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Biogenic isoprene fluxes were measured onboard the CIRPAS Twin Otteraircraft as part of the California Airborne Biogenic volatile organiccompound (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 massspectrometer (PTR–MS) and a wind radome probe to directly determine fluxesof isoprene over 7400 km of flight paths focusing on areas of Californiapredicted to have the largest emissions. The fast Fourier transform (FFT)approach was used to calculate fluxes of isoprene over long transects ofmore than 15 km, most commonly between 50 and 150 km. The continuous wavelettransformation (CWT) approach was used over the same transects to alsocalculate instantaneous isoprene fluxes with localization of bothfrequency and time independent of non-stationarities. Fluxes were generallymeasured by flying consistently at 400 m ± 50 m (a.g.l.) altitude, andextrapolated to the surface according to the determined flux divergencedetermined in the racetrack-stacked profiles. The wavelet-derivedsurface fluxes of isoprene averaged to 2 km spatial resolution showed goodcorrespondence to basal emission factor (BEF) land-cover data sets used todrive BVOC emission models. The surface flux of isoprene was close to zeroover Central Valley crops and desert shrublands, but was very high (up to15 mg m<sup>−2</sup> h<sup>−1</sup>) above oak woodlands, with clear dependence ofemissions on temperature and oak density. Isoprene concentrations of up to 8ppb were observed at aircraft height on the hottest days and over thedominant source regions.  Even though the isoprene emissions from agricultural crop regions,shrublands, and coniferous forests were extremely low, observations at theWalnut Grove tower south of Sacramento demonstrate that isoprene oxidationproducts from the high emitting regions in the surrounding oak woodlandsaccumulate at night in the residual layer above the valley and mix down intothe valley in the morning. Thus, the isoprene emissions surrounding thevalley have relevance for the regional photochemistry that is notimmediately apparent solely from the direct emission flux distribution.  This paper reports the first regional observations of fluxes from specificsources by eddy covariance from an aircraft which can finally constrainstatewide isoprene emission inventories used for ozone simulations by stateagencies. While previously there was no available means to constrain thebiogenic models, our results provide a good understanding of what the majorsources of isoprene are in California, their magnitude, and how they aredistributed.  This data set on isoprene fluxes will be particularly useful for evaluatingpotential model alternatives which will be dealt with in a separate paper toassess isoprene emission models and their driving variable data sets

Airborne flux measurements of biogenic isoprene over California