Mixed fossil and biofuel aerosols in the UK enhance light absorption

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(Left panel): A measurement station in Detling, UK, is one of several deployed in the UK during the study. (Right panel): Manvendra Dubey, Allison Aiken, and Kyle Gorkowski (Earth System Observations, EES-­‐14)work inside the station.

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Smoke from biomass burning and fossil fuel combustion alter climate by changing the solar energy balance of earth. Current climate models consider two types of carbonaceous particles – organic carbon (OC) and black carbon (BC) – that scatter and absorb sunlight, respectively to cool and warm climate. However, the net effect of the components is not additive due to photochemical reactions between them and with other atmospheric constituents. Moreover, there is evidence for an organic carbon called brown carbon (BrC), which is produced from residential wood combustion and forest fires. Current climate models due not include of effect of the short wavelength light absorption of sunlight by brown carbon. Recent assessments suggest that black carbon could be the second most important anthropogenic-warming agent, but this is very uncertain. Therefore, climate models need to capture the complex and dynamic evolution of optical properties of smoke to determine their impact on climate. The Laboratory led an international research team, which provided observational evidence that complex regional processes increase the light absorption by smoke particles and exacerbate their warming effect. The journal in Nature Communications published the results.

The team used using state-of-the-art instrumentation to perform an in-depth field study of optical, chemical and microphysical properties of particles emitted by fossil and residential fuel burning in the UK in winter. The measurements were part of the 2012 Clean Air for London (ClearfLo) project at Detling, a rural site 45 km southeast and downwind of London.

Scientists have recognized that coatings on black carbon focus sunlight (the lensing effect), which can enhance the light absorption cross-section. However, the effect had not been observed in the field previously. The team directly measured this lensing enhancement factor for black carbon by removing the coatings via heating the particles to 250 C. They determined that the ratio of the light absorption by ambient black carbon to the heated bare black carbon (termed the enhancement factor) has a mean value of about 1.4 at 780 nm. The enhancement factor increases with the coating thickness. The data reveal that coatings are secondary products of oxidized organics and nitrate that increase with age. The aged emissions contain significant amounts of brown carbon that are stable to heat and absorb light at 405 nm. The directly measured enhancement factor is suppressed at 405 nm because they are low volatility. The study successfully matched the observed enhanced absorption using optical models from climate models and the chemical data. Single-particle morphological analysis provided mechanistic insight of the enhancement factor in comparison with previous studies. The researchers conclude that the enhancement factor from the carbon particles is source and regionally dependent.

A previous study did not observed significant lensing enhancement in Sacramento, CA during a field campaign in the summer. The key difference is the widespread use of biofuels for residential heating in winter in UK. This paper is the first field demonstration of biofuel burning emitting organic species that coat BC particles produced by diesel combustion, resulting in an increase in light absorption. This is a twofold effect: 1) the organics amplify BC warming by lensing, and 2) stable BrC causes additional warming. The results underscore the importance of understanding and treating mixed emissions that are region specific and dynamic in climate assessments, and also provide a framework to implement this information.

Reference: “Enhanced Light Absorption by Mixed Source Black and Brown Carbon Particles in UK Winter,” Nature Communications 6, 8435 (2015); doi: 10.1038/ncomms9435. Manvendra Dubey (Earth System Observations, EES-14) led the project. The LANL team included Shang Liu (now at University of Colorado – Boulder), Allison Aiken (EES-14), Kyle Gorkowski (now a graduate student at Carnegie Mellon University), and Dubey. Collaborators: Aerodyne Research Inc., Georgia Institute of Technology, Michigan Technological University, University of California – Davis, and researchers from the UK, Switzerland, and Germany.

The DOE Office of Science, Office of Biological and Environmental Research, Atmospheric Science Program funded the Los Alamos work. Aiken received a Los Alamos Director’s Postdoctoral Fellowship, funded by the Laboratory Directed Research and Development (LDRD) program. The work supports the Lab’s Energy and Global Security mission areas and the Science of Signatures science pillar by improving the capability to quantify past, present, and future contributions of these particles to climate change models. Technical contact: Manvendra Dubey

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(Left panel): Mass fraction of the non-­‐refractory components internally mixed with black  carbon(shaded areas) and black carbon core median volume-­‐weighted diameter (open circles) as a function of refractory black carbon (RBC). The colors represent nitrate (blue), ammonium (orange), sulfate (red), chloride (purple), oxygenated organic aerosol factor (pink), solid fuel organic aerosol factor (brown), and hydrocarbon-­‐like organic aerosol factor (grey). The error bars depict standard deviation of the values for each RBC interval. (Right panel): Representative electron microscopy images of black carbon-­‐containing particles collected at the Detling site for embedded (top left), partly coated (top right), thinly coated (bottom left) and partially encapsulated and/or surface attached (bottom right) black carbon particle types. The size of each panel is 1 μm by 1μm.