Cooperative Institute for Research in Environmental Sciences

Abstracts: 33

Direct Measurements of the Fluxes of Biogenic VOCs from Agricultural Crops to the Atmosphere

Carsten Warneke

There is interest in and significant uncertainty about the emissions of oxygenated volatile organic compounds (VOCs) from vegetation to the atmosphere. For many types of vegetation including agricultural species, the biogenic emissions are dominated by oxygenated VOCs, which can be highly reactive and contribute to the formation of free radicals in the lower atmosphere.

Over the last few years, proton-transfer-reaction mass spectrometry (PTR-MS) has emerged as a useful tool to study the emissions and atmospheric chemistry of VOCs. In PTR-MS, atmospheric VOCs are ionized by reactions with H3O+ ions and detected with a mass spectrometer. The technique combines a high sensitivity with a fast time response, which allows the instrument to be used in rapidly changing air masses such as encountered in airborne experiments or in measurements close to emission sources.

In this work, we measured the fluxes of selected oxVOCs from an alfalfa field, before, during and after cutting, using a novel combination of disjunct eddy covariance and PTR-MS. Over the course of one day, a significant methanol flux of 4 mg m-2 h-1 was observed from undisturbed alfalfa with a maximum at 8 a.m., possibly caused by the evaporation of dew. A smaller release of hexenals during this day (0.04 mg m-2 h-1) demonstrated the sensitivity of the method. Other results suggested that acetaldehyde and acetone were released in the afternoon, but were lost by dry deposition in the evening and morning; deposition velocities were estimated to be 0.2 cm s-1 (acetaldehyde) and 0.09 cm s-1 (acetone). After the alfalfa was cut, the emissions of methanol, acetaldehyde, acetone and hexenals were significantly enhanced and remained high for three days during which the alfalfa was drying. This work suggests that the global source of oxVOCs due to the production of hay is of minor importance. The emission flux of methanol from vegetation during the growing season may be very large on a global basis.