R.W. Todd

EFFECT OF WIND TUNNEL AIR VELOCITY ON VOC FLUX FROM STANDARD SOLUTIONS AND CAFO MANURE/WASTEWATER

Researchers and practitioners have used wind tunnels and flux chambers to quantify the flux of volatile organic compounds (VOCs), ammonia, and hydrogen sulfide and estimate emission factors from animal feeding operations (AFOs) without accounting for effects of air velocity or sweep air flow rate. Laboratory experiments were conducted using a small rectangular wind tunnel (30.5 cm length, 15.2 cm width, 5.1 cm height). The objectives of the research were to (1) quantify the effect of wind velocity on VOC flux rates, (2) compare and contrast a two-film model with different wind speed corrections, and (3) provide insight into methods for either selecting appropriate wind tunnel velocities or conducting post-sampling wind velocity corrections to simulate field emission rates. Fluxes were measured on standard solutions and on manure/wastewater from beef cattle and dairy AFOs. Volumetric air exchange rates were varied between 0.6 and 44 exchanges per minute, corresponding to calculated longitudinal air velocities of 0.003 to 0.23 m s-1. Exhaust air was sampled using stainless steel sorbent tubes and analyzed for eleven volatile organic compounds comprised of seven volatile fatty acids (VFAs: acetic, propionic, isobutyric, butyric, isovaleric, valeric, and hexanoic) and four heavier molecular weight semivolatile organic compounds (sVOCs: phenol, p-cresol, indole, and skatole) using gas chromatography/mass spectrometry. Sulfur-containing VOCs were quantified using a portable total reduced sulfur meter. Flux rates for VOCs with small dimensionless Henrys law constants (i.e., those found at AFOs) increased with increasing air velocity. The two-film model with an experimentally derived reference gas-film transfer coefficient was found to reliably predict VOC flux at velocities between 0.003 and 0.23 m s-1. However, the two-film model did not reliably predict VOC flux with other air velocity correction formulae, an indication that flux is a function of wind tunnel geometry and turbulence factors, and not just average air velocity or sweep air flow rate. These results corroborate other studies that show that air velocity is a major factor affecting VOC fluxes from AFOs, verifying that an air velocity correction factor is required for estimating accurate VOC emission factors using wind tunnels and flux chambers.

CONTINUOUS AMMONIA EMISSION MEASUREMENTS FROM A COMMERCIAL BEEF FEEDYARD IN TEXAS

Ammonia emissions from cattle feedlots pose the potential to react with other compounds such as oxides of nitrogen and sulfur, which lead to detrimental environmental effects. Ambient ammonia (NH3) concentrations were measured continuously at a beef cattle feedyard for 12 months beginning in March 2007. Concentrations were measured every 5 min, 24 hours per day, at a sample intake height of 3.3 m using a chemiluminescence analyzer. On-site weather data were collected concurrently. Modeled emissions of NH3 were compared with the mass balance of N for the feedyard. Mean annual NH3 concentrations were 0.57 ppm, with a monthly average low of 0.37 ppm in December 2007 and a monthly average high of 0.77 ppm in August 2007. Flux densities were calculated using a backward Lagrangian stochastic model (WindTrax 2.0.7.8). Mean annual flux density was 70.7 µg m-2 s-1 (2.2 kg m-2 year-1). Mean monthly flux density ranged from 42.7 to 123.1 µg m-2 s-1 (0.11 to 0.32 kg m-2 month-1) in November and April 2007, respectively. Both concentration and flux density had a diel distribution with minima during the nighttime hours and maxima during the early afternoon. On an annual basis, 48.8% of fed N was volatilized as NH3. The inverse modeled daily ammonia production per head was 85.3 g NH3-N (head fed)-1 d-1.

Chemical Composition of Pen Surface Layers of Beef Cattle Feedyards1

The biological, physical, and chemical characteristics of beef cattle feedyard pen surfaces may affect nutrient transformations and losses to the atmosphere, ground water, or surface water. Feedyard pen surfaces can typically segregate into 3 or 4 layers. The purpose of this study was to determine if there were seasonal, within-pen location, days-on-feed, or urine effects on the chemical composition of the pen surface layers of feedyards. Samples were collected from 5 locations in 9 pens at 3 feedyards in each season and were analyzed for gravimetric water, pH, electrical conductivity (EC), nitrate + nitrite-N (NOx-N), ammonia + ammonium-N (NHx-N), N, C, and P. The percentage of water increased (P < 0.01) with depth among the manure layers and decreased in the soil. The pH of the manure layers increased with depth (P < 0.01) from approximately 7.6 to 8.2. The EC of the manure layers was greater (P < 0.01) than the EC of the soil layer, whereas the NOx-N concentration was greater (P < 0.01) in the soil layer. The NHx-N concentrations were lowest in the soil layer (P < 0.01). Total C and N concentrations decreased (P < 0.01) with sample depth. The composition of the layers was affected by season and location within the pen. Recent urine deposition did not affect the lower layers. The NHx-N concentration of the layers increased with days on feed. The differences in the chemical and physical properties of the layers in a feedlot pen may potentially affect nutrient losses to the atmosphere and to groundwater.

Auditing and Assessing Air Quality in Concentrated Feeding Operations123

The potential adverse effects of con- centrated animal feeding operations (CAFO) on the environment are a grow- ing concern. The air quality issues of most concern to CAFO vary but gener- ally include ammonia, hydrogen sul- fide, particulate matter, volatile organic compounds, greenhouse gases, and odors. Air pollutants may be regulated by federal and state laws or by nui- sance complaints. The United States En- vironmental Protection Agency, and poultry, swine, and dairy industries re- cently agreed to the National Air Emis- sions Monitoring System to fund re- search on atmospheric emissions from production farms in the United States. Air quality regulations may be based on actual emissions, atmospheric concentra- tions, or human perception, or via lim- iting the size or location of CAFO. Mea- suring the concentrations or emissions of most air pollutants is expensive and complex. Because of spatial and tempo- ral variability, concentrations and emis- sions must be measured continuously over an extended period of time. Be- cause different methods or models can give different results with the same data set, a multitude of methods should be used simultaneously to assure emis- sions are reasonable. The best method to measure concentrations and emissions will depend upon atmo- spheric concentrations, cost, facility characteristics, objectives, and other fac- tors. In the future, requirements for monitoring of air emissions from CAFO will probably increase. Reliable process- based models need to be developed so that emissions of air pollutants can be estimated from readily obtained diet, animal, facility, and environmental variables. Auditors will need to be trained in a variety of disciplines includ- ing animal sciences, chemistry, engi- neering, micrometeorology, instrumenta- tion, modeling, and logic.

Characterization of Ammonium Sorption by Beef Cattle Feedyard Manure

Ammonia (NH3) emissions from beef cattle feedyards represent a loss of agronomically important N and can potentially affect the environment. Sorption of ammonium (NH4+) and NH3 by mineral and organic solids decreases the proportion of free, aqueous ammoniacal N, which reduces volatilization potential. In the U.S., the High Plains region of Texas is subject to arid conditions with widely fluctuating temperatures. Furthermore, feedyard manure contains little soil and has high dry matter (DM) content; therefore, it is unclear if sorption parameters determined for soils or liquid manure systems are valid for feedyards. Our objectives were to use batch equilibration experiments to characterize NH4+ sorption by two feedyard manures. Kinetic and isotherm studies with (NH4)2SO4/0.01 M CaCl2 solutions revealed that feedyard manures could sorb up to 227 cmol kg-1 NH4+-N and that sorption was rapid and linearly related to NH4+ concentration. The Freundlich partitioning coefficient (Kp) for NH4+ sorption averaged 13.3 L kg-1. From 58% to 96% of the sorbed NH4+ was readily desorbable with 0.01 M CaCl2, and up to 81% was volatilized as NH3 when manures were air-dried. Temperature influenced sorption, with 112% more NH4+ sorption at 4°C than at 22°C. These results indicate that NH4+ sorption by manure on feedyard surfaces may temporarily reduce NH3 volatilization, particularly during winter.

Standardization of Flux Chamber and Wind Tunnel Flux Measurements for Quantifying Emissions from Area Sources at Animal Feeding Operations

A variety of wind tunnels and flux chambers have been used to measure fluxes of volatile organic compounds (VOC) and ammonia (NH3) at animal feeding operations (AFO). However, there has been little regard to the extreme variation and inaccuracy caused by inappropriate air velocity or sweep air flow rates. There is a need to standardize flux chamber and wind tunnel measurements. In this paper, we present results of evaporative and VOC flux measurements with the EPA flux chamber and a small wind tunnel. Emissions of gas-film controlled compounds like NH3 and VOC at AFOs are positively correlated with evaporation rates. We propose a simple methodology for standardization and comparison of different chamber types by measuring water evaporation using a mass balance approach. This same methodology holds promise for correcting field-measured chamber flux measurements resulting in more accurate emission estimates.