ARS | Publication request: A model for predicting VOC emission from silage

Submitted to: Atmospheric Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 2, 2012
Publication Date: April 3, 2012
Citation: Hafner, S.D., Montes, F., Rotz, C.A. 2012. A model for predicting VOC emission from silage. Atmospheric Environment. 54:134-140.

Interpretive Summary: Smog is a widespread form of air pollution in the United States (US) and world wide. Smog contains a mixture of pollutants that is usually quantified by measuring ozone gas, a dominant irritant in smog. Smog and ozone are created in the atmosphere when volatile organic compounds (VOCs) react with oxides of nitrogen (NOx). Reductions in VOC and NOx emissions from vehicles and industry over the past 40 years have resulted in a decline in ozone concentrations in the US. However, other sources of VOCs may be important in some areas. Recent measurements indicate that silage, a fermented cattle feed, may be an important source of VOC emissions on dairy farms in California. However, no tools exist for predicting emissions under particular conditions, or the effect of management practices on emissions. Furthermore, the only emission measurements currently available are from wind tunnels or chambers, both of which may produce inaccurate estimates of emissions under farm conditions. This work addresses both of the limitations. In it, we present a model that can serve as a tool for predicting emission, although additional parameter estimation is probably necessary. Additionally, we present measurements of VOC emission from silage made using a mass balance approach, which provides estimates of emission from loose silage under farm conditions.

Technical Abstract: Silage has been shown to be an important source of emissions of volatile organic compounds (VOCs), which are precursors to ground-level ozone. Measurements show that environmental conditions and silage properties influence emission rates, making it difficult to assess the contribution of silage to 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 overpredicted alcohol and acetaldehyde 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 may approach, or in some cases exceed, 50% of the initial mass, while relative losses of acetaldehyde will be greater.