Bidirectional air–surface exchange of ammonia (NH₃) has been neglected in many air quality models. In this study, we implement the bidirectional exchange of NH₃ in the GEOS-Chem global chemical transport model. We also introduce an updated diurnal variability scheme for NH₃ livestock emissions and evaluate the recently developed MASAGE_NH₃ bottom-up inventory. While updated diurnal variability improves comparison of modeled-to-hourly in situ measurements in the southeastern USA, NH₃ concentrations decrease throughout the globe, up to 17 ppb in India and southeastern China, with corresponding decreases in aerosol nitrate by up to 7 μg m⁻³. The ammonium (NH₄⁺) soil pool in the bidirectional exchange model largely extends the NH₃ lifetime in the atmosphere. Including bidirectional exchange generally increases NH₃ gross emissions (7.1 %) and surface concentrations (up to 3.9 ppb) throughout the globe in July, except in India and southeastern China. In April and October, it decreases NH₃ gross emissions in the Northern Hemisphere (e.g., 43.6 % in April in China) and increases NH₃ gross emissions in the Southern Hemisphere. Bidirectional exchange does not largely impact NH₄⁺ wet deposition overall. While bidirectional exchange is fundamentally a better representation of NH₃ emissions from fertilizers, emissions from primary sources are still underestimated and thus significant model biases remain when compared to in situ measurements in the USA. The adjoint of bidirectional exchange has also been developed for the GEOS-Chem model and is used to investigate the sensitivity of NH₃ concentrations with respect to soil pH and fertilizer application rate. This study thus lays the groundwork for future inverse modeling studies to more directly constrain these physical processes rather than tuning bulk unidirectional NH₃ emissions.