Chemical/Optical Properties of Oliktok Aerosols Field Campaign Report
The arctic environment is changing at an alarming rate. During this time of escalation, it is important to understand the impacts of aerosols, both natural and anthropogenic. To support this effort, an aerosol sampling campaign was conducted at the U.S. Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) Climate Research Facility third Mobile Facility (AMF3) at Oliktok Point on the North Slope of Alaska (NSA). The specific objectives of the campaign were to collect aerosol particles as a function of aerodynamic particle size in order to better understand the varying chemical and optical properties of the aerosols in the different particle size modes. A DRUM-style cascading impactor (Davis Rotating-Unit for Monitoring) was deployed at the beginning of March 2017 through the end of May 2017, collecting samples in eight size bins for each 12-hour period, thereby obtaining a diurnal signature. On March 6, a fire burned down the power generation shed at the sampling site. No samples were collected again until March 10. Due to DRUM sampler rotation issues, samples were collected intermittently until March 13. From March 14 through the end of the study, samples were collected with little interference. The size bins of the instrument range from approximately 10 to 0.09 μm; however, deployment at the Oliktok Point facility required an inlet configuration that effectively filtered out the coarsest size bin. Therefore, the reported size bins are 5.0 to 2.5, 2.5 to 1.15, 1.15 to 0.75, 0.75 to 0.56, 0.56 to 0.34, 0.34 to 0.26, and 0.26 to 0.09 μm. It is also possible that ultrafine particles were removed by electrostatic charging of the high-density polyethylene inlet tubing; however, confirmation of this would require further testing. The collected samples were returned to the University of California (UC), Davis for offline analysis. Particle mass concentrations were measured by soft-beta ray attenuation while optical attenuation was determined by broadband transmission spectrometry (10 wavelengths from 349 to 849 nm in 50 nm increments, minus 649 nm). The samples were then taken to the Advanced Light Source facility at Lawrence Berkeley National Laboratory for synchrotron-induced X-ray fluorescence spectrometry (SXRF). Thirty elements were detected, from sodium to molybdenum plus lead. The polarized nature of the synchrotron beam eliminates 97.5% of the primary Compton background, providing approximately a factor of 10 greater sensitivity than unpolarized X-ray instruments (Barberie et al. 2013). The data were reduced using custom software packages and analyzed for any sources of contamination, sample damage, or other potential causes of data invalidation. The interpretation of the final data set is ongoing and only early results will be described in this report. In spite of power and instrumental issues at the start, the field campaign was largely successful. Excluding the first stage that did not receive sample and power/rotational issues, the remaining stages were 99.7% complete (number of valid measurements/total number of measurements made). The percent of measurements below detection were < 30% for all species with the exception of sodium, magnesium, arsenic, and scandium. Sodium and magnesium were expected to be difficult to measure using SXRF due to low sensitivity at that energy range. Scandium is very rare in ambient samples unless near a very strong source. Arsenic is fairly common in urban settings but is not expected in the arctic environment.