Widespread Pollution From Secondary Sources of Organic Aerosols During Winter in the Northeastern United States

Secondary organic aerosol (SOA) from pollution sources is thought to be a minor component of organic aerosol (OA) and fine particulate matter beyond the urban scale. Here we present airborne observations of OA in the northeastern United States, showing that 58% of OA over the region during winter is secondary and originates from pollution sources. We observed a doubling of OA mass from SOA formation in aged emissions, with unexpected similarity to OA growth observed in polluted areas in the summer. A regional model with a simple SOA parameterization based on summer measurements reproduces these winter observations and shows that pollution SOA is widespread, accounting for 14% of submicron particulate matter in near‐surface air. This source of particulate matter is largely unaccounted for in air quality management in the northeastern United States and other polluted areas. Plain Language Summary Organic aerosol is a major contributor to fine particulate matter concentrations. The wintertime sources of organic aerosols in polluted areas have remained uncertain because of a lack of regional‐scale measurements. We made aircraft‐based observations of organic aerosols over the northeastern United States during winter. We observed that a majority of organic aerosols consist of secondary organic aerosols that form in the atmosphere from gases emitted from pollution sources. Prior to our measurements, the expectation was that wintertime pollution sources of secondary organic aerosols are minor outside urban areas. Our results show that their influence is ubiquitous over the entire eastern United States, and a better understanding of these sources can help in developing effective policies to reduce wintertime air pollution.