The Sierra Nevada snowpack supplies a major share of freshwater for California and parts of Nevada, yet new measurements show that even the cleanest storms now carry microscopic plastic particles down from the atmosphere. University of California, Riverside researchers sampled fresh snowfall at seven sites during active storms and found plastic fibers at every location, including remote areas reached only by skis or chairlift. ([1])
Atmospheric transport reaches remote peaks
Lead researcher Aleksandra Karapetrova and her team collected samples over six months to capture deposition during actual snow events. They chose sites that ranged from isolated backcountry locations to the more developed Mammoth Mountain ski area. The work revealed that snow acts as an efficient scavenger, pulling suspended microplastics out of the air as flakes form and fall.
Back-trajectory modeling pointed to agricultural and suburban regions upwind as likely source areas. Particles traveled long distances before settling with the snow, showing that local human activity is not the only contributor.
Measured deposition rates
At the most remote sites, snow deposited between 270 and 3,200 plastic particles per square meter per day during peak storm periods. The Mammoth Mountain location recorded roughly three times those amounts, with daily rates between 2,600 and 7,500 particles per square meter. Even the higher figures translate to very small total mass: an acre of snow at the upper rate would add about 1.5 grams of plastic per day.
Most particles were polyethylene, polypropylene, and polyester, materials commonly used in clothing and packaging. Concentrations in fresh snow ranged from roughly 12 to 93 particles per liter depending on the site.
Implications for water and land
Because the Sierra Nevada snowpack feeds rivers and reservoirs that millions rely on, the presence of microplastics raises questions about long-term accumulation in soil and waterways. The particles that fall with snow eventually melt into the same systems that support agriculture, ecosystems, and drinking water supplies downstream.
Researchers noted that the actual mass remains tiny, yet the consistent delivery across every sampled storm underscores how widespread atmospheric plastic has become. The study, published in Environmental Science & Technology, provides one of the first event-scale records of this process in a major North American mountain range.
What remains to be learned
Scientists still need to determine how much of the plastic survives melting and transport into rivers and soil, and whether certain particle types break down differently in cold, high-elevation environments. Continued monitoring during future winters will help track whether deposition rates change with shifts in weather patterns or regional plastic use.
The findings illustrate that even landscapes long considered pristine now receive a steady, if invisible, input of synthetic material from the air above.
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