The NASA Atmospheric Tomography (ATom) mission built a photochemical climatology of air parcels based on in situ measurements with the NASA DC-8 aircraft along objectively planned profiling transects through the middle of the Pacific and Atlantic Oceans.  ATom measured numerous gases and aerosols, particularly the gaseous species driving the chemical budgets of O₃ and CH₄:  i.e., O₃, CH₄, CO, C₂H₆, higher alkanes, alkenes, aromatics, NO_x, HNO₃, HNO₄, peroxyacetylnitrate, other organic nitrates, H₂O, HCHO, H₂O₂, and CH₃OOH.  From the 10 s (2 km) merged observations, a modeling data stream (MDS) based on observations of the core species, consisting of 146,494 distinct air parcels has been constructed from the 4 ATom deployments, providing a continuous data stream for initializing global chemistry models and calculating the 24-hour chemical tendencies.  Tendencies derived from 6 chemistry models using the ATom-1 MDS tend to agree and show a highly heterogeneous troposphere where globally 10% of the parcels control as much as 40% of the budget of O₃ and CH₄.  Surprisingly, modeled probability distributions (100-km cells) match ATom statistics (2 km parcels), indicating that the majority of the observed heterogeneity can be resolved with current global chemistry models.  On the other hand, the models' own chemical climatologies underestimate O₃ production below 4 km in both Pacific and Atlantic basins because they have lower NO_X levels than observed.