This study investigates the formation of secondary organic aerosol (SOA) from isoprene under low- and high-NOx conditions, focusing on the role of two key reactive intermediates: epoxiols of isoprene (IEPOX) and methacryloylperoxynitrate (MPAN). Under low-NOx conditions, the presence of acidified sulfate seed aerosol enhances isoprene SOA formation, with IEPOX playing a crucial role in this enhancement through acid-catalyzed particle-phase reactions. Under high-NOx conditions, SOA formation is dominated by the oxidation of MPAN, with the composition of the resulting SOA resembling that formed from isoprene and methacrolein. The study suggests that reactions of IEPOX and MPAN in the presence of anthropogenic pollutants, such as acidic aerosol produced from the oxidation of SO2 and NOx, could contribute significantly to "missing urban SOA" not included in current atmospheric models.This study investigates the formation of secondary organic aerosol (SOA) from isoprene under low- and high-NOx conditions, focusing on the role of two key reactive intermediates: epoxiols of isoprene (IEPOX) and methacryloylperoxynitrate (MPAN). Under low-NOx conditions, the presence of acidified sulfate seed aerosol enhances isoprene SOA formation, with IEPOX playing a crucial role in this enhancement through acid-catalyzed particle-phase reactions. Under high-NOx conditions, SOA formation is dominated by the oxidation of MPAN, with the composition of the resulting SOA resembling that formed from isoprene and methacrolein. The study suggests that reactions of IEPOX and MPAN in the presence of anthropogenic pollutants, such as acidic aerosol produced from the oxidation of SO2 and NOx, could contribute significantly to "missing urban SOA" not included in current atmospheric models.