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. Following wood pyrolysis, tar ball aerosols were laboratory generated fromwood tar separated into polar and nonpolar phases. Chemical information offresh tar balls was obtained from a high-resolution time-of-flight aerosolmass spectrometer (HR-ToF-AMS) and single-particle laser desorption/resonanceenhanced multiphoton ionization mass spectrometry (SP-LD-REMPI-MS). Theircontinuous refractive index (RI) between 365 and 425 nm was retrieved usinga broadband cavity enhanced spectroscopy (BBCES). Dynamic changes in theoptical and chemical properties for the nonpolar tar ball aerosols in NOx -dependent photochemical process were investigated in anoxidation flow reactor (OFR). Distinct differences in the chemicalcomposition of the fresh polar and nonpolar tar aerosols were identified.Nonpolar tar aerosols contain predominantly high-molecular weightunsubstituted and alkyl-substituted polycylic aromatic hydrocarbons (PAHs),while polar tar aerosols consist of a high number of oxidized aromaticsubstances (e.g., methoxy-phenols, benzenediol) with higher O : C ratiosand carbon oxidation states. Fresh tar balls have light absorptioncharacteristics similar to atmospheric brown carbon (BrC) aerosol with higherabsorption efficiency towards the UV wavelengths. The average retrieved RI is 1.661+0.020i and 1.635+0.003i for the nonpolar and polar tar aerosols,respectively, with an absorption Angstrom exponent (AAE) between 5.7and 7.8 in the detected wavelength range. The RI fits a volume mixing rulefor internally mixed nonpolar/polar tar balls. The RI of the tar ballaerosols 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 theO : C ratio of the tar balls. The photobleaching via OH radical initiatedoxidation is mainly attributed to decomposition of chromophoric aromatics,nitrogen-containing organics, and high-molecular weight components in theaged particles. Photolysis of nitrous oxide ( N2O ) was used tosimulate NOx -dependent photochemical aging of tar balls inthe OFR. Under high- NOx conditions with similar OH exposure,photochemical aging led to the formation of organic nitrates, and increasedboth oxidation degree and light absorption for the aged tar ball aerosols.These observations suggest that secondary organic nitrate formationcounteracts the bleaching by OH radical photooxidation to eventually regainsome absorption of the aged tar ball aerosols. The atmospheric implicationand climate effects from tar balls upon various oxidation processes arebriefly discussed.

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