Atmospheric Chemistry and Physics | 2019
Dynamic changes in optical and chemical properties of tar ball aerosols by atmospheric photochemical aging
Abstract
Abstract. Following wood pyrolysis, tar ball aerosols were laboratory generated from\nwood tar separated into polar and nonpolar phases. Chemical information of\nfresh tar balls was obtained from a high-resolution time-of-flight aerosol\nmass spectrometer (HR-ToF-AMS) and single-particle laser desorption/resonance\nenhanced multiphoton ionization mass spectrometry (SP-LD-REMPI-MS). Their\ncontinuous refractive index (RI) between 365 and 425\u2009nm was retrieved using\na broadband cavity enhanced spectroscopy (BBCES). Dynamic changes in the\noptical and chemical properties for the nonpolar tar ball aerosols in\n NOx -dependent photochemical process were investigated in an\noxidation flow reactor (OFR). Distinct differences in the chemical\ncomposition of the fresh polar and nonpolar tar aerosols were identified.\nNonpolar tar aerosols contain predominantly high-molecular weight\nunsubstituted and alkyl-substituted polycylic aromatic hydrocarbons (PAHs),\nwhile polar tar aerosols consist of a high number of oxidized aromatic\nsubstances (e.g., methoxy-phenols, benzenediol) with higher O\u2009:\u2009C ratios\nand carbon oxidation states. Fresh tar balls have light absorption\ncharacteristics similar to atmospheric brown carbon (BrC) aerosol with higher\nabsorption efficiency towards the UV wavelengths. The average retrieved RI is\n 1.661+0.020i and 1.635+0.003i for the nonpolar and polar tar aerosols,\nrespectively, with an absorption Angstrom exponent (AAE) between 5.7\nand 7.8 in the detected wavelength range. The RI fits a volume mixing rule\nfor internally mixed nonpolar/polar tar balls. The RI of the tar ball\naerosols decreased with increasing wavelength under photochemical oxidation.\nPhotolysis by UV light (254\u2009nm), without strong oxidants in the system,\nslightly decreased the RI and increased the oxidation state of the tar balls.\nOxidation under varying OH exposure levels and in the absence of\n NOx diminished the absorption (bleaching) and increased the\nO\u2009:\u2009C ratio of the tar balls. The photobleaching via OH radical initiated\noxidation is mainly attributed to decomposition of chromophoric aromatics,\nnitrogen-containing organics, and high-molecular weight components in the\naged particles. Photolysis of nitrous oxide ( N2O ) was used to\nsimulate NOx -dependent photochemical aging of tar balls in\nthe OFR. Under high- NOx conditions with similar OH exposure,\nphotochemical aging led to the formation of organic nitrates, and increased\nboth oxidation degree and light absorption for the aged tar ball aerosols.\nThese observations suggest that secondary organic nitrate formation\ncounteracts the bleaching by OH radical photooxidation to eventually regain\nsome absorption of the aged tar ball aerosols. The atmospheric implication\nand climate effects from tar balls upon various oxidation processes are\nbriefly discussed.