Distributions and Chemistry of NOx in the Remote Troposphere and the Validity of the Photo-Stationary State Approximation

The reactive nitrogen compounds NO and NO₂ (collectively termed NO_x) play a fundamental role in atmospheric chemistry through modulation of HO_x cycling and production of tropospheric ozone. NO_x is primarily emitted to the atmosphere as NO by both anthropogenic and natural sources, and is then relatively quickly converted to NO₂ through reaction with ozone (or peroxy radicals); during daylight hours, NO₂ is then photolyzed back to NO. The NO-O₃-NO₂ cycle forms the basis for the simple photostationary state (PSS), or Leighton, approximation. A requirement of the PSS approximation is that O₃ dominates the conversion of NO to NO₂. Thus, while this approximation has been shown to be adequate in polluted, urban settings, numerous studies have noted discrepancies between measured NO₂ and PSS-predicted NO₂ in rural and remote locations. This is particularly true in the far remote marine troposphere where measured NO₂ is nearly always greater than PSS-predicted NO₂. Several past studies have attributed these discrepancies as evidence for an NO₂ measurement artifact and have suggested that thermally-labile nitrate species are responsible. As measurements have improved, however, this discrepancy persists, and it is becoming less probable that significant measurement artifacts are the cause. In this work, we present global-scale NO_x measurements from the remote troposphere collected during the recent NASA Atmospheric Tomography missions and compare the measurements to PSS-predicted NO₂. Using a combination of observations and chemistry models, we evaluate the accuracy and validity of the PSS NO₂ approximation across a variety of conditions. We discuss discrepancies between measurements and PSS predictions across a range of latitudes, altitudes, and seasons. Particular emphasis is placed on observed NO_x enhancements in the tropical marine boundary layer (MBL) where the speculated NO₂ measurement artifact should be minimal. Potential sources of additional NO_x to the MBL, including oceanic emissions and renoxification on aerosols, and the chemical impacts of those additional sources, are investigated with 0-D and 3-D chemical models.