We found a match
Your institution may have rights to this item. Sign in to continue.
- Title
Dynamic changes of optical and chemical properties of tar ball aerosols by atmospheric photochemical aging.
- Authors
Chunlin Li; Quanfu He; Schade, Julian; Passig, Johannes; Zimmermann, Ralf; Laskin, Alexander; Rudich, Yinon
- Abstract
Following wood pyrolysis, tar ball aerosols were generated from wood tar separated into polar and nonpolar phases. Chemical information of fresh tar balls was obtained from the high-resolution time-of-flight aerosol mass spectrometer (HiRes-ToF-AMS) and laser desorption/resonance enhanced multiphoton ionization mass spectrometry (LD-REMPI-MS), and their refractive index between 365 and 425 nm were retrieved using a broadband cavity enhanced spectroscopy. Dynamic changes of the optical and chemical properties for the nonpolar tar ball aerosols in NOx-dependent photochemical process were investigated in an oxidation flow reactor (OFR). Distinct differences in the chemical composition of the polar and nonpolar tar aerosols were identified. Nonpolar tar aerosols contain predominantly high-molecular weight unsubstituted and alkyl-substituted polycylic aromatic hydrocarbons (PAHs), while polar tar aerosols consist of a high number of oxidized aromatic substances (e.g., methoxy-phenols, benzenediol) with higher O : C ratio and carbon oxidation state. Fresh tar aerosols have light absorption characteristics similar to atmospheric BrC with higher absorption efficiency towards the UV wavelengths. The average retrieved refractive index (RI) are 1.661 + 0.020i and 1.635 + 0.003i for the nonpolar and polar tar aerosols, respectively, with absorption Ångström exponent (AAE) between 5.7 and 7.8 in the wavelength region 365-425 nm. The RI fits a volume mixing rule for internally mixed nonpolar/polar tar aerosols. The RI of the tar aerosols decreased with increasing wavelength under photochemical oxidation. Photolysis by UV light (254 nm), without strong oxidants in the system, slightly decreased the RI and increased the oxidation state of the tar balls. Oxidation under varying OH exposure levels and in the absence of NOx diminished the absorption (bleaching), and increased the O : C ratio. The photobleaching of tar ball aerosols via photochemically induced OH-oxidation is mainly attributed to decomposition of chromophoric aromatics, nitrogen-containing organics, and high-molecular weight components. Photolysis of nitrous oxide (N2O) was used to simulate NOx-dependent photochemical aging of tar balls in the OFR. Under high NOx conditions, photochemical aging lead to the formation of organic-nitrates, increased oxidation degree and increased absorption for the tar ball aerosols. These observations suggest that secondary organic nitrate formation compensates the bleaching by photolysis and OH radical photooxidation to eventually regain some absorption of aged tar balls aerosols. The atmospheric implication and climatic effects from tar balls upon various oxidation processes are briefly discussed.
- Subjects
PYROLYSIS; ATMOSPHERIC aerosols; WOOD tar; MASS spectrometers; MULTIPHOTON ionization; CHEMICAL processes; POLYCYCLIC aromatic hydrocarbons; PHOTOLYSIS (Chemistry)
- Publication
Atmospheric Chemistry & Physics Discussions, 2018, p1
- ISSN
1680-7367
- Publication type
Article
- DOI
10.5194/acp-2018-882