Trends in PM2.5 transition metals in urban areas across the United States

Using data from the Environmental Protection Agency's Chemical Speciation Network, we have characterized trends in PM2.5 transition metals in urban areas across the United States for the period 2001-2016. The metals included in this analysis-Cr, Cu, Fe, Mn, Ni, V, and Zn-were selected based upon their abundance in PM2.5 , known sources, and links to toxicity. Ten cities were included to provide broad geographic coverage, diverse source influences, and climatology: Atlanta (ATL), Baltimore (BAL), Chicago (CHI), Dallas (DAL), Denver (DEN), Los Angeles (LA), New York City (NYC), Phoenix (PHX), Seattle (SEA), and St. Louis (STL). The concentrations of V and Zn decreased in all ten cities, though the V decreases were more substantial. Cr concentrations increased in cities in the East and Midwest, with a pronounced spike in concentrations in 2013. The National Emissions Inventory was used to link sources with the observed trends; however, the causes of the broad Cr concentration increases and 2013 spike are not clear. Analysis of PM2.5 metal concentrations in port versus non-port cities showed different trends for Ni, suggesting an important but decreasing influence of marine emissions. The concentrations of most PM2.5 metals decreased in LA, STL, BAL, and SEA while concentrations of four of the seven metals (Cr, Fe, Mn, Ni) increased in DAL over the same time. Comparisons of the individual metals to overall trends in PM2.5 suggest decoupled sources and processes affecting each. These metals may have an enhanced toxicity compared to other chemical species present in PM, so the results have implications for strategies to measure exposures to PM and the resulting human health effects.