Enhanced Isoprene Secondary Organic Aerosol Formation with C₅-alkene Triols Newly Added to Current Chemical...

Particulate 2-methyltetrols (2-MT), 2-methylglyceric acid (2-MG), and C₅-alkene triols serve as critical tracers for identifying isoprene-derived secondary organic aerosols (SOAs). Field observations have identified significant concentrations of C₅-alkene triols, yet no air quality models currently account for the formation pathways of these compounds. Simultaneously, the 2-MT concentrations are usually overestimated by 4-5 times compared to field observations due to the lack of the C₅-alkene triols formation pathway. In this study, we expanded the isoprene-SOA scheme and implemented it into the Community Multiscale Air Quality (CMAQ) model with the latest Community Regional Atmospheric Chemistry Multiphase Mechanism version 2 (CRACMM2) mechanism. The missing formation of C₅-alkene triols via the low ${\text{NO}}_x$ isoprene epoxydiols (IEPOX) pathway, the gas-particle partitioning of isoprene-derived SOA tracers, and the high NOx SOA formation pathway are comprehensively considered in this expanded isoprene-SOA scheme. Results indicate that the C₅-alkene triols contribute to 68% of all isoprene-derived SOA tracers, with approximately 50% in the gas phase and the remaining in the particle phase. The 2-MT concentrations are better represented in the new scheme compared to the default CRACMM2 based on the field observations. The overall SOA is improved by $0-4\mu \text{g}/{\text{m}}^3$ in China with the expanded scheme. The isoprene-derived SOA is highly influenced by anthropogenic emissions, especially ${\text{SO}}_x\left({\text{SO}}_2+{{\text{SO}}_4}^{2-}\right)$ , and the reduction in ${\text{SO}}_x$ reduces both isoprene aerosol tracers and organosulfates significantly, while a similar reduction in ${\text{NO}}_x$ leads to small increases in these species for both particle and gas phases.