Brown carbon from biomass burning imposes strong circum-Arctic warming

Abstract

•Brown carbon imposes strong Arctic warming•Warming effect of water-soluble brown carbon is ∼30% relative to black carbon•Biomass burning contributes ∼60% of the warming effect of brown carbon•Warming climate leads to increased wildfires that reinforce Arctic warming Rapid Arctic warming and associated glacier and sea ice melt have a great impact on the global environment, with implications for global temperature rise and weather patterns, shipping routes, local biodiversity, and methane release. Greenhouse gases and black carbon aerosols are well-known warming agents that accumulate in the Arctic atmosphere, but full warming agent picture remains incomplete, preventing accurate forecasts. The effects of brown carbon—an aerosol derived from biomass and fossil fuel burning—are particularly unclear. Through observations from a circum-Arctic cruise and numerical model simulations, we show that light-absorbing brown carbon, mainly from biomass burning, can impose a strong warming effect in the Arctic, especially in the summertime. If, as predicted, the frequency, intensity, and spread of wildfires continues to increase, this may reinforce circum-Arctic warming and further contribute to global warming, forming a positive feedback. In light of these results, the careful management of vegetation fires, especially in the mid- to high latitudes of the Northern Hemisphere, will prove important in mitigating the warming in the Arctic region. Rapid warming in the Arctic has a huge impact on the global environment. Atmospheric brown carbon (BrC) is one of the least understood and uncertain warming agents due to a scarcity of observations. Here, we performed direct observations of atmospheric BrC and quantified its light-absorbing properties during a 2-month circum-Arctic cruise in summer of 2017. Through observation-constrained modeling, we show that BrC, mainly originated from biomass burning in the mid- to high latitudes of the Northern Hemisphere (∼60%), can be a strong warming agent in the Arctic region, especially in the summer, with an average radiative forcing of ∼90 mW m−2 (∼30% relative to black carbon). As climate change is projected to increase the frequency, intensity, and spread of wildfires, we expect BrC to play an increasing role in Arctic warming in the future. Rapid warming in the Arctic has a huge impact on the global environment. Atmospheric brown carbon (BrC) is one of the least understood and uncertain warming agents due to a scarcity of observations. Here, we performed direct observations of atmospheric BrC and quantified its light-absorbing properties during a 2-month circum-Arctic cruise in summer of 2017. Through observation-constrained modeling, we show that BrC, mainly originated from biomass burning in the mid- to high latitudes of the Northern Hemisphere (∼60%), can be a strong warming agent in the Arctic region, especially in the summer, with an average radiative forcing of ∼90 mW m−2 (∼30% relative to black carbon). As climate change is projected to increase the frequency, intensity, and spread of wildfires, we expect BrC to play an increasing role in Arctic warming in the future. The Arctic is warming at an excessive rate of more than twice as fast as the rest of the globe,1Graversen R.G. Mauritsen T. Tjernström M. Källén E. Svensson G. Vertical structure of recent Arctic warming.Nature. 2008; 451: 53-56https://doi.org/10.1038/nature06502Crossref PubMed Scopus (400) Google Scholar,2Ballinger T.J. Overland J.E. Wang M. 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The light absorption of BrC was measured for the water extracts of aerosol samples (particulate diameter <10 μm [PM10]; n = 25) collected from late July to September 2017 during a circum-Arctic cruise (north of 60°N; Figure S1; Table S1). Source-specific, light-absorbing properties of BrC (i.e., the imaginary part of refractive index [Ri]) are derived and used as constraints in the Community Earth System Model coupled with IMPACT aerosol model (CESM/IMPACT) to evaluate the overall and source-specific impact of BrC on circum-Arctic warming. Molecular-level measurements of particle composition by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) are used to support the diagnosis of the major sources of the light absorption of BrC. Note that, in this study, we mainly focus on the warming effect of water-soluble BrC; however, water-insoluble BrC may also contribute significantly to light absorption and further increase the importance of BrC in the Arctic warming (see the discussion in the conclusions section). As shown in Figure 1, the light absorption coefficient of water-soluble BrC at 365 nm (babs-365) varies spatially from 0.02 to 0.26 Mm−1 in the circum-Arctic (Table S2). The average babs-365 of water-soluble BrC (0.10 ± 0.05 Mm−1) over the Arctic region is higher than the 0.04 M m−1 observed at Alert (82.5°N) from May to early June12Yue S. Bikkina S. Gao M. Barrie L.A. Kawamura K. Fu P. Sources and radiative absorption of water-soluble brown carbon in the high Arctic atmosphere.Geophys. Res. Lett. 2019; 46: 14881-14891https://doi.org/10.1029/2019GL085318Crossref Scopus (11) Google Scholar and lower than the 0.20 M m−1 observed at Utqiaġvik, Alaska (71.3°N) in August-September.35Barrett T.E. Sheesley R.J. 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The large spatial variation in source-specific mass absorption efficiency at 365 nm (MAE365) (Figure S3) reflects various contributing sources and mixtures of water-soluble BrC components in the Arctic. Here, water-soluble BrC is an operational term that refers to all the light-absorbing carbons in the water-soluble organic carbon (WSOC) (see experimental procedures). The sources of water-soluble BrC are resolved by apportioning WSOC via positive matrix factorization of typical tracers for fossil fuel combustion, biomass burning, secondary formation, primary biological particles, and marine aerosols (see experimental procedures).39Laskin A. Laskin J. Nizkorodov S.A. Chemistry of atmospheric brown carbon.Chem. Rev. 2015; 115: 4335-4382https://doi.org/10.1021/cr5006167Crossref PubMed Scopus (814) Google Scholar, 40Zhu C.-S. Zhang Z.-S. Tao J. Qu Y. Cao J.-J. Indication of primary biogenic contribution to BrC over a high altitude location in the southeastern Tibet.Atmos. 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To estimate the warming effect of BrC in the Arctic, we constrain the Ri of both water-soluble BrC and BC in the model based on our circum-Arctic measurements. The regression coefficients (i.e., MAE365) for each source (Table S7) obtained from the multivariate linear regression are within the variations of those measured for source samples and field measurements at typical global hotspots of BrC39Laskin A. Laskin J. Nizkorodov S.A. Chemistry of atmospheric brown carbon.Chem. Rev. 2015; 115: 4335-4382https://doi.org/10.1021/cr5006167Crossref PubMed Scopus (814) Google Scholar (Figure S7 and references therein). The wavelength-dependent imaginary Ris of the source-specific BrC are calculated from their MAEs and then used to constrain the model for assessing their climatic impact (experimental procedures). The wavelength dependencies of Ri for BrC from fossil fuel combustion, biomass burning, BSOA, and marine primary emission in the Arctic are determined by exponential fitting as shown in Figure S10. Note that we did not simulate bioaerosol in the model. While adding some uncertainty, its effect is not large, since the contribution from bioaerosol is very small, as shown above. The Ri of BC is constrained to be 0.62 according to our measurement of optical properties of BC for the same samples during the circum-Arctic cruise (experimental procedures). We then used the CESM/IMPACT model with the measurement-based absorption properties of BrC and BC to examine the radiative effect of water-soluble BrC in the whole Arctic (north of 60°N) (experimental procedures). The model reliably reproduces the spatial distribution of OC concentrations compared with our measurements on the circum-Arctic cruise (Figure S11A). The average modeled OC concentration (0.60 ± 0.35 μgC m−3) is only 5% smaller than that of the observed OC (0.63 ± 0.42 μgC m−3; Table S2). Moreover, the modeled OC concentration shows a similar monthly variation as the observations at the Arctic Station Alert (Figure S11B).12Yue S. Bikkina S. Gao M. Barrie L.A. Kawamura K. Fu P. Sources and radiative absorption of water-soluble brown carbon in the high Arctic atmosphere.Geophys. Res. Lett. 2019; 46: 14881-14891https://doi.org/10.1029/2019GL085318Crossref Scopus (11) Google Scholar The radiative absorption effect (RAE) of water-soluble BrC is calculated as the difference in the incoming radiation flux at the top of atmosphere between model runs that include and exclude the light absorption of water-soluble BrC, respectively (experimental procedures). The annual average RAE of water-soluble BrC in the Arctic is 41 ± 15 mW m−2, which varies from 16 to 128 mW m−2 (Figure 2A ). Spatially, the strongest effect occurs in Siberia (up to ∼100 mW m−2) while the weakest is over Greenland (Figure 2A). The warming effect of water-soluble BrC is 26% ± 5.7% (Figure 2B) relative to that of BC (160 ± 49 mW m−2; Figure S12). Compared with BC, the strongest relative effect of BrC occurs in Siberia (∼40%), while the weakest is in western Europe (<20%; Figure 2B). The RAE of water-soluble BrC in the Arctic has a remarkable seasonal variation, with the strongest warming effect in summer (June to August) due to the increase in OC concentrations as well as in solar radiation. The average RAE in the Arctic in July is up to 95 mW m−2, which is ∼50 times that in December (2.0 mW m−2) and dominates the annual warming (Figure 2C). In addition, the relative absorption effect from water-soluble BrC to that from BC increases to 31% in summer (Figure 2C). This is consistent with a case study at the Alert site from mid-May to early June (∼34%).12Yue S. Bikkina S. Gao M. Barrie L.A. 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These results demonstrate a strong impact of water-soluble BrC on circum-Arctic warming. Different sources of water-soluble BrC contribute differently to its radiative absorption. During the observation period, the simulated RAE of water-soluble BrC at our sample locations is attributed to ∼50% from fossil fuel combustion and ∼40% from biomass burning, and the rest ∼10% are from BSOA and marine primary emissions (Table S8), which is comparable to measurement results (Table S7). For the whole circum-Arctic, although in winter (December to February), fossil fuel combustion can be the dominant contributor (78%) to the RAE of water-soluble BrC, the total forcing is small compared with the warmer seasons. In summer, biomass burning contributes the most to the RAE of water-soluble BrC (up to ∼70%), due to the high frequency of biomass burning in the mid- to high latitudes of the Northern Hemisphere (Figure S14; Table S10). On an annual average, water-soluble BrC from biomass burning contributes the largest fraction (∼60%) to the total RAE in the circum-Arctic, while ∼30% is attributed to fossil fuel combustion (Figure 2D). This highlights the strong impact of biomass burning on the Arctic warming, as summer is more relevant to ice-sheet melting. The lowest Arctic sea ice coverage usually occurs in summer and early fall and is therefore more sensitive to warming.46Pistone K. Eisenman I. Ramanathan V. Observational determination of albedo decrease caused by vanishing Arctic sea ice.Proc. Nat. Acad. Sci. 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In contrast, carbohydrate-like, unsaturated hydrocarbons, aromatic structures, highly oxygenated, tannin-like, and other compounds only contribute a small fraction (<3% in total). This contribution profile is consistent among samples (Figure S16). The three dominant molecular classes can be emitted in large amounts by biomass burning.51Nozière B. Kalberer M. Claeys M. Allan J. D’Anna B. Decesari S. Finessi E. Glasius M. Grgić I. Hamilton J.F. et al.The molecular identification of organic compounds in the atmosphere: state of the art and challenges.Chem. Rev. 2015; 115: 3919-3983https://doi.org/10.1021/cr5003485Crossref PubMed Scopus (308) Google Scholar,52Tang J. Li J. Su T. Han Y. Mo Y. Jiang H. Cui M. Jiang B. Chen Y. 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This molecular-level analysis by FT-ICR MS is consistent with the source contribution from the modeling, supporting the importance of biomass burning in the circum-Arctic warming. Climate change in the Arctic is rapid and important to the entire world. This study highlights the large contribution of water-soluble BrC to Arctic warming using a model based on measurements of the optical parameters of water-soluble BrC in the circum-Arctic. We find that the radiative absorption effect of water-soluble BrC is strong in the Arctic, with an annual average of 26% ± 5.7% compared with BC and as high as 31% in summer. The model simulation indicates that the effect of water-soluble BrC on Arctic warming is dominated by BrC from biomass burning (∼60%; annual average), especially in summer (up to ∼70%), when the Arctic is more sensitive to warming (Figure 2D).