Humic-like Substances In Aerosols Research Articles

Characterization of Seasonal Difference of HULIS-C Sources from Water Soluble PM2.5 in Seoul, Korea: Probing Secondary Processes

HUmic-LIke Substances (HULIS) consists of a significant fraction of water-soluble organic carbon (WSOC) and optically active brown carbon. Understanding seasonality in their abundance and sources is an important step to quantifying their impacts on air quality and climate. In this study, we characterized the seasonal variation of HULIS-C (the carbon component of HULIS) and identified primary and secondary sources contributing to HULIS-C at an urban site in Seoul, Korea by analyzing 131 PM2.5 samples collected over a year from October 2012 to September 2013. The HULIS-C in Seoul had an annual average of 2.0 µg m–3, and displayed a seasonality of highest abundance in the winter (2.7 ± 1.3 µg m–3) and the lowest in the summer (1.7 ± 0.8 µg m–3). Its fraction in WSOC reached up to 0.96 in the summer while remained relatively constant in the range of 0.4–0.6 in other seasons. Sources of HULIS-C were resolved using Positive Matrix Factorization (PMF) and checked using inter-species relationships between HILIS-C and aerosol components. Specifically, secondary processes including aging and heterogeneous reaction (26%) contributes to the year-round background concentration of HULIS-C and fresh biomass burning (38%) and incomplete combustion (16%) were the major sources of underlying seasonal variation. We also found aging process in the secondary process had an important role in HULIS-C formation and was a dominant contributor in the summer while heterogenous reaction contributed considerably in other seasons. It implies that the different secondary processes in forming HULIS should be considered when the radiative forcing of brown carbon is estimated. Because these processes can lead the different physicochemical properties of HULIS in aerosols.

Humic-like substances global levels and extraction methods in aerosols

The abundance of humic-like substances in the atmosphere has received considerable attention since these substances play an important role in various atmospheric processes. The wide variety of quantitative techniques used to study humic-like substances produce dissimilar results, making data comparison difficult. Also, global background concentrations and the transfer of atmospheric humic-like substances are poorly known. Here, we compared resins to extract humic-like substances in aerosols, and we measured contents in aerosols from Mt. Fuji. Results show that diethylaminoethyl cellulose resins extracted more humic-like substances than the diethylaminoethyl resin (hydroxylated methacrylic polymer). The mean humic-like substances concentrations in the free troposphere in East Asia is similar to that in Europe, suggesting that the global background level of humic-like substances is 0.05 μg C m−3, based on the humic-like substances concentrations on Mt. Fuji and Mt. Sonnblick. Humic-like substances concentrations, especially fulvic acids concentration, at the summit of Mt. Fuji increased when air masses came from the continent along with the carbon monoxide and ozone.

Water soluble organic compounds formed by oxidation of soot

The water soluble organic compounds (WSOC) in soot samples as a function of the extent of ozone oxidation have been measured by a new methodology which utilises ion exchange chromatography, total carbon analysis and proton nuclear magnetic resonance. These results have been compared with the same analyses of various atmospheric aerosol samples. The WSOC produced from oxidation of soot particles increase rapidly with ozone exposure and consist primarily of aromatic polyacids which are found widely in atmospheric aerosols and which are frequently referred to as macromolecular humic-like substances (HULIS). This work demonstrates that the atmospheric oxidation of soot can produce HULIS in aerosols. The cloud condensation nuclei effectiveness of soot aerosol likely has its origin in these oxidation processes.