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| Titel |
Source apportionment of methane and nitrous oxide in California's San Joaquin Valley at CalNex 2010 via positive matrix factorization |
| VerfasserIn |
A. Guha, D. R. Gentner, R. J. Weber, R. Provencal, A. H. Goldstein |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 15, no. 20 ; Nr. 15, no. 20 (2015-10-29), S.12043-12063 |
| Datensatznummer |
250120131
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| Publikation (Nr.) |
 copernicus.org/acp-15-12043-2015.pdf |
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| Zusammenfassung |
Sources of methane (CH4) and nitrous oxide (N2O) were investigated
using measurements from a site in southeast Bakersfield as part of the CalNex
(California at the Nexus of Air Quality and Climate Change) experiment from
mid-May to the end of June 2010. Typical daily minimum mixing ratios of CH4
and N2O were higher than daily minima that were simultaneously observed
at a mid-oceanic background station (NOAA, Mauna Loa) by approximately
70 ppb and 0.5 ppb, respectively. Substantial enhancements of CH4 and
N2O (hourly averages > 500 and > 7 ppb,
respectively) were routinely observed, suggesting the presence of large
regional sources. Collocated measurements of carbon monoxide (CO) and a range
of volatile organic compounds (VOCs) (e.g., straight-chain and branched
alkanes, cycloalkanes, chlorinated alkanes, aromatics, alcohols, isoprene,
terpenes and ketones) were used with a positive matrix factorization (PMF)
source apportionment method to estimate the contribution of regional sources
to observed enhancements of CH4 and N2O.
The PMF technique provided a "top-down" deconstruction of ambient
gas-phase observations into broad source categories, yielding a seven-factor
solution. We identified these emission source factors as follows: evaporative
and fugitive; motor vehicles; livestock and dairy; agricultural and soil
management; daytime light and temperature driven; non-vehicular urban; and
nighttime terpene biogenics and anthropogenics. The dairy and livestock
factor accounted for the majority of the CH4 (70–90 %) enhancements
during the duration of experiments. The dairy and livestock factor was also
a principal contributor to the daily enhancements of N2O (60–70 %).
Agriculture and soil management accounted for ~ 20–25 % of
N2O enhancements over a 24 h cycle, which is not surprising given
that organic and synthetic fertilizers are known to be a major source of
N2O. The N2O attribution to the agriculture and soil management
factor had a high uncertainty in the conducted bootstrapping analysis. This
is most likely due to an asynchronous pattern of soil-mediated N2O
emissions from fertilizer usage and collocated biogenic emissions from crops
from the surrounding agricultural operations that is difficult to apportion
statistically when using PMF. The evaporative/fugitive source profile,
which resembled a mix of petroleum operation and non-tailpipe evaporative
gasoline sources, did not include a PMF resolved-CH4 contribution that
was significant (< 2 %) compared to the uncertainty in the
livestock-associated CH4 emissions. The uncertainty of the CH4
estimates in this source factor, derived from the bootstrapping analysis, is
consistent with the ~ 3 % contribution of fugitive oil and
gas emissions to the statewide CH4 inventory. The vehicle emission
source factor broadly matched VOC profiles of on-road exhaust sources. This
source factor had no statistically significant detected contribution to the
N2O signals (confidence interval of 3 % of livestock N2O
enhancements) and negligible CH4 (confidence interval of 4 % of
livestock CH4 enhancements) in the presence of a dominant dairy and
livestock factor. The CalNex PMF study provides a measurement-based
assessment of the state CH4 and N2O inventories for the southern
San Joaquin Valley (SJV). The state inventory attributes ~ 18 % of
total N2O emissions to the transportation sector. Our PMF analysis
directly contradicts the state inventory and demonstrates there were no
discernible N2O emissions from the transportation sector in the
southern SJV region. |
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