Fig. 6
The levels of strontium (SR), barium (Ba), lead (Pb), and copper (Cu) as measured at EPA speciation sites across the U.S. from 1999 to 2014 [27]. The Y-axis is μg/m3 as measured in the ambient air. Red circles indicate samples taken on or around holidays (i.e., July 4 or 5; December 31 or January 1)