• secondary organic aerosol;
  • photochemistry;
  • urban

[1] Photochemically processed urban emissions were characterized at a mountain top location, free from local sources, within the Mexico City Metropolitan Area. Analysis of the Mexico City emission plume demonstrates a strong correlation between secondary organic aerosol and odd oxygen (O3 + NO2). The measured oxygenated-organic aerosol correlates with odd oxygen measurements with an apparent slope of (104–180) μg m−3 ppmv−1 (STP) and r2 > 0.9. The dependence of the observed proportionality on the gas-phase hydrocarbon profile is discussed. The observationally-based correlation between oxygenated organic aerosol mass and odd oxygen may provide insight into poorly understood secondary organic aerosol production mechanisms by leveraging knowledge of gas-phase ozone production chemistry. These results suggest that global and regional models may be able to use the observed proportionality to estimate SOA as a co-product of modeled O3 production until more complete models of SOA formation become available.