Exploratory Ice Nucleation Measurements at Oliktok Field Campaign Report
Ice nucleating particles (INPs)—such as mineral dust and bacteria (1)—are a crucial source of seeds for cloud ice crystal formation, and can subsequently impact cloud radiative properties, lifetime, and precipitation formation processes (2-4). The presence of such particles in the atmosphere can be rare, but large concentrations are not required to have substantial impacts on cloud microphysics (3). However, knowledge of INPs, particularly in the Arctic, is limited due to the dearth of measurements in time and space. It is especially important to evaluate the sources and efficiency of INPs in the Arctic due to the role mixed-phase clouds play in facilitating the surface energy budget and thus energy reaching the sea ice and snow surfaces (4, 5). The purpose of this campaign, called INPOP (Ice Nucleating Particles at Oliktok Point), is to address the issue of limited information of Arctic INPs by providing time- and size-resolved INP concentrations over the course of a 3-month springtime campaign. This period is subject to Arctic haze, whereby pollutants from mid-latitudes are frequently transported to the Arctic and influence the relatively clean Arctic atmosphere (6-9). One unique aspect of this study is that the measurements were conducted at the U.S. Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) Climate Research Facility third ARM Mobile Facility (AMF3) in Oliktok Point, North Slope of Alaska (NSA), which is located in the northwest edge of the Prudhoe Bay oilfield. Typically, oil and natural gas extraction emissions are not thought to serve as a significant source of INPs, since naturally produced primary aerosols are the most efficient and abundant INPs (e.g., 1). However, the goal of INPOP was to evaluate INPs in the context of supporting aerosol and meteorological observations to assess: (1) if local industrial activities due to the oilfield operations can serve as a source of INPs (e.g., such as road dust from routine vehicular operations) and (2) if other regional or long-range transported sources impact a polluted Arctic location by introducing higher concentrations of more efficient INPs (e.g., transport of primary biological aerosol particles (PBAPs) from open ocean water or Arctic haze INPs). Unlike other Arctic coastal locations in the spring, Oliktok Point has a distinct combination of aerosol sources including long-range transported haze, regional/local marine emissions, and the local oilfield activities. Ground-based, immersion-mode, ice nucleation measurements were conducted on samples collected at the AMF3 in Oliktok Point from 1 March to 31 May, 2017. Samples were collected using a Davis Rotating-drum Unit for Monitoring (DRUM; DRUMAir, LLC) cascading impactor in 4 stages (i.e., size bins). Instrument information on the model used, the DA400, can be found at the DRUMAir website (http://www.drumair.com/). Stages A, B, C, and D, had particle size cuts at 2.96, 1.21, 0.34, and 0.15 μm particle diameter. Strips of sample substrate (PFA, perfluoroalkoxy; McMaster) were adhered to each disc in each stage and coated with petrolatum to enable particles to stick on impact. The discs move slowly over time, such that aerosol loading is streaked onto the PFA. Every 24 hours, a blank spot is created on the PFA to separate daily samples. The pump typically pulled 25 lpm at the inlet with all the discs in place in each stage. Daily samples were collected for each of the four stages, equaling 36,000 total L of air per sample. Discs rotated for approximately 24 days before the sampling per strip was complete and a new disc was installed by the ARM operators. Discs were stored frozen after collection until the end of the study.