Aerosol Interactions in Complex Mountainous Terrain

Aerosols are central to understanding surface-atmosphere exchanges and the water cycle within mountainous regions due to aerosol-cloud-precipitation interactions. Since they are distributed vertically within the column, they can impact large-scale circulation, precipitation, and land-atmosphere interactions. Aerosols also impact the surface energy by redistributing energy in the atmosphere as well as impact radiation budgets and snowmelt when deposited on the surface. Deposition via wet and dry processes further impacts the hydrological and biological cycles by delivering nutrients to surface and groundwater supplies that impact overall health of the regional ecosystem, e.g., soil, microbial processes, plant biology and growth, etc. Furthermore, depending on the aerosol physical and chemical properties, some are more ideal than others for forming warm-phase clouds (cloud condensation nuclei) and cold-phase clouds (ice nuclei). For these reasons, it is important to know not only the physical and optical properties of the particles, but also the chemical composition to understand the full cycle and influence within the integrated mountain hydroclimate.

Here, we report on the aerosol properties that were observed during the Surface Atmosphere Integrated field Laboratory (SAIL) campaign, a 21-month deployment (September 2021 - June 2023) of the Atmospheric Radiation Measurement (ARM) Facility, to the East River Watershed near Crested Butte, Colorado. This is to our knowledge the first time such large-scale atmospheric measurements have been coupled with surface hydrology in the mountains of the United States. The Aerosol Observing System (AOS) collected a comprehensive suite of aerosol measurements on Crested Butte Mountain while the Tethered Balloon System (TBS) collected data vertically within the boundary layer during different seasons. There are many aerosol processes that are hypothesized to be occurring above the surface in the East River Watershed that have large implications for bedrock-to-canopy research. We also present high-time resolution observations of bioaerosol made during the biologically active period of two sequential years during the campaign sampled at over 10,000 ft. We believe these to be the first measurements of this kind within mountainous terrain in the U.S. We show trends in different types of particles based on size, chemistry and optical properties sampled during different seasons and discuss relationships with large scale meteorology to understand sources and sinks. Transported aerosol events observed during snow-covered periods will also be presented, e.g. transported absorbing supermicron dust and biomass burning, as they can impact the timing of snowmelt and surface hydrology. In summary, we know that aerosols within the integrated mountain hydroclimate impact cloud formation, precipitation events and deposition of nutrients to the surface as well as alter the radiative budget in the atmosphere and on the surface. During the SAIL campaign in Colorado, we collected a large dataset of in situ aerosol measurements at the surface and various altitudes aloft during different seasons. Here, we will discuss some of the early findings coming out of the campaign to enhance the overall understanding of aerosol processes and regimes and their interactions within complex mountainous terrain.