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 radiationtrace gas measurements: observing the Earth’s backscattered uv/visible radiation

Sunlight passes through the atmosphere, reflectsoff clouds and the surface, and is scattered backinto the instrument field of view. Molecular spectralabsorption is proportional to the concentration ofthe gas doing the absorbing along the path.

Sun

Observing Principle for OMI

13 km))2600 km

12 km/24 km (binned & co-added)

flight direction» 7 km/sec

viewing angle± 57 deg

2-dimensional CCD

wavelength ~ 780 pixels

swath~ 580 pixels

GOME

1 TOMS

Sciamachy

OMI

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

stolen from Platt group

MF-DOAS: geometryfor sky-viewing mode

Spectrograph

CCD Controller

CCD

OpticsHolder FilterWheels

ISPower

Supplies

FW Controller Heat Exchanger

Telescope

Scattered Sky Light Direct SunLight

Solar Tracker/Positioner

WSUMFDOAS instrument

12

10

8

6

4

2

0

NO2 Differential Slant Column Density x10

16

molecules/cm

2

06:00 08:00 10:00 12:00 14:00 16:00 18:00

Pacific Standard Time

12

10

8

6

4

2

0

x10

16

1086420

x10

16

1086420

x10

16

Figure 3. MFDOAS NO2 Differential Slant Column: 9 May 2006

South West North East

Elevation View Angle 5o

Elevation View Angle 15o

Zenith

Elevation View Angle 45o

WSU MFDOASdata during INTEX B atPNNL, Richland, WA

46.8

46.7

46.6

46.5

46.4

46.3

46.2

46.1

46.0

45.9

Latitude

-119.6 -119.4 -119.2 -119.0 -118.8Longitude

Richland, WA

Kenewick, WA

Pasco, WA

4 3.8

3.6 3.4

3.2

3.2

3

3

2.8

2.6

2.6

2.4

2.2

2.2

2

2 2

1.8

1.8

1.6

1.6

1.4

1.4

1.2

1.2

1

1

0.8

0.6

0.6

0.4

4.4 4.2

3.6

3.2

3.2

3

3

2.8

2.6

2.4

Figure 7. OMI Tropospheric NO2 Vertical Column Density: 9 May 2006

543210

NO2 Column Density x 1015

molecules/cm2

OMI data during INTEX over PNNL, WA

spring 2006

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• many tropospheric trace gases have complicated molecular spectra allowing

identification and quantification of concentration, e.g.

• 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.

6.6

6.4

6.2

6.0

5.8

5.6

cross section (e19) [cm^2]

414.90414.80414.70wavelength (nm) [Harder, Brault, Johnston, and Mount, 1997]

NO2

photoabsorptioncross section

• 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

0.3

0.2

0.1

0.0

slant column NO

2

[arbitrary units]

20181614121086

time of day [PDST]

5° elev 15° 45° north east south west5° elev

15° el

9 May 2006slant column NO2 relative

to local noon zenith

45° el

6

5

4

3

2

1

0OMI Tropospheric NO

2 VCD x 10

15

molecules/cm

2

30-Apr-061-May-062-May-063-May-064-May-065-May-066-May-067-May-068-May-069-May-0610-May-0611-May-0612-May-0613-May-0614-May-06

Date

10

8

6

4

2

0

MFDOAS NO

2

SCD x 10

16molecules/cm

2

Figure 8. Comparison of OMI Tropospheric NO2 Vertical Column with MFDOAS NO2 Differential Slant Column Density

OMI Tropospheric NO2 Vertical Column Density

(integrated over several km south from PNNL)

MFDOAS Slant Column Density (Veiw elevation 5o South)

MFDOAS observation Time = OMI time over Tri-Cities

One negative OMI value (-0.725) found along the path was excluded

WSU MFDOASdata from PNNLduring INTEX

DOAS Theory

• 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

PC

SpectrographAtmosphereLight

Source

DetectorI o ( )

I ( )

DOAS Theory: Beer-Lambert Law

• Theory

• Reality

Air Mass Factors: Geometrical Approach

Recent Developments in DOAS: an Overview. Ulrich Platt. Institut für Umweltphysik, Universität Heidelberg

Total Slant Column