Abstracts of Annual Meeting of the Geochemical Society of Japan

Atmospheric aerosols impact the Earth's climate system, directly by scattering and absorbing solar radiation and indirectly by acting as cloud condensation nuclei (CCN) from regional to global scale. Fine aerosols that abundantly contain organic material also influence air quality and thus affect human health. Water-soluble organic carbon (WSOC) that is considered to be associated with a major fraction of secondary organic aerosol can significantly alter the hygroscopic behavior and enhance the capability of particles to act as CCN and thereby play an important role in climatic impacts. Although anthropogenic emissions are significant in Asia and high aerosol loadings are prevalent in the Asian atmosphere, relative contributions of different sources, particularly fossil and biospheric carbon, to organic aerosols are poorly understood. In this study, we measured radiocarbon (δ₁₄C), a unique tracer to distinguish fossil and modern carbon, and stable carbon (δ₁₃C) isotopic ratios in total carbon (TC) and WSOC in atmospheric aerosols from Sapporo, northern Japan (43.07°N, 141.36°E) over one-year period (2 September 2009 to 5 October 2010). The percent of modern carbon (pMC) ranged from 27% to 85% (ave. 57%) in TC and 58% to 114% (ave. 82%) in WSOC, with elevated levels in spring and lowest levels in winter, whereas δ₁₃C values varied between -25.6⁰/₀₀ and -23.1⁰/₀₀ (ave. -24.8) in TC and -26.7⁰/₀₀ and -21.2⁰/₀₀ (ave. -24.2⁰/₀₀) in WSOC, with a peak in winter. This study clearly demonstrates that WSOC is more enriched with modern carbon than TC in northern Japan. Our findings, followed by air mass trajectory analyses, reveal that fossil fuel combustion and biomass burning are two major sources of organic aerosols in winter whereas emissions of primary biogenic aerosols and the secondary organic aerosol formation due to the photo-oxidation of biogenic volatile organic compounds are important in spring (and also summer).

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Atmospheric aerosols modulate directly the radiative energy balance of the earth by absorbing and scattering the solar and terrestrial radiation. Understanding the physical and chemical properties of background aerosols is useful for determining source regions, elucidating the mechanism of long-range transport of anthropogenic pollutants. Molecular level studies of organic aerosols are limited because of their complexity and difficulties in the analyses. Aerosol samples were collected from a high elevation mountain site (Nainital, India; 1958 m amsl) in the central Himalayas which provide an isolated platform above the planetary boundary layer to better understand the composition of the remote continental troposphere.

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The aerosol samples were analyzed for water-soluble dicarboxylic acids (C2–C12) and related compounds (ketocarboxylic acids and α-dicarbonyls), as well as organic carbon, elemental carbon and water soluble organic carbon. The contributions of total diacids to total aerosol carbon content during wintertime were 1.7 % and 1.8%, for day and night, respectively; whereas during summer the contributions were significantly reduced. Molecular distributions of diacids demonstrated that oxalic (C2) acid was the most abundant species followed by C4 and C3 diacids. The annual average concentrations of total diacids (433 ± 108 ng m^-3), ketoacids (48 ± 23 ng m^-3), and α-dicarbonyls (9 ± 4 ng m^-3) were comparable to those from other Asian megacities such as Tokyo, Beijing and Hong Kong. During summer season most of the organic species were several times more abundant than in winter. Phthalic acid, which originates from oxidation of polycyclic aeromatic hydrocarbons such as naphthalene was found to be 7 times higher in summer. This feature has not been reported previously in atmospheric aerosols. Glyoxal and glyoxylic acid are intermediates in the formation of oxalic acid for winter months. In contrast, during summer months, succinic acid was found to be decomposed to malonic acid and then to oxalic acid following the oxidative degradation of biogenic unsaturated fatty acids. Based on molecular distributions and backward air mass trajectories, we found that diacids and related compounds in Himalayan aerosols were influenced by the anthropogenic activities from highly populated Indo-Gangetic plain areas.

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Water-soluble organic compounds including dicarboxylic acids, ketocarboxylic acids, and α-dicarbonyls are important components of atmospheric aerosols. They have strong impacts on radiative forcing of climate, human health effects, pollution transport, and visibility degradation. Total dicarboxylic acids account for ˃10% of the total particulate carbon in the remote marine atmosphere. The main purpose of this study is to obtain the atmospheric situation in Okinawa, which is located on the outflow region of Asian countries including China, in order to better understand the origin and transformation of diacids and related compounds in their transport process.Marine aerosol samples were collected in 2009 October to 2010 October at Cape Hedo station of National Institute of Environmental Studies, Okinawa (128.28° E, 26.87° N), an outflow region of Chinese aerosols, using high-volume air sampler and pre-combusted quartz fiber filters for 7 days at a flow rate of 60 m3 h-1. The filters were extracted with Milli Q water (10 mL X 3). The extracts were passed through a glass column (Pasteur pipette) packed with quartz wool to remove particles and then concentrated near dryness using a rotary evaporator under a vacuum and were derivatized with 14% BF3 in n-butanol at 100°C for one hour. The derived dibutyl esters and dibutoxy acetals were extracted with n-hexane. The hexane layer was concentrated to ~50 μL using a rotary evaporator, transferred to small vial (1.5 mL), dried to almost dryness by N2 blow down, and dissolved in known volume of n-hexane and the samples were analysed using gas chromatography and GC/MS.Homologous series of n-diacids (C2-C12), branched diacids, unsaturated diacids, and multifunctional diacids were detected as well as ω-oxoacids (C2-C9), glyoxal and methyl glyoxal. Oxalic acid was found as the dominant species in all four seasons followed by malonic, glyoxylic and succinic acids. The highest concentrations of diacids were found in spring (ave. 88± 28 ng m^-3) followed by winter season (72± 33 ng m^-3). The lowest diacids concentrations were found in summer (30± 30 ng m^-3). Ketoacids also followed similar trend, highest in spring (8± 4 ng m^-3) followed by winter (7± 2 ng m^-3). In contrast the highest concentrations of total α-dicarbonyls were found in winter (4± 2 ng m^-3).This study reported the chemical compositions and seasonal distributions of water-soluble organic aerosols over Okinawa, suggesting an influence of long-range transport of air masses from different region in each season. Higher concentrations of diacids and related compounds in spring followed by winter suggest that the airflow from Asian continent with biogenic emission in spring and anthropogenic emissions in winter. The lower concentrations were found in summer probably due to significant contribution of clean marine air masses. Interestingly, phthalic acid/azelaic acid (Ph/C9) ratios were also found to be higher in winter (ave. 4.8) followed by autumn (1.7) and spring (1.2) and lowest in summer (0.5), indicating the highest contribution of anthropogenic sources in winter.

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