Changes in Source Specific PM2.5: Unintended Consequences of Policy

Although the United States has been actively improving its air quality since 1970s, it has been important to further reduce exposures to PM2.5 to improve public health. Thus, regulations have been implemented to reduce emissions with particular focus on motor vehicles and electricity generation units (EGUs). To support the implementation of the National Ambient Air Quality Standards for PM2.5, the U.S. Environmental Protection Agency developed a national chemical speciation network to provide the necessary data for source apportionments to support control plans and provide accountability of the effectiveness of these actions. Thus, source apportionments and trend analyses of the PM2.5 compositional data obtained between 2005 and 2019 have been performed for multiple cities across New York State. The identified sources included secondary sulfate (SS), secondary nitrate (SN), spark-ignition vehicles (GAS), compression-ignition vehicles (DIE), road dust (RD), residual oil combustion (RO), fresh (FSS) and aged (AGS) sea salt, biomass burning (BB), and pyrolyzed organic carbon (OP). These source types are associated with secondary organic aerosol (SOA) formation (GAS) or redox-active transition metals (DIE, RD, RO). During this period the US EPA had imposed new controls on vehicular emissions (Tier 2), but in the same period, gasoline vehicle technology shifted from port fuel injection (PFI) to gasoline direction injection (GDI) and gasoline was reformulated to reduce its benzene content by substituting intermediate volatility organic compounds (IVOCs) as replacement anti-knock agents. Both of these changes led to more ambient SOA formation and an increase in GAS concentrations was observed beginning around 2015 after a sufficient number of GDI vehicles entered the fleet. GAS was highly correlated with secondary organic carbon. The introduction of Tier 3 vehicle emissions controls on new vehicles sold after January 1, 2017 was expected to reduce emissions and SOA formation. However, the penetration of these vehicles into the on-road fleet was insufficient to produce observable changes by 2019. Reductions in GAS may be observed during the 2023 to 2025 period when a sufficient fraction of Tier 3 vehicles are on the road.