Impact of Wildfire‐Emitted Black Carbon on Snowmelt Across Western North America

Abstract

Wildfires are a major source of black carbon (BC) and their increasing frequency and intensity have enhanced BC deposition onto snowpack across western North America, which accelerates snowmelt and triggers multiple feedbacks that can substantially alter atmospheric composition and regional hydrology. Using the Weather Research and Forecasting model coupled with Chemistry (WRF‐Chem) integrated with the Snow, Ice, and Aerosol Radiative (SNICAR) scheme, we investigate the impacts of fire‐emitted BC on snowmelt following the high‐emission 2021 fire season. Summertime wildfires generate elevated near‐surface BC concentrations, accounting for more than 80% of total near‐surface BC across Alaska, British Columbia, and the Pacific Northwest. During autumn and winter, fire‐emitted BC continues to account for over 50% of atmospheric BC across California, Alaska, and British Columbia. Fire‐emitted BC mass in snow column reaches seasonal domain maximum of 2,828 μg m ⁻² in winter and 2,978 μg m ⁻² in spring, accumulating persistently over the western Rockies and coastal mountains. This deposition reduces seasonal mean surface albedo by up to 0.01 in spring across western North America, especially over the Alaska Range, the Rockies, and the Sierra Nevada. The resulting albedo reduction increases solar radiation absorption by the snowpack, reducing seasonal mean snow water equivalent (SWE) by up to 87.3 kg m ⁻² and enhancing snowmelt by up to 4.28 kg m ⁻² in spring across the domain, with stronger impacts occurring in spring when increasing solar insolation accelerates snow aging and melt, re‐exposes BC at snow surface, and amplifies absorption through a positive feedback loop.