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. 2025 Feb 25;59(9):4571–4577. doi: 10.1021/acs.est.4c10040

Longer Lifetime of BC from Fossil Fuel Combustion than from Biomass Burning: Δ14C Evidence

Chaoliu Li †,⊥, Zhaofu Hu †, Shichang Kang †,∥,*, Elena N Kirillova ‡,#, Fangping Yan †,‡, Pengfei Chen †, Guofeng Shen §, Thompson T Jake ⊥, Örjan Gustafsson ‡,*
PMCID: PMC11912310  PMID: 39999098

Abstract

graphic file with name es4c10040_0005.jpg

Black carbon (BC) significantly contributes to atmospheric warming and glacier melting. However, the atmospheric lifetime of BC from different fuel sources remains poorly constrained. By analyzing Δ14C of BC in PM2.5 and precipitation samples collected for three years at a remote site in the Tibetan Plateau, we found that BC from fossil fuel contribution (ffossil BC) in PM2.5 exhibited greater seasonal variation than those from South Asia and emission inventories. Precipitation-induced fractionation between fossil fuel combustion-derived BC (BCff) and biomass burning-derived BC (BCbb) resulted in an increase of ffossil BC to 68 ± 7% during the wet monsoon season, which is significantly higher than levels measured at a background site in South Asia and in simultaneously collected precipitation samples. Our findings provide direct evidence that the lifetime of BCff is longer than that of BCbb during the monsoon season. These results emphasize the increased climate forcing of BCff relative to BCbb at remote sites receiving long-range transported BC.

Keywords: Carbonaceous Aerosol, Fractionation, δ14C, Precipitation, Tibetan Plateau

Short abstract

Our study provides compelling evidence that BC sourced from fossil fuel burning has a longer lifetime than that emitted from biomass combustion, which is important for understanding behavior of BC in the atmosphere.

1. Introduction

Black carbon (BC) aerosols are considered one of the most significant atmospheric warming agents due to their strong climate forcing potential.1 When deposited on snow and ice, BC reduces surface albedo, which accelerates glacier melt by absorbing sunlight.2,3 This process affects the evolution of freshwater downstream from glaciers. However, identifying the sources of atmospheric BC, especially in remote regions, remains challenging due to limited in situ data, hindering efforts to mitigate BC emissions.

This challenge is particularly notable in the Himalayas and the Tibetan Plateau (TP)—one of the most remote regions on Earth, characterized by extreme terrain variations. Numerous studies have shown that BC is a critical driver of glacier retreat in the TP, particularly in the Himalayas, which are directly impacted by substantial BC emissions from heavily polluted South Asia.3,4 This makes investigating BC sources in the TP a major research focus.5−7 However, there is ongoing debate over the dominant sources of BC in the TP. Some studies suggest that BC from outside the TP is the primary contributor to both atmospheric and glacier-deposited BC in the region.8 Others argue that locally sourced BC from within the TP itself also plays an important role.5,7,9

Radiocarbon (Δ14C) has been shown to be an effective method for distinguishing BC from biomass combustion and fossil fuel combustion sources (ffossil BC),10−13 especially in remote regions such as the Arctic14,15 and the TP.5,16 Additionally, studying the behavior of BC during the wet deposition process is an important scientific question related to its lifetime in the atmosphere. Therefore, selecting a representative remote site in the TP with minimal local influence is essential for investigating the sources of BC and the potential factors affecting its deposition.

Nam Co station is considered one of the most typical and representative remote stations in the inner TP17 (Figure 1). It has the lowest Aerosol Optical Depth (AOD) in the TP,18 and its annual BC concentration is among the lowest across the entire TP.19,20 The climate at Nam Co station is characterized by distinct wet monsoon and dry nonmonsoon seasons. Therefore, the clean atmosphere, the reception of BC from long distances, and the clear differentiation between wet and dry seasons make the Nam Co station an ideal site for investigating BC sources.

Figure 1.

Figure 1

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Location of the study site and related sites in South Asia and the Himalayas. The background map shows predicted monthly mean BC emissions of South Asia and East Asia (https://gems.pku.edu.cn).

In addition to BC sources, this study offers a valuable opportunity to explore the fractionation of fossil fuel combustion-derived BC (BCff) and biomass burning sourced BC (BCbb) during wet deposition. BCff is generally considered more hydrophobic and has a smaller particle size than that of BCbb, making it more likely to be transported over long distances.21 So far, the distinct mixing states of BC from these two fuel types have not been explicitly incorporated into most climate models.22 A previous study at Nam Co station provided direct evidence of lower ffossil in water-insoluble particulate carbon (WIPC) in precipitation samples compared to aerosols.23 A similar phenomenon was also observed in the remote Arctic, where the ffossil BC in snow was lower than in aerosols, implying fractionation between BCff and BCbb during wet deposition.24 Therefore, a similar fractionation mechanism should occur at other sites around the world. Proving this will have significant implications for understanding the atmospheric lifetime and different climate forcing of BC from fossil fuel and biomass combustion sources. Thus, the remote Nam Co station provides an ideal site for investigating this mechanism. We observed greater seasonal variations in ffossil BC at Nam Co station compared to a background site in South Asia. Additionally, the ffossil BC value at Nam Co station during the monsoon season was higher than those of the background site of South Asia and estimations from emission inventories. We propose that this discrepancy is primarily due to fractionation between BCff and BCbb, mainly driven by wet deposition. As a result, precipitation increases the BCff/BCbb ratio in the atmosphere, compared to what is predicted from emission inventories. This phenomenon is particularly pronounced at remote site in the world.

2. Materials and Methods

2.1. Study Sites

Aerosol and precipitation samples were collected at the Nam Co station from December 2016 to July 2019 (Figure 1). Nam Co station is located in an open area of the inner TP (30°46′23.80″N, 90°57′48.88″E, 4747 m a.s.l.). It is considered one of the most typical and representative remote stations in the inner TP.17 Nam Co station experiences two distinct climate regimes: the nonmonsoon season from October to May, dominated by westerly winds, and the monsoon season from June to September, governed by the Indian monsoon system. The nonmonsoon season is typically dry, with occasional dust storms, while the monsoon season brings heavy precipitation from South Asia. Traditionally, local residents migrate from the south to the north bank of Nam Co at the end of September to avoid cold weather and heavy snow, making the Nam Co station a typical remote site during this time. In early April, residents return to the south bank to access better grazing for their yaks and sheep (Figure S1). Because local emissions contribute to the atmosphere at some sites on the TP,9 biomass combustion from local residents may influence BC at Nam Co station from April to September. However, the absence of residents on the north bank during this period eliminates local emissions there. Therefore, PM2.5 samples were collected simultaneously at both the Nam Co station and the north bank from April to September to assess potential local contributions by comparing Δ14C values of BC between these two sites. To investigate the different behaviors of BCbb and BCff transported from South Asia to the TP, ffossil BC data from total suspended particulates (TSP) samples collected at Jomsom25 and Sinhagad Observatory Station (SINH)26 were included for comparison with BC composition in the Indian source region (Figure 1). Since ffossil BC exhibits relatively consistent values across different sites in South Asia,27 ffossil BC of SINH is expected to be representative of that in the Indo-Gangetic Plain (IGP). The ffossil BC values from Jomsom and SINH were obtained using consistent protocols, ensuring comparability with the data from Nam Co. It is worth noting that ffossil BC values from TSP samples were generally slightly lower than those from the PM2.5 samples. Therefore, the ffossil BC values from Jomsom and SINH could be slightly underestimated if they were also based on PM2.5 samples.

2.2. PM2.5 and Precipitation Sample Collection

To minimize bias in BC measurements due to relatively high mineral dust content in TSP samples at Nam Co station,19 we focused on PM2.5 samples. A total of 30 PM2.5 samples were collected using 90 mm preburned (550 °C, 6 h) quartz fiber filters (Whatman Corp) with an aerosol cyclone equipped with a flow meter that record the volume of air passed through the filters at standard conditions (TH150-A, Wuhan Tianhong INST Group, China) from December 2016 to July 2019. Additionally, six PM2.5 samples were collected from June 2018 to September 2019 on the roof of a local resident’s home at the north bank (30°50′04.40″N, 91°06′49.41″E, 4764 m a.s.l.) (Figure S1; Table S1). Due to low BC concentration in the atmosphere, PM2.5 samples were collected over extended periods (around 2 weeks), similar to methods used in Arctic studies.14,28 Two field blank filters were also collected during each season by exposing the filters in each sampler without pumping. For comparison, precipitation samples were also collected at the Nam Co station. Because of the low BC concentrations in precipitation29 and the limited precipitation amount per event, multiple precipitation events over a continuous period were combined to obtain enough BC for Δ14C analysis (Table S2). Despite these limitations, the potential fractionation between BCff and BCbb can still be effectively assessed by treating the monsoon season as a single entity. A total of six filter samples containing BC from precipitation were collected during the monsoon season.

2.3. Analytical Methods

The BC mass concentrations in filtered precipitation and PM2.5 samples were pretreated and quantified using the methods described in our previous work.5 In brief, a 1 cm2 punch of each collected sample was exposed to 37% hydrochloric acid (HCl) for 24 h to remove inorganic carbon. Residual acid was then evaporated at 60 °C for 2 h. Elemental carbon (EC, chemically equivalent to BC) concentration was measured by using the NIOSH 5040 protocol. For 14C measurements, the required filter area was calculated based on BC concentration and subjected to the same protocol. The CO2 produced was cryotrapped for measurement. Δ14C of trapped CO2 was measured at the Tandem Laboratory in Uppsala University, Sweden.27 Due to the inert nature of BC, no significant isotopic fractionation is expected during its long-range transport during the dry, low-precipitation nonmonsoon season.30,31 Similar analysis methods have been widely used in previous studies for both aerosol10−12,31,32 and snow/ice core samples.5,33 The potential impact of organic carbon charring on the estimated Δ14C of BC was considered negligible.12

2.4. BC Concentration and Δ14C Calculation

BC concentration in the atmosphere was calculated as

2.4. 1

where BCC is BC concentration in the atmosphere; BCN is BC concentration measured by NIOSH protocol; A is the area of particle loaded on the filter (50.24 cm2); V is standard air volume passed through the filter for each sample.

Δ14C of BC was calculated as

2.4. 2

where Δ14CBC is the measured radiocarbon content of BC; Δ14Cfossil is the fossil fuel combustion end member (−1000‰), and Δ14Cbiomass is the biomass burning end member (+70‰).5

3. Results and Discussion

3.1. ffossil BC in PM2.5 and Precipitation Samples

Despite potential influences of local sources (e.g., human activities near Nam Co station), ffossil BC in PM2.5 samples at Nam Co station were similar to those measured at the north bank, suggesting a minimal impact from local emissions during the study period (Figure S2). Therefore, BC collected at Nam Co station represents a well-mixed sample from both local and remote sources. This suggests that the data obtained at Nam Co station are representative of the remote inner TP, consistent with findings from a previous study.17 A similar limited spatial variation in ffossil BC is also found in snowpit samples from different glaciers in the inner TP. For instance, ffossil BC in snowpit samples from Zhadang glacier and Xiaodongkemadi glacier, located approximately 330 km apart, were 31 ± 1% and 29 ± 10%, respectively5 (Figure 1), implying well mixed BC in the atmosphere and consistent ffossil values at different sites within the remote inner TP.

The average BC concentration at the study site during the study period was 72 ± 34 ng m–3, consistent with previous measurements by an aethalometer34 and filter samples at this site.19 BC concentrations were generally low during the monsoon season and high during the premonsoon season, similar to earlier findings at this site34 (Figure 2b). This may reflect the combined effects of BC source emissions and the deposition rates. For instance, the precipitation amount and BC emissions reached their highest and lowest values, respectively, during monsoon period around Nam Co station (Figure 1c, Table S3). Both factors contribute to the observed low BC concentrations during this period. The average ffossil BC in PM2.5 samples at Nam Co station was 49 ± 15%, consistent with our previously reported value at the same site (44 ± 6%),5 indicating stable BC fuel sources across different years. However, ffossil BC in PM2.5 samples at the Nam Co station increased to 68 ± 7% during the monsoon season, significantly higher than that in precipitation samples (35 ± 13%) (Figure 3). This difference highlights the preferential removal of BCbb during wet deposition, leaving a higher proportion of BCff in the atmosphere. A similar phenomenon has been observed at other sites worldwide. For instance, ffossil BC in precipitation samples from Dübendorf, Switzerland, was 52 ± 7%,35 significantly lower than the 83 ± 10% observed in aerosols in nearby Zürich.11 Similarly, the Δ14C value of BC in snow samples from spring (−323.2 ± 115.5‰) at the remote site of Alert, Canada was lower than that of aerosol (−581 ± 78.7 ‰) collected simultaneously.24 Therefore, the preferential wet removal of BCbb over BCff is a widespread phenomenon occurring in both remote and urban areas worldwide.

Figure 2.

Figure 2

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Seasonal variations of ffossil BC (a), BC concentration in PM2.5 samples (b), and precipitation amounts (c) during the study period at Nam Co Station.

Figure 3.

Figure 3

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Comparison of ffossil BC and ffossil WIPC23 between precipitation/snow samples and PM2.5 samples during the monsoon season at Nam Co station.

3.2. Seasonal Variations of ffossil BC in PM2.5 Samples and Potential Triggers

Comparisons of ffossil BC values among this study, Jomsom, SINH and emission inventories are presented in Figure 4. It should be highlighted that there are three potential sources of uncertainties when these comparisons: First, aerosol samples collected in this study, as well as those from Jomsom and SINH, reflect localized site characteristics, whereas emission inventories represent broader regional averages. Second, the sampling periods for these study sites differ from the temporal scope of the emission inventories. Third, differences in the residence times of BC from various sources can lead to discrepancies between source-region inventories and the measurements at receptor sites. Despite these uncertainties, the comparisons provide valuable insights into source contributions and the consistency of ffossil BC estimates across different approaches.

Figure 4.

Figure 4

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Seasonal variations of ffossil BC at Nam Co station (a), Jomsom (b),25 and SINH (c).26 Note: ffossil BC of the south part of the TP and South Asia was calculated from the PKU emission inventories (Table. S3).

Distinct seasonal variations in ffossil BC were observed in PM2.5 samples at the Nam Co station. During the monsoon season, ffossil BC in PM2.5 samples at Nam Co station were significantly higher than those recorded in SINH, Jomsom and Nam Co station itself during the nonmonsoon season (Figure 4). Meanwhile, ffossil BC at Nam Co station during the monsoon season was higher than estimates from emission inventories for both South Asia and the TP (Figure 4a). This suggests that fractionation between BCbb and BCff during wet deposition plays a crucial role in the observed seasonal trends, as further supported by the clear difference between ffossil BC in precipitation and PM2.5 samples (Figure 3). Consequently, it is proposed that more BCbb than BCff is preferentially removed from the atmosphere during precipitation events, resulting in a peak in ffossil BC at the Nam Co station during the monsoon season. BCff, being more hydrophobic and smaller in particle size than that of BCbb, is more likely to be transported over long distance.21 Meanwhile, BCbb tends to be surrounded by a higher proportion of other components, such as brown carbon, compared to BCff, making BCbb heavier than the BCff aerosol. This could also cause fractionation between these two types of BC through gravitational settling over long distances.36 In fact, a similar phenomenon was also observed at Jomsom and SINH. While local sources, such as vehicles and cooking emissions, may contribute to the aerosols collected at these two sites,25 increased precipitation events during monsoon season also play an important role in the elevated ffossil BC during this period (Figure 4). This assumption is also supported by the significant relationship between ffossil BC in PM2.5 and precipitation amount during the collection period (Figure S3), similar to that reported for ffossil of WIPC in PM2.5 samples collected at Nam Co station,23 indicating a consistent wet deposition mechanism for both WIPC and BC. Additionally, lower active fire spots during the monsoon season (Figure S4c) is also consistent with slightly lower BCbb (and commensurately higher ffossil BC) in this period.

IGP is generally considered the primary source region for BC aerosol in the TP.37 However, the seasonal variation of ffossil BC at SINH was different with that of Nam Co station (Figure 4). While the ffossil BC values at both sites were similar during the nonmonsoon season, the corresponding values at Nam Co station during the monsoon season were significantly higher than those at SINH (Figure 4).26 For instance, the ratio of ffossil BC between the monsoon season and nonmonsoon seasons at SINH is 1.22, lower than the ratio of 1.47 observed at Nam Co station. Jomsom showed an intermediate value of 1.42, indicating an increasing seasonal variation trend of ffossil BC from the background sites of South Asia to the remote Himalayas and the inner TP. This may be caused by the cumulative impact caused by multiple precipitation events that BC undergoes as it is transported from source regions to remote sites. This refers to the fact that BC emitted from IGP can be transported over long distances and undergo multiple precipitation deposition events before reaching the Nam Co station. In other words, BC collected at the Nam Co station is subjected to longer transport distances and more frequent precipitation events compared to BC collected at SINH, which is closer to the emission sources. This is further supported by comparing the seasonal variations in ffossil BC with those from the emission inventories38 (Figure 4). First, ffossil BC estimates in South Asia and the TP based on emission inventories showed little seasonal variation, which is consistent with the ffossil BC at three sites during the nonmonsoon season, as all regions are dominated by biomass burning emissions. Second, the difference between the ffossil BC from emission inventories and observed values was smaller during the nonmonsoon season than that during the monsoon season at all three sites. Because ffossil BC at the Nam Co station during the monsoon season was significantly higher than the estimates from both the TP and South Asia emission inventories (Figure 4a), the most likely explanation is the fractionation of BCff and BCbb due to increased precipitation events experienced during transportation. Therefore, BCff has a much longer atmospheric lifetime than that of BCbb in the wet season. Based on ffossil BC in precipitation and aerosols, it is estimated that the scavenging ratio of BCbb was around 4.47 times higher than that of BCff during the monsoon season at Nam Co station. Despite the fact that the scavenging ratio of BCbb and BCff likely varies spatially, the atmospheric lifetime of BCff should be longer than that of BCbb at both source and receptor regions during the wet season.

In conclusion, heavy precipitation is the primary factor driving high ffossil BC in PM2.5 at the Nam Co station during the monsoon season. Due to its longer transport potential, BCff is more likely to reach remote regions during the monsoon season with frequent precipitation. Additionally, ffossil BC in PM2.5 samples collected at Nam Co station in January and March were comparable to or even lower than levels in South Asia (Figure 4a), likely reflecting contributions from biomass combustion emissions within the TP itself (Figure S4). For example, seasonal variation of ffossil BC at the Nam Co station more closely resembles that of emissions from the TP than from South Asia (Figure 4a). Typically, the hydrophilicity of aged BC increases compared to freshly emitted aerosols as it becomes coated or internally mixed with non-BC components during atmospheric transport.39 This enhanced hydrophilicity likely contributes to the observed lower contribution of IGP-sourced BC during monsoon season compared to the nonmonsoon season at the study site. Despite this, the ffossil BC at Nam Co station and the other two sites (Figure 4) showed a significant influence from IGP emissions during the non–monsoon season, mainly due to sparse precipitation and prevailing winds from South Asia (Figure S4). Similar conclusions were also reached from studies on WIPC in PM2.5 samples at Nam Co station23 and the aerosol transportation model.40 Another study proposed that decreased precipitation greatly increased transport efficiency of BC from South Asia to the Himalayas,41 further supporting this assumption.

3.3. Implication

In this study, we comprehensively investigated ffossil BC in PM2.5 samples collected from a remote site in the inner TP. We found that ffossil BC in precipitation samples was significantly lower than that in PM2.5 samples, due to the fractionation between BCbb and BCff caused by precipitation. Specifically, during the rainy monsoon season, ffossil BC in PM2.5 samples reached as high as 80.35%, much higher than that observed and estimated from emission inventories in potential source regions (i.e., IGP). These results suggest that BC transported from South Asia to TP undergoes extensive wet deposition during the monsoon season, leading to a relative enrichment of ffossil BC in the atmosphere. In contrast, due to sparse precipitation during the dry nonmonsoon season, BC emitted from South Asia can be effectively transported to the inner TP.42 This study provides strong geochemical evidence that cross-boundary transport of BC from heavily polluted South Asia to the remote inner TP occurs year-round, with much higher efficiency during the non–monsoon season. Normally, wet deposition accounts for a large fraction of the total deposition of BC.43 In this study, we found that the marked seasonal fluctuations of ffossil BC in PM2.5 samples at this remote site are mainly influenced by precipitation patterns rather than changes in source regions.

Emission inventories are crucial for studying climate forcing and the transport of BC in the atmosphere.30 However, this study shows that the seasonal variation of ffossil BC in the atmosphere at the Nam Co station differs more significantly from both emission inventories and observations at background sites in South Asia. Because BCbb is removed from the atmosphere more efficiently than BCff during transport from source regions to remote sites, the significance of BCbb in climate forcing in remote regions is likely lower than suggested by emission inventories. Consequently, BCbb and BCff should be treated separately in atmospheric transport models to improve the accuracy of BC fate modeling and their influence on climate forcing.

Acknowledgments

This study was supported by the Second Tibetan Plateau Scientific Expedition and Research Program (STEP) (2019QZKK0605), the Strategic Priority Research Program of the Chinese Academy of Sciences, the Pan-Third Pole Environment Study for a Green Silk Road (Pan-TPE) (XDA20040501), CAS “Light of West China” Program (E0900104), and the State Key Laboratory of Cryospheric Science (SKLCS-ZZ-2023). Further funding was received from the Swedish Research Council VR (Grant 2017-01601 to Ö.G.). This study is part of a framework across the Himalayas and Tibetan Plateau: Atmospheric Pollution and Cryospheric Change (APCC). C.L. shows his thanks to resources from Prof. Peter A. Raymond and Prof. Noah Planavsky.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.est.4c10040.

  • Data of aerosol samples of study site; data of precipitation samples of study site; data of BC from emission inventories; location of study site; ffossil BC between PM2.5 samples collected at study site and its North bank; relationship between ffossil BC in PM2.5 samples and precipitation amount; five-day backward air mass trajectories in different seasons at study site (PDF)

Author Contributions

S.K, Ö.G., and C.L. designed the experiment. C.L. and S.K. wrote the paper. F.Y.P. and Z.H. collected samples. C.L., E.K., and Z.H. did the experiment. G.S. provided emission inventories data. T.J. improved the English. The authors declare no conflict of interest.

The authors declare no competing financial interest.

Supplementary Material

es4c10040_si_001.pdf (648.4KB, pdf)

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