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Speciated PM_2.5 (particulate matter with an aerodynamic diameter ≤2.5 μm) data has been collected for about 20 yr (1990–present) at a rural location in the Lake Tahoe Basin (Bliss State Park) and about 15 yr (1989–2004) at an urban site in South Lake Tahoe. The Bliss State Park site is representative of the Desolation Wilderness, a Class I air quality area with visibility protection under the Clean Air Act. Carbonaceous aerosol dominated reconstructed fine mass at both sites, with 58% at Bliss State Park (BLIS) and 68% at South Lake Tahoe (SOLA). Fine mass at SOLA is 2.5 times that at BLIS, mainly due to enhanced organic and elemental carbon (OC and EC). SOLA experiences a winter peak in PM_2.5 mainly due to OC and EC from residential wood combustion, whereas BLIS experiences a summer peak in PM_2.5 mainly due to OC and EC from wildfires. Carbonaceous aerosol dominates visibility impairment …