A simplified isoprene oxidation mechanism for fast global chemistry transport modeling and emission inversion

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Abstract. We introduce the Simplified Isoprene Chemistry for MAGRITTE (SICMA), a compact chemical mechanism designed for computationally efficient global chemistry transport modeling 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 five lumped reactions. The SICMA parameters (rate coefficients and product yields) are optimized using box-model simulations across multiple NOx regimes to reproduce cumulative formaldehyde (HCHO) production and HOx concentrations from the full mechanism. The simplified scheme successfully captures the NOx-dependent branching of isoprene oxidation and reproduces HCHO production and oxidant recycling with high fidelity. Implemented in the global MAGRITTE model, SICMA reproduces the monthly HCHO vertical columns from the full chemistry run within 10 % over most continental regions. Larger discrepancies occur over boreal forests and remote oceans, mainly due to the simplified treatment of monoterpene oxidation and 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 nearly identical global totals when using SICMA or the full chemistry (568.0 and 568.4 Tg yr-1, respectively). SICMA therefore provides a robust and computationally efficient alternative to detailed isoprene mechanisms for large-scale modeling and emission inversion applications.

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