Airborne Eddy Covariance Fluxes Provide Novel Constraints on Sources and Sinks of Reactive Gases in the Planetary Boundary Layer

Atmospheric composition in the planetary boundary layer is dictated by the interplay of emissions, chemistry, transport, deposition and entrainment. Significant uncertainties surround each of these processes, especially in forested environments and chemical regimes defined by high isoprene and low NO_x. During the 2013 SEAC⁴RS (Studies of Emissions and Atmospheric Composition, Clouds and Climate Coupling by Regional Surveys) mission, the NASA DC-8 flew a set of four low-level transects over the Ozark Mountains. Known colloquially as the “isoprene volcano,” this region is a dense oak forest with few local anthropogenic emissions. This flight afforded a unique opportunity – perhaps the first ever – to calculate eddy covariance fluxes of a wide suite of reactive gases, including isoprene and its oxidation products, H₂O₂, ozone and NO_x. We demonstrate that synergistic information is gained when fluxes are simultaneously derived for multiple reactive species and at multiple heights in the boundary layer. These measurements can provide quantitative constraints on numerous chemical and physical parameters, including emission rates, oxidant concentrations, reaction branching ratios, deposition velocities and entrainment rates. In some instances, it is also possible to spatially resolve fluxes and derived quantities through application of wavelet transforms. As a near-direct measurement of underlying process rates, airborne flux observations may offer a powerful new tool in future efforts to improve biogenic emissions inventories, photochemical mechanisms and deposition parameterizations.