Earth
Science Reference Handbook
[ Missions: Aura ] 103
sphere has maintained a delicate natural balance between ozone
formation and destruction.
In recent years, however, man–made chemicals, such as
chlorofluorocarbons (CFCs), have altered the natural balance of
ozone chemistry. International agreements, such as the Montreal
Protocol and its amendments, have been enacted to control these
ozone–destroying chemicals, and recent data show that ozone is
being depleted at a slower rate than a decade ago. However, it is
too soon to tell if this trend is a result of the international protocols
restricting CFC production or whether other factors explain the
reduction in the rate of ozone loss.
Information returned from Aura’s four instruments helps us
answer these questions about ozone. The instruments measure the
total ozone column, ozone profiles, and gases important in ozone
chemistry. They also measure important sources, radicals, and
reservoir gases that play active roles in ozone chemistry. The data
gathered help to improve our ability to predict ozone changes and
also help us better understand how transport and chemistry impact
ozone trends.
What are the Processes Controlling Air Quality?
Agricultural and industrial activity have grown dramatically along
with the human population. Consequently, in parts of the world,
increased emissions of pollutants have significantly degraded air
quality. Respiratory problems and even premature deaths due to air
pollution occur in urban and some rural areas of both industrial-
ized and developing countries. Widespread burning for agricultural
purposes (biomass burning) and forest fires also contribute to poor
air quality, particularly in the tropics. The list of culprits contribut-
ing to the degradation of air quality includes tropospheric ozone,
a toxic gas, and other chemicals whose presence, accompanied by
the right atmospheric conditions, leads to the formation of ozone.
These so–called ozone precursors include oxides of nitrogen (NO
x
),
carbon monoxide (CO), methane (CH
4
), and other hydrocarbons.
Human activities such as biomass burning, inefficient coal com-
bustion, other industrial activities, and vehicular traffic all produce
ozone precursors.
The U.S. Environmental Protection Agency (EPA) has iden-
tified six pollutants: carbon monoxide, nitrogen dioxide, sulfur
dioxide, ozone, lead, and particulates (aerosols). Of these six
pollutants, ozone has proven the most difficult to control. Ozone
chemistry is complex, making it difficult to quantify the contri-
butions to poor local air quality. Pollutant–emission inventories
needed for predicting air quality are uncertain by as much as 50%.
Also uncertain is the amount of ozone that enters the troposphere
from the stratosphere.
For local governments struggling to meet national air–quality
standards, knowing more about the sources and transport of air pol-
lutants has become an important issue. Most pollution sources are
local, but satellite observations show that winds can carry pollutants
great distances, for example from the western and mid–western
states to the east coast of the United States and sometimes even from
one continent to another. We have yet to quantify accurately the
extent of inter–regional and inter–continental pollution transport.
Aura Instruments
HIRDLS
High Resolution Dynamics Limb
Sounder
HIRDLS is an advanced 21–channel
infrared radiometer measuring the
6–17–µm thermal emission of Earth’s
limb designed to provide critical infor-
mation on atmospheric chemistry and
climate. It provides accurate measure-
ments of trace gases, temperature,
and aerosols in the upper troposphere,
stratosphere, and mesosphere, with
daily global coverage at high vertical
resolution.
MLS
Microwave Limb Sounder
MLS is an advanced microwave
radiometer that measures microwave
emission from the Earth’s limb in five
broad spectral bands. MLS measures
trace gases at lower altitudes and with
better precision and accuracy than
its predecessor on UARS. MLS can
provide reliable measurements even in
the presence of cirrus clouds and dense
volcanic aerosols.
OMI
Ozone Monitoring Instrument
OMI is a nadir–viewing wide–field imag-
ing spectroradiometer that serves as
Aura’s primary instrument for tracking
global ozone change and continues
the high quality column–ozone record
begun in 1970 by the Nimbus–4 BUV.
OMI measures backscattered ultravio-
let and visible radiation and provides
daily global coverage of most of Earth’s
atmosphere. Data returned permit mod-
eling of air pollution on urban–to–su-
per–regional scales.
TES
Tropospheric Emission Spectrometer
TES is a high–spectral–resolution infra-
red–imaging Fourier–transform spec-
trometer that generates three–dimen-
sional profiles on a global scale of most
infrared–active species from Earth’s
surface to the lower stratosphere.
Earth Science Reference Handbook
104 [ Missions: Aura ]
The Aura instruments are designed to study tropo-
spheric chemistry. Together these instruments provide
global monitoring of air pollution on a daily basis and
measure five out of the six criteria pollutants identified
by the Environmental Protection Agency. Aura provides
data of suitable accuracy to improve industrial emission
inventories and also helps distinguish between industrial
and natural sources. Information provided by Aura may
lead to improvements in the accuracy of air–quality fore-
cast models.
How is Earth’s Climate Changing?
Carbon dioxide and other gases trap infrared radiation
that would otherwise escape to space. This phenomenon,
called the greenhouse effect, contributes to making the
Earth habitable. Increased atmospheric emissions of trace
gases that trap infrared radiation from industrial and agri-
cultural activities are causing climate change. Quantities
of many of these gases in the atmosphere have increased
and this has added to the greenhouse effect. During the
20
th
century, the global mean lower tropospheric tem-
perature increased by more than 0.4° C. This increase is
thought to be greater than during any other century in the
last 1000 years.
Improved knowledge of the sources, sinks, and
the distribution of greenhouse gases is needed for ac-
curate predictions of climate change. Aura measures
greenhouse gases such as methane, water vapor, and
ozone in the troposphere and lower stratosphere. Aura
also measures both absorbing and reflecting aerosols in
the lower stratosphere and lower troposphere, measures
upper tropospheric water-vapor and cloud-ice concentra-
tions, and makes high-vertical-resolution measurements
of some greenhouse gases in a broad swath (down to the
clouds) across the tropical upwelling region. All of these
measurements contribute key data for climate modeling
and prediction.
Instrument Descriptions
Each of Aura’s four instruments makes important contri-
butions to answering the three science questions described
above. The goal with Aura is to achieve ‘synergy’—mean-
ing that more information about the condition of the Earth
is obtained from the combined observations of the four
instruments than would be possible from the sum of the
observations taken independently. The four instruments
were carefully designed to achieve the overall mission
objectives. Each has different fields of view and comple-
mentary capabilities.
HIRDLS
High Resolution Dynamics Limb Sounder
HIRDLS Background
HIRDLS is an advanced
21–channel infrared ra-
diometer observing the
6–17 μm thermal emis-
sion of the Earth’s limb
and designed to provide
critical information on
atmospheric chemistry
and climate. It provides
accurate measurements of trace gases, temperature, and
aerosols in the upper troposphere, the stratosphere, and
the mesosphere, with daily near–global coverage at high
vertical resolution. HIRDLS looks backward and to the
side away from the Sun and scans vertically. Very precise
gyroscopes provide instrument–pointing information for
HIRDLS. To detect the weak infrared radiation from the
Earth’s limb, HIRDLS detectors must be kept at tem-
peratures below liquid nitrogen. An advanced cryogenic
refrigerator is used to keep the detectors cool.
The University of Colorado, Oxford University
(U.K.), the National Center for Atmospheric Research
(NCAR), and Rutherford Appleton Laboratory (U.K.)
designed the HIRDLS instrument. Lockheed Martin built
and integrated the instrument subsystems. The National
Environmental Research Council funded the U.K.’s par-
ticipation.
HIRDLS Contributions to Science
Questions
Is the Stratospheric Ozone Level Recovering?
The largest ozone depletions occur over the poles in the
lower stratosphere during winter. Therefore, HIRDLS
makes concentrated measurements in the northern polar
region and retrieves high–vertical–resolution daytime and
nighttime ozone profiles over the poles.
HIRDLS also measures NO
2
, HNO
3
, and CFCs, all
gases that play a role in stratospheric ozone depletion.
Although international agreements have banned their
production, CFCs are long–lived and will remain in the
stratosphere for several more decades. By measuring pro-
files of the long–lived gases, from the upper troposphere
into the stratosphere, HIRDLS can assess the transport of
air from the troposphere into the stratosphere.