Eddy Correlation Flux Measurements of Volatile Organic Compounds by Disjunct Eddy Sampling and Ion Trap Mass Spectrometry Analysis

The present technical capability to accurately measure the fluxes of volatile organic compounds (VOC) is one of the major limitations towards advancing our understanding of photochemical generation of tropospheric ozone and aerosols and strategies for their controls. We demonstrate a new analytical technique for the detection and direct flux measurement of VOC from both biogenic and anthropogenic sources. Air is collected instantaneously and subsequently analyzed in-situ with an ion trap mass spectrometer (ITMS). Samples are taken periodically rather than continuously, and correlated with the corresponding wind speed and direction at the time of sampling (disjunct eddy sampling). In order to calculate flux values with this technique, the deviations from the temporal means of the vertical wind speed and the VOC mixing ratio are correlated (Rinne et al., 2000). A volume of atmospheric air containing target VOC among a mixture is quickly sampled into an evacuated 0.5 l sample reservoir in appr. 0.2 s. Wind speed and direction at the sampling time are recorded with a 3-D sonic anemometer. From the sample reservoir, a 25 ml aliquot is directed through a short section of a Porous Layer Open Tubular (PLOT) column for enrichment of VOC and separation from air. The choices of PLOT column material and adsorption parameters determine the focusing efficiency of the enrichment trap with respect to the species of interest. The PLOT column is subsequently flash-heated, VOC are backflushed with helium and injected into the ITMS for analysis. Qualitative and quantitative analysis of target VOC from the mixture is accomplished by applying selective ionization, ion storage and fragmentation techniques (MS/MS) with the ITMS. The entire process of sample collection, focusing, desorption and analysis can be repeated every 20-30 seconds, and is automated and software driven. Compared to continuous eddy correlation measurements, this method increases the uncertainty of the flux measurement by reducing the sampling frequency, but allows enough time for enrichment and analysis by ITMS in order to achieve required detection limits. Up to 180 samples can be analyzed every hour, which has been demonstrated to be sufficient for surface-atmosphere VOC flux measurements (Rinne et al., 2000). This presentation describes the analytical approach and results of experiments towards applying this technique for ambient VOC flux measurements. References: Rinne H.J.I., Delany A.C., Greenberg J.P. and Guenther A.B. (2000), A true eddy accumulation system for trace gas fluxes using disjunct eddy sampling method. J. Geophys. Res. 105, 24791-24798.