Submitted to: Clean (Soil Air Water)
Publication Type: Proceedings
Publication Acceptance Date: 7/17/2009
Publication Date: 9/6/2009
Citation: Parker, D.B., Cole, N.A., Casey, K.D., Galvin, G., Ormerod, R., Paris, C.S., Caraway, E.A., Rhoades, M.B. 2009. Wind tunnels vs. flux chambers: Area source emission measurements and the necessity for VOC and odour correction factors. In: Proceedings of 19th International Clean Air and Environment Conference, September 6-9, 2009, Perth, Australia. 9 p. Interpretive Summary: Measurement of emissions of volatile organic compounds (VOC), odors, and ammonia (NH3)from livestock feeding operations is difficult. Wind tunnels and flux chambers have been used to measure fluxes of VOC, odors, and NH3. However many researchers have failed to account for effects of the air velocity or sweep air flow rates in the chambers on emissions. As a result, flux measurements have been highly variable and scientists have been in disagreement as to the better methodology. We conducted a series of laboratory and field experiments to compare flux measurements made at varying sweep air flow rates with a variety of chamber and wind tunnel configurations. The flux of NH3 and several VOC increased with increasing airflow rate in all chamber configurations. Using mass balance studies, we determined that chamber methods usually greatly underestimate the true flux of these compounds. Because emissions of these compounds are highly correlated with measured evaporation rates we demonstrated that a two-film model with wind speed and temperature adjustments corresponding to field water evaporation rates are a potential means to correct fluxes determined using chambers and wind tunnels.
Technical Abstract: Wind tunnels and flux chambers have been used to measure fluxes of volatile organic compounds (VOC), odour, and ammonia (NH3) with little regard to air velocity or sweep air flow rates. As a result, flux measurements have been highly variable and scientists have been in disagreement as to the better methodology. Over five years we have conducted a multitude of laboratory and field experiments to compare flux measurements at varying sweep air flow rates with a variety of chamber configurations including the 'USEPA' flux chamber, modified USEPA flux chamber, NCSU flux chamber, UNSW wind tunnel, and WTAMU mini wind tunnel. Flux of gas-film limited compounds increased with increasing airflow rate in all chamber configurations. Through mass balance studies, we have learned that chamber methods most often underestimate the true flux for gas-film limited compounds. Emissions of compounds commonly found at feedlots are linearly correlated with measured evaporation rates. The two-film model with empirically-derived, evaporation-based, chamber-specific reference gas-film transfer coefficient effectively modelled emission rates at a variety of air flow rates. We show that wind speed and temperature adjustments corresponding to equivalent field evaporation rates are a justified method for making flux corrections to those gas-film limited compounds typically responsible for odour at feedlots. Post-measurement corrections show promise to correct chamber measurements to real-world emissions.