SICMA: a simplified isoprene oxidation chemistry for the MAGRITTE chemistry transport model with application to source inversion

Accurate representation of volatile organic compound (VOC) chemistry remains a major challenge due to its complexity, particularly for isoprene oxidation, which strongly controls tropospheric formaldehyde (HCHO) and oxidant budgets. Full chemical mechanisms are computationally expensive, which limits their applicability in long simulations and inverse modeling frameworks. In this work, we present the Simplified Isoprene Chemistry for MAGRITTE (SICMA), a newly developed simplified chemical mechanism designed to efficiently represent isoprene oxidation while preserving key features of HCHO production and HOx recycling.
The SICMA isoprene chemistry scheme consists of 4 lumped reactions involving 4 organic species with parametrized yield coefficients and reaction rates, enabling a compact yet physically consistent representation of the dominant isoprene oxidation pathways. The coefficients and rates are optimized through box-model experiments constrained by the MAGRITTEv1.2 chemistry scheme. The SICMA chemistry is implemented within the MAGRITTEv1.2 chemistry-transport model and is evaluated using global simulations against the full chemical mechanism.
The simplified chemistry reproduces the main spatial patterns and seasonal variability of HCHO with good agreement relative to the full mechanism, while significantly reducing computational cost. The SICMA scheme also provides a good match for isoprene, HOx, and NOx. Our results demonstrate that SICMA provides a robust compromise between chemical realism and computational efficiency. The scheme is well suited for large-scale applications such as data assimilation, emission inversion, and sensitivity studies, where traditional full chemistry approaches are often prohibitive. SICMA thus offers a practical pathway towards improved exploitation of satellite HCHO observations for constraining isoprene emissions and understanding tropospheric oxidation chemistry. SICMA chemistry has been developed as part of the EU Horizon Europe CAMEO project with application to the IFS-COMPO model.