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周二的作业来啦，大家保持队形完成作业哦。
加油！
Beijing's air pollution
Blackest day
[attachimg=595,399]112967[/attachimg]
【Time1】
ON January 12th of last year, in an article in the print edition of The Economist, we reported that the public outcry over Beijing’s atrocious air quality was putting pressure on officials to release more data about more kinds of pollutants. We also noted that Chinese authorities had already embarked on a wide range of strategies to improve air quality, and that they probably deserve more credit than either foreign or domestic critics tend to give them. But we concluded with the sad reality that such work takes decades, and that “Beijing residents will need to wait before seeing improvements.”
On January 12th of this year, Beijing residents got an acrid taste of what that wait might be like, as they suffered a day of astonishingly bad air. Pollution readings went, quite literally, off the charts. Saturday evening saw a reading of 755 on the Air Quality Index (AQI). That index is based on the recently revised standards of the American Environmental Protection Agency (the EPA), which nominally maxes out at 500. For more perspective, consider that any reading above 100 is deemed “unhealthy for sensitive groups” and that anything above 400 is rated “hazardous” for all.
Like many Beijing residents, your correspondent has mobile-phone apps that keep up with the pollution readings. At an otherwise pleasant Saturday-evening meal with friends, he joined his companions in compulsively checking for updates.
Those previously unseen numbers were hard to believe, but they did seem to match up well enough with the noxious soup we could see, smell and taste outside. We are all far more familiar with the specifics of air-quality measurement than we would like to be. Apart from the AQI readings above 700, we were quite struck to see the readings for the smallest and most dangerous sort of particulate matter, called PM 2.5, which can enter deep into the respiratory system. These are named for the size, in microns, of the particles. A reading at a controversial monitoring station run by the American embassy showed a PM 2.5 level of 886 micrograms per cubic metre; Beijing’s own municipal monitoring centre acknowledged readings in excess of 700 micrograms.
For perspective on that set of figures, consider that the guideline values set by the World Health Organisation regard any air with more than 25 micrograms of PM 2.5 per cubic metre as being of unacceptable quality.
【395】
【Time2】
Chinese authorities have complained about the American embassy's insistence on independently monitoring—and publicly reporting—Beijing’s air quality. And sometimes much is made of the vast differences between those readings and China’s own official ones, which are often less dire. Indeed, a key feature of one of those mobile-phone apps is the side-by-side comparison of those competing data-sets. (It is of course a bad sign that people here need more than one app to keep up with all this.)
But on a day like Saturday, the discrepancy between official readings and independent ones hardly seemed to matter; you didn't need a weatherman to know which way the ill wind blew. Or failed to blow, as the case may have been. One expert quoted by Chinese media attributed this spike in pollution to a series of windless days that allowed pollutants to accumulate.
But wind can be a problem when it does blow, too. In the outlying provinces that are part of Beijing’s airshed, there is a great deal of heavy industry. Pollution regulations are much harder to enforce there. And, in this colder-than-average winter, people have been burning more coal and wood than usual.
It is likely to be many more Januarys to come before China gets the upper hand on its air-pollution problems. Indeed, as we mentioned last January 12th, after nearly sixty years of trying and a vast amount of progress, the city of Los Angeles has yet to meet America's federal air-quality standards. If there is any consolation to what Beijing had to endure this January 12th, it is that it should lend urgency to the public outcry, and help speed things in the right direction.
The other consolation is that readings like the ones showing now on Monday midday (in the mid 300s, merely “hazardous” and “severely polluted”) feel fine by comparison.
【306】
NASA Rules out Earth Impact in 2036 for Asteroid Apophis
[attachimg=300,319]112968[/attachimg]
【Time3】
NASA scientists at the agency's Jet Propulsion Laboratory in Pasadena, Calif., effectively have ruled out the possibility the asteroid Apophis will impact Earth during a close flyby in 2036. The scientists used updated information obtained by NASA-supported telescopes in 2011 and 2012, as well as new data from the time leading up to Apophis' distant Earth flyby Jan. 9, 2013.
Discovered in 2004, the asteroid, which is the size of three-and-a-half football fields, gathered the immediate attention of space scientists and the media when initial calculations of its orbit indicated a 2.7 percent possibility of an Earth impact during a close flyby in 2029. Data discovered during a search of old astronomical images provided the additional information required to rule out the 2029 impact scenario, but a remote possibility of one in 2036 remained -- until Wednesday.
"With the new data provided by the Magdalena Ridge [New Mexico Institute of Mining and Technology] and the Pan-STARRS [Univ. of Hawaii] optical observatories, along with very recent data provided by the Goldstone Solar System Radar, we have effectively ruled out the possibility of an Earth impact by Apophis in 2036," said Don Yeomans, manager of NASA's Near-Earth Object Program Office at JPL. "The impact odds as they stand now are less than one in a million, which makes us comfortable saying we can effectively rule out an Earth impact in 2036. Our interest in asteroid Apophis will essentially be for its scientific interest for the foreseeable future."
【245】
【Time4】
The April 13, 2029, flyby of asteroid Apophis will be one for the record books. On that date, Apophis will become the closest flyby of an asteroid of its size when it comes no closer than 19, 400 miles (31,300 kilometers) above Earth's surface.
"But much sooner, a closer approach by a lesser-known asteroid is going to occur in the middle of next month when a 40-meter-sized asteroid, 2012 DA14, flies safely past Earth's surface at about 17,200 miles," said Yeomans. "With new telescopes coming online, the upgrade of existing telescopes and the continued refinement of our orbital determination process, there's never a dull moment working on near-Earth objects."
NASA detects and tracks asteroids and comets passing close to Earth using both ground and space-based telescopes. The Near-Earth Object Observations Program, commonly called "Spaceguard," discovers these objects, characterizes a subset of them and plots their orbits to determine if any could be potentially hazardous to our planet.
The Near-Earth Object Program Office at JPL manages the technical and scientific activities for NASA's Near-Earth Object Program of the Science Mission Directorate in Washington. JPL is a division of the California Institute of Technology in Pasadena.
【194】
Ladybugs Diet Influences Effectiveness as Biocontrol Agent
【Time5】
By examining what lady bugs eat, U.S. Department of Agriculture (USDA) scientists are learning more about the movement of these beneficial insects in farm fields -- and whether they'll actively feed on crop pests.
Agricultural Research Service (ARS) entomologist Jonathan Lundgren at the agency's North Central Agricultural Research Laboratory in Brookings, S.D., and former ARS entomologist Michael Seagraves were part of a team of ARS and university scientists that examined how a lady beetle's diet alters its feeding patterns and physiology. ARS is USDA's chief intramural scientific research agency.
Lady beetles are deployed as biological controls of insect pests like aphids and Colorado potato beetles. Understanding the feeding behavior of these important beneficial insects will help researchers find ways to most effectively use the lady beetles as biocontrol agents.
In laboratory feeding tests, the researchers found that a lady beetle species called Coleomegilla maculata consumes two to three times more plant tissue after being fed a prey-only diet than after being fed a mixed diet of prey and plant tissue. This suggests that plant material is providing some key nutrients lacking in prey-only diets. It is important to recognize that non-prey foods contain different nutrients than insect prey, and that beetles that are fed mixed diets are often healthier that those fed only on prey, according to Lundgren.
【218】
【Rest】
In a followup study, Lundgren and his colleagues found that sugar consumption by lady beetles allows females to survive and produce more eggs than those denied this sweet treat. Foods like sugar and pollen are important components of their diets, and it is thought that lady beetles rely heavily on sugar resources in the field. In this study, Lundgren and Seagraves applied sugar sprays to soybeans and quantified the frequency of sugar feeding by analyzing the gut contents of common lady beetles in South Dakota, Maryland and Kentucky.
According to Seagraves, all the tested lady beetles regularly consumed sugar-like nectar in soybean fields, even when it wasn't applied as a supplement. However, the sugar-sprayed plots had more lady beetles than the untreated plots. This research makes the case that sugar-feeding is very important for lady beetle populations in cropland and suggests a possible way to help maintain beneficial species in agroecosystems.
【151】
【Obstacle】
Galaxy's Gamma-Ray Flares Erupted Far from Its Black Hole
[attachimg=300,162]112969[/attachimg]
In 2011, a months-long blast of energy launched by an enormous black hole almost 11 billion years ago swept past Earth. Using a combination of data from NASA's Fermi Gamma-ray Space Telescope and the National Science Foundation's Very Long Baseline Array (VLBA), the world's largest radio telescope, astronomers have zeroed in on the source of this ancient outburst.
Theorists expect gamma-ray outbursts occur only in close proximity to a galaxy's central black hole, the powerhouse ultimately responsible for the activity. A few rare observations suggested this is not the case.
The 2011 flares from a galaxy known as 4C +71.07 now give astronomers the clearest and most distant evidence that the theory still needs some work. The gamma-ray emission originated about 70 light-years away from the galaxy's central black hole.
The 4C +71.07 galaxy was discovered as a source of strong radio emission in the 1960s. NASA's Compton Gamma-Ray Observatory, which operated in the 1990s, detected high-energy flares, but the galaxy was quiet during Fermi's first two and a half years in orbit.
In early November 2011, at the height of the outburst, the galaxy was more than 10,000 times brighter than the combined luminosity of all of the stars in our Milky Way galaxy.
"This renewed activity came after a long slumber, and that's important because it allows us to explicitly link the gamma-ray flares to the rising emission observed by radio telescopes," said David Thompson, a Fermi deputy project scientist at NASA's Goddard Space Flight Center in Greenbelt, Md.
Located in the constellation Ursa Major, 4C +71.07 is so far away that its light takes 10.6 billion years to reach Earth. Astronomers are seeing this galaxy as it existed when the universe was less than one-fourth of its present age.
At the galaxy's core lies a supersized black hole weighing 2.6 billion times the sun's mass. Some of the matter falling toward the black hole becomes accelerated outward at almost the speed of light, creating dual particle jets blasting in opposite directions. One jet happens to point almost directly toward Earth. This characteristic makes 4C +71.07 a blazar, a classification that includes some of the brightest gamma-ray sources in the sky.
Boston University astronomers Alan Marscher and Svetlana Jorstad routinely monitor 4C +71.07 along with dozens of other blazars using several facilities, including the VLBA.
The instrument's 10 radio telescopes span North America, from Hawaii to St. Croix in the U.S. Virgin Islands, and possess the resolving power of a single radio dish more than 5,300 miles across when their signals are combined. As a result, The VLBA resolves detail about a million times smaller than Fermi's Large Area Telescope (LAT) and 1,000 times smaller than NASA's Hubble Space Telescope.
In autumn 2011, the VLBA images revealed a bright knot that appeared to move outward at a speed 20 times faster than light.
"Although this apparent speed was an illusion caused by actual motion almost directly toward us at 99.87 percent the speed of light, this knot was the key to determining the location where the gamma-rays were produced in the black hole's jet," said Marscher, who presented the findings Monday at the American Astronomical Society meeting in Long Beach, Calif.
The knot passed through a bright stationary feature of the jet, which the astronomers refer to as its radio "core," on April 9, 2011. This occurred within days of Fermi's detection of renewed gamma-ray flaring in the blazar. Marscher and Jorstad noted that the blazar brightened at visible wavelengths in step with the higher-energy emission.
During the most intense period of flaring, from October 2011 to January 2012, the scientists found the polarization direction of the blazar's visible light rotated in the same manner as radio emissions from the knot. They concluded the knot was responsible for the visible and the gamma-ray light, which varied in sync.
This association allowed the researchers to pinpoint the location of the gamma-ray outburst to about 70 light-years from the black hole.
The astronomers think that the gamma rays were produced when electrons moving near the speed of light within the jet collided with visible and infrared light originating outside of the jet. Such a collision can kick the light up to much higher energies, a process known as inverse-Compton scattering.
The source of the lower-energy light is unclear at the moment. The researchers speculate the source may be an outer, slow-moving sheath that surrounds the jet. Nicholas MacDonald, a graduate student at Boston University, is investigating how the gamma-ray brightness should change in this scenario to compare with observations.
"The VLBA is the only instrument that can bring us images from so near the edge of a young supermassive black hole, and Fermi's LAT is the only instrument that can see the highest-energy light from the galaxy's jet," said Jorstad.
NASA's Fermi Gamma-ray Space Telescope is an astrophysics and particle physics partnership. Fermi is managed by NASA's Goddard Space Flight Center. It was developed in collaboration with the U.S. Department of Energy, with contributions from academic institutions and partners in France, Germany, Italy, Japan, Sweden and the United States.
The VLBA is operated by the National Radio Astronomy Observatory, a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.
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发表于 2013-1-14 16:40:38
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