The solar radiation reaching the earth surface supports life on the earth and a small change in intensity would change the earth climate to a large extent. The observed surface solar radiation shows a decreasing trend from the 1950s to 1980s and an increasing trend since then (Global dimming and brightening, GDB). This transition can affect any surface physical processes and climate changes, such as global warming, the hydrological cycle, the cryosphere, and the terrestrial biosphere. Since GDB is recognized under both cloudy and cloud-free skies, the changes in clouds and aerosols, especially anthropogenic aerosols, are considered to be one of the key factors. In this paper, observational evidences, causes, General Circulation Model (GCM) simulations, and impacts of GDB are reviewed.
The aerosol impacts on warm clouds (aerosol indirect effects) are generally categorized into the cloud albedo effect (the Twomey effect) and the effects including cloud feedbacks, such as the cloud lifetime effect. As evidence of the Twomey effect, a number of in-situ observations over the world, including those over the Western Pacific, has shown an increase in cloud droplet number concentration near the cloud base in response to an increase in aerosol number concentration below cloud. Cloud feedbacks have also been intensively studied, however, they appear to change depending on meteorological regimes (called a buffered system) and are not as simple as hypothesized initially as the cloud lifetime effect. Because of a unique meteorology in addition to the world highest aerosol loading over the East Asia-Western Pacific region, further studies should be made to understand how aerosols can affect cloud lifecycle (such as precipitation) and macro-structure of cloud (such as cloud liquid water path and cloud fraction) in this region.
As one of mechanisms of the influence of variability in solar activity on the global climate, the atmospheric ionization by reaction of the galactic cosmic rays with atmosphere has an important role. A correlation between the galactic cosmic-ray intensity and the global cloud cover has been observed and a hypothesis is suggested that the variability in the solar activity controls global climate through the cosmic rays. It is necessary to clarify the mechanism of the correlation as well as verification of observed quantities. Here we describe how the cosmic rays produce aerosol particles, which originate cloud condensation nuclei. We also introduce the present status of laboratory experiments to verify these processes.
One hypothesis regarding global climate regulation by marine phytoplankton was proposed in 1987. This is called the CLAW hypothesis that describes the negative feedback loop through controlling the emission of biogenic sulfur compounds, dimethylsulfide (DMS) as responses to climate parameter changes such as solar radiation and sea surface temperature, resulting in controlling non sea-salt sulfate aerosols, cloud condensation nuclei (CCN) and cloud albedo, thus eventually mitigating the initial changes. However, verification of this hypothesis has not yet been concluded. This is mainly due to lack of our understandings both on the processes and responses described in this hypothesis, because of complexities in the processes of DMS production and loss in the seawater, and in those of aerosols and CCN formations in the atmosphere. In this paper, the progress of recent research on these processes and responses is briefly reviewed, and contribution of DMS to the climate regulation is discussed. Although research on climate regulation by DMS has not yet completed, on the basis of discussion with updated results, it can be concluded that this contribution is not sufficient to regulate the global climate.
Super-eruption release voluminous sulfuric gas into the atmosphere, which produce a large amount of sulfuric acid aerosol in the stratosphere. The sulfuric acid aerosol reflects incoming solar radiation, giving rise to the rapid climatic cooling called the “volcanic winter” on the Earth’s surface. The Toba super-eruption 74,000 years ago, the largest eruption in a million years on the Earth, erupted as much as 2,800 km3 of felsic magma. The average global temperature is estimated to have lowered more than 10°C by the Toba super-eruption, which is inferred to have caused a severe bottleneck of the human population. Modern civilized human society will also suffer fatal damage by “volcanic winter”, if a super-eruption were to occur today.
The eolian dust in the sediments of North Pacific and marginal seas of the northwestern Pacific has potential to record past climate changes. For example, the variability of eolian dust flux estimated from North Pacific sediments have been interpreted as recording past aridity changes of the eolian source region of the East Asia. However, recent observations revealed large effects of the wind-systems on the amount of dust transported to the North Pacific and marginal seas. Therefore, it is important to consider both effects of the wind-systems and aridity changes when we interpret eolian dust parameters. Recently, we estimated provenance of eolian dust in Japan Sea sediments and reconstructed its variations during the last glacial period based on a newly developed provenance-tracing method, a combination of electron spin resonance intensity and the crystallinity of quartz. The result revealed millennial-scale provenance changes in accordance with so-called Dansgaard-Oeschger (D-O) events, with dominance of quartz from the Mongolian Gobi during cold intervals of D-O events, whereas dominance of quartz from the Taklimakan Desert during warm intervals of D-O events. These provenance variations seem to mainly represent latitudinal displacement of the subtropical jet path in harmony with D-O events.
DSMC (Direct Simulation Monte Carlo) technique is a direct approach to solve the Boltzmann equation. To simulate small scale phenomena of gas mixture close to molecular size, such as behavior of nano-particles, particle formation from gas, etc., DSMC technique was extended to the rarefied gas mixture. This advanced technique was applied to the fundamental problems, such as Rayleigh’s problem, velocity distribution of gas molecules in equilibrium gas mixture, and time-dependent change of distribution of diffused gas molecules. Simulated results using DSMC technique extended to gas mixture showed a good agreement with theories and previous studies. Also, characteristics of Bird-scheme and Nanbu-scheme which have been widely applied to treat collisions between molecules in DSMC method were studied. Finally, a practical problem, the deposition of sputtered molecule, was simulated, and this technique was proved to be a powerful tool for solving this sort of problems.
We describe the design and performance of an aerosol spectrometer that simultaneously measures the size-dependent number concentration and chemical composition of particulate organic matter. The spectrometer consists of a differential mobility analyzer (DMA) for size classification and a gas chromatograph-mass spectrometer (GC-MS) for composition analysis. Size-classified particles of an aromatic hydrocarbon were directly introduced into a thermal desorption (TD) tube packed with adsorbent. Then, thermally desorbed aromatic hydrocarbon molecules were introduced into the GC-MS. Comparisons between the number concentrations of the classified particles measured with a Faraday cup electrometer and the peak area of nonylbenzene in the chromatogram of the GC-MS determined the detection limit of our spectrometer. For a particle size of 330 nm, the lower detection limit of the spectrometer was found to be approximately 2×103 particles/cm3 for an adsorption time of 30 seconds. This value was approximately 1/25 of the value of 5×104 particles/cm3 reported as the lower detection limit in our previous work (Okada et al., 2010).