1. Monitoring of tropospheric methane from space: problems and solutions Leonid Yurganov, Larrabee Strow, Scott Hannon University of Maryland Baltimore County, Joint Center for Earth Technology (affiliated with NASA), Baltimore, USA Thanks to AIRS CH 4 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 2010

2. Outline Terminology and definitions Thermal vs solar Measurements: CH 4 vs CO The report is based on AIRS and IASI sensors: Siberia: vertical extent of surface influence Wetlands, global & hemispheric CH 4 patterns CH 4 distributions in the high NH

3. Atmosphere Altitude, km Boundary layer Stratosphere Free troposphere 10 1.5 0 Tropopause 1000 250 850 Air pressure, hPa (mb)

4. Solar and thermal IR radiation Solar Solar radiance has a maximum near λ~ 0.5 μm, and it is attenuated by scattering, clouds, aerosol, etc Thermal Thermal (or terrestrial) radiance has a maximum near λ~10 μm (ν=1/λ=1000 cm -1 ) T ~ 6000 K T ~ 270-300 K

5. absorption lines There are methane absorption lines in both spectral intervals. Important 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)

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

7. Volume mixing ratio (VMR), dry Volume mixing ratio (VMR), ppm or ppb, molecules per molecules of dry air Total column amount (TC), Total column amount (TC), molecules/cm 2. A convenient formula to convert VMR in TC: TC = Σ ( VMR(ppb) * Δ P(mb) * 2.12E13) XCH 4 = XCH 4 = TC ch4 /TC air Mean atmospheric VMR

8. Carbon 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.

10. Can we measure CH 4 with a similar time resolution in the Arctic or anywhere? 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).

11. Vertical sensitivity: averaging kernel (AK) Matrix equation 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

12. IASI: Thermal vs sol/therm channels Averaing kernels for CH 4 TC, IASI, AK should be 1.0 Thermal channels near 7.7 μm Thermal/solar channels near 3.6 μm

13. Validations and comparisons Validation ♦Validation 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). What is the vertical extent of wetland methane emission influence? ♦What is the vertical extent of wetland methane emission influence?

14. Northern wetlands Surgut Alert Fraserdale

15. Wetland methane Vertical Distribution of Greenhouse Gases above Western Siberia by the Long-Term Measurement Data, Arshinov et al., Atm. Ocean. Opt. (2009) Magnitude and seasonality of wetland methane emissions, C. A. Pickett- Heaps et al., ACPD (2010) Wetland Arctic Blue – surface measurements Siberia Canada

16. should Surgut: what AIRS should see? Measured methane profiles were convolved with AKs and a-priori profiles (just a computer simulation)

17. really What does AIRS really see? Convolved aircraft Retrieved from spectra

18. This comparison allows us to assume that AIRS's CH 4 reported for the 200, 300, and 400 hPa levels are sensitive also to lower atmospheric levels (namely ~ 3-5 km asl, or even below that). This comparison allows us to assume that AIRS's CH 4 reported for the 200, 300, and 400 hPa levels are sensitive also to lower atmospheric levels (namely ~ 3-5 km asl, or even below that). Furher AIRS data will be compared with measurements at the NOAA surface network

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

20. ESRL/NOAA has global CH4 surface record since 1983. AIRS started in 2002. (referenced to 2003-2007) ESRL data courtesy of Ed Dlugokencky, NOAA

21. e

24. In summer 2010 West Siberian CH4 dropped to mean values of 2003 – 2007 due to low temperature Surgut CH 4 methane anomaly Surgut

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

30. Antarctic enigma: methane anomaly appears in West Antarctica every November and disappers afterwards AIRS, retrieval for 400 mb, December 2008

31. Conclusions AIRS V5 standard methane retrievals are sensitive not only to the upper troposphere, but also to the lower part of the troposphere. AIRS V5 standard methane retrievals are sensitive not only to the upper troposphere, but also to the lower part of the troposphere. Methane, after a period of growing, reached a peak in the middle of 2009, declined, and now is stable. Methane, after a period of growing, reached a peak in the middle of 2009, declined, and now is stable. Thank you!