This study identifies two key reactive intermediates, epoxydiols of isoprene (IEPOX) and methacryloylperoxynitrate (MPAN), in the formation of secondary organic aerosol (SOA) from isoprene under low- and high-NOx conditions, respectively. Under low-NOx conditions, IEPOX is enhanced in the presence of acidified sulfate seed aerosol, leading to higher SOA mass yields. Acid-catalyzed particle-phase reactions of IEPOX explain this enhancement. Under high-NOx conditions, SOA formation occurs through the oxidation of MPAN, a second-generation product of isoprene. The composition of SOA formed from MPAN photooxidation is similar to that from isoprene and methacrolein, indicating MPAN's role in high-NOx SOA formation. Reactions of IEPOX and MPAN with anthropogenic pollutants could contribute significantly to "missing urban SOA" not included in current atmospheric models. Isoprene is a major nonmethane hydrocarbon emitted into the atmosphere, contributing significantly to atmospheric organic aerosol. Its photooxidation plays a key role in balancing hydrogen oxide radicals in vegetated areas and influences urban ozone formation. The oxidation of isoprene produces low-volatility compounds, which are the largest source of atmospheric organic aerosol. The photooxidation of unsaturated VOCs proceeds through hydroxy peroxy radicals, whose fate depends on nitrogen oxide concentrations. Under low-NOx conditions, RO2 radicals react primarily with HO2, producing lower-volatility oxidation products. Under high-NOx conditions, RO2 radicals react with NO to produce alkoxy radicals or organic nitrates. Despite extensive study of isoprene SOA, the chemical pathways under both low- and high-NOx conditions remain unclear. The study shows that under low-NOx conditions, acid-catalyzed particle-phase reactions of IEPOX enhance SOA formation. IEPOX is formed from the photooxidation of isoprene and further oxidized by OH radicals. The study also identifies MPAN as a key intermediate in high-NOx SOA formation. MPAN is formed from the oxidation of methacrolein and plays a critical role in SOA production. The study confirms that MPAN is the key intermediate in the formation of 2-methylglyceric acid and its corresponding low-volatility oligoesters in the aerosol phase. The study provides insights into the chemical mechanisms of isoprene SOA formation under low- and high-NOx conditions. These findings have significant implications for atmospheric chemistry, particularly in understanding the sources and sinks of aerosol particles. The results suggest that the chemistry of isoprene in regional and global SOA models could help resolve discrepancies in atmospheric measurements. The study also highlights the importance of aerosol acidity in determining SOA formation and compositionThis study identifies two key reactive intermediates, epoxydiols of isoprene (IEPOX) and methacryloylperoxynitrate (MPAN), in the formation of secondary organic aerosol (SOA) from isoprene under low- and high-NOx conditions, respectively. Under low-NOx conditions, IEPOX is enhanced in the presence of acidified sulfate seed aerosol, leading to higher SOA mass yields. Acid-catalyzed particle-phase reactions of IEPOX explain this enhancement. Under high-NOx conditions, SOA formation occurs through the oxidation of MPAN, a second-generation product of isoprene. The composition of SOA formed from MPAN photooxidation is similar to that from isoprene and methacrolein, indicating MPAN's role in high-NOx SOA formation. Reactions of IEPOX and MPAN with anthropogenic pollutants could contribute significantly to "missing urban SOA" not included in current atmospheric models. Isoprene is a major nonmethane hydrocarbon emitted into the atmosphere, contributing significantly to atmospheric organic aerosol. Its photooxidation plays a key role in balancing hydrogen oxide radicals in vegetated areas and influences urban ozone formation. The oxidation of isoprene produces low-volatility compounds, which are the largest source of atmospheric organic aerosol. The photooxidation of unsaturated VOCs proceeds through hydroxy peroxy radicals, whose fate depends on nitrogen oxide concentrations. Under low-NOx conditions, RO2 radicals react primarily with HO2, producing lower-volatility oxidation products. Under high-NOx conditions, RO2 radicals react with NO to produce alkoxy radicals or organic nitrates. Despite extensive study of isoprene SOA, the chemical pathways under both low- and high-NOx conditions remain unclear. The study shows that under low-NOx conditions, acid-catalyzed particle-phase reactions of IEPOX enhance SOA formation. IEPOX is formed from the photooxidation of isoprene and further oxidized by OH radicals. The study also identifies MPAN as a key intermediate in high-NOx SOA formation. MPAN is formed from the oxidation of methacrolein and plays a critical role in SOA production. The study confirms that MPAN is the key intermediate in the formation of 2-methylglyceric acid and its corresponding low-volatility oligoesters in the aerosol phase. The study provides insights into the chemical mechanisms of isoprene SOA formation under low- and high-NOx conditions. These findings have significant implications for atmospheric chemistry, particularly in understanding the sources and sinks of aerosol particles. The results suggest that the chemistry of isoprene in regional and global SOA models could help resolve discrepancies in atmospheric measurements. The study also highlights the importance of aerosol acidity in determining SOA formation and composition