Chemical cycling drives the production and loss of many important atmospheric constituents. The speed of atmospheric chemical cycling is a particularly valuable indicator for characterizing and measuring the effects of such cycles on oxidant chemistry, air quality, and climate. Here, we apply graph theoretical methods to explicitly quantify and analyze the characteristic timescales of chemical cycles in the atmosphere, as simulated by the GEOS-Chem chemical mechanism. We identify all two-, three-, and four-reaction cycles in the mechanism and calculate a characteristic timescale for each individual cycle. We find that the speed of chemical cycling varies by orders of magnitude at any given location but tends to be faster in urban- and biogenically-dominated chemical regions, and slower during the night. We further quantify the fraction of cycling that contains a rate-determining step, and explicitly demonstrate the large potential for mechanisms to recycle oxidants like OH.

Nickel (Ni) is a micronutrient that plays a role in nitrogen uptake and fixation in the modern ocean may have impacted rates of methanogenesis on geological timescales. Here we present the results of a diagnostic model of global ocean Ni fluxes which addresses key questions about the biogeochemical processes which cycle Ni in the modern oceans. Our approach starts with extrapolating the sparse available observations of Ni data from the GEOTRACES project into a global gridded climatology of ocean Ni concentrations. Three different machine learning techniques were tested, each relying on marine tracers with better observational coverage such as macronutrient concentrations and physical parameters. The ocean transport of this global Ni concentration field is then estimated using the OCIM2 ocean circulation inverse model, revealing regions of net convergence or divergence. These diagnostics are not based on any assumption about Ni biogeochemical cycling, but their spatial patterns can be interpreted as reflecting biogeochemical processes. We find that the spatial pattern of Ni uptake in the surface ocean is similar to phosphate (P) uptake, but not silicate (Si) uptake, suggesting that Ni is not incorporated into diatom frustules. We find that Ni:P ratios at uptake do not decrease with Ni concentrations approaching 2 nM, which challenges the hypothesis of a ~2 nM pool of non-bioavailable Ni in the surface ocean. Finally, the net regeneration of Ni occurs deeper in the ocean than P remineralization, which could be explained by reversible scavenging or the presence of a refractory Ni phase.