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Declining ambient PM 2.5 concentrations have been attributed to fuel consumption standards and emission controls of secondary sulfate and nitrate aerosol precursors from transportation and industrial sectors. As a result, the relative contribution of PM 2.5 sources is modified, shifting PM 2.5 trends, physicochemical characteristics, and health effects. Carbonaceous fine aerosol account for most of PM 2.5 mass in the US. This study aims to examine the spatiotemporal trends of ambient PM 2.5 levels and their association with primary PM 2.5 emissions from anthropogenic activities and fires in the New York/New Jersey metropolitan statistical area (NYNJ MSA) airshed. PM 2.5 mass concentrations were obtained from the U.S. Environmental Protection Agency (USEPA) Air Data. Ambient PM 2.5 mass levels declined on average by 47%, at a rate of −0.61 ± 0.01 μg/m 3 /year in urban locations and −0.25 ± 0.01 μg/m 3 /year in upwind and peri-urban locations over the 2007 to 2017 period. The strong spatial gradient in 2007, with high PM 2.5 levels in urban locations and low PM 2.5 levels in peri-urban locations gradually weakened by 2013 but re-appeared in 2017. Over the same period, primary PM 2.5 emissions declined by 52% from transportation, 15% from industrial, and 8% from other anthropogenic sources corresponding to a decrease of 0.8, 0.9, and 0.6 μg/m 3 on ambient PM 2.5 mass, respectively. Wildland and prescribed fires emissions increased more than 3 times adding 0.8 μg/m 3 to ambient PM 2.5 mass. These results indicate that (i) fire emissions may impede the effectiveness of existing policies to improve air quality and (ii) the chemical content of PM 2.5 may be changing to an evolving mixture of aromatic and oxygenated organic species with differential toxicological responses as compared to inert ammonium sulfate and nitrate salts.