<p>Biogenic Volatile Organic Compound (BVOC) fluxes were measured onboard the CIRPAS Twin Otter aircraft as part of the California Airborne BVOC Emission Research in Natural Ecosystem Transects (CABERNET) campaign during June 2011. The airborne virtual disjunct eddy covariance (AvDEC) approach used measurements from a PTR-MS and a wind radome probe to directly determine fluxes of isoprene, MVK + MAC, methanol, monoterpenes, and MBO over 10 000 km of flight paths focusing on areas of California predicted to have the largest emissions of isoprene. The Fast Fourier Transform (FFT) approach was used to calculate fluxes 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 &quot;instantaneous&quot; fluxes with localization of both frequency and time independent of non-stationarities. Vertical flux divergence of isoprene is expected due to its relatively short lifetime and was measured directly using &quot;racetrack&quot; profiles at multiple altitudes. It was found to be linear and in the range 5% to 30% depending on the ratio of aircraft altitude to PBL height (z / zi). Fluxes were generally measured by flying consistently at 400 &plusmn; 50 m (a.g.l.) altitude, and extrapolated to the surface according to the determined flux divergence. The wavelet-derived surface fluxes of isoprene averaged to 2 km spatial resolution showed good correspondence to Basal Emission Factor (BEF) landcover datasets used to drive biogenic VOC (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&minus;2 h&minus;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. While isoprene emissions from agricultural crop regions, shrublands, and coniferous forests were extremely low, high concentrations of methanol and monoterpenes were found above some of these regions. These observations demonstrate the ability to measure fluxes from specific sources by eddy covariance from an aircraft, and suggest the utility of measurements using fast response chemical sensors to constrain emission inventories and map out source distributions for a much broader array of trace gases than was observed in this study. This paper reports the first regional direct eddy covariance fluxes of isoprene. The emissions of VOCs measured from aircraft with 2 km spatial resolution can quantify the distribution of major sources providing the observations required for testing statewide emission inventories of these important trace gases. These measurements will be used in a future study to assess BVOC emission models and their driving variable datasets.</p>

<p>Numerous volatile organic compounds (VOCs) exist in Earth&#39;s atmosphere, most of which originate from biogenic emissions. Despite VOCs&#39; critical role in tropospheric chemistry, studies for evaluating their atmosphere-ecosystem exchange (emission and deposition) have been limited to a few dominant compounds owing to a lack of appropriate measurement techniques. Using a high-mass resolution proton transfer reaction-time of flight-mass spectrometer and an absolute value eddy-covariance method, we directly measured 186 organic ions with net deposition, and 494 that have bidirectional flux. This observation of active atmosphere-ecosystem exchange of the vast majority of detected VOCs poses a challenge to current emission, air quality, and global climate models, which do not account for this extremely large range of compounds. This observation also provides new insight for understanding the atmospheric VOC budget.</p>

<p>Airborne flux measurements of isoprene were performed over the Californian oak belts surrounding the Central Valley. The authors demonstrate for the first time 1) the feasibility of airborne eddy covariance measurements of reactive biogenic volatile organic compounds; 2) the effect of chemistry on the vertical transport of reactive species, such as isoprene; and 3) the applicability of wavelet analysis to estimate regional fluxes of biogenic volatile organic compounds. These flux measurements demonstrate that instrumentation operating at slower response times (e.g., 1&ndash;5 s) can still be used to determine eddy covariance fluxes in the mixed layer above land, where typical length scales of 0.5&ndash;3 km were observed. Flux divergence of isoprene measured in the planetary boundary layer (PBL) is indicative of OH densities in the range of 4&ndash;7 &times; 106 molecules per cubic centimeter and allows extrapolation of airborne fluxes to the surface with Damköhler numbers (ratio between the mixing time scale and the chemical time scale) in the range of 0.3&ndash;0.9. Most of the isoprene is oxidized in the PBL with entrainment fluxes of about 10% compared to the corresponding surface fluxes. Entrainment velocities of 1&ndash;10 cm s&minus;1 were measured. The authors present implications for parameterizing PBL schemes of reactive species in regional and global models.</p>