Biosphere-atmosphere exchange of volatile organic compounds over C4 biofuel crops

Abstract

Significant amounts of ethanol are produced from biofuel crops such as corn and, in the future, likely switchgrass. The atmospheric effects of growing these plant species on a large scale are investigated here by measuring the plant-atmosphere exchange of volatile organic compounds (VOCs). Field grown corn and switchgrass emit VOCs at flux rates of 4.4 nmolC m−2 s−1 (10−9 mol carbon per square meter leaf area per second) and 2.4 nmolC m−2 s−1, respectively. Methanol contributes ∼60% to the molar flux but small emissions of carbonyls, aromatic compounds and terpenoids are relatively more important for potential air quality impacts. Switchgrass can act as a sink for carbonyls and aromatic compounds with compensation points of a few hundred pptv. In switchgrass moderate drought stress may induce enhanced emissions of monoterpenes, carbonyls and aromatics. Per liter of fuel ethanol produced, the estimated VOC emissions associated with the biomass growth of corn (7.8 g l−1) or switchgrass (6.2 g l−1) are in the same range as the VOC emissions from the use of one liter gasoline in vehicle engines. VOC emissions from the growing of biofuel crops can therefore be a significant contributor to the VOC emissions in the life cycle of biofuels. The VOC emissions from corn and switchgrass are small compared to those of tree species suggested as biofuel crops. Due to their reactivity with respect to OH the emissions from corn and switchgrass are not likely to have a significant impact on regional ozone formation.