Abstract We analyze the diurnal variability of atmospheric CO2 ${\text{CO}}{2}$, O2 ${\mathrm{O}}{2}$, and δ13 ${\delta }^{13}$CO2 above the canopies of two contrasting ecosystems: the Amazon tropical forest and the Loobos temperate forest. Using a coupled forest‐atmosphere model constrained by tower‐based and aircraft observations, we quantify the role of atmospheric processes—including entrainment, subsidence, and cloud ventilation—in shaping the diurnal amplitude, or diurnal range (DR), of carbon‐cycle tracers. Our results show that atmospheric processes can contribute more than twice as much as surface processes to DR. Misrepresenting these influences leads to substantial errors in interpreting observations and modeling tracer variability. We propose using DR as a metric to evaluate atmospheric tracer transport models and to compare site‐level measurements. We present a roadmap to identify which atmospheric or surface processes are poorly represented when modeled and observed DR diverge.

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