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| Titel |
Airborne flux measurements of biogenic isoprene over California |
| VerfasserIn |
P. K. Misztal, T. Karl, R. Weber, H. H. Jonsson, A. B. Guenther, 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 ; 14, no. 19 ; Nr. 14, no. 19 (2014-10-10), S.10631-10647 |
| Datensatznummer |
250119088
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| Publikation (Nr.) |
 copernicus.org/acp-14-10631-2014.pdf |
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| Zusammenfassung |
Biogenic isoprene fluxes were measured onboard the CIRPAS Twin Otter
aircraft as part of the California Airborne Biogenic volatile organic
compound (BVOC) Emission Research in Natural Ecosystem Transects (CABERNET)
campaign during June 2011. The airborne virtual disjunct eddy covariance
(AvDEC) approach used measurements from a proton transfer reaction mass
spectrometer (PTR–MS) and a wind radome probe to directly determine fluxes
of isoprene over 7400 km of flight paths focusing on areas of California
predicted to have the largest emissions. The fast Fourier transform (FFT)
approach was used to calculate fluxes of isoprene over long transects of
more than 15 km, most commonly between 50 and 150 km. The continuous wavelet
transformation (CWT) approach was used over the same transects to also
calculate instantaneous isoprene fluxes with localization of both
frequency and time independent of non-stationarities. Fluxes were generally
measured by flying consistently at 400 m ± 50 m (a.g.l.) altitude, and
extrapolated to the surface according to the determined flux divergence
determined in the racetrack-stacked profiles. The wavelet-derived
surface fluxes of isoprene averaged to 2 km spatial resolution showed good
correspondence to basal emission factor (BEF) land-cover data sets used to
drive BVOC emission models. The surface flux of isoprene was close to zero
over Central Valley crops and desert shrublands, but was very high (up to
15 mg m−2 h−1) above oak woodlands, with clear dependence of
emissions on temperature and oak density. Isoprene concentrations of up to 8
ppb were observed at aircraft height on the hottest days and over the
dominant source regions.
Even though the isoprene emissions from agricultural crop regions,
shrublands, and coniferous forests were extremely low, observations at the
Walnut Grove tower south of Sacramento demonstrate that isoprene oxidation
products from the high emitting regions in the surrounding oak woodlands
accumulate at night in the residual layer above the valley and mix down into
the valley in the morning. Thus, the isoprene emissions surrounding the
valley have relevance for the regional photochemistry that is not
immediately apparent solely from the direct emission flux distribution.
This paper reports the first regional observations of fluxes from specific
sources by eddy covariance from an aircraft which can finally constrain
statewide isoprene emission inventories used for ozone simulations by state
agencies. While previously there was no available means to constrain the
biogenic models, our results provide a good understanding of what the major
sources of isoprene are in California, their magnitude, and how they are
distributed.
This data set on isoprene fluxes will be particularly useful for evaluating
potential model alternatives which will be dealt with in a separate paper to
assess isoprene emission models and their driving variable data sets. |
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