Annual Report

2006	2005	2004	2003
2002	2001	2000	1999
1998	1997	1996	1995

Annual Report 2006

By K. Hirose, Director, Geochemical Research Department (GRD)

The Annual Report 2006, a summary of geochemical studies on chemical substances in the atmosphere and ocean related to environment and climate change performed by researchers of GRD/ Meteorological Research Institute and collaborated researchers are presented here, referring to papers published in 2006.

1. Atmospheric chemistry

1.1     Global model simulations of the transport of Asian and Sahara dust: total deposition of dust mass in Japan .

   Lee, H.N., Y. Igarashi, M. Chiba, M. Aoyama, K. Hirose and T. Tanaka

The global transport of dust emitted from major dust regions on earth was simulated by using a global three-dimensional chemical transport model. A modified dust emission flux and an advanced nonlocal diffusion scheme for accurately determining the atmospheric eddy diffusivity in the atmospheric boundary layer were implemented to improve the chemical transport model. Analyzed dust storms occurred in China and northern Africa, in which the Loess region, the Gobi, Takla Makan and Sahara deserts were taken into account as four source regions. The simulated results of dust transport were compared with observed dust deposition in Japan . The dust from the Sahara desert reached Japan in 9-10 days, whereas Gobi and Loess region dust took 2 to 3 days and 4 days for Takla Makan dust. The model-calculated total deposition of dust mass from all major dust source regions was in agreement with observation in Japan .

1.2     Seasonal and spatial variation of atmospheric ²¹⁰Pb and ⁷Be deposition: features of the Japan Sea side of Japan

    Yamamoto, M., A. Sakaguchi, K. Sasaki, K. Hirose, Y. Igarashi and C.K. Kim

Monthly deposition of ²¹⁰Pb and ⁷Be at Tatsunokuchi , Japan , located in the Japan Sea side, were determined during the period from 1991 to 2002. The results were compared with that of other sites in Japan and Korea ; 11 Japanese sites from Ishigaki to Wakkanai and Taejon in Korea . The monthly depositions of both ²¹⁰Pb and ⁷Be at Tatsunokuchi showed a seasonal variation with high in winter and low in summer. The deposition amounts of these radionuclides in the Japan Sea side sites were much greater than that in the Pacific Ocean sites.

1.3     Re-construction and updating our understanding on the global weapons tests ¹³⁷Cs fallout

    Aoyama, M., K. Hirose and Y. Igarashi

Global nuclear weapons testing fallout of ¹³⁷Cs in the globe has been documented in the UNSCEAR reports. However, many questions have arisen during the past three to four decades: a typical example is that the water column inventories of ¹³⁷Cs in the North Pacific were two or three times higher than the cumulative decay corrected fallout at the same latitude as stated in the UNSCAER reports. Here we re-construct more precious spatial distribution of global ¹³⁷Cs fallout with 10° x 10° grids primarily on the basis of global measurements of rain, seawater and soil. A typical feature of geographical distribution is that two high global ¹³⁷Cs fallout areas exist in the northern hemisphere, where the highest ¹³⁷Cs was observed in the globe.

1.4     Seasonal variations in SO₂ plume transport over Japan : Observations at the summit of Mt. Fuji from winter to summer

Igarashi, Y., Y. Sawa, K. Yoshioka, H. Takahashi, H. Matsueda, Y. Dokiya

Continuous measurements of SO₂ concentrations at the summit of Mt. Fuji were carried out during the period January to July, 2004. This is the first data set that covers the previous gap in SO₂ time series for springtime middle troposphere over Japan . The time series data show trends in high- SO₂ long-range transport events from winter to spring. Elevated SO₂ concentrations are always accompanied by elevated CO and ²²²Rn concentrations, and backward trajectories for such events indicate a continental origin. These data display clear sporadic signatures of Asian outflow over Japan to the North Pacific. Such high- SO₂ events decline in strength and frequency during spring. The seasonality of SO₂ was discussed with the prevailing wind direction and relative humidity.

1.5     Fog and rain water chemistry at Mt. Fuji : A case study during the September 2002 campaign

Watanabe, K., T. Takeba, N. Soda, Y. Igarashi, H. Takahashi, Y. Dokiya

Chemical constituents in fog and rain collected at the summit of Mt. Fuji , as well as at Tarobo, the ESE slope of Mt. Fuji were measured in September 2002. The pH in fog and rain waters at Mt. Fuji varied over a range of 4.0-6.8. Acidic fogs (pH<5) occurred at the summit when the air mass came from the industrial region of the Asian continent. High concentrations of Na⁺ and Cl^- were observed in the rain sample at the summit, possibly because of the long-range transport of sea-salt particles produced by a typhoon.

1.6     Seasonal and long-term variations in atmospheric CO₂ and ⁸⁵Kr in Tsukuba, central Japan

Inoue, H.Y., H. Matsueda, Y. Igarashi, Y. Sawa, A. Wada, K. Nemoto, H. Sartorius, C. Schlosser

Since February, measurements of atmospheric CO₂ at 200 m above the ground level have been carried out using a meteorological tower in Tsukuba, central Japan . We have examined seasonal/long-term variations in atmospheric CO₂, based on ⁸⁵Kr, which is a noble gas with well-known source and sink terms, monitored continuously at weekly intervals in Tsukuba since May 1995. During the annual cycle, the daily mean values of atmospheric CO₂ in the afternoons showed maximum in late April, except in 2002, and minimum in late August or early September. The seasonal variations of atmospheric ⁸⁵Kr with low in summer and high in winter were caused by the seasonal variations in the large-scale atmospheric circulation, and latitudinal distribution of atmospheric ⁸⁵Kr. Comparing with atmospheric ⁸⁵Kr, the draw-down and buildup for atmospheric CO₂ occurred about 1 week and 3 week later, respectively. A decrease of the seasonal CO₂ minimum was accompanied with a decrease of the atmospheric ⁸⁵Kr concentration, showing the effect of air mass transport from different origins. Unlike the minimum, the maximum CO₂ did not vary as expected from the air mass transport.

2. Marine chemistry

2.1     A new method to determine depth-dependent carbon export fluxes using vertical 230Th profiles

Hirose, K.

Because of their particle-reactive properties, radiogenic thorium isotopes in oceanic waters have been used in studies for the particle removal processes. In particular, ²³⁴Th has extensively been utilized to determine the carbon export flux from the euphotic zone. On the other hand, ²³⁰Th in seawater has so far mainly provided information on residence times, settling velocities and adsorption-desorption rates of ²³⁰Th in the water column. Here we introduce a new model to calculate the carbon export flux from vertical ²³⁰Th profiles. According to this model, the carbon export flux is directly related to the POC/²³⁰Th ratio in sinking or suspended particulate matter. The carbon export flux can be directly calculated from vertical profiles of POC and particulate ²³⁰Th, which should become an effective means to constrain ocean biogeochemical models.

2.2     Plutonium isotopes in seawater of the North Pacific: effect of close-in fallout

    Hirose, K., M. Aoyama, C.S. Kim, C.K. Kim, P. P. Povinec

The behavior of ^239,240Pu in marine environments was described as well as recent ^239,240Pu concentrations in seawater of the North Pacific. The ^239,240Pu concentrations in surface waters of the North Pacific ranged from 1 to 10 mBq m^-3 in 2001 and 2002. Vertical ^239,240Pu profiles in the mid-latitude of the eastern North Pacific and the equatorial western North Pacific showed a typical pattern with surface minimum, mid-depth maximum and gradual decrease with increasing depth, although temporal variations of ^239,240Pu profiles depended on sea areas. ²⁴⁰Pu/²³⁹Pu atom ratios in seawater samples, ranging from 0.17 to 0.28, were within values of global fallout and close-in fallout. The plutonium isotope signature suggests that major sources of plutonium in the western North Pacific were both global fallout and close-in fallout. Plutonium concentrations in shallower layers (<1000 m) of the North Pacific varied temporally by both physical and biogeochemical processes, whereas its behavior is rather conservative in deep layers (>2000 m).

2.3     Southern hemisphere ocean tracer study (SHOTS): an overview and preliminary results

    Aoyama, M., M. Fukasawa, K. Hirose, R.F.C. Mantoura, P.P. Povinec, C.S. Kim and K. Komura

About 900 seawater samples for anthropogenic radionuclide analysis were collected in the subtropical gyres of the South Pacific, the South Atlantic and the Indian Ocean during the BEAGLE2003 conducted in 2003-2004. As a preliminary result, we presented longitudinal distributions of ¹³⁷Cs and ^239,240Pu and vertical profiles of ¹³⁷Cs and ^239,240Pu in one site in the South Pacific. A typical feature is that surface ¹³⁷Cs and ^239,240Pu concentrations in the South Pacific were similar to those in the North Pacific and another feature is that the ¹³⁷Cs in the South Pacific showed a smaller effect of subduction of water mass transport comparing with that in the North Pacific. The SHOTS collaboration will produce a comprehensive data set on anthropogenic radionuclides in the Southern Hemisphere Oceans.

2.4     Chemical speciation of trace metals in seawater: a review

Hirose, K.

The recent development of the chemical speciation of trace metals in seawater is reviewed. The speciation studies reveal that metal ion complexation is one of the most important processes in seawater; especially, most bioactive trace metals, such as Fe(III) and Cu, exist as complexes with ligands in dissolved organic matter. The organic ligands in seawater are characterized with metal ions selected by the HSAB concept. A strong organic ligand, which originates from marine microorganisms, is classified as a hard base including carboxylates. The free organic ligand concentrations in seawater are buffered by complexation with excess amounts of Ca and Mg in seawater. The chemical equilibrium model suggested that the concentrations of bioactive free metal ions are at an optimal level to activities of marine microorganisms. For chemical speciation, it is important to have a better understanding of the ecological roles of trace metals in seawater.

2.5     Interannual variability of winter oceanic CO₂ and air-sea CO₂ flux in the western North Pacific for 2 decades.

Midorikawa, T., M. Ishii, K. Nemoto, H. Kamiya, A. Nakadate, S. Masuda, H. Matsueda, T. Nakano, H.Y. Inoue

The 2-decade records of the partial pressure of CO2 in surface water and the resulting air-sea CO₂ flux in the extensive subtropical to the equatorial region along 137°E in the western North Pacific in winter were analyzed. The result revealed presence of significant interannual variations that differed in different region. The larger variation of pCO₂^sea in the equatorial region is related to ENSO events. The magnitude of the variations differed year by year, corresponding to the different degrees of compensation between the sea surface temperature (SST) and dissolved inorganic carbon (DIC) effects on pCO₂^sea dring the events. Small pCO₂^sea variations in the subtropical gyre north of 23°N were due to highly counteracting effects between anticorrelated SST and DIC anomalies through the entrainment process. In contrast, a low negative correlation existed between SST and DIC, associated with the lateral advection in the region restricted around 15°N and 18°N in the North Equatorial Current, resulting in a large amplitude of variations of pCO₂^sea and hence CO₂ influx. It is important to monitor how the contributions of different properties to the CO₂ flux could change in the North Pacific, responding to possible shifts of the ENSO conditions and the future progress of global warming.

2.6     The latest batch-to-batch difference table of standard seawater and its application to the WOCE onetime sections

Kawano, T., M. Aoyama, T. Joyce, H. Uchida, Y. Takatsuki, M. Fukasawa

An updated batch-to-batch difference table of IAPSO standard seawater (SSW) up to P145 is proposed. Obtained batch-to-batch differences range from 1.2x10^-3 to ?1.9x10^-3 with reference to the average of those from P91 to P102. Batch-to-batch differences from P29 to P145 with reference to the recent batches and this average over every 5 years since 1960 are also presented, together with standard deviation. This reveals that inconsistency among batches had improved since the 1980s. Proposed batch-to-batch differences were applied to the observational results of the WOCE hydrographic onetime section in the Indian Ocean.

2.7     Bottom water warming along the pathway of lower circumpolar deep water in the Pacific Ocean

Kawano, T., M. Fukasawa, S. Kouketsu, H. Uchida, T. Doi, I. Kaneko, M. Aoyama, W. Schneider

Repeat trans-Pacific hydrographic observations along the pathway of Lower circumpolar Deep Water (LCDW) have been carried out. The result reveals that bottom water has warmed by about 0.005 to 0.01°C in recent decades. The warming is probably not from direct heating of LCDW, but is manifest as a decrease of the coldest component of LCDW evident at each hydrographic section. This finding is consistent with numerical model results of warming associated with decreased bottom water formation rates around Antarctica .

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