The reactive nitrogen compounds NO and NO2 (collectively termed NOx) play a fundamental role in atmospheric chemistry through modulation of HOx cycling and production of tropospheric ozone. NOx 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 NOx 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 NOx enhancements in the tropical marine boundary layer (MBL) where the speculated NO2 measurement artifact should be minimal. Potential sources of additional NOx 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.
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