China's spectacular economic growth during the last decadehas brought many benefits – and some challenges. Global atmosphericmapping of nitrogen dioxide pollution performed by ERS-2's GOME andEnvisat's SCIAMACHY reveals the world's largest amount of NO2 hangingabove Beijing and northeast China, as reported in Nature this week.
Aspart of ESA's Dragon Programme, European and Chinese researchers areusing results returned from the Global Ozone Mapping Experiment (GOME)on ERS-2 and the Scanning Imaging Absorption Spectrometer forAtmospheric Chartography (SCIAMACHY) on Envisat to monitor and forecastChinese air quality.
In this context, researchers at theUniversity of Bremen, the Max-Planck Institute of Meteorology inHamburg and France's Centre National de la Recherche Scientifique(CNRS) have been studying the retrieval of nitrogen dioxide variabilityfrom space and modelling its global behaviour.
The team havepublished an article in the 1 September 2005 edition of the sciencejournal Nature about the global changes in nitrogen dioxide observed inthe last decade from space and highlighted the dramatic changes overChina.
Nitrogen dioxide (NO2) is associated with nitrogen oxide(NO) in the atmosphere and the sum of the two is called NOX. This isreleased into the troposphere from power plants, heavy industry androad transport, along with biomass burning, lightning in the atmosphereand microbial activity in the soil. The emission of nitrogen oxides hasincreased about six-fold since pre-industrial times and in cities abovea thousand times more NOX is present than in the pristine and remotemarine boundary layer.
Exposure to nitrogen dioxide in largequantities is known to cause lung damage and respiratory problems,although little is known about the consequences of long term exposureto elevated atmospheric amounts. The presence of this gas is asignificant driver of the production of low-level ozone, which, withinthe troposphere (the lowest part of the atmosphere, extending eight to16 kilometres in height) is itself a harmful toxic pollutant, a majoringredient of photochemical smog.
"While nitrogen dioxidevertical column concentrations above central and eastern Europe andparts of the East Coast of the United States have been either static orexhibiting a small decrease, there is a clear and significant increaseover China," explains John Burrows of the University of Bremen'sInstitute of Environmental Physics, SCIAMACHY's Principal Investigator.
"BeforeSCIAMACHY was flying we previously retrieved NO2 data from itsprecursor instrument, GOME on ESA's ERS-2 mission. Although GOME hadlower resolution, the article shows that China's nitrogen dioxideretrievals from the two instruments overlap seamlessly.
"What thecombined data show are that nitrogen dioxide levels have risen byaround 50% since 1996, and this behaviour is continuing."
Space-basedsensors are the only way to carry out effective global and regionalmonitoring of the atmosphere. While GOME demonstrated the firstsatellite sensitivity to tropospheric nitrogen dioxide, SCIAMACHYpossesses superior performance, with a spatial resolution of 60 x 30kilometres compared to 320 x 40 km for its predecessor.
SCIAMACHYalso observes the atmosphere in two different ways – downwards ornadir-sounding' as well as 'limb-sounding' along the direction offlight – and with a larger spectral range than its predecessor.
Theincrease in nitrogen dioxide levels seen is an unfortunate side effectof economic success. China's industrial boom has seen it become theworld's largest consumer of copper, aluminium and cement and the secondbigger importer of oil. Car ownership within the country has beendoubling every few years.
"China's nitrogen dioxide concentrationvaries according to season," Burrows adds. "There is more in the winteras a result of differing emission patterns and meteorology. For examplemore fuel is burned for heating and nitrogen dioxide persists longer inthe atmosphere at that less sunny time of year – lasting around a dayrather than hours, as in the summer.
"Meteorology also plays a role. There is a peak before Christmas: thisis not because industrial activity, domestic heating or transportationis suddenly reduced after the holiday season but because there is aneastward outflow of air that was previously revolving around Asia. Thisis the same type of phenomenon that carries dust from the Gobi Desertacross to the West Coast of the US."
China is reliant on coal tomeet 75% of its national energy needs, and that means high levels ofanother atmospheric pollutant called sulphur dioxide (SO2) alsodetectable by SCIAMACHY. Large SO2 sources over China that overlap withnitrogen dioxide plumes are linked to power plants. Further to the westthere is also sulphur dioxide produced from smouldering undergroundcoal seam fires.
Burrows is the scientist who – supported by aninternational team – proposed both GOME and SCIAMACHY to national spaceagencies and ESA in the first place. He explained that the twoinstruments were originally chosen to fly because of their ability tomeasure stratospheric ozone, but were also selected in order toinvestigate the amount of useful information that could be retrievedfrom the troposphere.
"The instruments are now being used tomonitor a significant number of key tropospheric trace gases includingformaldehyde, methane, sulphur dioxide and carbon monoxide anddioxide," Burrows remembers. "Back when we were starting out, manypeople thought it would be impossible to get any useful results out ofthe troposphere. There are many important issues to deal with, such ascloud cover and the highly variable reflectivity of the surface, aswell as having the absorption or emission of stratospheric and upperatmospheric species situated between the troposphere and the instrument.
"Firstwith GOME and now much better with SCIAMACHY we are demonstrating itcan be done. The success so far is an important step on the way toestablishing an operational global observing system for the Earth'satmosphere. This is something we need as we enter the geological age ofthe Anthropocene, where the activities of mankind and its interactionswith natural phenomena are the driving force in global climate change.
"Nextwe are hoping for follow-up satellite missions, in particular fromgeostationary orbit to monitor atmospheric pollution, which has astrong diurnal variation and thereby determine objectively the changingatmospheric composition."
SCIAMACHY: surveying the world in six days
SCIAMACHY is aspectrometer, and it works by measuring sunlight – either transmitted,reflected or scattered by the Earth's atmosphere or surface in theultraviolet, visible and near infrared regions. Mathematical inversionof these data yields the amounts and distribution of trace gases, ozoneand related chemicals, clouds and dust particles throughout theatmosphere. With a 960-km swath and alternate limb and nadirobservations, SCIAMACHY covers the entire world every six days at theequator and more often at high latitudes.
This versatileinstrument represents a national contribution to ESA's Envisat mission.It was funded by the German government through the German AerospaceCenter (DLR), the Dutch government through the Netherlands Agency forAerospace Programmes (NIVR) and also the Belgian government through theBelgian Institute for Space Aeronomy (BIRA-IASB).
SCIAMACHY ispart of a family of atmospheric sounders that also includes GOME onERS-2 and also the forthcoming GOME-2 instrument due to launch nextyear aboard ESA's and EUMETSAT's first MetOp spacecraft.
The Dragon Programme is a joint undertakingbetween ESA, the Ministry of Science and Technology (MOST) of China andthe National Remote Sensing Centre of China (NRSCC). Its purpose is toencourage increased exploitation of ESA space resources within China aswell as stimulate increased scientific co-operation in the field ofEarth Observation science and applications between China and Europe.
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