2014 Progress Report: Investigating the Effects Of Atmospheric Aging on the Radiative Properties and Climate Impacts of Black Carbon Aerosol

EPA Grant Number: R835033
Title: Investigating the Effects Of Atmospheric Aging on the Radiative Properties and Climate Impacts of Black Carbon Aerosol
Investigators: Kroll, Jesse H. , Davidovits, Paul , Heald, Colette L.
Institution: Massachusetts Institute of Technology , Boston College
EPA Project Officer: Hunt, Sherri
Project Period: May 1, 2012 through April 30, 2015 (Extended to April 30, 2016)
Project Period Covered by this Report: May 1, 2014 through April 30,2015
Project Amount: $899,654
RFA: Black Carbon's Role In Global To Local Scale Climate And Air Quality (2010) RFA Text |  Recipients Lists
Research Category: Global Climate Change , Climate Change , Air

Objective:

The goal of this combined laboratory and modeling study is to better understand how the climate impact of black carbon (BC) particles changes upon atmospheric aging. The laboratory experiments utilize a suite of state-of-the-art aerosol instrumentation to measure the chemical and radiative properties of BC-containing particles as a function of photochemical processing. Flame-generated BC particles are characterized based on size, shape, chemical composition, and optical properties (extinction, scattering, and absorption), and these properties are tracked as a function of several aging pathways (the condensation of inorganic species, the condensation of secondary organic aerosol, and the heterogeneous oxidation of the BC surface). The changes to the chemistry and optical properties of brown carbon (BrC) particles are examined as well. The modeling component of this project involves the incorporation of these measured properties into a global model framework to investigate and quantify how such changes impact climate via direct radiative forcing (DRF).

Progress Summary:

Work Status/Overview

In Year 3 of this project, a major experimental intensive was carried out in order to measure the evolving chemistry, physical properties, and refractive indices of BC particles as a function of coating type and atmospheric aging. In addition, analysis of previous experiments on the aging of BrC was carried out, suggesting the importance of the “whitening” of BrC subsequent to emission. Modeling efforts have focused on exploring the observational constraints on BrC absorption and how this might be better represented in a global model (GEOS-Chem). A new method for estimating BrC absorption was developed from multi-wavelength absorption measurements, and applied to the analysis of AERONET and aethalometer data.

Year 4 of this project will involve the final analysis of the experimental (BC and BrC aging) data, the use of laboratory-derived aging parameterizations within the model framework for comparison against observations, and the preparation and submission of manuscripts describing this work. So far this project has led to seven publications in the peer-reviewed literature, with two more in preparation.

Results

Laboratory Studies of Black Carbon Aging. The primary experimental effort for Year 3 of this project was a major, multi-group laboratory intensive aimed at quantifying the changes to the optical, chemical, and physical properties of black carbon (BC) particles upon atmospheric aging. Experiments were carried out in March 2015 over a 3-week period, and researchers included the teams from MIT and BC, as well as collaborators from Aerodyne Research, Brookhaven National Laboratory, University of Georgia, and Universtiy of California-Davis. Black carbon particles were generated using an inverted diffusion flame, and, subsequent to denuding to remove gas phase co-products, were sent into a “potential aerosol mass” (PAM) chamber, for the exposure of the particles to the equivalent of hours to weeks of atmospheric oxidation. For most experiments, additional aerosol precursors (volatile organic compounds and/or sulfur dioxide) also were added, leading to the formation of coatings on the BC particles. After exiting the PAM, particles were sent into a wide array of analytical instruments, either as-is or after passing through a thermal denuder to remove the coatings. Analytical instruments are listed in the table following; together, they provide an extremely comprehensive description of the chemical composition, optical properties (scattering, absorption, extinction), and size distribution of the black carbon particles. A large number of experiments were carried out over the course of the intensive, covering different types of black carbon (methane vs. ethylene soot), coatings of secondary organic aerosol (SOA) from a number of precursors (α-pinene, isoprene, and naphthalene), sulfate coatings (from SO2 oxidation), and mixed coatings (different SOA precursors, SOA+sulfate). Analysis of this dataset is ongoing, and is a major effort in Year 4 of this project. Because of the richness of the dataset, a wide range of research avenues is being pursued by the various collaborators, but the primary objective for this project is the determination of wavelength-dependent complex refractive indices. These will be determined as a function of particle/coating type, as well as of the extent of atmospheric aging. Parameterization of these results is a major output of this project, due to the usefulness of such parameterizations for modeling the evolving optical properties of atmospheric BC particles. In addition, measured optical properties and particle morphologies (inferred from differences in measured mass and size distributions), when compared against Mie theory, will provide a direct measurement of how coatings by secondary species can influence BC optical properties. Specifically, this will provide insight into the conditions under which BC coatings form the uniform core-shell morphologies that lead to large absorption enhancements.

Instrument Particle Characteristics Measured
Aerosol mass spectrometer (AMS) Size-resolved composition of non-refractory aerosol components
Soot-particle aerosol mass spectrometer (SP-AMS) Size-resolved composition of black-carbon-containing particles
Centrifugal particle mass analyzer (CPMA) Particle mass distribution
Scanning mobility particle sizer (SMPS) Particle size distribution
Single particle soot photometer (SP2) Size, coatings of black-carbon-containing particles
UCD Cavity ringdown/photoacoustic spectrometer (CRD/PAS) Scattering and absorption at 405 nm and 532 nm, hygroscopicity
UGA UV-Vis photoacoustic spectrometer (PAS) Absorption at 8 wavelengths (301 nm, 314 nm, 364 nm, 405 nm, 436 nm, 546 nm, 578nm 687 nm)
Cavity-attenuated phase-shift monitor (CAPS) Particle extinction at 632 nm
CAPS/single-scattering albedo monitor (CAPS-SSA) Particle extinction and scattering at 532 nm

Experimental Studies on the Effects of Aging on the Composition and Properties of Brown Carbon Particles. A second major effort in Year 3 was analyzing a series of experiments (carried out in Year 2) that examined the oxidative aging of brown carbon (BrC) particles. Such experiments were unique in that only pure BrC particles were studied: by generating particles via smoldering, no black carbon was co-generated, simplifying the interpretation of the results dramatically relative to previous work. Experiments were carried out at Lawrence Berkeley National Laboratory (LBNL), in collaboration with the groups of Christopher Cappa (UC-Davis), Thomas Kirchstetter (LBNL and UC-Berkeley), and Kevin Wilson (LBNL). A large number of experiments were carried out with smoldering ponderosa pine needles as the BrC source, and aging of emissions was carried out using a photochemical oxidation reactor (similar to that used for the BC aging experiments, above) using either OH or O3 as the oxidant. Both heterogeneous oxidation and SOA formation were probed, by repeating experiments with and without a denuder to remove gas-phase species prior to oxidation.

The majority of the effort in Year 3 involved reducing/analyzing the large dataset. Data collected in each experiment (each of which was carried out over a range of oxidation) include high resolution aerosol mass spectra (using two ionization techniques, electron impact ionization and vacuum ultraviolet photoionization) and measurements of scattering and absorption at two wavelengths (405 nm and 532 nm). Analysis then focused on the correlation of aerosol composition (elemental ratios, intensities of key ions in the mass spectrum) with optical properties (real and imaginary components of the refractive index), for the full range of experimental parameters (extent of smoldering, denuded/non-denuded, oxidant type, oxidant exposure). A major complication is the variability in emissions: the BrC generated varies dramatically in composition and optical properties over the course of a single “burn,” and even differs significantly among replicate burns. This is an important result in itself, though it complicates analysis of the aging chemistry.

Despite these challenges, a consistent picture of the effects of BrC aging is emerging from the analysis (which is still ongoing). Shown in Figure 1 is the change to the imaginary component of the refractive index (corresponding to light absorption) of BrC particles as a function of OH exposure (spanning the equivalent of ~5.5 days of atmospheric oxidation). For the non-denuded experiments (black triangles), absorption drops dramatically with oxidation; this appears to be driven largely by the formation of SOA, which is considerably less absorbing (“whiter”) than the primary particles. In fact, this aerosol is exceedingly hygroscopic (far more than most laboratory-generated SOA), and its climate effect may be dominated by water uptake rather than light absorption. On the other hand, for the denuded case (red circles), no SOA is formed, so the chemistry is dominated by heterogeneous oxidation of the BrC particles themselves. Absorption at 405 nm decreases with oxidation, though not as dramatically as in the SOA-forming case, indicating that heterogeneous oxidative aging has the effect of degrading key chromophores in the aerosol. Correlations with aerosol mass spectra indicate that such changes are driven by degradation of aromatic/conjugated moieties within the particles. Overall these results indicate that the aging of biomass emissions – both by formation of secondary aerosol and the heterogeneous oxidation of primary particles – will lead to a considerable “bleaching” of BrC aerosol subsequent to emission, a result that is consistent with recent field measurements.


Figure 1: changes to the absorption properties of BrC (as given by k, the imaginary component of the refractive index) with oxidation by OH. When SOA is formed (black triangles), absorption decreases dramatically, indicating that the SOA is far less absorbing than the primary BrC. Absorption at 405 nm (bottom panel) also decreases even in the absence of SOA, indicating β€œbleaching” of the BrC by heterogeneous oxidation.

Global Modeling of Atmospheric Brown Carbon. The focus of the modeling work in Year 3 has been on improving our ability to estimate and simulate the absorption of BrC. We used the GEOS-Chem chemical transport model with an online radiative transfer model RRTMG (referred to as GC-RT), which was developed in Year 1 of the project, to explore a number of different approaches to modeling BrC aerosol. These initial efforts focused on the idea of linking the absorption of BrC to the emission type, including using the BC:OC as suggested by Saleh, et al. (2014). However, we found a wide range in simulated BrC using various approaches and given the high uncertainty on BC (the other key absorbing aerosol), we found that we could not use Absorption Aerosol Optical Depth (AAOD) measurements as an arbiter of model performance for BrC. Thus, we turned our attention to developing a method for estimating the absorption of BrC alone from measurements.

Unlike BrC, which absorbs primarily in the UV, BC absorbs at all visible-UV wavelengths. Previous studies have used this difference to try to extract BrC by the following procedure: first estimate the BC absorption at long wavelengths, and then assuming an AAE=1, scale this visible absorption to estimate the BC absorption at UV wavelengths, finally subtract the estimated BC UV absorption from the total absorption in the UV. The residual is the estimate of BrC absorption in the UV. However, field measurements suggest that AAE of BC varies from 0.8 to 1.4 (Lack and Langridge, 2013). We use Mie calculations for a range of sizes of BC, and under various coating assumptions to show that AAE=1 is a poor assumption for BC larger than 20 nm (Figure 2). Instead, we develop an approach that takes advantage of the wavelength dependence of absorption to separate BrC and BC.


Figure 2. The AAE of BC from Mie calculations under a range of size and coating assumptions, illustrating that the assumption that AAE=1 is not valid under many conditions.

This method requires measurements of absorption at at least one wavelength in the UV and two longer wavelengths. Based on these measurements (here taken at 440 nm, 675 nm, and 870 nm), we define the wavelength dependence of the AAE (WDA) as follows:

π‘Šπ·π΄=exp(𝐴𝐴𝐸440/870)exp(𝐴𝐴𝐸675/870)

Note that for AAE=1, the WDA would be equal to 1. We then use a Mie code to estimate the WDA for BC of varying sizes and coatings (shown as a blue envelope in Figure 3). On Figure 3 we also plot the annual mean AERONET observations of total absorption for 2014. Any data that fall above the blue envelope are indicative of a significant contribution of BrC (absorbing light more strongly at 440 nm than at longer wavelengths). Observations that fall within the blue envelope may contain BrC, but the contribution is likely small and cannot be estimated from our method. This method provides a more exact attribution of absorption from BC and BrC than previous approaches. The uncertainty in the estimated BrC absorption varies with AAE and total absorption but generally does not exceed 25%. We have applied this approach to estimate BrC absorption from AERONET observations and from aethalometer data from six surface sites. From our analysis we find that BrC generally contributes 10-30% of the absorption at 440 nm. Absorption at one wavelength is insufficient to estimate the radiative impacts of BrC, we also need to know something about the wavelength dependence. We use the aethalometer data as well as the UV absorption measurements from the OMI satellite in concert with the AERONET observations to estimate that the AAE of BrC is quite consistent globally and seasonally with a value of ~4. The exception to this is Europe, where the AAE is somewhat lower (~2), perhaps associated with a dominance of biofuel sources in this region. These results currently are being written up and we anticipate that a manuscript will be submitted for publication in early 2016.


Figure 3: Mie calculation of the wavelength dependence of the AAE of BC (blue envelope). Also shown are annual mean observed total absorption measurements at AERONET sites in 2014 (red crosses).

Future Activities:

In the final year of this project (Year 4), the analysis of the experimental work described above (BC and BrC aging experiments) will be concluded, and results will be written up for publication. In particular, the evolution of refractive indices for both particle types will be parameterized as a function of coating type/thickness and extent of atmospheric aging. For the modeling work, we will finalize our observational analysis of BrC absorption. Finally, the parameterizations of aging determined from the laboratory studies will be tested within the developed model framework, allowing for a comparison against various observational constraints. 

References:

R. Saleh, E. S. Robinson, D. S. Tkacik, A.T. Ahern, S. Liu, A. C. Aiken, R. C. Sullivan, A. A. Presto, M. K. Dubey, R. J. Yokelson, N. M. Donahue, & A. L. Robinson; Brownness of organics in aerosols from biomass burning linked to their black carbon content,Nature Geoscience, 2014; 7:647-650.

D. A. Lack and J. M. Langridge; On the attribution of black and brown carbon light absorption using the Ångström exponent, Atmos. Chem. Phys., 2013; 13:10535-10543.


Journal Articles on this Report : 7 Displayed | Download in RIS Format

Other project views: All 25 publications 9 publications in selected types All 9 journal articles
Type Citation Project Document Sources
Journal Article Browne EC, Franklin JP, Canagaratna MR, Massoli P, Kirchstetter TW, Worsnop DR, Wilson KR, Kroll JH. Changes to the chemical composition of soot from heterogeneous oxidation reactions. Journal of Physical Chemistry A 2015;119(7):1154-1163. R835033 (2014)
R835033 (Final)
  • Abstract from PubMed
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  • Journal Article Heald CL, Ridley DA, Kroll JH, Barrett SRH, Cady-Pereira KE, Alvarado MJ, Holmes CD. Contrasting the direct radiative effect and direct radiative forcing of aerosols. Atmospheric Chemistry and Physics 2014;14(11):5513-5527. R835033 (2013)
    R835033 (2014)
    R835033 (Final)
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  • Journal Article Lambe AT, Cappa CD, Massoli P, Onasch TB, Forestieri SD, Martin AT, Cummings MJ, Croasdale DR, Brune WH, Worsnop DR, Davidovits P. Relationship between oxidation level and optical properties of secondary organic aerosol. Environmental Science & Technology 2013;47(12):6349-6357. R835033 (2012)
    R835033 (2013)
    R835033 (2014)
    R835033 (Final)
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  • Journal Article Lambe AT, Ahern AT, Wright JP, Croasdale DR, Davidovits P, Onasch TB. Oxidative aging and cloud condensation nuclei activation of laboratory combustion soot. Journal of Aerosol Science 2015;79:31-39. R835033 (2014)
    R835033 (Final)
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  • Journal Article Onasch TB, Fortner EC, Trimborn AM, Lambe AT, Tiwari AJ, Marr LC, Corbin JC, Mensah AA, Williams LR, Davidovits P, Worsnop DR. Investigations of SP-AMS carbon ion distributions as a function of refractory black carbon particle type. Aerosol Science and Technology 2015;49(6):409-422. R835033 (2014)
    R835033 (Final)
    R833747 (Final)
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  • Journal Article Sedlacek III AJ, Lewis ER, Onasch TB, Lambe AT, Davidovits P. Investigation of refractory black carbon-containing particle morphologies using the Single-Particle Soot Photometer (SP2). Aerosol Science and Technology 2015;49(10):872-885. R835033 (2014)
    R835033 (Final)
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  • Journal Article Wang X, Heald CL, Ridley DA, Schwarz JP, Spackman JR, Perring AE, Coe H, Liu D, Clarke AD. Exploiting simultaneous observational constraints on mass and absorption to estimate the global direct radiative forcing of black carbon and brown carbon. Atmospheric Chemistry and Physics 2014;14(20):10989-11010. R835033 (2014)
    R835033 (Final)
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  • Progress and Final Reports:

    Original Abstract
    2012 Progress Report
    2013 Progress Report
    Final Report