Aerosol Science and Technology ISSN: 0278-6826 (Print) 1521-7388 (Online) Journal homepage: https://www.tandfonline.com/loi/uast20 Effect of heterogeneous oxidative aging on light absorption by biomass burning organic aerosol Eleanor C. Browne, Xiaolu Zhang, Jonathan P. Franklin, Kelsey J. Ridley, Thomas W. Kirchstetter, Kevin R. Wilson, Christopher D. Cappa & Jesse H. Kroll To cite this article: Eleanor C. Browne, Xiaolu Zhang, Jonathan P. Franklin, Kelsey J. Ridley, Thomas W. Kirchstetter, Kevin R. Wilson, Christopher D. Cappa & Jesse H. Kroll (2019) Effect of heterogeneous oxidative aging on light absorption by biomass burning organic aerosol, Aerosol Science and Technology, 53:6, 663-674, DOI: 10.1080/02786826.2019.1599321 To link to this article: https://doi.org/10.1080/02786826.2019.1599321 View supplementary material Accepted author version posted online: 26 Mar 2019. Published online: 15 Apr 2019. Submit your article to this journal Article views: 321 View Crossmark data Full Terms & Conditions of access and use can be found at https://www.tandfonline.com/action/journalInformation?journalCode=uast20 AEROSOL SCIENCE AND TECHNOLOGY 2019, VOL. 53, NO. 6, 663–674 https://doi.org/10.1080/02786826.2019.1599321 Effect of heterogeneous oxidative aging on light absorption by biomass burning organic aerosol Eleanor C. Brownea , Xiaolu Zhangb , Jonathan P. Franklinc, Kelsey J. Ridleyc, Thomas W. Kirchstetterd,e, Kevin R. Wilsonf , Christopher D. Cappag , and Jesse H. Krollc aDepartment of Chemistry and Cooperative Institute for Research in Environmental Sciences, University of Colorado Boulder, Boulder, Colorado, USA; bAir Quality Research Center University of California Davis, Davis, California, USA; cDepartment of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA; dEnergy Technologies Area, Lawrence Berkeley National Laboratory, Berkeley, California, USA; eDepartment of Civil and Environmental Engineering, University of California Berkeley, Berkeley, California, USA; fChemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, USA; gDepartment of Civil and Environmental Engineering, University of California Davis, Davis, California, USA ABSTRACT ARTICLE HISTORY Light-absorbing organic aerosol (brown carbon, BrC) impacts the radiative balance of the Received 17 November 2018 earth’s atmosphere; however, the magnitude of this impact is poorly constrained due to Accepted 23 February 2019 uncertainties in BrC sources, composition, and lifetime. In particular, the role of chemical EDITOR “aging” on the optical properties of BrC particles is poorly understood. Here we carry out laboratory studies aimed at understanding how one such aging process, heterogeneous oxi- Paul Ziemann dation, may affect the chemical and optical properties of biomass burning-derived BrC. We generate BrC from smoldering ponderosa pine needles, oxidize the BrC in a flow reactor, and use simultaneous measurements of aerosol optical properties and chemical composition to monitor changes upon oxidation. Under the set of conditions investigated here, we find that with increased oxidant exposure, the aerosol becomes more oxidized and less absorb- ing, presumably due to oxidative degradation of the chromophores. Both the kinetics and evolution of this process are oxidant dependent. While heterogeneous oxidation by ozone results in a rapid “bleaching” of the BrC (i.e., decrease in absorptivity), a substantial fraction of the BrC is resistant to bleaching by this mechanism. In contrast, bleaching due to hetero- geneous oxidation by OH in the presence of ozone remains active over long timescales (timescale of days), suggesting a sustained evolution of BrC optical properties throughout the aerosol atmospheric lifetime. 1. Introduction cooling (scattering) effect of OA (Stocker et al. 2013). Atmospheric aerosol particles can directly impact the Global model simulations have shown that BrC can Earth’s radiative balance via the scattering and absorp- indeed be an important contributor to overall aerosol tion of radiation (“aerosol–radiation interactions”) radiative forcing (Chung, Ramanathan, and Decremer (Stocker et al. 2013). As organic aerosol (OA) is a 2012; Feng, Ramanathan, and Kotamarthi 2013;Jo major component of submicron aerosol mass (Zhang et al. 2016; Lin et al. 2014; Saleh et al. 2015; Wang et al. 2007), understanding its optical properties is et al. 2014; Wang et al. 2018). However, such contri- critical for calculating aerosol radiative forcing butions, in both the present and future atmospheres, (Stocker et al. 2013). Due to the often weak absorp- remain highly uncertain, due to the high level of tion properties of many organic species (compared to chemical complexity and variability of BrC aerosols. strong absorbers such as black carbon and mineral Organic compounds contributing to light absorption dust), OA is traditionally considered to predominantly likely comprise a small mass fraction of total OA, and scatter light (Seinfeld and Pandis 2006). However, their chemical identity is complex and uncertain some components of OA may absorb UV and visible (Laskin, Laskin, and Nizkorodov 2015). Furthermore, light; this fraction, known as brown carbon (BrC), despite considerable study in recent years, virtually all may result in warming, somewhat offsetting the aspects of the BrC lifecycle and impacts (sources, CONTACT Eleanor C. Browne [email protected] Department of Chemistry and Cooperative Institute for Research in Environmental Sciences, University of Colorado Boulder, Boulder, CO 80309, USA. Supplemental data for this article can be accessed here. ß 2019 American Association for Aerosol Research 664 E. C. BROWNE ET AL. composition, properties, and lifetime) remain poorly oxidation, though the nature of the change (browning, constrained (Laskin, Laskin, and Nizkorodov 2015). bleaching, or one followed by the other) varies with Biomass and biofuel burning, an important global aging process and chemical system studied. source of OA in general (Bond et al. 2004; Hallquist These previous studies have focused primarily on et al. 2009), is also an important atmospheric source how chemical transformations within the particle can of primary BrC (Laskin, Laskin, and Nizkorodov affect BrC absorption. There has been considerably 2015). With future increases in biomass burning emis- less direct study on how heterogeneous oxidation sions projected (Pechony and Shindell 2010), con- reactions may affect light absorption properties of BrC straining biomass burning BrC is important for particles. The only exceptions are recent studies by projections of radiative forcing. Understanding the cli- Sumlin et al. (2017) and Li et al. (2019). Sumlin et al. mate impacts of primary biomass burning BrC, how- (2017) exposed particles generated from smoldering ever, requires not only knowledge of the emissions peat to high levels of ozone and OH and found small and properties and BrC particles, but also knowledge decreases to light absorption from heterogeneous oxi- of their atmospheric lifetimes. Examples of biomass dation by OH (the effects of ozone was not quanti- burning BrC chromophores include polycyclic aro- fied), though such changes were found to occur only matic hydrocarbons, phenols, aromatic acids, and after the equivalent of several days of atmospheric oxi- nitroaromatic compounds (Lin et al. 2017; Lin et al. dation. Likewise, Li et al. (2019) oxidized tar balls by 2018; Bluvshtein et al. 2017; Mohr et al. 2013). As exposure to high levels of OH and ozone and found atmospheric processing is known to alter the chemical bleaching to occur. These results stand in contrast to composition of OA (Jimenez et al. 2009), it is likely the more immediate changes observed in field studies that BrC chromophores will be altered as well. (Forrister et al. 2015; Xuan Wang et al. 2018). Various aging processes could potentially make the In this work, we investigate further the effects of BrC more absorbing (“browner”) or less absorbing heterogeneous oxidation on the absorption of biomass (“whiter”), and thus it may not be appropriate to burning-derived BrC by adding additional constraints assign a single set of optical properties to BrC over its on oxidation by OH and also investigating the role of entire atmospheric lifetime. Furthermore, if whiten- ozone. BrC is generated from the smoldering of pon- ing/bleaching occurs, the lifetime of biomass burning- derosa pine needles, which generates OA and BrC but derived primary BrC may be substantially different no black carbon (soot). The generated particles are from the lifetime of biomass burning OA overall. then sent into an oxidation flow reactor to simulate Indeed, field measurements of ambient BrC in bio- hours to days of atmospheric oxidation, first by ozone mass burning plumes support the view of BrC as a alone and then by ozone and OH. A particular focus dynamic system. Studies tracking individual plumes of this work is the kinetics of the changes to absorp- find that light absorption by BrC drops rapidly after tion properties of the BrC. This focus provides context emission, with lifetimes on the order of a single day for how heterogeneous oxidation, under the condi- (Adler et al. 2011; Forrister et al. 2015; Wang et al. tions studied here, may affect BrC optical properties 2018), suggesting substantial bleaching with atmos- during atmospheric transport. pheric age. However, these studies also indicate a per- sistent lower level of