Seasonal and diurnal variations of ¹³C¹⁸O¹⁶O in air: Initial observations from Pasadena, CA

The budget of atmospheric CO₂ is widely studied using records of temporal and spatial variations of concentrations, δ¹³C and δ¹⁸O values. However, the number and diversity of sources and sinks prevents these alone from fully constraining the budget. Molecules containing two rare isotopes can serve as additional tracers and potentially provide additional, independent, constraints. We present data documenting seasonal and diurnal variations of CO₂ having a mass of 47 u (mostly ¹³C¹⁸O¹⁶O) in air from Pasadena, CA. We report these data using the 'mass 47 anomaly' (Δ₄₇), which is defined as the deviation of R⁴⁷(=[47]/[44]) from that expected for a random distribution of isotopologues. Between February 2004 and December 2005, Δ₄₇ showed a seasonal pattern that differed significantly from that expected based on thermodynamic equilibrium. During the year 2004 Δ₄₇ was 0.76‰ in winter, increased to 0.87‰ in summer and gradually decreased through the autumn to 0.81‰ at the end of the year. Δ₄₇ then increased again through the winter and spring of 2005 to 0.97‰ in summer followed by a decrease to 0.88‰ at the end of 2005. The seasonal variations cannot be accounted for by variations in the relative contribution of local fossil fuel sources. Diurnal variations were the combined effect of both fuel combustion and respiration having Δ₄₇ values of 0.41‰ and ca. 0.77‰, respectively. The seasonal cycle may be interpreted as a competition between low Δ₄₇ values in respiration and higher Δ₄₇ values resulting from CO₂-water exchange in photosynthesis.