HR: 1330h
AN: A32A-0111    [PDF]
TI: Methanol and Acetone Fluxes from Agricultural Soil During the Summer 2003 European Heat Wave
AU: * Custer, T G
EM: thcuster@iup.physik.uni-bremen.de
AF: Universitat Bremen, Institute of Environmental Physics, University of Bremen, NW1, Otto-Hahn-Allee 1, Bremen, 28359 Germany
AU: Schade, G W
EM: gws@iup.physik.uni-bremen.de
AF: Universitat Bremen, Institute of Environmental Physics, University of Bremen, NW1, Otto-Hahn-Allee 1, Bremen, 28359 Germany
AB: Field measurements of volatile organic compound (VOC) fluxes have often been carried out using enclosure measurements on single branches or young whole plants. Micrometeorological methods have been restricted to the flux gradient method or, more recently, to the relaxed eddy accumulation (REA) method, because fast response VOC sensors have not been available. In addition, most VOC flux measurements have been carried out above forested regions, while little is known about the VOC flux characteristics of savannas, grasslands, pastures, and grain production. We used the novel Proton-Transfer-Reaction Mass Spectrometer (PTRMS) as a fast VOC sensor to measure methanol and acetone mixing ratios used for eddy covariance analysis to yield methanol and acetone fluxes above an agricultural field prior to seeding. The field site belongs to the German federal agricultural research station (FAL), Braunschweig, and is 81 m a.s.l. Annual mean temperature at the site is $8.8\deg$ C with an average precipitation of 618 mm. The soil type is cambisol / loamy sand (pH 6.5; organic matter content ~1.4 %). First measurements were carried out during the European heat wave in August 2003. Daytime air temperatures between August 8 and 13 reached up to $40\deg$ C and rarely fell below $20\deg$ C at night. Sensible heat flux exceeded 300 W m$^{-2}$ at times. Both methanol and acetone were measured at 4 Hz for 30 min on an alternating half-hour schedule. Mixing ratios (up to 35 ppb for methanol and 10 ppb for acetone) showed diurnal variation with higher values during low turbulence times, especially at night, clearly indicating regional sources. Fluxes were always positive and reached up to 0.35 mg C m$^{-2}$ h$^{-1}$ for methanol and 0.1 mg C m$^{-2}$ h$^{-1}$ for acetone. Methanol fluxes showed a typical temperature dependence with a {\it $\beta$}-factor around 0.1, while acetone fluxes did not. Though the highest methanol fluxes occurred during the highest sub-soil temperatures, a significant correlation to the soil temperature at either five or ten centimeters depth was not found. Rather, the highest flux preceded the highest air temperature by approximately two hours, indicating that topsoil temperatures, measured to exceed $50\deg$ C at times, were the major driver of methanol fluxes.
DE: 0315 Biosphere/atmosphere interactions
DE: 0365 Troposphere--composition and chemistry
DE: 0368 Troposphere--constituent transport and chemistry
SC: Atmospheric Sciences [A]
MN: 2003 Fall Meeting