This paper demonstrates how important mass fixers can be in the simulation of long-lived greenhouse gases using transport models based on the highly efficient semi-Lagrangian advection scheme. Mass fixers can have a large impact on the representation of the inter-hemispheric gradient of CO₂ and CH₄, a crucial feature of their distribution. This work is relevant for models simulating atmospheric composition that use semi-Lagrangian advection schemes both for climate and air quality applications.