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Light Absorption Properties and Radiative Effects of Primary Organic Aerosol Emissions Zifeng Lu, David G. Streets, Ekbordin Winijkul, Fang Yan, Yanju Chen, Tami C. Bond, Yan Feng, Manvendra K. Dubey, Shang Liu, Joseph P. Pinto, and Gregory R. Carmichael Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b00211 • Publication Date (Web): 26 Mar 2015 Downloaded from http://pubs.acs.org on March 29, 2015
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Light Absorption Properties and Radiative Effects of Primary Organic
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Aerosol Emissions
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Zifeng Lu,*,† David G. Streets,† Ekbordin Winijkul,† Fang Yan,† Yanju Chen,‡ Tami C.
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Bond,‡ Yan Feng,§ Manvendra K. Dubey,║ Shang Liu,║ Joseph P. Pinto,┴ and Gregory R.
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Carmichael# †
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‡
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Energy Systems Division, Argonne National Laboratory, Argonne, Illinois 60439, United States
║
Department of Civil and Environmental Engineering, University of Illinois at UrbanaChampaign, Urbana, Illinois 61801, United States
Environmental Science Division, Argonne National Laboratory, Argonne, Illinois 60439, United States
Earth and Environmental Sciences Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States
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┴
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#
National Center for Environmental Assessment, U.S. EPA, Research Triangle Park, North Carolina 27709, United States
Department of Chemical and Biochemical Engineering, The University of Iowa, Iowa City, Iowa 52242, United States
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Revised Manuscript Submitted to
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Environmental Science & Technology
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March 20, 2015
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The submitted manuscript has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory (“Argonne”). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC02-06CH11357. The U.S. Government retains for itself, and others acting on its behalf, a paid-up nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government.
* Corresponding Author: E-mail:
[email protected]; phone: (630) 252-9853; fax (630) 252-8007.
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ABSTRACT
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Organic aerosols (OA) in the atmosphere affect Earth’s energy budget by not only scattering but
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also absorbing solar radiation due to the presence of the so-called “brown carbon” (BrC)
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component. However, the absorptivities of OA are not or poorly represented in current climate
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and chemical transport models. In this study, we provide a method to constrain the BrC
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absorptivity at the emission inventory level using recent laboratory and field observations. We
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review available measurements of the light-absorbing primary OA (POA), and quantify the
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wavelength-dependent imaginary refractive indices (kOA, the fundamental optical parameter
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determining the particle’s absorptivity) and their uncertainties of the bulk POA emitted from
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biomass/biofuel, lignite, propane, and oil combustion sources. In particular, we parameterize the
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kOA of biomass/biofuel combustion sources as a function of the black carbon (BC)-to-OA ratio,
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indicating that the absorptive properties of POA depend strongly on burning conditions. The
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derived fuel-type-based kOA profiles are incorporated into a global carbonaceous aerosol
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emission inventory, and the integrated kOA values of sectoral and total POA emissions are
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presented. Results of a simple radiative transfer model show that the POA absorptivity warms
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the atmosphere significantly and leads to ~27% reduction in the amount of the net global average
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POA cooling compared to results using the non-absorbing assumption.
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INTRODUCTION Atmospheric carbonaceous aerosols (i.e., black carbon, BC, and organic aerosol, OA)
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affect Earth’s energy budget by scattering and absorbing solar radiation. While BC is commonly
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considered to be the second most important global warming agent in the atmosphere (after only
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CO2),1 OA is typically treated as a cooling agent that purely scatters solar radiation. However,
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recent studies showed that OA components can also contribute substantially to light absorption.
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In contrast to BC, which absorbs light throughout the ultraviolet (UV)-visible spectrum, the so-
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called “brown carbon” (BrC) component in OA absorbs mostly in the near-UV and blue
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wavelength and exhibits stronger wavelength-dependence.2 BrC has been widely observed in
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biomass/biofuel3-18 and fossil-fuel14,19-23 combustion sources, and including its contribution in
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climate and chemical transport models has been shown to improve the simulations of aerosol
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light absorption significantly.24,25 BrC has been reported to be responsible for from ~20% to
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>50% of the aerosol light absorption at UV wavelengths23-29 and thus could perturb the radiative
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balance and influence tropospheric photochemistry.
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Knowledge of the optical properties of primary OA (POA) emissions is essential in
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modeling their climate effects. The complex refractive index (m=n−ik) is the fundamental
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parameter determining the particles’ optical properties, where the real (n) and the imaginary (k)
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parts represent the scattering and absorbing components, respectively. For example, the mass
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absorption and scattering cross-sections (MAC and MSC) of spherical particles—the two most
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important optical properties—can be derived from m and the particles’ size distribution through
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Mie theory.30 In current climate and chemical transport models, light absorption properties of
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POA are poorly represented. Most models still treat POA as non-absorbing (i.e., k values of OA,
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kOA=0) and underestimate the total warming effect of carbonaceous aerosols. For the limited
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studies that took into account the BrC effects, the treatments were also too simplistic to reflect
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the variety of reported kOA values, which typically span two orders of magnitude. For example,
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Park et al.24 calculated the light absorption coefficients of BrC over East Asia in the GEOS-
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Chem model as a product of the simulated BC concentration, a BrC-to-BC ratio of one, and a
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fixed MAC of 3.8 m2 g−1. Feng et al.26 treated 66% of the total POA from biofuel/biomass
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emissions as BrC, whereas Lin et al.31 assumed that all POA from biomass, biofuel, and
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residential coal burning were BrC. Both studies used spectrally dependent kOA from Kirchstetter
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et al.3 and Chen and Bond7 as high- and low-absorbing scenarios in their models to evaluate the
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influence of BrC on the global distribution of radiative forcing (RF). Clearly, better
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parameterization of the light-absorbing properties of POA is urgently needed in models.
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Since BrC is a complex mixture encompassing a large group of organic compounds with
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various absorptivities and lacking a formal analytical definition,1,2 we focus on the light-
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absorbing properties of the bulk POA instead of treating the BrC separately as a POA subset. We
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first review the currently available kOA-related measurements and develop fuel-type-based kOA
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spectral profiles, as well as their uncertainties, for POA emitted from both biofuel/biomass and
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fossil-fuel combustion sources. Then these wavelength-dependent kOA profiles are incorporated
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into a global POA emission inventory and the integrated light absorption properties of sectoral
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and total POA emissions are estimated. We also show how the POA radiative effect changes
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when accounting for the POA absorption.
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METHODS AND DATA
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Data Sets of Biomass/Biofuel Combustion Sources. Currently, the majority of light-
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absorbing POA studies have focused on the biomass/biofuel combustion sources, because POA
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emissions from these sources are highly absorptive2 and contribute ~90% of the global POA
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budget.1,32 However, reported kOA values varied substantially among different studies due to the
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complex and poorly understood chemical nature of the light-absorbing compounds comprising
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POA. Recently, Saleh et al.14 found that kOA of biomass burning emissions can be parameterized
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as a function of the BC-to-OA ratio. Following the same idea, we reviewed the available
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measurements of absorbing OA from not only biomass burning but also biofuel combustion
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sources and examined whether the absorption properties of OA from these sources can be linked
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to the BC-to-OA ratio.
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Data sets of biomass/biofuel combustion considered in this work3-18 are shown in Figure 1.
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We used laboratory OA measurements of fresh and chemically aged emissions and field OA
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measurements near intensive biomass/biofuel emission sources. Ambient and remote sensing
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measurements were discarded since they may be influenced by emissions from a variety of
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sources and by the presence of secondary organic aerosols (SOA). We also included a series of
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well-controlled FLAME (Fire Lab at Missoula Experiment) measurements of biomass
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smoke.12,13,17 Since the reported optical properties of these measurements were for the bulk
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carbonaceous aerosols, only experiments with the OA fraction >95% of the total aerosol mass
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were included. These data were utilized in Figure 1c only to show the wavelength dependence of
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the light-absorbing OA in a wide range of BC-to-OA ratios.
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Figure 1
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We assumed that the kOA has a power-law wavelength dependence14,33,34 and can be
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expressed as:
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kOA, k OA,550 550 /
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where kOA,550 is kOA at the wavelength (λ) of 550 nm and w is the wavelength dependence of kOA.
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For those studies that measure the OA absorption coefficients/efficiencies, w is often described
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using the absorption Ångström exponent (AAE), where AAE=w+1 for small particles.14 We
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made the following corrections/calculations to reported results not giving kOA,550 and/or w
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directly:
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w
(1)
for studies reporting the kOA (or absorption coefficients/efficiencies) at multiple
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wavelengths, w (or AAE) was determined from the linear regression fit of log(kOA) (or
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log(absorption coefficients/efficiencies)) versus log(λ);
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reported or calculated AAE values were converted to w by subtracting one;
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for kOA values reported at wavelengths other than 550 nm, kOA,550 values were calculated
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using Eq (1);
for studies measuring the density-normalized absorption coefficient (σliquid=α/ρ) from
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organic liquid (e.g., methanol and acetone) extracts of POA, kOA was calculated by:34
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kOA, liquid, / 4
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where ρ is the density of the dissolved compounds and was fixed to 1.2 g cm−3.35 Note
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that σliquid is different from the particulate MAC although they have the same units of m2
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g−1;34
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(2)
kOA,550 and w of Kirchstetter et al.’s3 results were corrected to 0.022 and 3.3, respectively (S1 in the Supporting Information, SI);
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when converting the mass of OC (organic carbon) to OA, an OA-to-OC ratio of 2.0 was used, as suggested by Turpin and Lim35 for biofuel/biomass burning sources. Monte Carlo Simulations of Ångström Attribution of POA Absorption for Lignite
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Combustion Sources. Although most of the reported kOA values were focused on
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biomass/biofuel burning sources, some studies did show that POA emissions from
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coal,19,20,23,36,37 oil,14,21 and propane22 combustion contain light-absorbing components. We focus
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on coal in this section and show the treatments of others in the next section.
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Bond et al. measured the light-absorbing properties of carbonaceous aerosols emitted from
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a small industrial boiler burning lignite19,37 and residential stoves burning both bituminous coal
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and lignite.20 They found that light absorption by particles from these low-temperature coal
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combustion sources exhibited a strong spectral dependence with AAE up to 2.9. They
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hypothesized that a spectrally dependent k of co-emitted OA is the most plausible explanation of
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this phenomenon.36 Unfortunately, they did not provide absorption properties of OA, but only of
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the bulk carbonaceous aerosols; and to our knowledge, there is currently no study reporting the
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kOA directly for coal combustion sources. Here, we applied the widely-used absorption
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attribution method3,11,27,38 to the measurements of Bond et al. for the lignite-burning industrial
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boiler19, in order to separate the light absorption by OA from the total aerosol absorption, and
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retrieved the kOA values through the Mie calculation. This particular dataset was chosen because
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the emissions were well characterized, and most of the relevant information can be found in a
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series of subsequent publications.37,39-41
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The experiments of Bond et al. were conducted at a lignite-burning plant in Leipzig,
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Germany. MACs for the bulk submicrometer particles were given at four wavelengths (450, 550,
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750, and 1000 nm). We assumed that BC and POA were the only light absorbers in the emitted
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particles. Therefore, the measured MAC at λ can be expressed as 7 ACS Paragon Plus Environment
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MAC f BC MACBC, f POA MACPOA,
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where fBC and fPOA are the mass fractions of BC and POA in the total submicrometer particles,
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respectively. The light absorption was assumed to be attributed solely to (“encapsulated”) BC at
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1000 nm,
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MAC1000 f BC MACBC,1000
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Assuming that the spectral absorption of (“encapsulated”) BC follows the power law relationship
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( AAE BC )38,42, MACBC at a shorter wavelength is given by:
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MACBC, MACBC,1000 1000
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Combining Eq (3)−(5), MACPOA at λ was determined as 1
(3)
(4)
AAEBC
(5)
AAEBC MAC MAC1000 1000
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MACPOA,
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We then performed Mie calculations and optimized the kOA,λ until the Mie-predicted MACPOA,λ
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matched the one estimated by Eq (6). Finally, w was determined from the linear regression fit of
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log(kOA,λ) versus log(λ), and kOA over the whole spectrum was derived from Eq (1).
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f POA
(6)
We used a Monte Carlo approach to simulate the above processes and determine the
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uncertainties. The Monte Carlo method is a problem-solving technique that approximates the
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probability of certain outcomes by running multiple simulations with random variables. In this
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work, we assigned appropriate probability distributions to all input parameters and performed
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10,000 simulations. Unless specified otherwise, the term “uncertainty” in this article refers to
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one standard deviation (±1σ) or the coefficient of variation (CV, σ divided by the mean)
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expressed as a percentage. We applied normal distributions for MAC measurements with
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uncertainties of 30% provided by Bond et al.19 As a key input to the Mie model, POA size
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distribution was assumed to be similar to the bulk emission with a count mean diameter (CMD)
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of 50±5 nm and a geometric standard deviation (GSD) of 1.45±0.05.39-41 We ignored both the
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ultrafine (CMD ~4 nm) and the accumulation mode (CMD ~350 nm) detected in the
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experiments, since particles in these two modes together accounted for
