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Mapping CH_4 : CO_2 ratios in Los Angeles with CLARS-FTS from Mount Wilson, California

Wong, K. W. and Fu, D. and Pongetti, T. J. and Newman, S. and Kort, E. A. and Duren, R. and Hsu, Y.-K. and Miller, C. E. and Yung, Y. L. and Sander, S. P. (2015) Mapping CH_4 : CO_2 ratios in Los Angeles with CLARS-FTS from Mount Wilson, California. Atmospheric Chemistry and Physics, 15 (1). pp. 241-252. ISSN 1680-7316. doi:10.5194/acp-15-241-2015.

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The Los Angeles megacity, which is home to more than 40% of the population in California, is the second largest megacity in the United States and an intense source of anthropogenic greenhouse gases (GHGs). Quantifying GHG emissions from the megacity and monitoring their spatiotemporal trends are essential to be able to understand the effectiveness of emission control policies. Here we measure carbon dioxide (CO_2) and methane (CH_4) across the Los Angeles megacity using a novel approach – ground-based remote sensing from a mountaintop site. A Fourier transform spectrometer (FTS) with agile pointing optics, located on Mount Wilson at 1.67 km above sea level, measures reflected near-infrared sunlight from 29 different surface targets on Mount Wilson and in the Los Angeles megacity to retrieve the slant column abundances of CO_2, CH_4 and other trace gases above and below Mount Wilson. This technique provides persistent space- and time-resolved observations of path-averaged dry-air GHG concentrations, XGHG, in the Los Angeles megacity and simulates observations from a geostationary satellite. In this study, we combined high-sensitivity measurements from the FTS and the panorama from Mount Wilson to characterize anthropogenic CH_4 emissions in the megacity using tracer–tracer correlations. During the period between September 2011 and October 2013, the observed XCH_4 : XCO_2 excess ratio, assigned to anthropogenic activities, varied from 5.4 to 7.3 ppb CH_4 (ppm CO_2)^(−1), with an average of 6.4 ± 0.5 ppb CH_4 (ppm CO_2)^(−1) compared to the value of 4.6 ± 0.9 ppb CH_4 (ppm CO_2)^(−1) expected from the California Air Resources Board (CARB) bottom-up emission inventory. Persistent elevated XCH_4 : XCO_2 excess ratios were observed in Pasadena and in the eastern Los Angeles megacity. Using the FTS observations on Mount Wilson and the bottom-up CO_2 emission inventory, we derived a top-down CH_4 emission of 0.39 ± 0.06 Tg CH_4 year^(−1) in the Los Angeles megacity. This is 18–61% larger than the state government's bottom-up CH_4 emission inventory and consistent with previous studies.

Item Type:Article
Related URLs:
URLURL TypeDescription
Pongetti, T. J.0000-0001-9465-0853
Newman, S.0000-0003-0710-995X
Kort, E. A.0000-0003-4940-7541
Duren, R.0000-0003-4723-5280
Miller, C. E.0000-0002-9380-4838
Yung, Y. L.0000-0002-4263-2562
Sander, S. P.0000-0003-1424-3620
Alternate Title:Mapping CH4 : CO2 ratios in Los Angeles with CLARS-FTS from Mount Wilson, California
Additional Information:© Author(s) 2015. This work is distributed under the Creative Commons Attribution 3.0 License. Received: 23 May 2014 – Discussion started: 26 Jun 2014 – Revised: 16 Sep 2014 – Accepted: 18 Oct 2014 – Published: 12 Jan 2015. The authors thank our colleagues at JPL, Q. Zhang (California Institute of Technology), D. Wunch (California Institute of Technology), P. Wennberg (California Institute of Technology), C. Roehl (California Institute of Technology), J. Stutz (University of California, Los Angeles) and G. Keppel-Aleks (University of Michigan) for helpful comments. Support from the NASA Postdoctoral Program, California Air Resources Board, NOAA Climate Program, NIST GHG and Climate Science Program and JPL Earth Science and Technology Directorate is gratefully acknowledged. Y. L. Yung was supported in part by NASA grant NNX13AK34G to the California Institute of Technology and the KISS program of Caltech. Edited by: R. Harley
Group:Keck Institute for Space Studies
Funding AgencyGrant Number
NASA Postdoctoral ProgramUNSPECIFIED
California Air Resources BoardUNSPECIFIED
National Oceanic and Atmospheric Administration (NOAA)UNSPECIFIED
National Institute of Standards and Technology (NIST)UNSPECIFIED
Keck Institute for Space Studies (KISS)UNSPECIFIED
Issue or Number:1
Record Number:CaltechAUTHORS:20181016-153903592
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Official Citation:Wong, K. W., Fu, D., Pongetti, T. J., Newman, S., Kort, E. A., Duren, R., Hsu, Y.-K., Miller, C. E., Yung, Y. L., and Sander, S. P.: Mapping CH4 : CO2 ratios in Los Angeles with CLARS-FTS from Mount Wilson, California, Atmos. Chem. Phys., 15, 241-252,, 2015.
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:90300
Deposited By: George Porter
Deposited On:17 Oct 2018 15:47
Last Modified:16 Nov 2021 03:31

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