The results of Pb isotopic analyses of the PM2.5 samples are provided in Supporting Table S1 and displayed in Fig. 2. Most GBNP samples fall between an array representing California (Western U.S.) PM2.5 sources and an array representing Asian, specifically Chinese, sources (arrays from Ewing et al. (2010)). PM2.5 samples collected by Ewing et al. (2010) in spring 2008 at Mt. Tamalpais and Chabot (Oakland, CA) fall in a similar region in Fig. 2, as do data from filters collected in Central California during the springs of 2003, 2004 and 2005. Some GBNP samples fall along or just to the right of the Asian array indicating that Icotinib those samples essentially consist entirely of Asian (Chinese) sourced Pb, while a few fall along the California array. Eleven of the group 1 (high Pb) samples fall near the Asian array, along with 1 sample from group 3 (high altitude (HA)), and four samples from group 6 (control samples). The fraction of the total Pb sourced in Asia (FAsiaPb) can be calculated based on the isotopic composition of the sample (see SI and (Ewing et al., 2010)). The FAsiaPb value for each sample can then be used to calculate the average concentration in air (ng/m3) during the 24-hour sampling period of Pb sourced in Asia and transported across the Pacific; we call this parameter Trans-Pac Pb with units of ng/m3 (SI Table 1).