Fine particulate matter and visibility in the Lake Tahoe Basin: Chemical characterization, trends, and source apportionment

Speciated PM2.5 (particulate matter with an aerodynamic diameter <= 2.5 mu m) 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 PM2.5 mainly due to OC and EC from residential wood combustion, whereas BLIS experiences a summer peak in PM2.5 mainly due to OC and EC from wildfires. Carbonaceous aerosol dominates visibility impairment, causing about 1/2 the reconstructed aerosol light extinction at BLIS and 70% at SOLA. Trend analysis (19902009) showed statistically significant decreases in aerosol extinction at BLIS on 20% best and 60% middle visibility days and statistically insignificant upward trends on 20% worst days. SOLA (1990-2003) showed statistically significant decreases in aerosol extinction for all day categories, driven by decreasing OC and EC. From the regional haze rule baseline period of 2000-2004 until 2005-2009, BLIS saw 20% best days improving and 20% worst days getting worse due to increased wildfire effects. Receptor modeling was performed using positive matrix factorization (PMF) and chemical mass balance (CMB). It confirmed that (1) biomass burning dominanted PM2.5 sources at both sites with increasing importance over time; (2) low combustion efficiency burning accounts for most of the biomass burning contribution; (3) road dust and traffic contributions were much higher at SOLA than at BLIS; and (4) industrial combustion and salting were minor sources. Implications: Visibility on the 20% worst visibility days decreased at Bliss State Park, which represents the Desolation and Mokelumne Wilderness Class I areas. This decrease in visibility, contrary to Regional Haze Rule requirements, was mainly due to increased wildfire activity. Increased sulfate-caused light extinction on worst visibility days may be due to long-range transport from Asia. These factors may make it difficult to achieve reasonable progress toward the national visibility goal. Methods need to be developed to account for the effect of wildfires and intercontinental transport when evaluating progress toward the national goal.