Total Versus Speciated Monoterpene Concentrations and Fluxes: Do Speciated Measurements Underestimate Emissions?

Concentrations and fluxes of speciated and total monoterpenes were measured simultaneously above a ponderosa pine plantation at Blodgett Forest, in the Sierra Nevada, CA. Speciated measurements were made using relaxed eddy accumulation (REA) coupled with a gas chromatograph with flame ionization detectors (GC-FID), and total monoterpene measurements were made using eddy covariance coupled with a proton transfer reaction mass spectrometer (PTR-MS). Ambient, in situ measurements of monoterpene concentrations and fluxes are typically limited to a few monoterpene species. Because simultaneous measurements of both total and speciated monoterpenes have never been made, it is not known whether speciated monoterpene measurements adequately represent the impact of total monoterpene emissions to the atmosphere. We present results from the first field experiments comparing simultaneous measurements of concentrations and fluxes of seven commonly emitted monoterpene species with new measurements of total monoterpene concentrations and fluxes. As all monoterpene species share the same mass, the PTR-MS, coupled with the eddy covariance technique, allowed us to make real time, in situ concentration and flux measurements of the sum of all monoterpene species in the atmosphere. These two flux measurement techniques were in good agreement on the temporal pattern of monoterpene flux, but showed that measurements of speciated monoterpenes underestimated the flux of total monoterpenes to the atmosphere. As our GC-FID system is comparable to other speciated monoterpene concentration measurement systems typically used, the underestimation of emissions made by our speciated measurements suggest that models based on speciated monoterpene flux measurements may be underestimating regional and global monoterpene emissions and their impacts on atmospheric chemistry.