Eri Tachibana

Fluorescent water-soluble organic aerosols in the High Arctic atmosphere

Organic aerosols are ubiquitous in the earth’s atmosphere. They have been extensively studied in urban, rural and marine environments. However, little is known about the fluorescence properties of water-soluble organic carbon (WSOC) or their transport to and distribution in the polar regions. Here, we present evidence that fluorescent WSOC is a substantial component of High Arctic aerosols. The ratios of fluorescence intensity of protein-like peak to humic-like peak generally increased from dark winter to early summer, indicating an enhanced contribution of protein-like organics from the ocean to Arctic aerosols after the polar sunrise. Such a seasonal pattern is in agreement with an increase of stable carbon isotope ratios of total carbon (δ13CTC) from −26.8‰ to −22.5‰. Our results suggest that Arctic aerosols are derived from a combination of the long-range transport of terrestrial organics and local sea-to-air emission of marine organics, with an estimated contribution from the latter of 8.7–77% (mean 45%).

Seasonal cycles of water-soluble organic nitrogen aerosols in a deciduous broadleaf forest in northern Japan

The seasonal variations in aerosol water-soluble organic nitrogen (WSON) concentrations measured in a deciduous forest canopy over an approximately 30 month period were investigated for possible sources in the forest. The WSON concentrations (average 157 ± 127 ng N m−3) and WSON/water-soluble total nitrogen mass fractions (average 20 ± 11%) in the total suspended particulate matter exhibited a clear seasonal cycle with maxima in early summer. The WSON mass was found to reside mostly in the fine-mode size range (Dp   1.9 µm), which was similar to the size distributions of sugar compounds, indicating that the major WSON sources in autumn are associated with primary biological emissions. The vertical differences in WSON concentrations suggest that the water-soluble organic aerosol is enriched with nitrogen below the canopy level relative to the forest floor. The WSON concentration increased with enhanced hydrogen ion concentrations in aerosol in the early summer, indicating that aerosol acidity associated with anthropogenic sources outside the forest likely plays an important role in the formation of WSON in that season. The study suggests that multiple sources of WSON within the forest canopy may dominate over others in specific seasons, providing insights into WSON formation processes in forest environments.

Stable carbon and nitrogen isotopic compositions of tropical atmospheric aerosols: sources and contribution from burning of C 3 and C 4 plants to organic aerosols

In this paper, we report for the first time the δ13C and δ15N data for PM2.5 and PM10 aerosols collected in Tanzania during May–August 2011 together with total carbon (TC) and nitrogen (TN) contents. Mean TC concentrations were 6.5±2.1 µg m−3 in PM2.5 and 9.2±3.5 µg m−3 in PM10. δ13C of TC ranged from −26.1 to −20.6‰ with a mean of −23.6‰ in PM2.5 and from −24.4 to −22.4‰ with a mean of −23.6‰ in PM10. We found substantially greater δ13C values in PM2.5 samples during dry season as well as strong positive correlation between levoglucosan (and nss-K+) and TC in PM2.5. These results suggest a significant contribution from burning of C4 plants to fine organic aerosol formation. TN contents showed a mean of 0.7±0.3 µg m−3 in PM2.5 and 0.8±0.2 µg m−3 in PM10. δ15N ranged from +13.4 to +22.1‰ with a mean of +16.2±2.7‰ in PM2.5 and +10.4 to +18.7‰ with a mean of +13.7±2.2‰ in PM10. δ15N showed higher ratios in fine particles than coarse particles in both wet and dry season. The relatively high δ15N values suggest isotopic enrichment of 15N in aerosols via exchange reaction between NH3 (gas) and (particle). We found a strong correlation between TN and (r 2=0.94 in PM2.5 and r 2=0.86 in PM10) and (r 2=0.48 in PM2.5 and r 2=0.55 in PM10). We also found that organic nitrogen is less significant than inorganic nitrogen in the Morogoro aerosols. Based on stable carbon isotopic composition, contributions of burning C3 plants to TC were estimated to range from 42 to 74% in PM2.5 and from 39 to 64% in PM10, whereas those of C4 ranged from 26 to 58% in PM2.5 and from 36 to 61% in PM10. These results suggest that burning activities of C3 and C4 plants contribute to organic aerosol formation in Tanzania.

Stable carbon isotopic compositions of low-molecularweight dicarboxylic acids, glyoxylic acid and glyoxal in tropical aerosols: implications for photochemical processes of organic aerosols

Tropical aerosols of PM2.5 and PM10 were collected at a rural site in Morogoro, Tanzania (East Africa), and analysed for stable carbon isotopic composition (δ13C) of dicarboxylic acids (C2–C9), glyoxylic acid (ωC2) and glyoxal (Gly) using gas chromatography/isotope ratio mass spectrometer. PM2.5 samples showed that δ13C of oxalic (C2) acid are largest (mean, −18.3±1.7‰) followed by malonic (C3, −19.6±1.0‰) and succinic (C4, −21.8±2.2‰) acids, whereas those in PM10 are a little smaller: −19.9±3.1‰ (C2), −20.2±2.7‰ (C3) and −23.3±3.2‰ (C4). The δ13C of C2–C4 diacids showed a decreasing trend with an increase in carbon numbers. The higher δ13C values of oxalic acid can be explained by isotopic enrichment of 13C in the remaining C2 due to the atmospheric decomposition of oxalic acid or its precursors. δ13C of ωC2 and Gly that are precursors of oxalic acid also showed larger values (mean, −22.5‰ and −20.2‰, respectively) in PM2.5 than those (−26.7‰ and −23.7‰) in PM10. The δ13C values of ωC2 and Gly are smaller than those of C2 in both PM2.5 and PM10. On the other hand, azelaic acid (C9; mean, −28.5‰) is more depleted in 13C, which is consistent with the previous knowledge; that is, C9 is produced by the oxidation of unsaturated fatty acids emitted from terrestrial higher plants. A significant enrichment of 13C in oxalic acid together with its negative correlations with relative abundance of C2 in total diacids and ratios of water-soluble organic carbon and organic carbon further support that a photochemical degradation of oxalic acid occurs during long-range transport from source regions.

Identification of hydroxy‐ and keto‐dicarboxylic acids in remote marine aerosols using gas chromatography/quadruple and time‐of‐flight mass spectrometry

RATIONALE The identification of hydroxy- and keto-dicarboxylic acids (diacids) in remote marine aerosol samples is important for a better understanding of the composition of organic particulate matter, as this chemical composition is essential for predicting the effects on climate, air quality, and human health. Molecular characterization of these compounds provides insights into sources and formation pathways of organic aerosols. METHODS The method of chemical derivatization followed by gas chromatography-flame ionization detection (GC-FID), gas chromatography/quadruple mass spectrometry (GC/QMS) and gas chromatography/time-of-flight mass spectrometry (GC/TOFMS) was used to identify hydroxy- and keto-diacids in remote marine aerosols. Atmospheric samples were collected at Chichijima Island in the western North Pacific and the diacids and related compounds were extracted with organic-free ultrapure water. A two-step derivatization technique was employed, using 14% BF3 /n-butanol for the butylation of carboxyl groups and acidic ketones followed by N,O-bis(trimethylsilyl)trifluoroacetamide (BSTFA) for the trimethylsilylation of hydroxyl groups. RESULTS Several new peaks were detected in the gas chromatogram after trimethylsilylation of the dibutyl ester fraction. Based on mass spectral interpretation with authentic standards, we successfully identified and quantified a homologous series of hydroxydiacids, including tartaric and oxaloacetic acids. In addition, transformation of oxaloacetic acid into its enol form was elucidated. CONCLUSIONS Utilizing GC-FID, GC/QMS and GC/TOFMS, hydroxy- and keto-diacids were identified in the remote marine aerosols. A complete structural characterization was achieved with extensive mass spectral analysis. Molecular distributions of hydroxydiacids generally showed the predominance of malic acid followed by tartronic acid. We consider that these hydroxydiacids are important intermediates in the atmospheric oxidation of organic aerosols to result in smaller diacids. Copyright

Evidence of a reduction in cloud condensation nuclei activity of water-soluble aerosols caused by biogenic emissions in a cool-temperate forest

Biogenic organic aerosols can affect cloud condensation nuclei (CCN) properties, and subsequently impact climate change. Large uncertainties exist in how the difference in the types of terrestrial biogenic sources and the abundance of organics relative to sulfate affect CCN properties. For the submicron water-soluble aerosols collected for two years in a cool-temperate forest in northern Japan, we show that the hygroscopicity parameter κCCN (0.44 ± 0.07) exhibited a distinct seasonal trend with a minimum in autumn (κCCN = 0.32–0.37); these κCCN values were generally larger than that of ambient particles, including water-insoluble fractions. The temporal variability of κCCN was controlled by the water-soluble organic matter (WSOM)-to-sulfate ratio (R2 > 0.60), where the significant reduction of κCCN in autumn was linked to the increased WSOM/sulfate ratio. Positive matrix factorization analysis indicates that α-pinene-derived secondary organic aerosol (SOA) substantially contributed to the WSOM mass (~75%) in autumn, the majority of which was attributable to emissions from litter/soil microbial activity near the forest floor. These findings suggest that WSOM, most likely α-pinene SOA, originated from the forest floor can significantly suppress the aerosol CCN activity in cool-temperate forests, which have implications for predicting climate effects by changes in biogenic emissions in future.

Chemical transfer of dissolved organic matter from surface seawater to sea spray water-soluble organic aerosol in the marine atmosphere

It is critical to understand how variations in chemical composition in surface seawater (SSW) affect the chemistry of marine atmospheric aerosols. We investigated the sea-to-air transfer of dissolved organic carbon (DOC) via cruise measurements of both ambient aerosols and SSW in the Oyashio and its coastal regions, the western subarctic Pacific during early spring. Sea spray aerosols (SSAs) were selected based on the stable carbon isotope ratio of water-soluble organic carbon (WSOC) (δ13CWSOC) and concentrations of glucose as a molecular tracer in marine aerosols together with local surface wind speed data. For both SSA and SSW samples, excitation-emission matrices were obtained to examine the transfer of fluorescent organic material. We found that the ratios of fluorescence intensity of humic-like and protein-like substances in the submicrometer SSAs were significantly larger than those in the bulk SSW (~63%). This ratio was also larger for the supermicrometer SSAs than for the SSW. The results suggest significant decomposition of protein-like DOC on a timescale of <12–24 h and/or preferential production of humic-like substances in the atmospheric aerosols regardless of the particle size. This study provides unique insights into the complex transfer of DOC from the ocean surface to the atmosphere.