Chemical Reactivity of the Remote Troposphere Derived from Atmospheric Tomography Mission (ATom) Measurements

The NASA Atmospheric Tomography (ATom) mission has built a photochemical climatology of the remote troposphere based on objectively planned profiling transects over the Pacific and Atlantic Oceans. ATom measured numerous gases and aerosols, providing information on the chemical evolution of the atmosphere and the scope of pollution in the vast remote troposphere over the ocean basins and Antarctica. Here, we focus on the core set of species that control the primary reactivity of the remote troposphere, specifically the chemical tendencies of O₃ and CH₄. This core list includes O₃, CH₄, CO, C₂H₆, higher alkanes, alkenes, aromatics, NO_x, HNO₃, HO₂NO₂, PAN, other organic nitrates, H₂O, HCHO, H₂O₂, CH₃OOH. All four ATom deployments are now complete, and data from the first three (ATom-1, ATom-2, and ATom-3) have been released as of this talk and are available at the Oak Ridge National Laboratory DAAC*.

A modeling data stream (MDS) based on the 10-second merged observations has been constructed for the ATom-1 deployment (August 2016) for the purpose of consistent photochemical computations. (Other analyses, such as species covariance, should use the instrument data files or the primary 10-second-merge files*.) The MDS contains 31,429 air parcels of approximate size 2.5 km horizontal by 160 m vertical**. The MDS provides a continuous 10-second data stream of the core species to initialize chemical models for calculating the reactivities (P-O3, L-O3, and L-CH4, all in ppb/day) in each MDS air parcel. It also includes alternative parcel streams to investigate the relative importance of specific measurement uncertainties. Results from 6 global 3D models and a photochemical box model, shown here, are generally consistent indicating that different model formulations of the chemistry agree when the core species are specified. Thus, we can use the ATom measurements to identify the most reactive parcels, and find that the hottest 10% drive almost half of the total chemistry of the remote regions. Standard model climatologies underestimate the extent of heterogeneity and thus underestimate these low probability, highly important air masses. With the extensive list of other trace species, ATom measurements will be able to identify the source and even chemical processing of the hot parcels and possibly understand why the models do not represent them well in the remote troposphere. A deeper problem will be to understand the size of these hot spots and assess whether or how the global models with horizontal scales of order 100 km can possibly represent their occurrence. The ATom measurements provide a starting point for evaluating what is really important in tropospheric chemistry in terms of which air matters and must be represented in the global models.

* https://doi.org/10. 3334/ORNLDAAC/1581

** https://espoarchive.nasa.gov/archive/browse/atom/DC8/MDS.