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

Abstract Satellite‐based measurements of the column CH 2 O/NO 2 ratio have previously been used to estimate near‐surface ozone (O 3 ) sensitivity (i.e., NO x or VOC limited), and the forthcoming launch of air quality‐focused geostationary satellites provides a catalyst for reevaluating the ability of satellite‐measured CH 2 O/NO 2 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‐NO x interactions (referred to as LRO x /LNO x ) provides a good indicator of maximum O 3 production along NO x 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 CH 2 O/NO 2 can be used to indicate O 3 sensitivity, but there is an important “transition/ambiguous” range whereby CH 2 O/NO 2 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 CH 2 O/NO 2 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 CH 2 O/NO 2 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 CH 2 O/NO 2 ratio. The results of these studies suggest that although satellite measurements of CH 2 O/NO 2 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.