Particle Acidity and Seed Aerosol Drive the Spatiotemporal Dynamics of Isoprene-Epoxydiol-Derived Secondary Organic Aerosols in Eastern China

Isoprene, a major global precursor to secondary organic aerosol (SOA), affects air quality and radiative forcing. A key SOA formation pathway is the reactive uptake of isoprene epoxydiol (IEPOX), a process influenced by multiple environmental factors. Here, we simulate the spatial distribution and interannual trend of IEPOX-SOA over Eastern China from 2014 to 2019 using CMAQ and quantify the contributions of different factors. Our results indicate that ambient IEPOX-SOA concentrations are elevated in central and northeast China, averaging 135 ng·m^-3 and with a recent declining trend between -19.1 and -34.8 ng·m^-3·yr^-1. Long-term trends of IEPOX-SOA are primarily driven by changes in sulfate (-50.5 ng·m^-3·yr^-1), which serve as seed aerosols, while a decrease in organic carbon reduces the coating effects and partially offsets (+16.1 ng·m^-3·yr^-1) the decline. Aerosol acidity plays a key role in governing its spatial distribution by affecting reactive rates. In Eastern China, high ammonia levels effectively neutralize acidity, thereby suppressing abundant IEPOX-SOA formation, resulting in lower concentrations compared to other regions, such as the United States. Sensitivity tests indicate that substantial future NH₃ reductions in China could increase the IEPOX-SOA by up to 100%. This study clarifies the key chemical mechanisms governing IEPOX-SOA production and supports regional air pollution control strategies.