Transcript Ground Based NO2 Measurement Methods

Ground and Satellite Observations of
Atmospheric Trace Gases
George H. Mount
Laboratory for Atmospheric Research
WSU
13 April 2007
Outline
• Aura/OMI - Ozone Monitoring
Instrument on the Aura satellite
• Aura ground truth/validation of data
• MFDOAS instrument for urban airshed
trace gas measurements and satellite
validation
• NASA INTEX B results, PNNL, spring
2006
Aura/OMI
• WSU involvement began in 1997 at the inception
of the OMI project - Dutch instrument on NASA
bird, small US team
• double channel spectrograph covering 270 - 510
nm at 0.7nm resolution
• launched in July 2004
• measures column O3, NO2, BrO, OClO, CH2O,
SO2, aerosol indices
• data products:
–
–
–
–
atmospheric column of above gases
trop NO2
trop O3 - not yet routinely available
aerosol data
trace gas measurements: observing the
Earth’s backscattered uv/visible radiation
Sun
Sunlight passes through the atmosphere, reflects
off clouds and the surface, and is scattered back
into the instrument field of view. Molecular spectral
absorption is proportional to the concentration of
the gas doing the absorbing along the path.
Observing Principle for OMI
2-dimensional CCD
swath
~ 580 pixels
wavelength
~ 780 pixels
flight direction
» 7 km/sec
viewing angle
± 57 deg
12 km/24 km (binned & co-added)
2600 km
13 km
))
GOME
Sciamachy
OMI
1
TOMS
single pixel size for four satellite instruments
OMI single pixel
OMI pixel 12 km x 13 km superposed onto the Seattle airshed (zoom mode)
New WSU MFDOAS Instrument
• WSU was funded 3 years ago to develop a new ground based
instrument that would mimic the satellite measurements from the
ground & support validation of the Aura satellite data from the ground
• new instrument uses the molecular spectrum of the sky and direct sun
as light sources for measurement of urban air pollution
• scans the sky at low elevation angles where the tropospheric air mass
is significantly enhanced
• completing development at WSU as we speak
• ground based campaign at NASA Goddard Space Flight Center 7-21
May
• ground based campaign at NASA Jet Propulsion Laboratory 1-15 July
• was fielded in prototype form during the NASA INTEX at Pacific
Northwest National Laboratory in central Washington spring 2006
MF-DOAS: geometry
for sky-viewing mode
stolen from Platt group
ect
Lig Sun
ht
Dir
Sca
dS
ky
Lig
WSU
MFDOAS instrument
ht
IS
CCD Controller
Spectrograph
Heat Exchanger
Solar Tracker/
Positioner
Telesc
ope
Supplies
Power
Filter
Wheels
Optics
Holder
CCD
FW Controller
tter
e
16
x10 16
x10
Figure 3. MFDOAS NO2 Differential Slant Column: 9 May 2006
12
10
Elevation View Angle 5
o
16
NO2 Differential Slant Column Density x10 molecules/cm
2
8
6
4
2
0
12
South
West
North
East
10
8
Elevation View Angle 15
o
6
4
2
0
10
8
6
4
2
0
Elevation View Angle 45
10
8
6
4
2
0
o
Zenith
06:00
08:00
10:00
12:00
14:00
Pacific Standard Time
16:00
18:00
WSU MFDOAS
data during
INTEX B at
PNNL, Richland,
WA
Figure 7. OMI Tropospheric NO2 Vertical Column Density:
9 May 2006
46.8
1.6
0.6
1
1.2
Richland, WA
4.4
3.4
3.8
46.2
3
46.1
46.0
1.6
1
-119.6
0
3.2
3.2
2.6
1.8 2
-119.4
3
2.6
1
2
2.2
2
-119.2
-119.0
Longitude
3
15
4
NO2 Column Density x 10 molecules/cm
OMI data
during INTEX
over PNNL, WA
spring 2006
2.4
3.2
3.2 2.8
Pasco, WA
3.6 3
4
4.2
Kenewick, WA 3.6
3 2.8
46.3
45.9
1.2
1.4
0.6
46.5
Latitude
2
0.4
46.6
2.2
1.8
46.7
2.6
0.8
46.4
2.4
1.4
5
2
-118.8
comparison with
MFDOAS shows a
20% bias between
OMI and MFDOAS
with OMI
underestimating NO2
column. Other
validations show a
similar
underestimate.
Things to consider in using satellite data:
• basic limitations:
• can only observe when clear - problem in Pacific NW in winter
• tied to equator crossing time (1345h for Aura) for a polar orbiter
• cannot get more than a couple of orbits of data before the urban area rotates
out of the FOV - orbital period ~ 90 minutes
• cannot observe at night for most instruments
• accuracy of the trop result depends on removing the strat overburden (if exists)
• trace gases:
• ozone and NO2 total columns are done well from space
• problem is the stratospheric overburden --> must separate strat/trop
• this is not an easy problem - cloud slicing, use of other instruments onboard
• RT codes have advanced this a lot in the last 5 years (spherical codes)
• CO can be done fairly easily in the IR part of the spectrum
• SO2 and HCHO are very hard due to low levels in the trop - few ppbv sensitivity
• aerosols are measured by using uv radiances and work well
• footprint size and model grid size
• this has followed the technology with steady evolution to smaller footprints
• 2-D detectors --> large advances in footprint and simultaneous large swath angle
• OMI: 114° = 2600 km swath with 13 km pixels and a spectrum at central pixel
• scanning mirrors are a thing of the past for swath coverage (perp. to orbital track)
• temporal resolution
• orbital period of ~ 90 min --> get a picture each period until airshed
rotates out of the swath - larger swath angles are better --> information
on multiple orbits
• may take several days to build up an image due to spatial scanning swath
• satellite moves at 7 km/sec --> ground track always moves at that speed
• integration time to get good s/n - sets footprint in velocity vector
direction (NS)
• short int. times produce low s/n, especially in the Huggins O3 bands
•time to get complete global coverage - e,g, GOME 3d
to Farren’s talk
Spectroscopic Technique for Air Pollution Measurements
• measure the absorbed spectrum of sunlight reflected from the Earth’s surface
cross section (e19) [cm^2]
• many tropospheric trace gases have complicated molecular spectra allowing
identification and quantification of concentration, e.g.
6.6
6.4
NO2
photoabsorption
cross section
6.2
6.0
5.8
5.6
414.70
414.80
414.90
wavelength (nm)
[Harder, Brault, Johnston, and Mount, 1997]
• need a telescope to look down on the Earth
• to collect light
• to image the Earth surface onto the spectrograph over
a wide field of view (e.g. for OMI: 114° = 2600 km) at high
spatial resolution (e.g. for OMI: 12 km x 24 km pixel size in “global mode”)
• spectrograph to sort out the molecular spectra
• imaging detector which allows simultaneous detection ofa large spectral region for
each spatial resolution element over a wide swath of geography with high
spatial resolution. Older systems use a scan mirror to change the FOV.
• basic physics is very simple
• solar light traverses an atmospheric path: Sun --> Earth surface, then
reflecting Earth surface/clouds --> satellite sensor
• measurement of spectrum of incoming light - solar irradiance spectrum
• measurement of spectrum of light into sensor (reflected solar
spectrum from Earth = Earth radiance spectrum)
• ratio of Earth radiance spectrum to solar spectrum
• elimination of spectrum of solar spectrum (to first order)
• reveals absorption spectrum of atmospheric molecules of interest
• absorbance depth is proportional to the abundance of that molecule
along the absorption path
• note: this is NOT a tropospheric concentration
• technique only useful if there is “differential” absorption from
the molecule doing the absorbing - a smooth continuum spectrum with no
spectral structure only depresses the entire intensity of the spectrum
Current Satellite Data - TOMS, GOME, TES, MOPITT, Sciamachy
• spatial footprint
• currently operational satellite doing air pollution work
• Sciamachy footprint: ~ 30 km x 60 km - scanning mirror for swath
• GOME: ~ 80 km x 340 km - scanning mirror for swath
• TOMS: ~ 50 km x 200 km - scanning mirror for swath
• MOPITT: 22 km x 22 km - scanning mirror for swath - CO
• TES: 5 km x 9 km: ozone
• temporal resolution at time of overpass (typically about 1:30PM)
• get a picture each orbit of 90 min duration when airshed underneath
• with a large swath, get several orbits of data sequentially over airshed
• with small swath, may take several days to build a picture (GOME -3d)
• time of day is restricted: depends on equatorial time transit
• e.g. for Aura, it is 1345 h --> obsv at same time of day each day
0.4
9 May 2006
slant column NO 2 relative
slant column NO 2 [arbitrary units]
to local noon zenith
0.3
0.2
5° elev
0.1
0.0
5° elev
15°
45°
north
east
south
west
15° el
45° el
6
8
10
12
14
time of day [PDST]
16
18
20
10
OMI Tropospheric NO2 Vertical Column Density
(integrated over several km south from PNNL)
5
o
MFDOAS Slant Column Density (Veiw elevation 5 South)
8
MFDOAS observation Time = OMI time over Tri-Cities
4
6
4
16
SCD x 10 molecules/cm
3
2
2
1
14-May-06
13-May-06
12-May-06
0
11-May-06
10-May-06
9-May-06
8-May-06
7-May-06
6-May-06
5-May-06
4-May-06
3-May-06
2-May-06
1-May-06
0
2
One negative OMI value (-0.725) found
along the path was excluded
30-Apr-06
15
6
MFDOAS NO2
OMI Tropospheric NO2 VCD x 10 molecules/cm
2
Figure 8. Comparison of OMI Tropospheric NO2 Vertical Column
with MFDOAS NO2 Differential Slant Column Density
Date
WSU MFDOAS
data from PNNL
during INTEX
DOAS Theory
Detector
I o  
Light
Source
I 
Atmosphere
PC
Spectrograph
• Measure wavelength dependent light intensity (I[]) as light passes
through the air mass
• Initial intensity (Io []) decreases in the airmass due to


absorption by the trace gases,
scattering by molecules and aerosol particles
• trace gases can be detected in the ratio of I [] to Io[] as a function of
wavelength due to their unique absorption features
DOAS Theory: Beer-Lambert Law
• Theory
• Reality
Air Mass Factors: Geometrical Approach
Total Slant
Column
Recent Developments in DOAS: an Overview. Ulrich Platt. Institut für Umweltphysik, Universität Heidelberg