On a wind-scraped ridge near the summit of Mount Washington, where clouds tear through steel cables strung like harp wires, a quiet scientific treasure has been gathering for nearly two decades. From 1996 to 2014, researchers and hut caretakers at the Appalachian Mountain Club’s Lakes of the Clouds shelter captured not just rain, but the very mist that swirled over New Hampshire’s tallest peak—condensing droplets on Teflon strings, funneling them into vials, and preserving a 19-year record of the sky’s chemistry. Now, that collection—over 700 samples long—has revealed a crucial truth: where air comes from matters, but so does how much rain it’s already shed.
This distinction is vital for predicting how pollution travels across complex landscapes. Air masses don’t just carry pollutants like sulfate ions—they scrub them out as they rain. The more it rains upwind, the cleaner the air becomes by the time it reaches places like Mount Washington. But until now, most atmospheric models relied solely on wind origin to estimate pollution levels. By combining isotopic analysis of water molecules with sulfate measurements, researchers from the University of Michigan Engineering, the Appalachian Mountain Club, and Plymouth State University showed that including rainout history improves pollution predictions by nearly 16 percentage points. While wind origin alone explained 40% of sulfate variation, adding rainout pushed the model to 55.6% accuracy.
"It’s a valuable piece of the scientific puzzle to have these physical samples to directly tell you something about where the air came from and what pollutants it carried," said Adriana Bailey, assistant professor at U-M and co-author of the study. The team used sulfate (SO₄²⁻) as their pollution marker because of its strong signal and relevance to acid rain, which has long threatened high-elevation ecosystems. Isotopes of hydrogen and oxygen in the water revealed rainout history—clouds with a wet past carried fewer heavy water molecules, a fingerprint of prior precipitation.
Georgia Murray, senior scientist at the Appalachian Mountain Club, emphasized the real-world impact: the data helped federal land managers understand acid deposition and shaped public education on air quality. The samples, some stored for decades, underwent rigorous quality checks to remove evaporated or compromised vials before analysis.
This rare long-term dataset doesn’t just refine models—it reveals how mountain environments act as sentinels of atmospheric change. As climate patterns shift and pollution sources evolve, the lessons from Mount Washington offer a clearer lens for protecting vulnerable watersheds and forests downstream. In an era of digital simulations, the power of a physical sample—captured one cloud at a time—remains unmatched.