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

Abstract

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 NO_x isoprene epoxydiols (IEPOX) pathway, the gas-particle partitioning of isoprene-derived SOA tracers, and the high NO_x 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 remainder 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 μg/m³ in China with the expanded scheme. The isoprene-derived SOA is highly influenced by anthropogenic emissions, especially SO_x (SO₂ + SO₄^2–), and the reduction in SO_x reduces both isoprene aerosol tracers and organosulfates significantly, while a similar reduction in NO_x leads to small increases in these species for both particle and gas phases.