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| Titel |
Study of Differential Column Measurements for Urban Greenhouse Gas Emission Monitoring |
| VerfasserIn |
Jia Chen, Jacob K. Hedelius, Camille Viatte, Taylor Jones, Jonathan E. Franklin, Harrison Parker, Paul O. Wennberg, Elaine W. Gottlieb, Manvendra K. Dubey, Steven C. Wofsy |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250123906
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| Publikation (Nr.) |
 EGU/EGU2016-3244.pdf |
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| Zusammenfassung |
| Urban areas are home to 54% of the total global population and account for ∼ 70% of total
fossil fuel emissions. Accurate methods for measuring urban and regional scale carbon fluxes
are required in order to design and implement policies for emissions reduction
initiatives.
In this paper, we demonstrate novel applications of compact solar-tracking Fourier
transform spectrometers (Bruker EM27/SUN) for differential measurements of the
column-averaged dry-air mole fractions (DMFs) of CH4 and CO2 within urban areas. Our
differential column method uses at least two spectrometers to make simultaneous
measurements of CO2, CH4 and O2 column number densities. We then compute the
column-averaged DMFs XG for a gas G and the differences ΔXG between downwind and
upwind stations. By accurately measuring the small differences in integrated column amounts
across local and regional sources, we directly observe the mass loading of the atmosphere
due to the influence of emissions in the intervening locale. The inference of the
source strength is much more direct than inversion modeling using only surface
concentrations, and less subject to errors associated with modeling small-scale transport
phenomena.
We characterize the differential sensor system using Allan variance analysis and show
that the differential column measurement has a precision of 0.01% for XCO2 and XCH4
using an optimum integration time of 10 min, which corresponds to standard deviations of
0.04 ppm, and 0.2 ppb, respectively. The sensor system is very stable over time and
after relocation across the contiguous US, i.e. the scaling factors between the two
Harvard EM27/SUNs and the measured instrument line function parameters are
consistent.
We use the differential column measurements to determine the emission of an area
source. We measure the downwind minus upwind column gradient ΔXCH4 (∼ 2 ppb, 0.1%)
across dairy farms in the Chino California area, and input the data to a simple column model
for comparison with emission strengths reported in the literature. Our model assumes that air
parcels within the air column are transported with a mass-enhancement-weighed horizontal
wind velocity U, which is estimated using surface wind speeds measured at nearby airports
and assuming a wind profile power law up to the mixing height, to which CH4 emissions are
transported vertically by turbulent flow. The emission estimate using differential
column measurements is dominated by the uncertainty in the transport i.e. U, not the
differential column measurements themself. Furthermore, we derive spatial column
gradient ratios ΔXCH4/ΔXCO2 across Pasadena within the Los Angeles basin,
and determine values that are consistent with regional emission ratios from the
literature.
Our precise, rapid measurements allow us to determine short-term variations (5 to 10
minutes) of XCO2 and XCH4 in side-by-side measurements at Caltech and Harvard. Both
Harvard EM27/SUNs capture these fluctuations simultaneously, which represent geophysical
phenomena, not noise as might be assumed.
Overall, this study helps establish a range of new applications for compact solar-viewing
Fourier transform spectrometers. |
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