DOI: 10.25675/10217/211646
title: Supporting scanning transmission electron microscopy (STEM) data for manuscript titled "Observations of ice nucleating particles in the free troposphere from western U.S. wildfires"

description: Images, elemental maps, and energy dispersive x-ray spectra, sizing data and categorizations made on the basis of these data are assembled in this archive, which is associated with the above-noted publication. Data describe atmospheric particles collected within or around wildfire plumes from the NSF/NCAR C-130 aircraft during Western Wildfire Experiment for Cloud Chemistry, Aerosol Absorption, and Nitrogen (WE-CAN), under support of the National Science Foundation Atmospheric and Geospace Sciences award number 1650786. Based from Boise, ID, maps of the WE-CAN research flights will be published in the submitted publication reference listed at the end of this file. Particle collections were made onto 3-mm diameter transmission electron microscope supports (200 mesh) made of Cu or Ni, and coated with C-formvar films. Collections, as fully described in the publication, represent total particles in a size class from 0.13 to 0.55 microns (using an impactor system) from two different wildfire plume (so-called Sugarloaf fire, research flight 15) during two different plume penetrations (RF15A and RF15C), and ice nucleating particles (INPs) collected/integrated over a number of plume passes. The INPs were collected as activated ice crystals within the Colorado State University (CSU) continuous flow diffusion chamber (CFDC), simulating freezing under supercooled clouds conditions (cf., publication for details). Drs. Cynthia Twohy (NWRA) and Darin Toohey (University of Colorado) oversaw ambient particle collections. Dr. Paul DeMott, Dr. Ezra Levin, and Mr. Kevin Barry oversaw INP collections. Ambient plume particles and INPs processed in the CFDC were both collected from a solid diffuser inlet system on the NSF/NCAR C-130, and represent wildfire particles at ages of 1-2.5 hours aging. INPs were limited in size entering the CFDC at 2.5 microns, although the majority of these were in the same size range as the total particles. STEM data were collected at the Colorado State University Analytical Resources Core – Imaging and Surface Science facility. Dr. Roy Geiss (CSU Chemistry) conducted STEM analyses.
description: Wildfires in the western U.S. are large sources of particulate matter, and the area burned by wildfires is predicted to increase in the future. Some particles released from wildfires can affect cloud formation by serving as ice nucleating particles (INPs). INPs have numerous impacts on cloud radiative properties and precipitation development. Wildfires are potentially important sources of INPs, as indicated from previous measurements, but their abundance in the free troposphere has not been quantified. The Western Wildfire Experiment for Cloud Chemistry, Aerosol Absorption, and Nitrogen (WE-CAN) campaign sampled free tropospheric immersion-freezing INPs from smoke plumes near their source and downwind, along with widespread aged smoke. The results indicate an enhancement of INPs in smoke plumes relative to out-of-plume background air, but the magnitude of enhancement was both temperature and fire dependent. The majority of INPs were inferred to be predominately organic in composition with some contribution from biological sources at modest supercooling, and contributions from minerals at deeper supercooling. A fire involving primarily sagebrush shrubland and aspen forest fuels had the highest INP concentrations measured in the campaign, which is partially attributed to the INP characteristics of lofted, uncombusted plant material. Electron microscopy analysis of INPs also indicated tarballs present in this fire. Parameterization of the plume INP data on a per-unit-aerosol surface area basis confirmed that smoke is not an efficient source of INPs. Nevertheless, the high numbers of particles released from, and ubiquity of western U.S. wildfires in summertime, regionally elevate INP concentrations in the free troposphere.