Dynamic changes in optical and chemical properties of tar ball aerosols by atmospheric photochemical aging

Abstract. Following wood pyrolysis, tarball aerosols were laboratory generated from wood tarseparated into polar and nonpolar phases. Chemical information of fresh tarballs was obtained from a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) and single-particle laser desorption/resonance enhanced multiphoton ionization mass spectrometry (SP-LD-REMPI-MS). Their continuous refractive index (RI) between 365 and 425 nm was retrieved using a broadband cavity enhanced spectroscopy (BBCES). Dynamic changes in the optical and chemical properties for the nonpolar tarball aerosols in NO x -dependent photochemical process were investigated in an oxidation flow reactor (OFR). Distinct differences in the chemical composition of the fresh polar and nonpolar taraerosols were identified. Nonpolar taraerosols contain predominantly high-molecular weight unsubstituted and alkyl-substituted polycylic aromatic hydrocarbons (PAHs), while polar taraerosols consist of a high number of oxidized aromatic substances (e.g., methoxy-phenols, benzenediol) with higher O : C ratios and carbon oxidation states. Fresh tarballs have light absorption characteristics similar to atmospheric brown carbon(BrC) aerosol with higher absorption efficiency towards the UV wavelengths. The average retrieved RI is 1.661+0.020 i and 1.635+0.003 i for the nonpolar and polar taraerosols, respectively, with an absorption Angstrom exponent(AAE) between 5.7 and 7.8 in the detected wavelength range. The RI fits a volume mixing rule for internally mixed nonpolar/polar tarballs. The RI of the tarball 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 tarballs. Oxidation under varying OH exposure levels and in the absence of NO x diminished the absorption (bleaching) and increased the O : C ratio of the tarballs. The photobleaching via OH radical initiatedoxidation is mainly attributed to decomposition of chromophoric aromatics, nitrogen-containing organics, and high-molecular weight components in the aged particles. Photolysis of nitrous oxide ( N 2 O ) was used to simulate NO x -dependent photochemical aging of tarballs in the OFR. Under high- NO x conditions with similar OH exposure, photochemical aging led to the formation of organic nitrates, and increased both oxidation degree and light absorption for the aged tarball aerosols. These observations suggest that secondary organic nitrate formation counteracts the bleaching by OH radical photooxidation to eventually regain some absorption of the aged tarball aerosols. The atmospheric implication and climate effects from tarballs upon various oxidation processes are briefly discussed.