Yutaka Tobo

Phylogenetic analysis of atmospheric halotolerant bacterial communities at high altitude in an Asian dust (KOSA) arrival region, Suzu City.

The microbial communities transported by Asian desert dust (KOSA) events have attracted much attention as bioaerosols because the transported microorganisms are thought to influence the downwind ecosystems in Korea and Japan. However, the atmospheric microbial community has not been investigated at high altitude in the KOSA arrival area. In this study, to estimate the viability and diversity of atmospheric halotolerant bacteria, which are expected to resist to various environmental stresses as well as high salinities, bioaerosol samples were collected at 10 and 600 m above the ground within the KOSA arrival area, Suzu City, Japan, during KOSA events. During the sampling period, the particle numbers at 600 m were higher than those at 10 m, suggesting that large particles of aerosol fall from the high altitude of 600 m to the ground surface. The microorganisms in bioaerosol samples grew in media containing up to 15% NaCl concentrations demonstrating the viability of the halotolerant bacteria in bioaerosol samples. The PCR-DGGE analysis using 16S rDNA revealed that the bacterial species in NaCl-amended cultures were similar to the bacteria detected from the genomic DNA directly extracted from the bioaerosol samples. The 16S rDNA sequences of bacterial communities in bioaerosol samples were classified into 4 phylotypes belonging to the Bacilluscereus or Bacillussubtilis group. The bioaerosol samples collected at 600 m included 2 phylotypes belonging to B. subtilis, and one phylotype among all 4 phylotypes was identical between the samples at 10 and 600 m. In the atmosphere at 600 m, the halotolerant bacterial community was expected to remain viable, and the species composition was expected to include a few species of the genus Bacillus. During this investigation period, these atmospheric bacteria may have been vertically transported to the ground surface, where the long-range KOSA particle transport from China is frequently observed.

Asian dust particles converted into aqueous droplets under remote marine atmospheric conditions

The chemical history of dust particles in the atmosphere is crucial for assessing their impact on both the Earth’s climate and ecosystem. So far, a number of studies have shown that, in the vicinity of strong anthropogenic emission sources, Ca-rich dust particles can be converted into aqueous droplets mainly by the reaction with gaseous HNO3 to form Ca(NO3)2. Here we show that other similar processes have the potential to be activated under typical remote marine atmospheric conditions. Based on field measurements at several sites in East Asia and thermodynamic predictions, we examined the possibility for the formation of two highly soluble calcium salts, Ca(NO3)2 and CaCl2, which can deliquesce at low relative humidity. According to the results, the conversion of insoluble CaCO3 to Ca(NO3)2 tends to be dominated over urban and industrialized areas of the Asian continent, where the concentrations of HNO3 exceed those of HCl ([HNO3/HCl] > ∼ 1). In this regime, CaCl2 is hardly detected from dust particles. However, the generation of CaCl2 becomes detectable around the Japan Islands, where the concentrations of HCl are much higher than those of HNO3 ([HNO3/HCl] < ∼ 0.3). We suggest that elevated concentrations of HCl in the remote marine boundary layer are sufficient to modify Ca-rich particles in dust storms and can play a more important role in forming a deliquescent layer on the particle surfaces as they are transported toward remote ocean regions.

An improved approach for measuring immersion freezing in large droplets over a wide temperature range

Immersion freezing (ice nucleation by particles immersed in supercooled water) is a key process for forming ice in mixed-phase clouds. Immersion freezing experiments with particles in microliter-sized (millimeter-sized) water droplets are often applied to detecting very small numbers of ice nucleating particles (INPs). However, the application of such large droplets remains confined to the detection of INPs active at temperatures much higher than the homogeneous freezing limit, because of artifacts related to freezing of water droplets without added INPs at temperatures of −25 °C or higher on a supporting substrate. Here I report a method for measuring immersion freezing in super-microliter-sized droplets over a wide temperature range. To reduce possible artifacts, droplets are pipetted onto a thin layer of Vaseline and cooled in a clean booth. In the Cryogenic Refrigerator Applied to Freezing Test (CRAFT) system, freezing of pure (Milli-Q) water droplets are limited at temperatures above −30 °C. An intercomparison of various techniques for immersion freezing experiments with reference particles (Snomax and illite NX) demonstrates that despite the use of relatively large droplets, the CRAFT setup allows for evaluating the immersion freezing activity of the particles over almost the entire temperature range (about −30 °C to 0 °C) relevant for mixed-phase cloud formation.

Hunting the Snark: Identifying the Organic Ice Nuclei in Soils

The contribution of soil organic matter as a potential source of atmospheric ice nuclei (IN) has long been postulated. Rather surprisingly, considering the abundance of IN active at warm temperatures in many soils, it remains unresolved. This research aimed to identify sources of high-temperature, organic IN in a range of Wyoming and Colorado soils. Methods used included physical, chemical and enzymatic tests combined with quantitative PCR to estimate the number of ice nucleation active bacteria. All soils contained 106 to >107 IN active at −10°C. Reductions in IN after heating or digestion with hydrogen peroxide suggested that IN active >−15°C were effectively all organic. Ice nuclei active >−7°C appear to be primarily a mixture of biological macromolecules. At colder temperatures there was a large pool of organic IN that were quite resistant to most physico-chemical challenges.

Ice nucleation by particles containing long-chain fatty acids of relevance to freezing by sea spray aerosols

Heterogeneous ice nucleation in the atmosphere regulates cloud properties, such as phase (ice versus liquid) and lifetime. Aerosol particles of marine origin are relevant ice nucleating particle sources when marine aerosol layers are lifted over mountainous terrain and in higher latitude ocean boundary layers, distant from terrestrial aerosol sources. Among many particle compositions associated with ice nucleation by sea spray aerosols are highly saturated fatty acids. Previous studies have not demonstrated their ability to freeze dilute water droplets. This study investigates ice nucleation by monolayers at the surface of supercooled droplets and as crystalline particles at temperatures exceeding the threshold for homogeneous freezing. Results show the poor efficiency of long chain fatty acid (C16, C18) monolayers in templating freezing of pure water droplets and seawater subphase to temperatures of at least -30 °C, consistent with theory. This contrasts with freezing of fatty alcohols (C22 used here) at nearly 20 °C warmer. Evaporation of μL-sized droplets to promote structural compression of a C19 acid monolayer did not favor warmer ice formation of drops. Heterogeneous ice nucleation occurred for nL-sized droplets condensed on 5 to 100 μm crystalline particles of fatty acid (C12 to C20) at a range of temperatures below -28 °C. These experiments suggest that fatty acids nucleate ice at warmer than -36 °C only when the crystalline phase is present. Rough estimates of ice active site densities are consistent with those of marine aerosols, but require knowledge of the proportion of surface area comprised of fatty acids for application.

Biological ice nuclei and the impact of rain on ice nuclei populations

With 18% of total US landmass devoted to croplands, farmland is a potentially major source of biogenic particles to the atmosphere. We investigated two farms as potential sources of biological ice nuclei (IN). We found that each of these farms contained abundant INA bacteria on the vegetation; however, airborne ina gene concentrations were typically below detectable limits, demonstrating a disconnect between local vegetative sources and the air above them. The question remains, then, as to how biological IN are released into the atmosphere. In a second study, we investigated how precipitation impacted the concentration and composition of IN. Results from these measurements show that ground level IN concentrations were enhanced during rain events, and that some portion of these IN were biological. In this paper, we present results from both of these studies, and discuss modified measurement techniques aimed at characterizing the often very low number concentrations of biological IN.

Investigation of ice nucleation properties of mineral and soil particles

Number fractions of different types of submicron mineral (kaolinite) and soil (China loess soils and agricultural soils from different crops) particles capable of nucleating ice under mixed-phase cloud conditions were investigated using a continuous flow diffusion ice chamber (CFDC). We found that both China loess and agricultural soil particles are better ice nucleators than kaolinite particles particularly at warmer temperatures. We also found that although the ice nucleation properties of untreated China loess and agricultural soil particles are relatively similar to each other, agricultural soil particles lose their ice nucleating ability after heat treatment at 300°C. Our results suggest that agricultural soils contain rich organic matter, which can play a key role in enhancing their ice nucleating ability.

Composition of halophilic bacteria survived in bioaerosol

Teruya Maki, Shinzi Susuki, Fumihisa Kobayashi, Makiko Kakikawa, Maromu Yamada, Tomomi Higashi, Chunsang Hong, Yutaka Tobo, Hiroshi Hasegawa, Kazumasa Ueda and Yasunobu Iwasaka 1 Graduate School of Natural Science and Technology, Kanazawa University, Kakuma, Kanazawa, 920-1192, Japan. 2 Institute of Nature and Environmental Technology, Kanazawa University, Kakuma, Kanazawa, 920-1192, Japan. 3 Faculty of Environmental and Symbiotic Science, Prefectural University of Kumamoto.3-1-100 Tsukide, Kumamoto 862-8502, Japan 4 Hygiene, Kanazawa University School of Medicine, 13-1 Takara-machi, Kanazawa 920-8640, Japan 5 Frontier Science Organization, Kanazawa University, Kakuma, Kanazawa, 920-1192, Japan.

Ice Cloud Formation Mechanisms Inferred from in situ Measurements of Particle Number-size Distribution

4 Institute of Atmospheric Physics, Chinese Academy of Science, Beijing 100029, China Abstract Ice crystals in cirrus clouds are formed via homogeneous or heterogeneous pathway. This work addresses in situ measurements of ice crystal concentrations in the region that is thought to be in a favorable condition for ice nucleation. Based on a series of measurements by a balloon-borne optical particle counter, possible scenarios of ice cloud formations will be discussed.