Numerical Modeling of Global Tropospheric Ozone Effects from Isoprene Oxidation Chemistry in the INM-CM6.0 Earth System Model

Isoprene (C₅H₈), the most abundant biogenic volatile organic compound (400–600 Tg C yr⁻¹), significantly affects tropospheric chemical composition, yet its oxidation chemistry remains absent from most Russian climate models.

This study presents the first implementation of the Mainz Isoprene Mechanism (MIM1: 44 reactions, 16 species) in the atmospheric component of the INM-CM6.0 Earth system model. Two 12-year numerical experiments (2010–2019) were conducted: a control run without isoprene chemistry and an experiment with the activated MIM1 scheme.

Results reveal a NOₓ-dependent two-layer vertical structure of isoprene's effect on ozone. In the tropical lower troposphere (0–5 km, 20°S–20°N), low-NOₓ conditions (<100 ppt) cause ozone reduction of 10–20% through radical termination, accompanied by 15–30% OH decrease and 30–60% CO increase. In the middle troposphere (8–15 km), ozone increases by 10–20% due to thermal decomposition of PAN and MPAN convectively transported from the boundary layer. In the subtropics (20–35°) where NOₓ exceeds 500 ppt, isoprene promotes photochemical ozone production at all altitudes (+5–20%).

A spatial hierarchy of nitrogen reservoir species was identified: ISON and NALD operate at local scales (0–5 km), MPAN at regional scales (up to 8 km), and PAN at global scales (up to 15–18 km).

Comparison with CAMS, MERRA-2, and ERA5 reanalyses shows substantial improvement in model performance: tropical CO discrepancies decreased from 20–30% to 10–15%, OH biases reduced by factors of 2–3, and tropospheric O₃ overestimation dropped from 15–20 to 5–10 ppb.

These findings indicate that explicit isoprene chemistry is essential for accurate tropospheric composition simulation, particularly given projected emission increases of 21–57% by 2100.

This work was supported by the Russian Science Foundation, grant No. 23-77-30008 (https://rscf.ru/project/23-77-30008).