A specific subset of atmospheric aerosols can act as ice-nucleating particles (INPs) in shallow mixed-phase clouds and, ultimately, influence precipitation and earth’s radiative energy balance. Despite impending significance on climate, the total implications of trace metals, such as lead oxides, are not well defined. In 2014, there were 38 short tons (7.6 x 104 lbs.) of lead emitted from Texas (729 short tons in the U.S. total), mainly from anthropogenic sources including fuel combustion sources, chemical and petroleum refining, on-road vehicles, non-road engines and metal industry processing. Subsequently, lead transforms into series of oxides (e.g., PbO) when heated in air above 545oC at facilities, and PbO can be released into the atmosphere. In this study, the immersion freezing efficiency of lead-dominant powder sample is characterized using the synthesized PbO, which has known physical and chemical properties (ρ = 9.53 g cm-3, 1.19 ± 0.37 µm, 0.07 ± 0.02 m2 g-1). Our immersion freezing experiments were implemented using a droplet cooling assay, called the West Texas Cryogenic Refrigerator Applied to Freezing Test (WT-CRAFT). We measured the immersion freezing ability of lead oxide in the simulated cloud droplets in WT-CRAFT across a range of relevant temperatures (T > ~-30oC) and estimated its ice nucleation active surface site density, ns(T), over several orders of magnitudes of suspension concentrations ranging from 0.1 wt% (1 mg mL-1) to 0.000001 wt% (10-4 mg mL-1). Our preliminary results indicate an abrupt ice nucleation of PbO within a narrow temperature range around -18oC over the examined concentration range. Our test with other INPs, such as microcrystalline cellulose and illite NX, show that their ice nucleation efficiencies, examined with the same concentration and temperature range, are substantially lower than that of PbO, suggesting the need for further characterization of lead oxide to better understand the influence of lead-containing ambient particles on cloud and precipitation formation in shallow clouds. Complementary results of X-ray diffraction, energy dispersive X-ray spectroscopy and infrared spectroscopy measurements will also be presented along with data.
Acknowledgements: N. Hiranuma acknowledges financial aids by the Higher Education Assistance Fund (HEAF), WTAMU Office of Graduate School and Killgore Research Center. The authors acknowledge Craig Whiteside and Kimberly Cory for their technical support and useful discussion regarding WT-CRAFT.
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