We propose a new atmospheric coordinate (M_θe) based on equivalent potential temperature (θ_e) but with mass as the unit. This coordinate is useful in studying the spatial and temporal distribution of long-lived chemical tracers (CO₂, CH₄, O₂ / N₂, etc.) from sparse data, like airborne observation. Using this coordinate and sparse airborne observation (HIPPO and ATom), we resolve the Northern Hemisphere mass-weighted average CO₂ seasonal cycle with high accuracy.