A simplified isoprene oxidation mechanism for fast formaldehyde-based emission inversion of isoprene

We introduce the Simplified Isoprene Chemistry for MAGRITTE (SICMA), a compact chemical mechanism designed for computationally efficient global chemistry transport modeling of formaldehyde (HCHO) and adjoint-based emission inversions. The scheme reduces the isoprene oxidation network of the MAGRITTEv1.2 model from 93 organic species and 243 reactions to four organic species and six lumped reactions. The SICMA parameters (rate coefficients and product yields) are optimized using box-model simulations across multiple NO x regimes to reproduce cumulative HCHO production and HO x concentrations from the full mechanism. The optimization employs idealized diurnal biogenic emissions with a fixed monoterpene-to-isoprene emission ratio of 0.1 typical of tropical source regions. The simplified scheme successfully captures the NO x -dependent branching of isoprene oxidation and reproduces HCHO production and oxidant recycling with high fidelity. Implemented in the global MAGRITTE chemistry transport model, SICMA reproduces the monthly HCHO vertical columns from the full chemistry run within 5 % over most continental regions. Larger discrepancies occur over boreal forests and remote oceans, mainly due to the assumed monoterpene-to-isoprene ratio and the absence of organic nitrate chemistry. Despite these simplifications, the seasonal cycle and spatial distribution of HCHO columns remain in close agreement with both the full chemistry simulation and TROPOMI observations. Inversions of isoprene emissions constrained by TROPOMI HCHO columns yield similar global totals when using SICMA or the full chemistry (568 and 574 Tg yr −1 , respectively). SICMA therefore provides a robust and computationally efficient alternative to detailed isoprene mechanisms for large-scale modeling of HCHO and isoprene emission inversion applications.