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.