Author's personal copy A mass transfer model for VOC emission from silage Sasha D. Hafner * , Felipe Montes, C. Alan Rotz USDA-ARS, Pasture Systems and Watershed Management Research Unit, 3702 Curtin Rd., University Park, PA 16802, USA article info Article history: Received 10 November 2011 Received in revised form 1 March 2012 Accepted 2 March 2012 Keywords: Dairy farms Silage Volatile organic compounds Emission Model abstract Silage has been shown to be an important source of emissions of volatile organic compounds (VOCs), which contribute to the formation of ground-level ozone. Measurements have shown that environmental conditions and silage properties strongly influence emission rates, making it difficult to assess the contribution of silage in VOC emission inventories. In this work, we present an analytical convection-diffusion-dispersion model for predicting emission of VOCs from silage. It was necessary to incorporate empirical relationships from wind tunnel trials for the response of mass transfer parameters to surface air velocity and silage porosity. The resulting model was able to accurately predict the effect of temperature on ethanol emission in wind tunnel trials, but it over-predicted alcohol and aldehyde emission measured using a mass balance approach from corn silage samples outdoors and within barns. Mass balance results confirmed that emission is related to gas-phase porosity, but the response to air speed was not clear, which was contrary to wind tunnel results. Mass balance results indicate that alcohol emission from loose silage on farms may approach 50% of the initial mass over six hours, while relative losses of acetaldehyde will be greater. Published by Elsevier Ltd. 1. Introduction Silage on dairy farms has recently been identified as an impor- tant source of volatile organic compound emissions in the San Joaquin Valley of California, USA (Shaw et al., 2007; Chung et al., 2009; Howard et al., 2010; Malkina et al., 2011). Alcohols appear to be the most important compounds emitted from silage with respect to potential ozone formation (Howard et al., 2010). For corn silage, which dominates silage production in the US (Wilkinson and Toivonen, 2003), ethanol is generally the most concentrated alcohol (typically about 10 g kg 1 in corn silage, i.e., 1% of dry matter (Kleinschmit and Kung, 2006)), followed by 1-propanol and other alcohols. Understanding the impact of VOC emissions from silage on air quality will require accurate methods for esti- mating emissions. Measurements of VOC emission rates from silage have been made using the emission isolation flux chamber method (Alanis et al., 2008; Chung et al., 2009); large (room-sized) environ- mental chamber methods (Howard et al., 2010); and wind tunnel systems (Hafner et al., 2010; Montes et al., 2010). Wind tunnel measurements have demonstrated that ethanol emission is sensitive to surface air velocity and temperature, as well as to silage properties, including porosity and particle size (Hafner et al., 2010; Montes et al., 2010). Accurate prediction of emission of ethanol or other VOCs from silage will therefore require a model that incorporates these relationships. Our objectives were to develop a model for predicting VOC emission from silage, and to evaluate the model by comparing predicted emission to emission measurements made in barns and outdoors. 2. Methods 2.1. Model description We developed a one-dimensional transport and emission model that is similar to other models that have been developed for VOC transport in porous media (Jury et al., 1990). Our model was developed to predict VOC emission from a silage surface (such as the front of a bunker silo or from the upper surface of feed in a feed bunk or feed lane) that is exposed to moving air (Fig. S-1). VOC creation and destruction are not included, but rather our model captures processes that occur after fermentation is complete. Oxidation of alcohols in silage does occur when silage is exposed to air, but this process typically takes place over a period of days (Woolford, 1983; Spoelstra et al., 1988), while the emission processes of interest takes place over a period of hours. Oxidation of aldehydes and other VOCs may be more rapid, but measurements * Corresponding author. Present address: USDA-ARS, 10300 Baltimore Ave., Beltsville, MD 20705, USA. Tel.: þ1 301 504 7327. E-mail addresses: sasha.hafner@ars.usda.gov, sdh11@cornell.edu (S.D. Hafner). Contents lists available at SciVerse ScienceDirect Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv 1352-2310/$ e see front matter Published by Elsevier Ltd. doi:10.1016/j.atmosenv.2012.03.005 Atmospheric Environment 54 (2012) 134e140