Characterizing the Sources and Transport of Wintertime Ice-Nucleating Particles in Fairbanks, Alaska

 

The Arctic is warming at a rate several times faster than the global mean, a phenomenon commonly referred to as Arctic amplification. Short-lived climate forcers, particularly aerosols acting as ice-nucleating particles (INPs), may influence this amplification through aerosol-cloud indirect effects. During the polar night, INPs modulate the ratio of liquid-to-ice in mixed-phase clouds, altering their capacity to trap outgoing longwave radiation and warm the surface. Despite their importance, the sources and transport pathways of INPs in high-latitude regions remain poorly constrained. While truly pristine Arctic environments are rare, cold, polluted sub-Arctic regions such as interior Alaska provide natural laboratories for investigating INP populations under conditions that combine low temperatures with enhanced anthropogenic and regional aerosol influences. Such environments may be particularly relevant to Arctic locations experiencing episodic pollution, long-range aerosol transport, or increasing local emissions. While chemical fingerprinting provides critical insights into particle composition and local abundance, it cannot inherently resolve the geographic origins or transport history of air masses bringing INPs to a given region.

To address this limitation, we apply backward trajectory-based modelling in an attempt to link observed INPs to their potential source regions. We build on recent work investigating the sources of wintertime INPs in the sub-Arctic urban environment of Fairbanks, Alaska, using observations from the Alaskan Layered Pollution and Chemical Analysis (ALPACA) field campaign conducted in January and February 2022. During the campaign, Fairbanks experienced persistent surface-based temperature inversions and extreme cold events that favored the accumulation of locally emitted anthropogenic aerosols. Analysis of ALPACA-2022 data has reported INP concentrations significantly higher at relatively cold freezing temperatures than those typically observed at other high-latitude sites, consistent with three dominant INP classes: heat-labile biological particles, potentially associated with local vegetation such as lichens; organic particles linked to residential wood combustion, supported by correlations with levoglucosan; and a source attributed to road dust, possibly generated by the application of traction gravel on icy roads.

Using a backward trajectory modeling framework, we investigate the spatial origins and atmospheric transport of INP sourced from the Fairbanks region. Backward transport simulations are conducted using the FLEXible PARTicle dispersion model (FLEXPART), driven by 1.33 km resolution wind fields from the Weather Research and Forecasting (WRF) model, including assimilation of meteorological data from the ALPACA campaign. The surface influence and residence time of air masses arriving at the ALPACA measurement site in downtown Fairbanks are quantified. Potential Emission Sensitivity (PES) footprints are calculated by combining with high resolution emissions fields of potential INP sources, based on downscaling emissions using vegetation, road and building datasets. Interpreting PES fields, in conjunction with the observed INP analysis, allows characterization of both the INP sources and their transport pathways in Fairbanks. The results have broad implications for INP sources and aerosol-cloud indirect effects over the wider sub-Arctic and potentially Arctic region.