Earth Science Reference Handbook [ Missions: Aura ] 101

Earth Science Reference Handbook

[ Missions: Aura ]  105

What are the Processes Controlling Air Quality?

HIRDLS  measures  ozone,  nitric  acid,  and  water  vapor  in  the 

upper  troposphere  and  lower  stratosphere. With  these  measure-

ments, scientists can estimate the amount of stratospheric air that 

descends into the troposphere and this allows scientists to better 

distinguish between natural ozone sources and pollution originat-

ing from man–made sources. This is an important step forward in 

quantifying the level at which human activities are impacting the 

air we breathe.

How is Earth’s Climate Changing?

HIRDLS measures water vapor and ozone. Accurate measurement 

of greenhouse gases such as these are important because scientists 

input  this  information  into  models  they  use  to  predict  climate 

change. The  more  accurate  the  information  that  goes  into  these 

models, the more accurate and useful the resulting predictions will 

be. HIRDLS is also able to distinguish between aerosol types that 

absorb or reflect incoming solar radiation and can map high thin 

cirrus clouds that reflect solar radiation. This new information al-

lows scientists to better understand how to represent aerosols and 

thin cirrus clouds in climate models.

 

HIRDLS Principal Investigators

John Barnett, Oxford University (U.K.)

John Gille, University of Colorado and NCAR (U.S.)

HIRDLS URLs

www.eos.ucar.edu/hirdls/

www.atm.ox.ac.uk/hirdls/

 

 

MLS

Microwave Limb Sounder

MLS Background

MLS  is  an  advanced  microwave  radiom-

eter  that  measures  microwave  emission 

from the Earth’s limb in five broad spectral 

bands.  These  bands  are  centered  at  118 

(dual  polarization),  190,  240,  640  GHz, 

and 2.5 THz (dual polarization). MLS can 

measure trace gases at lower altitudes and 

with better precision and accuracy than its 

predecessor on UARS. In addition, MLS obtains trace–gas profiles 

with a typical vertical resolution of 3 km and has a unique ability 

to measure trace gases in the presence of ice clouds and volcanic 

aerosols. MLS pioneers the use of planar diodes and monolithic 

millimeter–wave integrated circuits to make the instrument more 

reliable and resilient to launch vibration. MLS looks outward from 

Key HIRDLS Facts

Heritage: Limb Radiance Inversion 

Radiometer (LRIR); Limb Infrared 

Monitor of the Stratosphere (LIMS) and 

Stratospheric and Mesospheric Sounder 

(SAMS); Improved Stratospheric 

and Mesospheric Sounder (ISAMS), 

and Cryogenic Limb Array Etalon 

Spectrometer (CLAES)

Instrument Type: Limb viewing vertical 

scanning radiometer* 

Spectral Range: 6–17 µm, using 21 

channels

Scan Type: Vertical limb scans at fixed 

position*

Scan Range: Elevation, 22.1° to 27.3° 

below horizontal, +43° on anti–sun side

Dimensions: 149.9 cm × 118.5 cm 

× 130.2 cm

Detector IFOV: 1.25 km vertical × 10 km 

horizontal

Duty Cycle: 100%

Power: 262 W (average), 365 W (peak)

Data Rate: 70 kbps

Thermal Control: Detector cooler, 

Stirling–cycle, heaters, surface coatings, 

radiator panel

Contributors

Instrument Design: University of 

Colorado, Oxford University (U.K.), Na-

tional Center for Atmospheric Research 

(NCAR), and Rutherford Appleton  

Laboratory (U.K.)

Instrument Assembly and Integration: 

Lockheed Martin built and integrated 

instruments

Funding: National Environmental  

Research Council (U.K.)

* HIRDLS was originally designed to scan 

vertically at seven different horizontal posi-

tions across the satellite track. Unfortunate-

ly, the horizontal scanning capability was 

lost during launch and the instrument now 

can only scan vertically at a fixed position. 

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Earth Science Reference Handbook

106  [ Missions: Aura ]

the front of the spacecraft and obtains vertical scans of the limb. 

NASA’s Jet Propulsion Laboratory (JPL) developed, built, tested, 

and operates MLS.

MLS Contributions to Science Questions

Is the Stratospheric Ozone Layer Recovering?

MLS continues the ClO and HCl measurements made by UARS. 

These  measurements  provide  important  information  on  the  rate 

at which stratospheric chlorine is destroying ozone and the total 

chlorine loading of the stratosphere. MLS provides the first global 

measurements of the stratospheric hydroxyl (OH) and hydroperoxy 

(HO

2

)  radicals  that  are  part  of  the  hydrogen  catalytic  cycle  for 

ozone destruction. In addition, MLS measures bromine monoxide 

(BrO), a powerful ozone–destroying radical with both manmade 

and naturally occurring sources. 

 

MLS measurements of ClO and HCl are especially important 

in the polar winters. Taken together, these measurements help sci-

entists determine what fraction of stable chlorine reservoirs (HCl) 

is converted to the ozone–destroying radicals (ClO). Since recent 

research results indicate that the Arctic stratosphere may now be at 

a threshold for more severe ozone loss due to climate change, the 

MLS data are of critical importance for understanding observed 

changes in Arctic winter ozone.

What are the Processes Controlling Air Quality?

MLS  measures  carbon  monoxide  (CO)  and  ozone  in  the  upper 

troposphere. CO is normally created in the lower troposphere by 

incomplete burning of hydrocarbons and is an ozone precursor. 

When  scientists  observe  heightened  concentrations  of  CO  and 

ozone at the higher levels of the troposphere, it is an indicator of 

strong  vertical  transport  in  the  troposphere. These  observations 

can serve as a useful tool for tracking the movement of polluted 

air masses in the atmosphere. 

How is Earth’s Climate Changing?

MLS makes measurements of upper tropospheric and lower strato-

spheric water vapor, ice content, and temperature. Accurate mea-

surements of all of these constituents are needed to help scientists 

create models that can predict how the Earth’s climate is likely to 

change in the future. MLS also measures ozone and nitrous oxide 

(N

2

O)—both  important  greenhouse  gases—in  the  upper  tropo-

sphere and lower stratosphere. 

MLS Principal Investigator

Nathaniel  Livesey,  NASA  Jet  Propulsion  Laboratory/California 

Institute of Technology

MLS URL

mls.jpl.nasa.gov/

Key MLS Facts

Heritage: Microwave Limb Sounder 

(MLS)

Instrument Type: Microwave radiometer

Scan Type: Vertical limb scan in plane of 

orbit, done by moving reflector antenna

Calibration: Views ‘blackbody’ target 

and ‘cold’ space with each limb scan

Spectral Bands: Broad bands centered 

near 118, 190, 240, and 640 GHz and 

2.5 THz

Spatial Resolution: Measurements are 

performed along the suborbital track, 

and resolution varies for different 

parameters; 5–km cross–track ×  

~200–km along–track × 3–km vertical 

are typical values

Dimensions: 150 cm × 190 cm ×  

180 cm (GHz sensor); 80 cm × 100 cm × 

110 cm (THz sensor); 160 cm × 50 cm × 

30 cm (spectrometer)

Mass: 455 kg

Duty Cycle: 100%

Power: 544 W full–on

Data Rate: 100 kbps

Thermal Control: Via radiators and 

louvers to space as well as heaters

Thermal Operating Range: 10–35° C

FOV: Boresight 60–70° relative to nadir, 

in plane of orbit 

Instantaneous FOV at 640 GHz: 1.5 km 

vertical × 3 km cross–track × 200 km 

along–track at the limb tangent point 

Pointing Requirements 

(platform+instrument, 3σ):

  Control: 36 arcsec

  Knowledge: 1 arcsec per s

  Stability: 72 arcsec per 30 s

  Jitter: 2.7 arcsec per 1/6 s

Contributor: NASA JPL developed, 

built, tested, and operated MLS; U.K. 

University of Edinburgh contributed 

to data processing algorithms and 

validation.

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