Nearly Half of US Breathes Unhealthy Levels of Pollution—Cars and Trucks Largely to Blame
Union of Concerned Scientists
MAY 21, 2025
referred to as secondary emissions – is formed in the atmosphere from precursor pollutants that also originate from the combustion in fossil-fuel powered engines, such as nitrogen oxides (NO x ), sulfur dioxide (SO 2 ) and volatile organic compounds (VOCs). But beyond this primary formation of PM 2.5 , even more PM 2.5
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