Size Distribution of Health-Relevant Trace Elements in Airborne Particulate Matter During a Severe Winter Stagnation Event: Implications for Epidemiology and Inhalation Exposure Studies

Size distributions for As, Cd, Bi, Br, Fe, Mn, S, Sb, Tl, K, V, Rb, elemental carbon (EC), organic carbon (OC), sulfate (SO ^2? ₄ ), chloride (Cl^?), and nitrate (NO^? ₃ ) were measured at Bakersfield and Modesto CA between 12/15/2000–01/07/2001. S and V size distributions were highly correlated with little diurnal variation suggesting that, during stagnant winter conditions, inhalation exposure and epidemiological studies can choose exposure/analysis times of several days during which population exposure will be relatively uniform. In contrast, the size distribution and temporal patterns of Fe are highly variable requiring more frequent measurements and shorter health analysis periods to adequately characterize exposure concentrations. K and Rb (tracers for wood smoke) exhibited nearly identical diurnal size distribution shifts (R ²  > 0.99) with smaller particles emitted at night and larger aged particles evident during the day. This pattern suggests that the health effects of fresh wood smoke could be studied through nighttime inhalation exposure experiments and aged wood smoke could be evaluated through daytime experiments. All of the components’ size distributions were consistent with production by combustion sources and/or secondary chemistry. Future inhalation exposure studies should use a combustion source to generate these particles. Lung deposition calculations predict that 32 ± 11% of PM deposition occurs in the pulmonary region, 61 ± 23% occurs in the respiratory region, and 6 ± 2% occurs in the tracheo-bronchial region for all components. Deposition patterns were highly correlated with PM _1.8 concentrations suggesting that exposure estimates can be developed with bulk filter samples during the current episode.