Light absorption of forest organic aerosol fractions with different polarity

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.

PM_0.95 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.

The mass absorption efficiency at 365 nm (MAE₃₆₅) for WISOM was highest among all OA fractions (mean ± standard deviation: 0.37 ± 0.22 m²g^-1), followed by the efficiencies for HULIS (0.14 ± 0.09 m²g^-1) and HP-WSOM (0.07 ± 0.05 m²g^-1). 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 (f) was analyzed, and it showed an increase with the decrease of polarity: on average, the f 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 MAE₃₆₅, which were higher in winter than in summer. Pearson’s correlation analyses between MAE₃₆₅ 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.