Ice Nucleation Activity of Dust Particles Emitted from a Cattle Feeding Operation in the Texas Panhandle

Agricultural dust particles from animal feeding operations have been long known to affect regional air quality in the Texas Panhandle. In particular, housing roughly one-third of the nation’s cattle, open-air feedlots within 100 miles of West Texas A&M University (WTAMU) represent a significant source of dust particles. Our previous studies show that 24-hour averaged concentration of <10 µm feedlot dusts (composed of hydrophobic humic acid, water soluble organics, less soluble fatty acids and those carbonaceous materials mixed with salts) can be as high as 1200 μg m⁻³ and consistently above the National Ambient Air Quality Standard limit, which is 150 μg m⁻³ (Hiranuma et al., 2011, ACP). However, their impact on the local weather (i.e., cloud and hail formation) and, ultimately, climate are still poorly constrained. Here we present the results of immersion mode freezing abilities of particles sampled at three different cattle feedlots, namely Feedlot I, II and III (>18,000 cattle), in the Texas Panhandle during July 9-11, 2017. Aerosol samples were collected using a swirling aerosol collector as well as 47 mm membrane filters at ~1.5 m above the ground of the upwind and downwind edges of each facility. Additionally, simultaneous particle size distribution and mass concertation measurements were also carried out at both edges. Afterwards, the water-suspended samples were analyzed to estimate their ice nucleation active surface site densities as a function of temperature, n_s(T), using a droplet freezing assay setup at WTAMU. Our preliminary n_s(T) results show that the bimodal ice nucleation behavior with the first onset at ~-7 °C and the second one of T < -20 °C. The n_s value at the highest observed nucleation temperature of n_s(-7 °C) ~ 5 x 10⁴ m^-2 suggests that feedlot dusts possess (1) efficient ice-nucleating particles (INPs) comparable to other agricultural soil dusts [O’Sullivan et al., 2014, ACP] and (2) substantial biological entity that is known to be highly ice nucleation active in the atmosphere. If true, the feedlot dust is expected to produce highly elevated INP concentrations in a wide range of temperatures (T < -7 °C). Lastly, we will discuss the comparison of our n_s(T) to other ice-nucleating efficiency metrics, such as ice nucleation efficiency per unit mass, n_m(T), the cumulative concentration of INPs per volume of liquid, C_INPs(T), and the number concentration of INPs per volume of air, n_INPs(T), to solicitously constrain the n_s framework. Overall, our investigation in agricultural dust INPs will be an important first step towards better understanding cloud microphysics and hail formation mechanisms in the Southern Great Plains and, perhaps, even in the larger spatial scale depending on meteorological conditions encountered.

Acknowledgements: C. Whiteside and N. Hiranuma thank for the funding support from Killgore Graduate Student Research Grant and Faculty Research Grant. N. Hiranuma acknowledges partial financial support by the Higher Education Assistance Fund (HEAF). C. Whiteside acknowledges Dr. William J. Rogers for partial financial support.