Shifts in global atmospheric oxidant chemistry from land cover change

Abstract. Human activities have profoundly altered natural vegetation, primarily by converting pristine land for agriculture and grazing. Land cover change (LCC) influences the Earth system through modifications of surface albedo, roughness length, evapotranspiration, and atmospheric composition. This work investigates how LCC-driven changes in BVOC fluxes, anthropogenic surface emissions, natural soil NO emissions, and O₃ deposition fluxes affect atmospheric chemistry. The chemistry–climate model EMAC was used to compare: (1) present-day land cover, which includes areas deforested for crops and grazing, with the potential natural vegetation (PNV) cover simulated by the model, and (2) an extreme reforestation scenario where grazing land is restored to natural vegetation. Our results show that the expansion of agricultural land reduces global BVOC emissions, leading to lower annual average surface OH concentrations (−5.7 %) and CO mixing ratios (−6.2 %), despite increased CO from agricultural burning. Meanwhile, NO_x mixing ratios increase (+7.8 %) due to enhanced anthropogenic and natural soil sources. While regional ozone responses vary, global ozone production sensitivity shifts from a NO_x- to a VOC-sensitive regime. These changes influence radiative forcing: reductions in tropospheric O₃ and CH₄ lifetimes exert a combined net cooling of −60 mW m⁻², partially offset by warming from reduced BVOC-driven SOA formation. Reforestation of grazing areas reverses these trends to some extent, though with a weaker response.