Changes in global atmospheric oxidant chemistry from land cover conversion

Abstract

Human activities have profoundly altered natural vegetation, primarily by converting pristine natural land to 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 biogenic volatile organic compound (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 larger annual average surface OH concentrations (+5.7 %) and lower CO mixing ratios (−6.2 %), despite increased CO from agricultural burning. Meanwhile, NOx 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 NOx- to a VOC-sensitive regime. These changes influence radiative forcing with reductions in tropospheric O3 and CH4 lifetimes exerting a combined radiative effect of −60 mW m−2 (cooling), partially offsetting the warming from reduced BVOC-driven aerosol formation. Reforestation of grazing areas reverses these trends to some extent, though with a weaker response.