Insights from Diurnal Cycle of O2 and CO2 Records Collected at Scripps Pier, La Jolla, California

The Scripps Pier (Scripps Institution of Oceanography) is one of the few sites globally with concurrent, continuous, in situ measurements of atmospheric O₂ and CO₂. Situated at a land ocean interface near a dense urban corridor, the site receives marine and continental air masses under transport conditions shaped by land sea breezes, boundary layer evolution, productive coastal waters, terrestrial biosphere and local fossil fuel fluxes. We find that variations of atmospheric O₂ and CO₂ were largely anticorrelated, but shifted in phase, a pattern consistent with opposite-sign responses to surface exchange processes and to modulation by atmospheric transport. Diurnal phase space relationships between O₂ and CO₂ often form closed loop structures that emerge from phase offsets between surface fluxes and transport pathways. The detailed structure of these phase relationships varied from day to day with changes in wind regimes and with varying contributions from urban fossil fuel emissions, terrestrial biosphere exchange, and air-sea fluxes. Back trajectory classification resolved these relationships into nocturnal offshore and daytime onshore flows with distinct O₂ to CO₂ slopes that indicated differing mixtures of the contributing processes. Many features of the observed patterns can be understood based on day-to-day variation of the relative amounts of different processes with fixed exchange ratios. This study also addresses the extent to which episodic variability in oceanic dissolved oxygen influences atmospheric O₂ variability at the site.