Abstract
Abstract. Following wood pyrolysis, tar ball aerosols were laboratory generated from wood tar separated into polar and nonpolar phases. Chemical information of fresh tar balls 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 tar ball aerosols in NOx-dependent photochemical process were investigated in an oxidation flow reactor (OFR). Distinct differences in the chemical composition of the fresh 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 ratios and carbon oxidation states. Fresh tar balls 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+0.020i and 1.635+0.003i for the nonpolar and polar tar aerosols, respectively, with an absorption Ångström 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 tar balls. The RI of the tar ball 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 of the tar balls. The photobleaching via OH radical initiated oxidation is mainly attributed to decomposition of chromophoric aromatics, nitrogen-containing organics, and high-molecular weight components in the aged particles. Photolysis of nitrous oxide (N2O) was used to simulate NOx-dependent photochemical aging of tar balls in the OFR. Under high-NOx 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 tar ball aerosols. These observations suggest that secondary organic nitrate formation counteracts the bleaching by OH radical photooxidation to eventually regain some absorption of the aged tar ball aerosols. The atmospheric implication and climate effects from tar balls upon various oxidation processes are briefly discussed.
Highlights
Organic aerosol (OA), which represent a ubiquitous and dominant burden of the tropospheric particulate pollutants, play important roles in atmospheric chemistry and balance of regional and global radiation (Jimenez et al, 2009; Kanakidou et al, 2005; Seinfeld and Pandis, 2016; Shrivastava et al, 2017)
The optical measurements reported for tar balls or other biomass burning brown carbon (BrC) were discrete over several wavelengths that were constrained by instruments measuring particle light coefficients, or indirectly inferred from calculations based on their electron energy-loss spectra or from UV–visible absorption of solutions containing dissolved tar balls (Alexander et al, 2008)
The signal at m/z 137 is much higher in the nonpolar-phase tar ball aerosols (1.0 % and 0.5 % for nonpolar and polar tar balls, respectively), and the fraction of fragment m/z 137 is consistent with reference values of 0.3 %–2.0 % (Li et al, 2012). m/z 44 (CO+2 ), a marker fragment of carboxylic acids, has been parameterized as f 44 to present the oxidation degree of organic aerosols (Aiken et al, 2008; Ng et al, 2010)
Summary
Organic aerosol (OA), which represent a ubiquitous and dominant burden of the tropospheric particulate pollutants, play important roles in atmospheric chemistry and balance of regional and global radiation (Jimenez et al, 2009; Kanakidou et al, 2005; Seinfeld and Pandis, 2016; Shrivastava et al, 2017). Better understanding of the optical properties of biomass burning BrC aerosols is crucial for constraining its atmospheric and climatic implications and Earth’s energy balance. Microanalysis has found that tar balls are homogeneous spherical carbonaceous particles with sizes ranging from tens to hundreds of nanometers These particles contribute a considerable fraction of the biomass burning BrC (Pósfai et al, 2004; Hand et al, 2005; Li et al, 2017). The optical measurements reported for tar balls or other biomass burning BrC were discrete over several wavelengths that were constrained by instruments measuring particle light coefficients, or indirectly inferred from calculations based on their electron energy-loss spectra or from UV–visible absorption of solutions containing dissolved tar balls (Alexander et al, 2008). The atmospheric implications and climate forcing due to atmospheric aging of tar aerosols and evolution of their optical properties were explored
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