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学术报告:How large is the cosmic dust flux into the earth’s atmosphere?
2016-04-06 | 编辑: | 【  】【打印】【关闭

  题目1:How large is the cosmic dust flux into the earth’s atmosphere?

  报告人:J. M. C. Plane(School of Chemistry, University of Leeds, United Kingdom)

  题目2:Global Atmospheric Chemical Modelling Studies:Surface to Thermosphere

  报告人:Wuhu Feng (NCAS, School of Earth and Environment; School of Chemistry, University of Leeds, United Kingdom)

  时间:2016年4月8日(周五) 上午9:00-11:00

  地点:综合楼七楼报告厅

  报告摘要:

  How large is the cosmic dust flux into the earth’s atmosphere?

  J. M. C. Plane

  School of Chemistry, University of Leeds, United Kingdom

  Cosmic dust particles are produced in the solar system from the sublimation of comets as they orbit close to the sun, and also from collisions between asteroids in the belt between Mars and Jupiter. Current estimates of the magnitude of the cosmic dust input rate into the Earth’s atmosphere range from 2 to well over 100 tons per day, depending on whether the measurements are made in space, in the middle atmosphere, or in polar ice cores. This nearly 2 order-of-magnitude discrepancy indicates that there are serious flaws in the interpretation of observations that have been used to make the estimates.

  Dust particles enter the atmosphere at hyperthermal velocities (11 – 72 km s-1), and mostly ablate at heights between 80 and 120 km in a region of the atmosphere known as the mesosphere/lower thermosphere (MLT). The resulting metal vapours (Fe, Mg, Si and Na etc.) then oxidize and recondense to form nm-size particles, termed “meteoric smoke”. These particles are too small to sediment downwards. Instead, they are transported by the general circulation of the atmosphere, taking roughly 5 years to reach the surface. There is great interest in the role smoke particles play as condensation nuclei - of noctilucent ice clouds in the mesosphere, and polar stratospheric clouds in the lower stratosphere - and in the ability of these particles to remove acidic gases. There are also potential implications for climate, as the input of bio-available cosmic Fe in the Southern Ocean can increase biological productivity and stimulate CO2drawdown from the atmosphere.

  In this colloquium I will describe three new estimates of the dust input, and attempt to reconcile them. The first estimate comes from a zodiacal dust cloud model which predicts that more than 90% of the dust entering the atmosphere comes from Jupiter Family Comets, and that the dust is mostly in a near-prograde orbit and should enter the atmosphere with an average velocity around 14 km s-1. However, relatively few of these slow particles are observed, even by the powerful Arecibo 430 MHz radar. Using coupled models of meteoroid differential ablation, ionization and radar detection to compute the probability of detecting a specified meteoroid in the Arecibo beam, an upper limit to the cosmic dust input of 16 t d-1has been obtained from the radar observations.

  The second method uses lidar measurements of the vertical Na and Fe atom fluxes in the MLT, combined with predictions of the relative geographic variations of the key wave-induced vertical transport processes from the Whole Atmosphere Community Climate Model (WACCM). The estimated global influx of cosmic dust is then between 50 and 150 t d-1.

  The final method is to model several of the mesospheric metal layers - Na, Fe, K and Ca - using WACCM with a full treatment of the gas-phase chemistry of these metals, together with the explicit formation and growth of meteoric smoke particles. The absolute densities of the metal layers can be satisfactorily modelled with a dust input of up to 25 t d-1, but only if the dust mass and velocity distribution is that predicted by the zodiacal dust cloud model referred to above. The zodiacal dust model also produces a flux of cosmic spherules to the surface which is in good agreement with that measured in ice at the South Pole.

  Underpinning this modelling work are new laboratory experiments at Leeds. Three systems will be described: a Meteor Ablation Simulator, which measures the evaporation of metals from cosmic dust particles that are flash heated to over 3000 K; a Time-of-Flight mass spectrometer with laser photo-ionization which is used to study the reactions of neutral metallic compounds in the gas phase; and a flowing afterglow experiment to study the dissociative recombination of metallic ions with electrons.

  Global Atmospheric Chemical Modelling Studies:Surface to Thermosphere

  Wuhu Feng

  NCAS, School of Earth and Environment, University of Leeds, United Kingdom

  School of Chemistry, University of Leeds, United Kingdom

  Three-dimensional chemical transport models (CTMs) and coupled chemistry-climate models (CCMs) have been widely used to study the dynamical and chemical processes which control the distributions of atmospheric compositions.In this talk, I will mainly focus on the stratospheric ozone depletion and mesospheric metal layers. I will also brief introduce two different models and their applications for the research purposes: one is our NCAS (National Centre for Atmospheric Science) community CTM (TOMCAT/SLIMCAT,www.see.leeds.ac.uk/slimcat) whichcontains a detailed description of stratospheric and tropospheric chemistry as well as a detailed aerosol module.The model has been widely used to study transport and chemistry in the upper troposphere and lower stratosphere (UTLS) and also forWMO ozone assessment. The other is our modelling developmentbased on the NCAR Community Earth System Model (CESM): global atmospheric model of meteoric metals and meteoric smoke particles (MSPs).

  ----

  National Centre for Atmospheric Science

  School of Earth and Environment, University of Leeds, Leeds, LS2 9JT

  Tel: +44 113 343 6408

  http://homepages.see.leeds.ac.uk/~earfw/








 
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