Monitoring of tropospheric methane from space: problems

and solutionsLeonid YurganovLeonid Yurganov, Larrabee Strow, , Larrabee Strow,

Scott HannonScott Hannon

University of Maryland Baltimore County, Joint Center for Earth Technology (affiliated with NASA), Baltimore, USA

Thanks to AIRS CH4 retrieval algorithm developers: Xiaozhen Xiong and Chris Barnett (NESDIS, NOAA); we grateful also to NASA and EUMETSAT for processing and archiving AIRS and IASI data

PERGAMON meeting, Brussels, 18 October 2010PERGAMON meeting, Brussels, 18 October 2010

OutlineOutline

Terminology and definitions Thermal vs solar Measurements: CH

4 vs CO

The report is based on AIRS and IASI The report is based on AIRS and IASI

sensors:sensors: Siberia: vertical extent of surface influence Wetlands, global & hemispheric CH

4 patterns

CH4 distributions in the high NH

AtmosphereAltitude, kmAltitude, km

Boundary layer

Stratosphere

Free troposphere

10

1.5

0

Tropopause

1000

250

850

Air pressure, hPa (mb)Air pressure, hPa (mb)

Solar and thermal IR radiationSolar and thermal IR radiationSolar Solar radiance has a

maximum near λ~ 0.5 μm, and it is attenuated by scattering, clouds, aerosol, etc

ThermalThermal (or terrestrial) radiance has a maximum near λ~10 μm (ν=1/λ=1000 cm-1)

T ~ 6000 K

T ~ 270-300 K

There are methane absorption linesabsorption lines in both spectral intervals.

Important parameters of spectrometersImportant parameters of spectrometers Spectral resolution Spectral resolution (the finer the better)

Spatial resolution Spatial resolution (the finer the better)

Swath Swath (the wider the better)

Signal to Noise Ratio Signal to Noise Ratio (the larger the better)

Vertical sensitivity Vertical sensitivity (the deeper the better)

AIRS and IASIAIRS and IASI

Both are at polar sun-synchronous orbits. AIRS/Aqua:AIRS/Aqua: Atmospheric Infrared Sounder,

since September 2002, grating spectrometer, resolution around 7.7 μm: 0.8 cm-1. Swath ± 670 km. Maximum spatial resolution 2.3 km.

IASI/Metop-AIASI/Metop-A: Infrared Atmospheric Sounding Interferometer, since July 2007. Resolution 0.5 cm-1. Swath ± 1066 km. Spatial resolution 18 km

Volume mixing ratio (VMR),Volume mixing ratio (VMR), ppm or ppb, molecules per molecules of drydry air

Total column amount (TC), Total column amount (TC), molecules/cm2. A convenient formula to convert VMR in TC:

TC = Σ ( VMR(ppb) * Δ P(mb) * 2.12E13) XCHXCH

44 = = TCch4

/TCair

Mean atmospheric VMR

Carbon monoxideCarbon monoxide

This is an example of easily measurable gas. It varyes in the normal, undisturbed troposphere between 50 ppb in the South Hemisphere (SH) and 120 ppb in the Northern Hemisphere (NH).

Forest fires in Russia in July – August 2010.Forest fires in Russia in July – August 2010.

Can we measure CH4 with a similar

time resolution in the Arctic or anywhere?

Main problems.Main problems. Low variability of methane, even near the

surface (less than 10%, usually 2-3% ). Low sensitivity of thermal spectra to the

boundary layer and water vapor spectral contamination.

Low Signal to Noise Ratio of solar spectra in some methane bands (low detector sensitivity) .

Vertical sensitivity: averaging kernel (AK)Vertical sensitivity: averaging kernel (AK)

Matrix equationMatrix equation

Retrieved = A-priori + AK * (True - A-priori) Retrieved = A-priori + AK * (True - A-priori) If AK=1, Retrieved = True; If AK=0, Retrieved = A-priori. If AK=1, Retrieved = True; If AK=0, Retrieved = A-priori.

6 km

0 km

IASI: Thermal vs sol/therm channels

Averaing kernels for CH4 TC, IASI, AK should be 1.0

Thermal channels near 7.7 μm

Thermal/solar channels near 3.6 μm

Validations and comparisons

♦ValidationValidation is the only way to decide if a space-based sensor supplies reasonable data. ♦Here we use excelent Russia/Japan long-term aircraft data series in West Siberia (see recent paper Vertical Distribution of Greenhouse Gases above Western Siberia by the Long-Term Measurement Data, by Arshinov et al., Atm. Ocean. Opt. (2009). ♦WWhat is the vertical extent of wetland methane hat is the vertical extent of wetland methane emission influence? emission influence?

Northern wetlands

Surgut

Alert

Fraserdale

Wetland methaneVertical Distribution of Greenhouse Gases Vertical Distribution of Greenhouse Gases

above Western Siberia by the Long-Term above Western Siberia by the Long-Term Measurement Data, Arshinov et al., Atm. Measurement Data, Arshinov et al., Atm. Ocean. Opt. (2009)Ocean. Opt. (2009)

Magnitude and seasonality of wetland Magnitude and seasonality of wetland methane emissions, C. A. Pickett-methane emissions, C. A. Pickett-Heaps et al., ACPD (2010)Heaps et al., ACPD (2010)

Wetland

Arctic

Blue – surface measurements

Siberia Canada

Surgut: what AIRS shouldshould see?Measured methane profiles were convolved with AKs and

a-priori profiles(just a computer simulation)

What does AIRS reallyreally see?Convolved aircraftRetrieved from spectra

This comparison allows us to assume that This comparison allows us to assume that AIRS's CHAIRS's CH

4 4 reportedreported for the 200, 300, and 400 for the 200, 300, and 400

hPa levels are sensitive also to lower hPa levels are sensitive also to lower atmospheric levels (namely ~ 3-5 km asl, or atmospheric levels (namely ~ 3-5 km asl, or even below that). even below that).

Furher AIRS data will be compared with measurements at the NOAA surface network

AIRS and surface NH measurements AIRS and surface NH measurements (seasonal cycles averaged over 2003-2009)(seasonal cycles averaged over 2003-2009)

AIRS NH

Surface NH

ESRL/NOAA has global CH4 surface record since 1983. ESRL/NOAA has global CH4 surface record since 1983. AIRS started in 2002. AIRS started in 2002.

(referenced to 2003-2007)

ESRL data courtesy of Ed Dlugokencky, NOAA

ee

In summer 2010 West Siberian CH4 dropped to mean values of 2003 –

2007 due to low temperature

Surgut CH4 methane anomaly

SurgutSurgut

Comparison of CH4 and CO for the period of Comparison of CH4 and CO for the period of Russian fires (July-August, 2010)Russian fires (July-August, 2010)

Antarctic enigma: Antarctic enigma: methane anomaly appears in West Antarctica every

November and disappers afterwards

AIRS, retrieval for 400 mb, December 2008

ConclusionsConclusions AIRS V5 standard methane retrievals are AIRS V5 standard methane retrievals are

sensitive not only to the upper troposphere, sensitive not only to the upper troposphere, but also to the lower part of the troposphere. but also to the lower part of the troposphere.

Methane, after a period of growing, reached a Methane, after a period of growing, reached a peak in the middle of 2009, declined, and now peak in the middle of 2009, declined, and now is stable .is stable .

Thank you! Thank you!