Observations of Ice Nucleating Particles in the Free Troposphere From Western US Wildfires

Abstract

Wildfires in the western United States 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 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 super cooling, and contributions from minerals at deeper super cooling. A fire involving primarily sagebrush shrub land 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 tar balls 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 US wildfires in summertime, regionally elevate INP concentrations in the free troposphere.

Ice Nucleating Particle Connections to Regional Argentinian Land Surface Emissions and Weather During the Cloud, Aerosol, and Complex Terrain Interactions Experiment

“…The IS is constructed using two aluminum blocks, machined to fit polymerase chain reaction (PCR) plates, encased by cold plates through which coolant is circulated. The IS produced immersion freezing spectra reaching to a lower limit of −27 to −30°C (±0.2°C) with a detection limit of ∼0.001 INPs L −1 and is supported with well-established experimental protocols applied in diverse scenarios (Barry, Hill, Jentzsch, et al, 2021;Barry, Hill, Levin, et al, 2021;Beall et al, 2017;DeMott et al, 2017;Hill et al, 2016;Hiranuma et al, 2015).…”

Ice‐Nucleating Particles That Impact Clouds and Climate: Observational and Modeling Research Needs

“…(2020) and Barry et al. (2021) show that organic components are a potentially useful predictor of INPs active at temperatures warmer than −25°C in biomass‐burning plumes, which may be associated with in situ production of tarballs. Another recent study by Jahl et al.…”

“…Additionally, field and laboratory studies have shown that biomass burning plumes undergo complex chemical transformations during transport that models cannot yet capture, which may result in modified ice-nucleating activity (Hodshire et al, 2019;McMeeking et al, 2009). For example, Schill et al (2020) and Barry et al (2021) show that organic components are a potentially useful predictor of INPs active at temperatures warmer than −25°C in biomass-burning plumes, which may be associated with in situ production of tarballs. Another recent study by Jahl et al (2021) finds surprising evidence that the ice-nucleation ability of biomass-burning aerosol may be enhanced during atmospheric transport and aging.…”

Biomass Burning Smoke and Its Influence on Clouds Over the Western U. S.

“…These particles were confirmed from images to be ice. Barry et al (2021) showed that smoke plumes measured during this project were associated with elevated ice nucleating particle concentrations. However, the limited sampling in clouds containing ice at a range of temperatures precludes robust conclusions on smoke impacts on ice formation during WE-CAN.…”

“…In addition, since wildfire smoke particles serve as ice nucleating particles (INPs) under some conditions (Barry et al, 2021;Levin et al, 2005;McCluskey et al, 2014;Sokolik et al, 2019), precipitation increases are possible through this route in deeper clouds as well. In fact, Barry et al (2021) found that INPs were enhanced in WE-CAN smoke plumes relative to background air outside plumes, and that organic INP dominated over biological and mineral dust under most conditions.…”