Given that the backgound concentrations are largely a result of 100 years unregulated industrial smog generation and use of coal fires then remediating now would make sense in that the sources of pollution are nowhere near as significant and will continue to decline as more buses burn chip oil, more houses burn gas and more electricity is non fossil fuel derived. Surely the modern loading from atmospheric deposition will be nothing compared to the period 1850 - 1950?
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