remote chemical sensing

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CHAPTER 4

REMOTE CHEMICAL SENSING

APPLICATIONFOR ATMOSPHERE MONITORING

Dong Jiang

Yaohuan Huang

Dafang Zhuang

1. INTRODUCTION

Accompanying the acceleration o urbanization and industrialization, air pollution has become one othe most serious environmental problems on Earth. It afects not only human health but also the health

o ecological systems. Te atmosphere protects lie on Earth by absorbing ultraviolet solar radiation,

warming the surace through heat retention, and reducing temperature extremes between day and night.

However, severe loss o stratospheric ozone has been detected in the high latitudes o the Northern

Hemisphere as well as over the Antarctic. At the same time, intensication o ultraviolet radiation has

been observed. Ultraviolet radiation is known to be a danger to human beings as well as having an efect

on agriculture, orests, and water ecosystems.

Global air pollution studies have been an important topic. Te observation and collection o reli-

able data on regional and global air quality has a rather brie history. Routine atmospheric measure-

ments o gas and particle concentrations have been conducted at sites with ground-based instruments

(such as atmospherically emitted radiance intererometers), which has severely restricted the area o

land that can be monitored. Te ground instruments are designed to monitor specic pollutants (e.g.,

carbon dioxide), and many o these instruments cannot provide an accurate description o the total

concentration o all pollutants at a regional level (Mark et al. 2004). Remote sensing satellites have many

advantages or monitoring air quality. Satellite observations can provide a complete survey o a region,


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128 CHEMICAL SENSORS. VOLUME 6: CHEMICAL SENSORS APPLICATIONS

showing the major sources o pollution and the distribution pattern (Xuemei et al. 2001). Since the

early 1970s, remote sensing instrumentation have been developed, abricated, and operated or remotelymeasuring several atmospheric parameters. Tey have proved to be e cient tools or atmospheric moni-

toring. Many new chemical remote sensing sensors, such as NASAs Aura (EOS-CHEM), have been

established in recent years. race gas and aerosol instrumentation have been developed and operated

to measure ambient concentrations o trace gases and aerosols and the exchange o trace gases with the

Earths surace. Such data and inormation support the research into the atmospheric energy balance, the

hydrological cycle, climate trends, and other aspects o the atmospheric system that are o vital interest

to us. Remote sensing is central to this efort because it is the only way we can obtain ull spatial and

temporal perspective needed to understand atmospheric processes (Michael 1993).

Tis chapter ocuses mainly on the remote sensing sensors onboard satellites. Satellite remote sens-

ing o trace gases and aerosols or air quality applications appeared in the middle o the last century.

In the 1970s, the global distributions o H2O, CH4, and HNO3 were obtained rom the U.S. GOES

meteorological satellite. Lyons et al. presented an image rom the GOES satellite showing a large area ohaze covering the Midwest United States (Lyons et al. 1976). Fraser et al. used GOES observations to

conduct the rst retrieval o aerosol optical depth over land and applied it to examine a haze event over

the eastern United States (Fraser et al. 1984). Ater that, the OMS instrument on board the Nimbus-7

meteorological satellite gathered important inormation about the O3 distribution in the troposphere.

In recent years, satellite remote sensing o air quality has evolved dramatically. Global observations are

now available or a wide range o species, including aerosols, tropospheric O3, tropospheric NO2, CO,

HCHO, and SO2 (Randall 2008).

2. TECHNIQUES AND INSTRUMENTS FOR

ATMOSPHERE MONITORING

2.1. TECHNIQUES FOR ATMOSPHERE MONITORING

Remote sensing in the most generally accepted meaning reers to instrument-based techniques employed

in the acquisition and measurement o spatially organized (most commonly, geographically distributed)

data/inormation on some properties (spectral, spatial, physical) o an array o target points (pixels)

within the sensed scene that correspond to eatures, objects, and materials, doing this by applying one

or more recording devices not in physical, intimate contact with the item(s) under surveillance (Xuemei

et al. 2001). Su ce to say that remote sensing is a tool or gathering inormation, usually about the sur-

ace o the Earth and the atmosphere. Remote chemical sensing techniques or retrieval o atmosphere

parameters all broadly into three categories.

2.1.1. Optical Remote Sensing

Optical remote sensing makes use o visible, near-inrared, and short-wave inrared sensors to orm

images o the Earths surace by detecting the solar radiation reected rom targets on the ground


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REMOTE CHEMICAL SENSING 129

(wavelength < 4 mm). Diferent materials reect and absorb diferently at diferent wavelengths. When

a remote sensing satellite sends a signal toward the Earth, the signal comes into contact with the atmo-sphere and is modied by the interaction between the radiation and the atmospheric components. Te

sensor on the satellite then records the modied signal and determines both the geometric and radio-

metric changes in the signal. Te change in the signal is due to particle absorption and elastic scattering.

By adding the scattering and absorption components, and integrating these components along the path

between the Earths surace and the satellites altitude, the particulate optical thickness can be calculated.

Additional corrections may be made to the particulate optical thickness (Chance 2006).

race gas remote sensing using solar backscatter takes advantage o attenuation in the intensity o

radiation traversing a medium. Tis attenuation is commonly expressed as Beers law(Randall 2008):

,0sI I e

-

= (4.1)

where I is the backscattered intensity observed by a satellite instrument at a specic wavelength , I,0

is the backscattered intensity that would be observed in the absence o absorption, is the absorp-

tion cross section o the trace gas, and s is the trace gas abundance over the atmospheric path length,

which is commonly reerred to as the slant column. race gas retrieval using solar backscatter exploits

the spectral variation in to iner s, including a spectral t to determine atmospheric abundance over

the radiation path and a radiative transer calculation to determine the path o radiation through the

atmosphere (Bowman et al. 2006; Randall 2008).

2.1.2. Thermal Infrared Remote Sensing

Atmosphere monitoring in the thermal inrared uses spectral variation in absorbed and emitted radia-

tion to iner trace gas abundance (spectral range: 450 mm). Te upwelling thermal intensity at the

top o the atmosphere is the sum o contributions rom the surace and the atmosphere. Te vertical

distribution o a trace gas can be obtained by exploiting the pressure dependence o the trace gas spectral

emission lines (Bowman et al. 2006).

race gas proles derived rom thermal inrared observations typically have little sensitivity near the

surace because inrared instruments depend on thermal contrast, although boundary-layer sensitivity is

possible under conditions with high contrast between the skin temperature and the air temperature, and

with enhanced boundary-layer concentrations (Randall 2008).

2.1.3. Active Remote Sensing

Active sensors onboard satellites, such as lidar, transmit energy downward and measure the backscatter.

Te diferential absorption lidar technique provides three-dimensional mapping o gas distributions in

the atmosphere. Pulses rom a tunable laser are transmitted into the atmosphere, and photons, elastically

backscattered rom aerosols and major constituents, are collected by an optical telescope, giving rise to


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an electrical transient ater detection in a photomultiplier tube. It has been ound that NOx, CO, CO2,

SO2, O3, etc., can be monitored using active laser remote sensing techniques (Edner et al. 1992).Laser radar monitoring o the environment is an application o time-resolved laser spectroscopy.

Diferential optical absorption as well as laser-induced uorescence can be used or this type o remote

sensing. Apart rom providing range-resolved data, the use o an active illumination source provides a

more accurate assessment than i just the ambient passive radiation is employed (Andersson 1997).

2.2. SENSORS FOR ATMOSPHERE MONITORING

As mentioned in Section 2.1, satellite remote sensing o the atmosphere alls broadly into three catego-

ries, so sensors or atmosphere monitoring can also be classied into three types accordingly, as optical

sensors, thermal sensors, and active laser radar sensors.

2.2.1. Optical Sensors

Te main optical sensors used or atmosphere monitoring are listed in able 4.1. Te objectives, spectral

characters, and main applications are described in detail in the ollowing paragraphs.

SPECTRALRANGESENSORS PLATFORM PERIOD (m) MAINAPPLICATIONS

OMS Nimbus-7 19781993 0.310.38 O3, aerosolMeteor-3 19962005AURA

GOME-1 ERS-2 19952003 0.230.79 NO2, HCHO, SO2, O3GOME-2 MetOP 2006 0.240.79 NO2, HCHO, SO2, O3MOPI erra 1999 4.7 COMODIS erra 1999 0.414.4 Aerosol

Aqua 2002SCIAMACHY ENVISA 2002 0.232.3 NO2, HCHO, SO2, O3OMI Aura 2004 0.270.50 NO2, HCHO, SO2, O3, aerosolPOLDER PARASOL 2004 0.441.0 Aerosol

Table 4.1. Optical sensors for atmosphere monitoring

2.2.1.1. TOTAL OZONE MAPPING SPECTROMETER (TOMS)

Te OMS program (otal Ozone Mapping Spectrometer), specializing in ozone retrieval, began with

the launch o OMS Flight Model #1 on the Nimbus-7 spacecrat on October 24, 1978. Te Nimbus-7

OMS instrument measures backscattered ultraviolet radiance rom Earth at wavelength bands cen-

tered at 312.5, 317.5, 331.3, 339.9, 360.0, and 380.0 nm. Te rst our wavelengths are sensitive to


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ozone; the two longer wavelengths are used or estimating the scene reectivity necessary or deriving

ozone amounts. Te OMS instrument ell silent in May 1993. It was the only source o high-reso-lution global inormation about ozone. Meteor-3, a Russian satellite, also carried a OMS. It stopped

operating on December 27, 1994. Earth Probe OMS, onboard AURA, was launched on July 2, 1996,

and continued to present OMS data until it experienced calibration problems in recent years. Te

Ozone Monitoring Instrument (OMI) onboard AURA is currently the only NASA spacecrat in orbit

that specializes in ozone retrieval (Qiu et al. 2008).

OMS measures the total solar radiance incident on the satellite andcompares it to the ultravio-

let radiation scattered back rom the atmosphere (40400 nm). otal column ozone is inerred rom

the diferential absorption o scattered sunlight in the ultraviolet using the ratio o two wavelengths,

312 and 331 nm, or instance, where one wavelength is strongly absorbed by ozone while the other is

weakly absorbed. Because it depends on scattered solar radiation, OMS does not work at night. Ozone

measurements given by OMS are in Dobson units and give the total ozone in a column (http://toms.

gsc.nasa.gov/index_v8.html).

2.2.1.2. GOME-1 AND GOME-2

GOME, a nadir-scanning ultraviolet and visible spectrometer or global monitoring o atmospheric

OZone, was launched onboard ERS-2 in April 1995. It measures solar backscatter with broad spectral

coverage (230790 nm) and moderate resolution (0.20.4 nm). A key eature o GOME is its ability

to detect other chemically active atmospheric trace gases as well as aerosol distribution (www.esa.int/

esaLP/ESAS5VYWC_LPmetop_0.html). GOME-1 has been measuring ozone (total column and

prole), nitrogen dioxide, and other minor trace gases since 1995. An advanced GOME-2 instrument

on the MEOP satellites will provide the input or the ozone data record in the timerame 20052020,

provided by the EUMESA Polar System. ropospheric NO2, HCHO, SO2, and tropospheric O3 can

be retrieved rom GOME-1 data. GOME-2 observes all the species o GOME-1. Inormation about the

spatial-temporal distribution o tropospheric trace gases has been presented based on GOME-2.

2.2.1.3. MOPITT

Te MOPI instrument onboard NASAs erra satellite is a nadir-viewing gas correlation radiometer

operating in the 4.7-m band, measuring tropospheric carbon monoxide on the global scale. MOPI

has been operational since March 2000. Te MOPI instrument measures upwelling inrared radi-

ances in absorption bands o both CO and methane using the technique o gas-lter correlation radi-

ometry. Ultimately, MOPI-retrieved CO proles are either analyzed directly or are assimilated intomodels to study the chemistry and dynamics o CO (and other constituents) in the lower atmosphere

(www.acd.ucar.edu/mopitt/concepts.shtml). MOPI retrievals o CO have been thoroughly validated

in a variety o geographical settings (Emmons et al. 2007). MOPI measurements enable scientists to

analyze the distribution, transport, sources, and sinks o CO, a trace gas produced by methane oxida-

tion, ossil uel consumption, and biomass burning.


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2.2.1.4. MODERATE-RESOLUTION IMAGING

SPECTRORADIOMETER (MODIS)

MODIS (Moderate Resolution Imaging Spectroradiometer) is a key instrument aboard the erra (EOS

AM) and Aqua (EOS PM) satellites. erras orbit around the Earth is timed so that it passes rom north to

south across the equator in the morning, while Aqua passes south to north over the equator in the ater-

noon. erra MODIS and Aqua MODIS are viewing the entire Earths surace every 12 days, acquiring

data in 36 spectral bands with varying spatial resolution o 250, 500, and 1000 m (http://modis.gsc.nasa.

gov/). Te channels span the spectral range rom 405 to 14,385 nm, and bandwidth varies rom channel

to channel. Aerosol retrievals over land rom MODIS were described originally by Kauman et al. (1997).

wo independent retrievals are conducted at 470 and 660 nm, and subsequently interpolated to 550

nm. Te surace reectance or the channels at 470 and 660 nm are estimated rom measurements at 2.1

mm using empirical relationships (Levy 2007). Te products rom MODIS or atmosphere monitoring

include aerosol products, water vapor products, and atmosphere prole products. Te aerosol productsrom MODIS include aerosol type, aerosol optical thickness, particle size distribution, aerosol mass con-

centration, optical properties, and radioactive orcing; Te water vapor product monitors atmospheric

water vapor and precipitable water. Te atmosphere prole product monitors proles o atmospheric

temperature and moisture, atmospheric stability, and total ozone burden (http://modis.gsc.nasa.gov).

2.2.1.5. SCANNING IMAGING ABSORPTION SPECTROMETER FOR

ATMOSPHERIC CARTOGRAPHY (SCIAMACHY)

Te SCanning Imaging Absorption spectroMeter or Atmospheric CartograpHY (SCIAMACHY) is an

imaging spectrometer whose primary mission objective are global measurements o trace gases in the tro-

posphere and in the stratosphere. Te solar radiation transmitted, backscattered, and reected rom the

atmosphere is recorded at relatively high resolution (0.21.5 nm) over the range 2401700 nm, and in se-

lected regions between 2.0 and 2.4 m (www.iup.uni-bremen.de/sciamachy). SCIAMACHY has three di-

erent viewing geometries, nadir, limb, and sun/moon occultation, which yield total column values as well

as distribution proles in the stratosphere and (in some cases) the troposphere or trace gases and aerosols.

Te large wavelength range o SCIAMACHY is also ideally suited or the detection o clouds and aerosols.

2.2.1.6. OMI

OMI measurements are one o the our instruments on the Aura platorm launched on July 15, 2004.

Te OMI instrument can distinguish among aerosol types, such as smoke, dust, and sulates, and mea-

sures cloud pressure and coverage, which provide data to derive tropospheric ozone. OMI continues

the OMS record or total ozone and other atmospheric parameters related to ozone chemistry and

climate (http://aura.gsc.nasa.gov/instruments/omi.html). OMI is a nadir-viewing imaging spectrometer

that uses two-dimensional CCD detectors to measure the solar radiation backscattered by the Earths

atmosphere and surace over 270500 nm with a spectral resolution o 0.5 nm (Randall 2008). Te OMI

instrument employs hyperspectral imaging in a push-broom mode to observe solar backscatter radiation


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in the visible and ultraviolet. Te hyperspectral capabilities improve the accuracy and precision o the total

ozone amounts and also allow or accurate radiometric and wavelength sel-calibration over the long term.

2.2.1.7. POLARIZATION AND DIRECTIONALITY OF THE EARTHS

REFLECTANCE (POLDER)

POLDER (Polarization and Directionality o the Earths Reectance) is settled on the PARASOL,

the second microsatellite in the Myriade series. POLDER is designed to improve our knowledge o

the radiative and microphysical properties o clouds and aerosols by measuring the directionality and

polarization o light reected by the Earthatmosphere system (http://smsc.cnes.r/PARASOL). Te

POLDER instrument consists o a digital camera with a CCD detector array, wide-eld telecentric op-

tics, and a rotating lter wheel enabling measurements in nine spectral channels rom blue (0.443 mm)

through to near-inrared (1.020 mm) and in several polarization directions. Polarization measurementsare perormed at 0.490, 0.670, and 0.865 mm. Te bandwidth is between 20 and 40 nm, depending on

the spectral band (Chance 2006; Randall 2008).

2.2.2. Thermal Sensors

Te main thermal sensors or atmosphere monitoring are listed in the able 4.2. Te objectives, spectral

characters, and main applications are described in detail in the ollowing paragraphs.

SPECTRALRANGESENSORS PLATFORM PERIOD (m) APPLICATION

OVS IROS 1978 3.515.5 O3CLAES UARS 19911993 3.512.9 O3, NO, NO2, HNO3, ClONO3IMG ADEOS 19961997 3.316.7 CO, HNO3 AIRS EOS 2002 3.715.4 O3, CO, CH4ES Aura 2004 2.3-15.3 O3, CO, CH4, NO, NO2, HNO3IASI MetOP 2006 3.6215.5 O3, CO, CH4

Table 4.2. Thermal sensors for atmosphere monitoring

2.2.2.1. TOVS

Te IROS Operational Vertical Sounder (OVS) aboard NOAAs IROS series o polar orbiting

satellites, launched in 1978, consists o three instruments: the High Resolution Inrared Radiation

Sounder (HIRS), the Microwave Sounding Unit (MSU), and the Stratospheric Sounding Unit (SSU).

Te MSU and SSU have been replaced with improved instruments, the AMSU-A and AMSU-B, on the

newer satellites (www.ozonelayer.noaa.gov/action/tovs.htm). OVS has a band at 9700 nm, an impor-

tant ozone-absorption band. Like the OMS data, OVS gives total ozone column concentrations in


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Dobson units, but the quality and accuracy o its data are dependent on cloud conditions. OVS data

are best when collected under cloudless conditions. Te NOAA satellite that carries OVS is a polarorbiter that passes close enough to the poles to give continuous data.

2.2.2.2. CRYOGENIC LIMB ARRAY ETALON SPECTROMETER (CLAES)

Te CLAES (Cryogenic Limb Array Etalon Spectrometer) instrument was launched on the Upper

Atmosphere Research Satellite (UARS) in September 1991. Instead o measuring reected radiation

by using spectroscopy, it determines the amount o ozone by measuring the radiance emitted at several

wavelengths. CLAES makes measurements o thermal emission rom the Earths limb in a number o

spectral regions which are then used to derive stratospheric altitude proles o temperature, pressure,

ozone (O3), water vapor (H2O), methane (CH4), nitrous oxide (N2O), nitrogen oxide (NO), nitrogen

dioxide (NO2), dinitrogen pentoxide (N2O5), nitric acid (HNO3), chlorine nitrate (ClONO2), CFCl3,and CF2Cl2. Aerosol extinction coe cients are also calculated or each spectral region (http://badc.nerc.

ac.uk/data/claes) Unlike data rom OMS, CLAES can provide data at night, since it measures emitted

radiation rather than solar radiance. However, its orbit prevents collection o data in the vicinity o the

poles (www.lmsal.com/9130.html).

2.2.2.3. INTERFEROMETRIC MONITOR FOR GREENHOUSE GASES (IMG)

Te IMG (Intererometric Monitor or Greenhouse gases) was launched as one o eight sensors boarding

the ADEOS satellite (Advanced Earth Observing Satellite) in August 1996. Te ADEOS satellite ceased

to collect and transmit data in June 1997 due to a power ailure in its solar panel. IMG is a Michelson-

type Fourier transorm spectrometer (FS) with two mirrors and a beam splitter. Te incident radiation

received rom the Earth is divided by the beam splitter into two paths. One mirror is moved so that

the two paths produce an intererence pattern when they are recombined. Te signal measured by the

detector, the intererogram, can be inverse Fourier-transormed to obtain the incident spectrum. Te

diameter o the entrance aperture or the optics is 10 cm. Te scanning mirror is suspended on magnetic

bearings and scans a 10-cm-long path in 10 s (www.eorc.jaxa.jp/AtmChem/IMG).

IMG was the rst high-resolution nadir inrared tropospheric sounder that allowed simultaneous

retrieval o several trace gases. IMG obtained detailed spectra o thermal inrared radiation rom the

Earths surace and the atmosphere. Termal inrared spectra include absorption and emission signatures

o many atmospheric gases. IMGs high-resolution spectra give atmospheric concentrations o water

vapor and other greenhouse gases, and also temperature proles.

2.2.2.4. ATMOSPHERIC INFRARED SOUNDER (AIRS)

Te Atmospheric Inrared Sounder (AIRS), an advanced sounder containing 2378 inrared channels,

our visible/near-inrared channels, and a 13.5-km nadir eld o view, aimed at obtaining highly accurate


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temperature proles within the atmosphere plus a variety o additional Earth/atmosphere products

(http://aqua.nasa.gov/about/instrument_airs.php). AIRS is the highlighted instrument in the AIRS/AMSU-A/HSB triplet centered on measuring accurate temperature and humidity proles throughout

the atmosphere. AIRS measures the Earths outgoing radiation at 0.41.0 m and at 3.715.4 m with

1 K temperature retrieval accuracy per 1-km layer in the troposphere (Liu 2008).

AIRS uses cutting-edge inrared technology to create three-dimensional maps o air and surace

temperature, water vapor, and cloud properties. With 2378 spectral channels, AIRS has a spectral reso-

lution more than 100 times greater than previous inrared sounders and provides more accurate inor-

mation on the vertical proles o atmospheric temperature and moisture. AIRS can also measure trace

greenhouse gases such as ozone, carbon monoxide, carbon dioxide, and methane (http://airs.jpl.nasa.

gov/overview/overview).

2.2.2.5. TROPOSPHERIC EMISSION SPECTROMETER (TES)

Te ropospheric Emission Spectrometer (ES) was launched into sun-synchronous orbit aboard Aura,

the third o NASAs Earth Observing System (EOS) spacecrat, in July 2004. Te primary objective o

ES is to make global, three-dimensional measurements o ozone and other chemical species involved

in its ormation and destruction.

ES is a Fourier-transorm inrared emission spectrometer with high spectral resolution (0.1 cm1)

and coverage over a wide spectral range (6503050 cm1) (Randall 2008). ES is a high-resolution imag-

ing inrared Fourier-transorm spectrometer that operates in both nadir and limb-sounding modes. ES

global survey standard products include prole measurements o ozone, water vapor, carbon monoxide,

methane, nitrogen dioxide, and nitric acid or 16 orbits every other day. ES Special Observations are

research measurements o targeted locations or regional transects which are used to observe specic

phenomena or to support local or aircrat validation campaigns (Beer 2006). ropospheric O3 and CO

are retrieved with an optimal estimation method. In cloud-ree conditions the vertical resolution o the

O3 estimate is about 6 km, with sensitivity to both the lower and upper troposphere but reduced sensi-

tivity in the boundary layer (Worden et al. 2004).

2.2.2.6. INFRARED ATMOSPHERIC SOUNDING INTERFEROMETER (IASI)

IASI (Inrared Atmospheric Sounding Intererometer) is a state-o-the-art, sophisticated sounding in-

strument that will be used or global measurements o atmospheric temperature and moisture with

unprecedented accuracy and spectral resolution to improve weather prediction. Te IASI instru-

ment consists o a Fourier-transorm spectrometer associated with an imaging system, designed tomeasure the inrared spectrum emitted by the Earth in the thermal inrared using a nadir geometry.

Te instrument is providing spectra o high radiometric quality at 0.5 cm1 resolution, rom 645 to

2760 cm1 (Randall 2008). Tis instrument is also destined to provide a wealth o data on various

components o the atmosphere to urther our understanding o atmospheric processes and the inter-

actions among atmospheric chemistry, climate, and pollution. In addition, the IASI will deliver data


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on land-surace emissive and sea-surace temperature (in cloud-ree conditions) (www.esa.int/esaLP/

SEMM36BUQPE_LPmetop_0.html).

2.2.3. Laser Radar Sensors

Laser radar sensors are more complex than optical sensors but provide more accuracy in retrieving atmo-

sphere parameters. A laser radar instrument was launched on the Space Shuttle in September 1994. Te

methodology o global cloud and aerosol monitoring has been tested and interesting results have been

achieved. Te main laser radar sensors include CALIOP and GLAS, with abilities o cloud monitoring

and aerosol prole retrieving.

2.2.3.1. CLOUD-AEROSOL LIDAR WITH ORTHOGONALPOLARIZATION (CALIOP)

Cloud-Aerosol LIdar with Orthogonal Polarization (CALIOP) is a two-wavelength polarization-

sensitive lidar that provides high-resolution vertical proles o aerosols and clouds as well as their opti-

cal and physical properties (Winker 2007). CALIOP utilizes three receiver channels: one measuring

the 1064-nm backscatter intensity and two channels measuring orthogonally polarized components

o the 532-nm backscattered signal. Dual 14-bit digitizers on each channel provide an efective 22-bit

dynamic range. Te receiver telescope is 1 m in diameter. A redundant laser transmitter is included in

the payload. Cloud and aerosol layers are discriminated using the magnitude and spectral variation o

the lidar backscatter (Randall 2008). Aerosol extinction proles are computed with a vertical resolution

o 120360 m rom an extinction-to-backscatter ratio, or lidar ratio. Aerosol layers can be detected with

su cient averaging (Winker 2004).

2.2.3.2. GLAS LIDAR

Te GLAS lidar (Geoscience Laser Altimeter System) onboard ICESat (the Ice Cloud and Elevation

Satellite) was launched in January 2003. It was the rst laser altimeter system onboard a spacecrat. Te

GLAS lidar was designed to measure ice sheet elevation, but it is being applied to retrieve aerosol pro-

les. GLAS make unique atmospheric observations, including measuring ice-sheet topography, cloud

and atmospheric properties, and the height and thickness o radioactively important cloud layers needed

or accurate short-term climate and weather prediction (Spinhirne 2005).

3. APPLICATIONS

3.1. AEROSOL RETRIEVAL

Aerosol thickness is an indicator o the overall pollution o an area. ropospheric aerosols are impor-

tant components o the earthatmosphereocean system (Kauman 2005), afecting climate through


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three primary mechanisms. First, it causes direct radiative orcing results when radiation is scattered or

absorbed by the aerosol itsel. Second, indirect radiative orcing results when enhanced concentrationso aerosol particles modiy cloud properties, resulting in more cloud drops, albeit smaller in size, that

generally increase the albedo o clouds in the Earths atmosphere. Finally, aerosol particles can have an

indirect efect on heterogeneous chemistry, which in turn can inuence climate by modiying the con-

centration o climate-inuencing constituents (such as greenhouse gases) (Song et al. 2007).

Te relative efects o aerosol optical thickness and single scattering albedo on satellite reection

unction measurements is the basis or remote sensing o aerosol optical thickness and single scattering

albedo rom reected solar radiation measurements. Te maximum sensitivity to aerosol optical thick-

ness occurs over dark suraces. For suraces brighter thanAg= 0.1, whereAg is the surace reectance,

the sensitivity is much reduced and depends on aerosol absorption. Tereore, measurements over ocean

suraces or dark targets over land are most requently used to detect aerosol optical thickness rom space-

based sensors, and a combination o dark and bright suraces are used to detect aerosol single scattering

albedo (Kauman et al. 1997). Many methods have been used over the past 30 years to monitor aerosolthickness, and atmospheric aerosols can be retrieved by diferent methods, which can be classied by

single- and multiple-channel reectance, multiangle reectance, the contrast-reduction method, and

polarization (Siakis, 1998).

Remote sensing o aerosol optical properties rom space has, in the past, been accomplished using

satellite data not explicitly designed with this application in mind. Tis has included AVHRR data,

whose primary purpose was the determination o sea surace temperature and vegetation index, and

OMS data, whose primary purpose was the derivation o total ozone content (Carlson 1977). Since

then, not only aerosols above seas but also aerosols above lands have been studied and tested. Sensors

applied or these studies include AVHRR, OMS, OVS, SeaWiFS, MERIS, GLI, OMI, etc. An in-

depth discussion o those sensors was presented by Michael et al. (1999).

Since the end o the last century, quite a ew satellite sensors, such as MODIS, HIRIS, PICASSO

(USANASA), ILAS (Japan), and POLDER (European Space Agency), have been launched success-

ully with the goal o monitoring cloud and aerosol at the global scale. Many types o international

scientic projects, such as IGAC (International Global Atmospheric Chemistry Project), APEX (Asian

Atmospheric Particulate Environment Change Studies), etc., had been conducted or global aerosol

monitoring with support o remote chemical sensing methods (Dubovik et al. 2008).

3.2. WATER VAPOR RETRIEVAL

Water vapor is one o the most important and most abundant greenhouse gases in the Earths atmo-

sphere, keeping the temperature o the Earths surace above the reezing level. Atmospheric water vapor

plays a key role in the hydrological cycle, whose distribution is essential in understanding weather and

global climate. Te distribution o water vapor varies greatly both in space and time, with values ranging

rom about 5 cm near the equator to less than one-tenth as much at the poles, which can lead to sudden

changes in local weather (www.ae.utexas.edu/courses/ase389p_gps/projects99/whitlock /intro.html). In

order to develop accurate weather prediction and global climate models, it is vital to monitor water

vapor as accurately as possible. Te radiosonde network has long been the primary in situ observing

system or monitoring atmospheric water vapor. Radiosondes provide vertical proles o meteorological


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Figure 4.1. Diagram of retrieval of water vapor from MODIS data.


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variables such as pressure, temperature, and relative humidity. Sometimes, wind inormation can be

obtained as well.Te Global Positioning System (GPS) is an increasingly operational tool or measuring precipitable

water vapor. GPS signals are delayed when propagating through the troposphere. Te total tropospheric

delay can be divided into a hydrostatic term (ZHD), caused primarily by dry gases in the atmosphere,

and a wet term (ZWD), caused by the reractivity due to water vapor (Zhenhong et al. 2008). GPS

measurements provide estimates o the total zenith delay (ZD) using mapping unctions. I surace air

pressure is known with an accuracy o 0.3 hPa or better, ZHD can be estimated. Te primary advantage

o GPS is that it makes continuous measurements possible. Furthermore, the spatial density o the cur-

rent Continuous GPS (CGPS) network is much higher than that o the radiosonde network, and its

capital and operational costs are much lower than or remote sensing (Zhenhong et al. 2008).

Te remote sensing method is based on detecting the absorption by water vapor o the reected

solar radiation ater it has transerred down to the surace and back up through the atmosphere. Te

near-inrared total-column precipitable water is very sensitive to boundary-layer water vapor, since itis derived rom attenuation o reected solar light rom the surace. Tis data product is essential to

understanding the hydrological cycle, aerosol properties, aerosolcloud interactions, energy budget, and

climate. O particular interest is the collection o water vapor data above cirrus cloudiness, which has

important applications to climate studies (http://modis-atmos.gsc.nasa.gov/MOD05_L2/index.html).

As a case in point, the technique implemented or the MODIS water vapor retrievals uses ratios o radi-

ance rom water vaporbsorbing channels centered near 0.905, 0.935, and 0.94 m with atmospheric

window channels at 0.865 and 1.24 m. Both the two-channel and three-channel ratioing techniques

are used to retrieve the water vapor or MODIS (http://modis-atmos.gsc.nasa.gov/MOD05_L2/index.

html). Figure 4.1 illustrates the steps in retrieving water vapor rom MODIS data.

Te output rom the Level 2 near-IR water vapor algorithm includes column water vapor amounts

on a pixel-by-pixel basis and an associated quality assurance parameter. In addition to the Level 2 near-

IR water vapor product, Level 3 (MOD43) gridded products are produced daily, every 8 days, and

monthly (Zhenhong et al. 2008).

3.3. ATMOSPHERIC TRACE GASES DETECTING

Oxides o carbon, sulur, nitrogen, and ozone are serious environmental pollutants produced by the

productive and social activities o humans. Hence real-time monitoring and comprehensive control o

these pollutants are very important. Atmospheric trace gases data, such as or ozone, are also important

or climate research and as an input to numerical weather prediction models. race gas measurements

will be important or monitoring the long-term efects o global climate change.

3.3.1. Ozone

Since the mid-1950s, total stratospheric ozone amounts have been regularly measured. In the early part

o this period, all measurements were made in situ by instruments released rom the ground. From 1979


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until today, a steady decrease in stratospheric ozone has been noted. Te decrease has been especially ob-

vious over Antarctica, where an ozone hole appears in the spring and disappears in the summer. Eachyear, this springtime hole covers a larger area than it had the previous year (www.cot.edu/ete/modules/

ozone/ozremote.html).

In 1978 NASA launched the Nimbus-7 satellite, equipped with a otal Ozone Mapping

Spectrometer (OMS). Several other satellites were launched subsequently. Since 1979, several satellites

have been equipped with sensors that collect data on ozone, as well as other atmospheric constituents

that afect the amounts o ozone present. American, Russian, and Japanese satellites have carried ozone

sensors. Te satellites that have own the otal Ozone Mapping Spectrometer (OMS) have included

Meteor-3, Nimbus-7, and, most recently, ADEOS and Earth Probe. Te Cryogenic Limb Array Etalon

Spectrometer (CLAES) is an instrument that has own on the Upper Atmosphere Research Satellite

(UARS) since 1991. Te IROS-N Operational Vertical Sounder (OVS) measures radiances.

Ozone retrieval is based on comparison between measured radiances and radiances based on ra-

diative transer calculations or diferent amounts o ozone in the atmosphere. Ozone absorbs stronglybetween 312 and 380 nm, in the ultraviolet region. Comparing what raction o the incoming radiance

in this band is reected, it is possible to relate this value to the total amount o ozone. NASA maintains

a OMS homepage, with an extensive database and additional inormation about the project. It is also

possible to retrieve the ozone concentration at any point and any time until the previous day (http://

toms.gsc.nasa.gov/index_v8.html).

3.3.2. Nitrogen Dioxide

Nitrogen dioxide (NO2) is one o the key species in atmospheric chemistry. In the stratosphere, it is

involved in catalytic ozone destruction, whereas the photolysis o tropospheric NO2

results in O3

orma-

tion. In addition, it is indirectly responsible or the atmosphere oxidizing capacity and contributes to

radiative orcing o climate (Burrows et al. 1999). However, or a long time, the global distribution o

NO2 could only be analyzed by global chemistry transport models, because ground-based or airborne

measurement campaigns were temporally and spatially limited.

Stratospheric NO2 has been measured by a number o satellite instruments, e.g., LIMS (Limb

Inrared Monitor o the Stratosphere), SME (Solar Mesosphere Explorer), SAGE-II/III (Stratospheric

Aerosol and Gas Experiment), ISAMS (Improved Stratospheric and Mesospheric Sounder), HALOE

(Halogen Occultation Experiment), and POAM (Polar Ozone and Aerosol Measurement). Despite the

global coverage o satellite observations, these measurements are characterized by the limited time sam-

pling and high uncertainty in the lower stratosphere (Ionov et al. 2006). Te Global Ozone Monitoring

Experiment (GOME) in 1995 was the rst satellite mission to provide a global picture o atmospheric

NO2 with reasonable spatial and temporal resolution. Unlike previous satellite systems, aiming at in-dividual NO2 vertical prole measurements, GOME is designed to map the global distribution o

the NO2 vertical column. Since then, similar instruments such as the SCanning Imaging Absorption

spectroMeter or Atmospheric CartograpHY (SCIAMACHY), Ozone Monitoring Instrument (OMI),

and GOME-2 have been launched into sky. Te tropospheric NO2 maps derived rom these instruments


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have been used to study many scientic applications, such as pollution emissions and pollutant distribu-

tions (Hans et al. 1987).Te NO2 inverse algorithm, the Diferential Optical Absorption Spectroscopy (DOAS), rom satel-

lite measurements are most popular retrieve methods. Te DOAS method determines the NO2 state

column density (SCD) along the light path through the atmosphere based on the Lambert-Beer law. It

makes use o a diferential absorption signal with respect to an extraterrestrial solar spectrum. Te rst

step o this technique is the removal o aerosol scattering and surace reecting efects by a low-order

polynomial unction, then the ring efects have to be considered, and nally the NO 2 SCD is derived

based on a spectral t o NO2 to a reectance spectrum. Based on the total SCD, the tropospheric NO2

SCD is calculated by subtracting the stratospheric NO2 concentration, and the tropospheric NO2 SCD

is converted to VCD by air mass actor (AMF) (Chen et al. 2009). Kokhanovsky and Rozanov (2009)

studied the accuracy o the retrieved NO2 vertical columns using satellite observations under cloudy

conditions using the radiative transer code SCIARAN. It was ound that the tropospheric nitrogen

dioxide columns can be retrieved in the case o thin clouds, i their optical properties and the altitudeare retrieved rom independent observations. A diagram o the retrieval o NO2 column concentration

inormation is shown in Figure 4.2.

3.3.3. Other Trace Gases

Most estimates o air quality rom satellite observations have ocused on ground-level aerosol mass

concentration o NO2 and O3. However, inormation on other trace gases (such as CO) concentra-

tion is becoming available, in part due to the increasing spatial resolution aforded by more recent

instrumentation.

Te MOPI instrument onboard erra is a nadir-viewing gas correlation radiometer operating

in the 4.7-mm band o CO (Drummond and Mand 1996). Satellite retrievals o CO exhibit strong

signals rom the ree troposphere due to broad averaging kernels o current instruments and reduced

thermal contrast near the surace. Nonetheless, enhanced signals in CO columns over cities are apparent

in long-term averages or SCIAMACHY and MOPI (Clerbaux et al. 2008). Furthermore, ground-

level CO concentrations in regional air quality models are sensitive to boundary conditions, which can

be constrained by satellite observations. CO retrievals eature lower tropospheric inormation in regions

with strong thermal contrast such as arid environments (Randall 2008).

Retrieved global SO2 slant columns rom GOME with su cient accuracy have been very useul

to study volcanic plumes and major pollution sources. In 2009 the satellite instruments OCO and

GOSA were launched, which promise a revolutionary improvement in our ability to monitor the

greenhouse gases CO2 and CH4. HCHO columns and SO2 columns or both absorption and extinc-

tion can be retrieved rom OMI (Randall 2008; Chen et al. 2009). o date, signicant uncertaintiesremain in our understanding o the global cycles o trace gases. Te large number o measurements and

global coverage as provided by satellite instruments could signicantly accelerate progress in process

understanding, which is urgently needed to understand the atmospheric evolution o trace gases, and

prerequisite to climate change prediction (Khokhar et al. 2005).


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Figure 4.2. Diagram of retrieval of NO2 column concentration.

Remote sensing images

Differential absorption

signal with respect to an extraterrestrial solar spectrum

Tropospheric and stratospheric trace-gas concentrations

Removal molecular O2 and N2Rotational Raman scattering by considered Ring effects

NO2 state column density (SCD)

Spectra fit of NO2 to a

reflectance spectrum

Air mass factor(AMF)

NO2 vertical column density (VCD)

Subtracting stratospheric

NO2 column concentration

Tropospheric NO2 column concentration


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4. CONCLUSION

One o the most important ecological issues or our planet is climate change. It is generally agreed

that the Earths climate will modiy in response to radiative orcing induced by changes in atmospheric

trace gases, cloud cover, cloud type, solar radiation, and tropospheric aerosols (liquid or solid particles

suspended in the air). In order to develop conceptual and predictive global climate models, it is vital to

monitor these properties. Unortunately, our knowledge o most climatic parameters is limited, so good

climate models exist only or very limited areas o the Earth. For example, atmospheric temperature

data, a good index or measuring global warming, are very limited over the ocean. Te distribution and

sources o greenhouse gases are other major unknowns (http://modis.gsc.nasa.gov).

Satellite-derived data are essential to obtain global knowledge about these parameters. Tis require-

ment led to the development o chemical sensors or monitoring atmosphere quality rom space. Remote

sensing satellites have many advantages or monitoring air quality. Satellite observations can provide a

complete survey o a region, show the major sources o pollution, and the distribution pattern. Sincethe early 1970s, remote sensing instrumentation had been developed or remotely measuring several

atmospheric parameters. Space-based monitoring is the only efective way to assess atmosphere contents

distribution on a global basis, and many new chemical remote sensing sensors have been developed in

recent years. race gas and aerosol instrumentation have been developed and operated to measure ambi-

ent concentrations o trace gases and aerosols and the exchange o trace gases with the Earths surace.

Such data and inormation support researches into the atmospheric energy balance, the hydrological

cycle, climate trends, and other aspects o the atmospheric system that are o vital interest to us.

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