Mapping CH<sub>4</sub> : CO<sub>2</sub> ratios in Los Angeles with CLARS-FTS from Mount Wilson, California
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 ess...
| Main Authors: | , , , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2015-01-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | http://www.atmos-chem-phys.net/15/241/2015/acp-15-241-2015.pdf |
| Summary: | 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<sub>2</sub>) and methane (CH<sub>4</sub>) 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<sub>2</sub>,
CH<sub>4</sub> 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<sub>4</sub> emissions in the megacity using
tracer–tracer correlations. During the period between September 2011 and
October 2013, the observed XCH<sub>4</sub> : XCO<sub>2</sub> excess ratio, assigned to
anthropogenic activities, varied from 5.4 to 7.3 ppb CH<sub>4</sub> (ppm
CO<sub>2</sub>)<sup>−1</sup>, with an average of 6.4 ± 0.5 ppb CH<sub>4</sub> (ppm
CO<sub>2</sub>)<sup>−1</sup> compared to the value of 4.6 ± 0.9 ppb CH<sub>4</sub> (ppm
CO<sub>2</sub>)<sup>−1</sup> expected from the California Air Resources Board (CARB)
bottom-up emission inventory. Persistent elevated XCH<sub>4</sub> : XCO<sub>2</sub> 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<sub>2</sub>
emission inventory, we derived a top-down CH<sub>4</sub> emission
of 0.39 ± 0.06 Tg CH<sub>4</sub> year<sup>−1</sup> in the Los Angeles megacity.
This is 18–61% larger than the state government's bottom-up CH<sub>4</sub> emission
inventory and consistent with previous studies. |
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| ISSN: | 1680-7316 1680-7324 |