<p>Organic aerosol (OA), a major component of atmospheric aerosol, is considered to be one of the key players in atmospheric radiative balance and climate change. Chromophoric OA, termed as brown carbon (BrC), is a component that can absorb solar radiation in the ultraviolet and short-wavelength visible regions and is composed of a wide range of poorly characterized compounds. Whereas light absorption properties were analyzed to characterize chromophoric OA, fluorescent properties also provide information on them. In this study, the fluorescence property of solvent extractable organics in submicron aerosol particles collected in a forest in the cool-temperate zone of northern Japan, was characterized.</p><p>Aerosol samples were collected on quartz filters (cut-off diameter: &#8804;0.95 micrometer) in Tomakomai Experimental Forest of Hokkaido University. Organic aerosol components in the samples were extracted and fractionated on the basis of their polarity by the combination of solvent extraction and solid-phase extraction methods. Water-soluble organic matter (WSOM) and water-insoluble organic matter (WISOM) were extracted sequentially by using multiple solvents. Two fractions, humic-like substance (HULIS) and highly-polar water-soluble organic matter (HP-WSOM), were fractionated from WSOM by solid phase extraction. The excitation&#8722;emission matrices (EEMs) were measured using a fluorescence spectrometer, and the fluorescence property of the extracts was characterized by the classification of EEM profiles using a Parallel Factor (PARAFAC) model.</p><p>From the PARAFAC analysis, five types of fluorescent components were identified for each of WSOM and WISOM fractions. A fluorescence component with the characteristics reported to be associated with (HULIS) accounted for large fractions of the fluorescence from WSOM and WISOM (mean: 68% and 84%, respectively). The relative contribution of the fluorescent components for WSOM shows a clear seasonal variation of the characteristics of WSOM. Furthermore, from each of HULIS and HP-WSOM fractions, five types of fluorescent components were identified. Fluorescence components with the characteristics of protein-like compounds identified in previous EEM studies accounted for a large fraction of the fluorescence from HP-WSOM (mean: 53%), whereas the contribution of protein-like compounds was smaller in the case of the HULIS fraction (mean: 23%).</p>
Atmospheric organic aerosol (OA) are considered as a significant contributor to the light absorption of OA, but its relationship with abundance, composition and sources are not understood well. In this study, the abundance, chemical structural characteristics, and light absorption property of HULIS and other low-to-high polar organics in PM0.95 collected in Tomakomai Experimental Forest (TOEF) were investigated with consideration of their possible sources. HULIS were the most abundant (51%), and correlation analysis revealed that biogenic secondary organic aerosols significantly contribute to HULIS. The mass spectra obtained using a high-resolution aerosol mass spectrometer (HR-AMS) showed that HULIS and highly polar water-soluble organic matter (HP-WSOM) were substantially oxygenated organic aerosol fractions, whereas water-insoluble organic matter (WISOM) had a low O/C ratio and more hydrocarbon-like structures. The WISOM fraction was the predominant light-absorbing organics. HULIS and WISOM showed a noticeable seasonal change in mass absorption efficiency (MAE365), which was highest in winter. Further, HULIS were shown to be less absorbing than those reported for urban sites. The findings in this study provide insights into the contribution of biogenic secondary OA on aerosol property and radiative forcing under varying contributions from other types of OA.
<p>Organic aerosol (OA) is a ubiquitous component of atmospheric aerosol and affects radiative forcing not only by scattering but also by absorbing solar radiation. The light absorption property of OA should vary depending on its composition, which is not well understood to date. Humic-like substances (HULIS), a medium polar part of OA, constitute significant part of water-soluble organic matter (WSOM) and have light-absorbing capacity. In addition, recent studies showed that less polar water-insoluble organic matter (WISOM) absorbed light stronger than WSOM. Knowledge on the light absorption property of all the parts of OA in atmospheric aerosols is important to understand their contribution to aerosol light absorption. In this study, the light absorption property of extractable organics with low-to-high polarity in submicron aerosols collected at a forest site was characterized.</p><p>PM<sub>0.95</sub> samples (particles with a diameter smaller than 0.95 mm) were collected on quartz filters in Tomakomai Experimental Forest of Hokkaido University, Japan, from June 2012 to May 2013. Organic aerosol components in the samples were extracted and fractionated by the combination of solvent extraction and solid-phase extraction methods. WSOM and WISOM were extracted sequentially by using multiple solvents. HULIS and highly-polar water-soluble organic matter (HP-WSOM) were fractionated from WSOM by solid-phase extraction. The light absorption by the OA fractions were measured using a UV-visible spectrometer. Further, a high-resolution time-of-flight aerosol mass spectrometer was used to quantify the OA fractions and to analyze the types of generated ions.</p><p>The mass absorption efficiency at 365 nm (MAE<sub>365</sub>) for WISOM was highest among all OA fractions (mean &#177; standard deviation: 0.37 &#177; 0.22 m<sup>2</sup>g<sup>-1</sup>), followed by the efficiencies for HULIS (0.14 &#177; 0.09 m<sup>2</sup>g<sup>-1</sup>) and HP-WSOM (0.07 &#177; 0.05 m<sup>2</sup>g<sup>-1</sup>). HULIS was shown to be whiter (more transparent) than that reported from previous studies. WISOM was the predominant light-absorbing OA fraction among three OA fractions. The absorption of solar radiation by the OA fractions relative to that by elemental carbon (<em>f</em>) was analyzed, and it showed an increase with the decrease of polarity: on average, the <em>f </em>values were 12%, 8%, and 2%, for WISOM, HULIS, and HP-WSOM, respectively, for the solar spectrum in a range from 300 to 500 nm. HULIS and WISOM showed noticeable seasonal changes in MAE<sub>365</sub>, which were higher in winter than in summer. Pearson&#8217;s correlation analyses between MAE<sub>365</sub> and ion groups of OA fractions indicate that organic compounds with N, O, and S atoms may contribute substantially to the light absorption of OA components.</p>
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