Thick haze plagued northeastern China in January 2013, strongly affecting both regional climate and human respiratory health. Here, we present dual carbon isotope constrained (Δ14C and δ13C) source ...apportionment for combustion-derived black carbon aerosol (BC) for three key hotspot regions (megacities): North China Plain (NCP, Beijing), the Yangtze River Delta (YRD, Shanghai), and the Pearl River Delta (PRD, Guangzhou) for January 2013. BC, here quantified as elemental carbon (EC), is one of the most health-detrimental components of PM2.5 and a strong climate warming agent. The results show that these severe haze events were equally affected (∼30%) by biomass combustion in all three regions, whereas the sources of the dominant fossil fuel component was dramatically different between north and south. In the NCP region, coal combustion accounted for 66% (46–74%, 95% C.I.) of the EC, whereas, in the YRD and PRD regions, liquid fossil fuel combustion (e.g., traffic) stood for 46% (18–66%) and 58% (38–68%), respectively. Taken together, these findings suggest the need for a regionally-specific description of BC sources in climate models and regionally-tailored mitigation to combat severe air pollution events in East Asia.
There is a wealth of studies of polychlorinated biphenyls (PCB) in surface water and biota of the Arctic Ocean. Still, there are no observation-based assessments of PCB distribution and inventories ...in and between the major Arctic Ocean compartments. Here, the first water column distribution of PCBs in the central Arctic Ocean basins (Nansen, Amundsen, and Makarov) is presented, demonstrating nutrient-like vertical profiles with 5–10 times higher concentrations in the intermediate and deep water masses than in surface waters. The consistent vertical profiles in all three Arctic Ocean basins likely reflect buildup of PCBs transported from the shelf seas and from dissolution and/or mineralization of settling particles. Combined with measurement data on PCBs in other Arctic Ocean compartments collected over the past decade, the total Arctic Ocean inventory of ∑7PCB was estimated to 182 ± 40 t (±1 standard error of the mean), with sediments (144 ± 40 t), intermediate (5 ± 1 t) and deep water masses (30 ± 2 t) storing 98% of the PCBs in the Arctic Ocean. Further, we used hydrographic and carbon cycle parametrizations to assess the main pathways of PCBs into and out of the Arctic Ocean during the 20th century. River discharge appeared to be the major pathway for PCBs into the Arctic Ocean with 115 ± 11 t, followed by ocean currents (52 ± 17 t) and net atmospheric deposition (30 ± 28 t). Ocean currents provided the only important pathway out of the Arctic Ocean, with an estimated cumulative flux of 22 ± 10 t. The observation-based inventory of ∑7PCB of 182 ± 40 t is consistent with the contemporary inventory based on cumulative fluxes for ∑7PCB of 173 ± 36 t. Information on the concentration and distribution of PCBs in the deeper compartments of the Arctic Ocean improves our understanding of the large-scale fate of POPs in the Arctic and may also provide a means to test and improve models used to assess the fate of organic pollutants in the Arctic.
Combustion-derived black carbon (BC) aerosols accelerate glacier melting in the Himalayas and in Tibet (the Third Pole (TP)), thereby limiting the sustainable freshwater supplies for billions of ...people. However, the sources of BC reaching the TP remain uncertain, hindering both process understanding and efficient mitigation. Here we present the source-diagnostic Δ(14)C/δ(13)C compositions of BC isolated from aerosol and snowpit samples in the TP. For the Himalayas, we found equal contributions from fossil fuel (46±11%) and biomass (54±11%) combustion, consistent with BC source fingerprints from the Indo-Gangetic Plain, whereas BC in the remote northern TP predominantly derives from fossil fuel combustion (66±16%), consistent with Chinese sources. The fossil fuel contributions to BC in the snowpits of the inner TP are lower (30±10%), implying contributions from internal Tibetan sources (for example, yak dung combustion). Constraints on BC sources facilitate improved modelling of climatic patterns, hydrological effects and provide guidance for effective mitigation actions.
The burial of terrestrial organic carbon (terrOC) in marine sediments contributes to the regulation of atmospheric CO
on geological timescales and may mitigate positive feedback to present-day ...climate warming. However, the fate of terrOC in marine settings is debated, with uncertainties regarding its degradation during transport. Here, we employ compound-specific radiocarbon analyses of terrestrial biomarkers to determine cross-shelf transport times. For the World's largest marginal sea, the East Siberian Arctic shelf, transport requires 3600 ± 300 years for the 600 km from the Lena River to the Laptev Sea shelf edge. TerrOC was reduced by ~85% during transit resulting in a degradation rate constant of 2.4 ± 0.6 kyr
. Hence, terrOC degradation during cross-shelf transport constitutes a carbon source to the atmosphere over millennial time. For the contemporary carbon cycle on the other hand, slow terrOC degradation brings considerable attenuation of the decadal-centennial permafrost carbon-climate feedback caused by global warming.
Remobilization to the atmosphere of only a small fraction of the methane held in East Siberian Arctic Shelf (ESAS) sediments could trigger abrupt climate warming, yet it is believed that sub-sea ...permafrost acts as a lid to keep this shallow methane reservoir in place. Here, we show that more than 5000 at-sea observations of dissolved methane demonstrates that greater than 80% of ESAS bottom waters and greater than 50% of surface waters are supersaturated with methane regarding to the atmosphere. The current atmospheric venting flux, which is composed of a diffusive component and a gradual ebullition component, is on par with previous estimates of methane venting from the entire World Ocean. Leakage of methane through shallow ESAS waters needs to be considered in interactions between the biogeosphere and a warming Arctic climate.
Water‐soluble organic carbon (WSOC) is a major constituent (~ 20–80%) of the total organic carbon aerosol over the Indian subcontinent during the dry winter season. Due to its multiple primary and ...secondary formation pathways, the sources of WSOC are poorly characterized. In this study, we present radiocarbon constraints on the biomass versus fossil sources of WSOC in PM2.5 for the 2010/2011 winter period for the megacity Delhi, situated in the northern part of the heavily polluted Indo‐Gangetic Plain. The fossil fuel contribution to Delhi WSOC (21 ± 4%) is similar to that recently found at two South Asian background sites. In contrast, the stable carbon isotopic composition of Delhi WSOC is less enriched in 13C relative to that at the two receptor sites. Although potentially influenced also by source variability, this indicates that near‐source WSOC is less affected by atmospheric aging. In addition, the light absorptive properties of Delhi WSOC were studied. The mass absorption cross section at 365 nm (MAC365) was 1.1–2.7 m2/g with an Absorption Ångström Exponent ranging between 3.1 and 9.3. Using a simplistic model the relative absorptive forcing of the WSOC compared to elemental carbon in 2010/2011 wintertime Delhi was estimated to range between 3 and 11%. Taken together, this near‐source study shows that WSOC in urban Delhi comes mainly (79%) from biomass burning/biogenic sources. Furthermore, it is less influenced by photochemical aging compared to WSOC at South Asian regional receptor sites and contributes with a relatively small direct absorptive forcing effect.
Key Points
The fossil source contribution to PM2.5 WSOC in New Delhi is 21 ± 4%
WSOC is less enriched in 13C in near‐source aerosols compared to aged aerosols
Delhi PM2.5 WSOC extracts demonstrate small light absorptive properties
Black carbon (BC) aerosols perturb climate and impoverish air quality/human healthaffecting ∼1.5 billion people in South Asia. However, the lack of source-diagnostic observations of BC is hindering ...the evaluation of uncertain bottom-up emission inventories (EIs) and thereby also models/policies. Here, we present dual-isotope-based (Δ14C/δ13C) fingerprinting of wintertime BC at two receptor sites of the continental outflow. Our results show a remarkable similarity in contributions of biomass and fossil combustion, both from the site capturing the highly populated highly polluted Indo-Gangetic Plain footprint (IGP; Δ14C-f biomass = 50 ± 3%) and the second site in the N. Indian Ocean representing a wider South Asian footprint (52 ± 6%). Yet, both sites reflect distinct δ13C-fingerprints, indicating a distinguishable contribution of C4-biomass burning from peninsular India (PI). Tailored-model-predicted season-averaged BC concentrations (700 ± 440 ng m–3) match observations (740 ± 250 ng m–3), however, unveiling a systematically increasing model-observation bias (+19% to −53%) through winter. Inclusion of BC from open burning alone does not reconcile predictions (f biomass = 44 ± 8%) with observations. Direct source-segregated comparison reveals regional offsets in anthropogenic emission fluxes in EIs, overestimated fossil-BC in the IGP, and underestimated biomass-BC in PI, which contributes to the model-observation bias. This ground-truthing pinpoints uncertainties in BC emission sources, which benefit both climate/air-quality modeling and mitigation policies in South Asia.
Emissions from biomass burning contribute significantly to water-soluble organic carbon (WSOC) and light-absorbing organic carbon (brown carbon). Ambient atmospheric samples were collected at an ...urban site in Beijing during winter and summer, along with source samples from residential crop straw burning. Carbonaceous aerosol species, including organic carbon (OC), elemental carbon (EC), WSOC and multiple saccharides as well as water-soluble potassium (K+) in PM2.5 (fine particulate matter with size less than 2.5 μm) were measured. Chemical signatures of atmospheric aerosols in Beijing during winter and summer days with significant biomass burning influence were identified. Meanwhile, light absorption by WSOC was measured and quantitatively compared to EC at ground level. The results from this study indicated that levoglucosan exhibited consistently high concentrations (209 ± 145 ng m−3) in winter. Ratios of levoglucosan/mannosan (L/M) and levoglucosan/galacosan (L/G) indicated that residential biofuel use is an important source of biomass burning aerosol in winter in Beijing. Light absorption coefficient per unit ambient WSOC mass calculated at 365 nm is approximately 1.54 ± 0.16 m2 g−1 in winter and 0.73 ± 0.15 m2 g−1 in summer. Biomass burning derived WSOC accounted for 23 ± 7% and 16 ± 7% of total WSOC mass, and contributed to 17 ± 4% and 19 ± 5% of total WSOC light absorption in winter and summer, respectively. It is noteworthy that, up to 30% of total WSOC light absorption was attributed to biomass burning in significant biomass-burning-impacted summer day. Near-surface light absorption (over the range 300–400 nm) by WSOC was about ∼40% of that by EC in winter and ∼25% in summer.
•High and relatively stable levoglucosan concentrations were found in Beijing in winter.•Light absorption from biomass burning to WSOC was evaluated in Beijing.•Light absorption (300–400 nm) by WSOC relative to EC was ∼40% (winter) and ∼25% (summer).•Residential biomass burning is non-negligible source to fine PM in Beijing winter.
The limited understanding of black carbon (BC) aerosol emissions from incomplete combustion causes a poorly constrained anthropogenic climate warming that globally may be second only to CO2 and ...regionally, such as over East Asia, the dominant driver of climate change. The relative contribution to atmospheric BC from fossil fuel versus biomass combustion is important to constrain as fossil BC is a stronger climate forcer. The source apportionment is the underpinning for targeted mitigation actions. However, technology-based “bottom-up” emission inventories are inconclusive, largely due to uncertain BC emission factors from small-scale/household combustion and open burning. We use “top-down” radiocarbon measurements of atmospheric BC from five sites including three city sites and two regional sites to determine that fossil fuel combustion produces 80 ± 6% of the BC emitted from China. This source-diagnostic radiocarbon signal in the ambient aerosol over East Asia establishes a much larger role for fossil fuel combustion than suggested by all 15 BC emission inventory models, including one with monthly resolution. Our results suggest that current climate modeling should refine both BC emission strength and consider the stronger radiative absorption associated with fossil-fuel-derived BC. To mitigate near-term climate effects and improve air quality in East Asia, activities such as residential coal combustion and city traffic should be targeted.
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