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Uncovering mechanisms of episodic methane sources observed by a very tall eddy covariance tower Ankur Desai, UW-Madison Atmospheric & Oceanic Sciences Weiguo Wang, NOAA EMC Bruce D Cook, NASA GSFC AMS 30 th AgForMet / 1 st Biogeosciences Conference Wed. May 30, 2012, Boston, MA Abstract #J6.3

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A. Desai J6.3 2/22 Why do we even have tall towers? Atmospheric and ecological research suffers from the perfect site bias High signal to noise ratio E.g., convection in southern great plains Pretty site bias (infrastructure, homogeneity) E.g., Even-aged homogenous flux towers Regional-scale flux measurements can address some of this bias

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A. Desai J6.3 3/22 A very tall tower!

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A. Desai J6.3 4/22 Pretty sites!

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A. Desai J6.3 5/22 The reality

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A. Desai J6.3 6/22 Long-term variability of CO 2 NEE

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A. Desai J6.3 7/22 Methane is a real bugger In late 2010, we added fast response methane measurements to the long running Ameriflux US-PFa (WLEF) tower

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A. Desai J6.3 8/22 And it surprised us!

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A. Desai J6.3 9/22 Different sources in winter and summer? Methane flux magnitudes do not change in magnitude from winter to growing season, but do change in quality.

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A. Desai J6.3 10/22 Environmental controls not evident CH 4 emissions regionally are only weakly correlated to temperature, unlike at plot scale. Winter CH 4 fluxes strongly correlated to small magnitude CO 2 fluxes.

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A. Desai J6.3 11/22 Questions Are there ecosystem-scale environmental controls on these relatively large bursts of methane inside and outside of the growing season? Which landscapes in the tower footprint are responsible for large CH 4 sources? Is there a shift in key regions for methane emission and consumption by season?

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A. Desai J6.3 12/22 Where are the methane sources? 30 meter land cover derived from hand analysis and ground truthing of Quickbird imagery

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A. Desai J6.3 13/22 Seeing more with less Reclassification of 34 land covers reveals importance of small- scale wetlands and forest type across footprint

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A. Desai J6.3 14/22 PBL footprint models required Surface-layer flux footprint models are not valid for tall towers Applied Wang et al. (2006) J. Atm. Ocean. Tech. CBL cross- wind integrated flux footprint model to one year at 122m measurement height at WLEF

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A. Desai J6.3 15/22 I see the forest and the swamp! Typical hourly flux footprint samples a wide range of wetland and forest types Variation with stability and wind direction allow us to evaluate sources

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A. Desai J6.3 16/22 Footprint sample bias is small Surprisingly, long-term footprint biases are small, so we are confident that tower samples regional flux over long-time periods

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A. Desai J6.3 17/22 No smoking gassy gun? Mean land cover by wind-direction segregated footprint hints but does not fully support strong CH4 sources from wetlands NW of tower

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A. Desai J6.3 18/22 Maybe pure footprints help? However, high methane emissions occur both in footprints with mostly upland influence (top) AND mostly wetland influence (bottom)

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A. Desai J6.3 19/22 Forests may have CH 4 sources Temperature response segregated by dominant footprint land cover reveals high CH 4 emission by deciduous forest dominant footprints!

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A. Desai J6.3 20/22 Coevolution of CO 2 and CH 4 sources? Alternative look some consistencies in difference in land cover influence by periods with high CO2 and/or high CH4

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A. Desai J6.3 21/22 Conclusions Regional fluxes arise from a a variety of non-pretty landscapes with low signal-to-noise Requires a more robust sample design for scaling from stand to region Episodic methane sources influence regional methane flux budgets and have a different pace and mechanism then CO 2, especially in growing season Flux footprint models and land cover maps hint at a wetland influence for CH 4 emissions, but not clearly Uncertainty in all three (flux, footprint, land cover) can be large and require evaluation Upland sources of methane cannot be ruled out Simple temperature response functions from the plot-scale do not necessarily pan out at the region -> implications for ecosystem model parameterization Current plans include development of soil survey chamber for CH 4 flux seeking advice!

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A. Desai J6.3 22/22 Acknowledgments $$: NSF CAREER DEB #0845166 WLEF/ Park Falls (US-PFa) tall tower research partners: NOAA ESRL (A. Andrews, J. Kofler), USFS NRS (M. Kubiske, D. Baumann), Penn State (K. Davis), WI ECB (J. Ayers), Ameriflux, Picarro, Inc., Los Gatos Research, Inc., P Weishampel (Northland College) Desai lab at UW Field site manager: Jonathan Thom http://flux.aos.wisc.edu desai@aos.wisc.edu 608-218-4208