XCO₂

The column-averaged dry-air mole fraction of carbon dioxide — the total CO₂ in a vertical column of atmosphere, averaged top to bottom, reported in parts per million.

Why it matters

XCO₂ is what space-based CO₂ missions (OCO-2, OCO-3) actually measure, and the quantity flux-inversion models use to locate sources and sinks.

In plain terms

Not the CO₂ right at your nose, but the average across the whole air column above a spot — like measuring a drink’s strength through the entire glass, not just the top sip.