Temporal trends of particle gross β-activity and PM_2.5 mass concentrations in USA during 2001-2017

This paper investigates the temporal trends of particle gross β-activity (PM-β) and PM_2.5 mass in the 50 contiguous United State of America during the period of 2001-2017. With the implementation of stringent air pollution control policies, national ambient PM_2.5 concentrations decreased by 38.5% during this period. However, a smaller decrease of 9.4% was observed for PM-β; PM-β is mostly associated with radon emissions, which do not change much with time. The largest PM_2.5 and PM-β reductions were observed in the Southeast, while the smallest were found in the West. The overall PM-β to PM_2.5 ratio was 0.77 (0.64-0.88; 25^th-75^th percentile) and was higher when PM_2.5 levels were lower. The highest mean PM-β /PM_2.5 ratios were found in Wyoming (0.69) and South Dakota (0.51), areas with higher radon, while the mean lowest (0.17) was in Delaware followed by New Jersey (0.18). These results show that there might be toxic, previously unaccounted, components which are still present at low PM_2.5 levels.