NO2 Vertical Profiles From the Ground to the Lower Stratosphere for Satellite Validation

NO₂ is an important trace gas that is an EPA criteria pollutant and controls ozone tendency in both the troposphere and stratosphere through a series of catalytic cycles. Thus, accurate measurements of NO₂ from the surface up through the stratosphere are important for diagnosing a variety of atmospheric chemistry processes. The capability of satellites to map NO₂ from space with high signal/noise has made NO₂ a particularly important and useful indicator of pollution trends and spatial variability. The global community has recently invested in a suite of new geostationary satellites, allowing for widespread mapping of NO₂ pollution throughout the day. The ability to perform these retrievals is dependent on assigning portions of the NO₂ column to the stratosphere and the free troposphere, which requires an accurate a priori vertical distribution of the trace gas. Recent analyses of aircraft vertical profiles of NO and NO₂ and the NO/NO₂ ratio to the upper troposphere has shown a discrepancy between measurements and theory, with NO/NO₂ ratios derived from measurements substantially lower than those calculated from the photostationary state or from GEOS-Chem above about 8 km. Concurrently, measured NO vertical profiles show good agreement with modeled profiles, while measured NO₂ vertical profiles are substantially higher above 8 km than modeled profiles. These discrepancies are important to resolve for interpreting the space-based NO₂ measurements, particularly in regions with lower NO₂ columns which are becoming increasingly common as NO_x sources decrease throughout the United States.

Here we present NO/NO₂ vertical profiles from three aircraft campaigns that combined, span measurements from the boundary layer up to the lower stratosphere. Two sets of measurements were performed on the NASA WB-57 between about 14 and 19 km, and the third set was a NASA DC-8 campaign measuring between 0.5 and 12 km. All measurements were made using the NOAA NOy-LIF instrument, which measures NO, NO₂, and NO_y using laser-induced fluorescence detection of NO. This instrument uses a unique photolytic converter design to minimize positive interferences for NO₂ that can occur when sampling air into a warm aircraft cabin prior to analysis. Results from these campaigns show good agreement between measured NO/NO₂ vertical profiles and those calculated from the photostationary state equation. This indicates that these measurements can be used to help reconcile the measurement/model disagreement and for NO₂ validation for the new NASA TEMPO instrument.