New insights into the column CH2O/NO2 ratio as an indicator of near-surface ozone sensitivity

Satellite-based measurements of the column CH2O/NO2 ratio have previously been used to estimate near-surface ozone (O-3) sensitivity (i.e., NOx or VOC limited), and the forthcoming launch of air quality-focused geostationary satellites provides a catalyst for reevaluating the ability of satellite-measured CH2O/NO2 to be used in this manner. In this study, we use a 0-D photochemical box model to evaluate O-3 sensitivity and find that the relative rate of radical termination from radical-radical interactions to radical-NOx interactions (referred to as LROx/LNOx) provides a good indicator of maximum O-3 production along NOx ridgelines. Using airborne measurements from NASA's Deriving Information on Surface Conditions from Column and Vertically Resolved Observations Relative to Air Quality (DISCOVER-AQ) deployments in Colorado, Maryland, and Houston, we show that in situ measurements of CH2O/NO2 can be used to indicate O-3 sensitivity, but there is an important "transition/ambiguous" range whereby CH2O/NO2 fails to categorize O-3 sensitivity, and the range and span of this transition/ambiguous range varies regionally. Then, we apply these findings to aircraft-derived column density measurements from DISCOVER-AQ and find that inhomogeneities in vertical mixing in the lower troposphere further degrades the ability of column CH2O/NO2 to indicate near-surface O-3 sensitivity (i.e., the transition/ambiguous range is much larger than indicated by in situ data alone), and we hypothesize that the global transition/ambiguous range is sufficiently large to make the column CH2O/NO2 ratio unuseful for classifying near-surface O-3 sensitivity. Lastly, we present a case study from DISCOVER-AQ-Houston that suggests that O-3 sensitivity on exceedance days may be substantially different than on nonexceedance days (which may be observable from space) and explore the diurnal evolution of O-3 sensitivity, O-3 production, and the column CH2O/NO2 ratio. The results of these studies suggest that although satellite measurements of CH2O/NO2 alone may not be sufficient for accurately classifying near-surface O-3 sensitivity, new techniques offered by geostationary platforms may nonetheless provide methods for using space-based measurements to develop O-3 mitigation strategies.