Abstract
Atmospheric particulate matter plays an important role in the Earth’s radiative balance. Over the past two decades, it has been established that a portion of particulate matter, black carbon, absorbs significant amounts of light and exerts a warming effect rivalling that of anthropogenic carbon dioxide1,2. Most climate models treat black carbon as the sole light-absorbing carbonaceous particulate. However, some organic aerosols, dubbed brown carbon and mainly associated with biomass burning emissions3,4,5,6, also absorbs light7. Unlike black carbon, whose light absorption properties are well understood8, brown carbon comprises a wide range of poorly characterized compounds that exhibit highly variable absorptivities, with reported values spanning two orders of magnitude3,4,5,6,9,10. Here we present smog chamber experiments to characterize the effective absorptivity of organic aerosol from biomass burning under a range of conditions. We show that brown carbon in emissions from biomass burning is associated mostly with organic compounds of extremely low volatility11. In addition, we find that the effective absorptivity of organic aerosol in biomass burning emissions can be parameterized as a function of the ratio of black carbon to organic aerosol, indicating that aerosol absorptivity depends largely on burn conditions, not fuel type. We conclude that brown carbon from biomass burning can be an important factor in aerosol radiative forcing.
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Acknowledgements
FLAME 4 and R.J.Y. were supported by NSF grant ATM-0936321. S.L., A.C.A. and M.K.D. thank DOE’s ASR programme F265 for financial support. Carnegie Mellon University team thanks DOE’s ASR programme (ER65296) and NSF (AGS-1256042) for financial support. The authors also thank the Fire Science Laboratory Staff and other FLAME 4 team members.
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R.S. performed the data analysis and wrote the manuscript. R.S., A.L.R., N.M.D., R.C.S. and A.A.P. designed the experiments. E.S.R., A.T.A., D.S.T. and R.S. performed smog chamber experiments. A.L.R., A.A.P., R.C.S., A.T.A., D.S.T., E.S.R. and R.S. built the smog chamber experimental set-up. E.S.R. analysed SP2 and HR-AMS data. S.L. and A.C.A. collected and analysed PASS-3 data. R.J.Y. organized FLAME 4. All authors discussed the data and commented on the manuscript.
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Saleh, R., Robinson, E., Tkacik, D. et al. Brownness of organics in aerosols from biomass burning linked to their black carbon content. Nature Geosci 7, 647–650 (2014). https://doi.org/10.1038/ngeo2220
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DOI: https://doi.org/10.1038/ngeo2220
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