Observed NO/NO2 Ratios in the Upper Troposphere Imply Errors in NO‐NO2‐O3 Cycling Kinetics or an Unaccounted NOx Reservoir

• Published in: Geophysical research letters 2018-05, Vol.45 (9), p.4466-4474
• Main Authors: Silvern, R. F., Jacob, D. J., Travis, K. R., Sherwen, T., Evans, M. J., Cohen, R. C., Laughner, J. L., Hall, S. R., Ullmann, K., Crounse, J. D., Wennberg, P. O., Peischl, J., Pollack, I. B.
• Format: Article
• Language: English
• Subjects: Atmospheric chemistry; Chemistry; Computer simulation; Cycles; Errors; GEOS‐Chem; Kinetics; Nitrogen compounds; Nitrogen dioxide; Nitrogen oxides; NOx; Oxidants; Oxidizing agents; Photochemicals; Photochemistry; Photolysis; Propulsion systems; Ratios; Reaction kinetics; Reservoirs; Satellite observation; Satellites; SEAC4RS; Troposphere; Tropospheric chemistry; Uncertainty; Upper troposphere
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2018GL077728

Observations from the SEAC4RS aircraft campaign over the southeast United States in August–September 2013 show NO/NO2 concentration ratios in the upper troposphere that are approximately half of photochemical equilibrium values computed from Jet Propulsion Laboratory (JPL) kinetic data. One possible explanation is the presence of labile NOx reservoir species, presumably organic, decomposing thermally to NO2 in the instrument. The NO2 instrument corrects for this artifact from known labile HNO4 and CH3O2NO2 NOx reservoirs. To bridge the gap between measured and simulated NO2, additional unaccounted labile NOx reservoir species would have to be present at a mean concentration of ~40 ppt for the SEAC4RS conditions (compared with 197 ppt for NOx). An alternative explanation is error in the low‐temperature rate constant for the NO + O3 reaction (30% 1‐σ uncertainty in JPL at 240 K) and/or in the spectroscopic data for NO2 photolysis (20% 1‐σ uncertainty). Resolving this discrepancy is important for understanding global budgets of tropospheric oxidants and for interpreting satellite observations of tropospheric NO2 columns. Plain Language Summary We identify large discrepancies between observed NO/NO2 ratios and models representing our best understanding of the chemistry controlling NO and NO2 in the upper troposphere over the southeast United States during August–September 2013. We suggest that either unrecognized chemistry or errors in modeled cycling between NO, NO2, and O3 could explain this discrepancy. Either explanation will have important implications for global tropospheric chemistry and for the interpretation of satellite observations of NO2. Key Points Large model overestimates of NO/NO2 concentration ratios in the upper troposphere imply errors in NO‐NO2‐O3 cycling kinetics or the presence of an unaccounted labile NOx reservoir The presence of an unaccounted labile NOx reservoir would affect the NOx lifetime in the upper troposphere and would suggest unrecognized, likely organic, chemistry Possible error in NO‐NO2‐O3 cycling kinetics would have large implications for global simulations of tropospheric ozone and for satellite retrievals of tropospheric NO2