, suggesting the important impact of regional atmospheric transportation on the BC source profile in the PRD region. Also, we further found that fossil fuel combustion BC contributed 84% to the total BC reduction during 2013-2018. The response of PM_2.5 concentration to the ¹⁴C-derived BC source apportionment is successfully fitted (r = 0.90) and the results predicted that it would take ∼6 years to reach the WHO PM_2.5 guideline value (10 μg m^-3) for the PRD region if the emission control measures keep same as they are at present. Taken together, our findings suggest that dual-carbon isotope is a powerful tool in constraining the source apportionment of BC for the evaluations of air pollution control and carbon emission measures.

Observational Evidence of Large Contribution from Primary Sources for Carbon Monoxide in the South Asian Outflow

South Asian air is among the most polluted in the world, causing premature death of millions and asserting a strong perturbation of the regional climate. A central component is carbon monoxide (CO), which is a key modulator of the oxidizing capacity of the atmosphere and a potent indirect greenhouse gas. While CO concentrations are declining elsewhere, South Asia exhibits an increasing trend for unresolved reasons. In this paper, we use dual-isotope (δ¹³C and δ¹⁸O) fingerprinting of CO intercepted in the South Asian outflow to constrain the relative contributions from primary and secondary CO sources. Results show that combustion-derived primary sources dominate the wintertime continental CO fingerprint (f_primary ∼ 79 ± 4%), significantly higher than the global estimate (f_primary ∼ 55 ± 5%). Satellite-based inventory estimates match isotope-constrained f_primary-CO, suggesting observational convergence in source characterization and a prospect for model-observation reconciliation. This "ground-truthing" emphasizes the pressing need to mitigate incomplete combustion activities for climate/air quality benefits in South Asia.

Wintertime Air Quality in Megacity Dhaka, Bangladesh Strongly Affected by Influx of Black Carbon Aerosols from Regional Biomass Burning

Clean air is a key parameter for a sustainable society, and currently, megacity Dhaka has among the worst air qualities in the world. This results from poorly constrained contributions of a variety of sources from both local emissions and regional influx from the highly polluted Indo-Gangetic Plain, impacting the respiratory health of the 21 million inhabitants in the Greater Dhaka region. An important component of the fine particulate matter (PM_2.5) is black carbon (BC) aerosols. In this study, we investigated the combustion sources of BC using a dual carbon isotope (δ¹³C and Δ¹⁴C) in Dhaka during the high-loading winter period of 2013/14 (regular and lockdown/hartal period) in order to guide mitigation policies. On average, BC (13 ± 6 μg m^-3) contributed about 9% of the PM_2.5 (145 ± 79 μg m^-3) loadings. The relative contribution from biomass combustion under regular conditions was 44 ± 1% (with the rest from fossil combustion), while during periods of politically motivated large-scale lockdown of business and traffic, the biomass burning contribution increased to 63 ± 1%. To reduce the severe health impact of BC and other aerosol pollution in Dhaka, mitigation should therefore target regional-scale biomass/agricultural burning in addition to local traffic.