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 (O₃) 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 CH₂O/NO₂ to be used in this manner. In this study, we use a 0-D photochemical box model to evaluate O₃ 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 O₃ 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 CH₂O/NO₂ can be used to indicate O₃ sensitivity, but there is an important 'transition/ambiguous' range whereby CH₂O/NO₂ fails to categorize O₃ 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 O₃ 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 O₃ sensitivity. Lastly, we present a case study from DISCOVER-AQ-Houston that suggests that O₃ sensitivity on exceedance days may be substantially different than on nonexceedance days (which may be observable from space) and explore the diurnal evolution of O₃ sensitivity, O₃ 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 O₃ sensitivity, new techniques offered by geostationary platforms may nonetheless provide methods for using space-based measurements to develop O₃ mitigation strategies. [ABSTRACT FROM AUTHOR]