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

Abstract

Satellite‐based measurements of the column CH2O/NO2 ratio have previously been used to estimate near‐surface ozone (O3) 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 O3 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 O3 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 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.

The Dawn of Geostationary Air Quality Monitoring: Case Studies From Seoul and Los Angeles

“…Whether considering daily measurements or analysis of long term monthly averages, instruments like OMPS NM provide a well-characterized, quantitatively stable measurement reflecting a balance of NO 2 emissions and removal at spatial scales of ∼25 km, with some limited information on pollutant transport (e.g., Beirle et al, 2011;Valin et al, 2013Valin et al, , 2014de Foy et al, 2016). As such, the measurements available from the past have not been sufficient to address the more pressing air quality management needs: the ability to distinguish sources within urban airsheds, characterization of local mesoscale flow patterns on pollutant transport, quantification of NO 2 removal mechanisms (e.g., Valin et al, 2013), or better characterization of photochemical ozone production to NO x (NO + NO 2 ) or VOC control strategies (e.g., Martin et al, 2004;Duncan et al, 2010;Jin et al, 2017;Schroeder et al, 2017).…”

Ozone seasonal evolution and photochemical production regime in the polluted troposphere in eastern China derived from high-resolution Fourier transform spectrometry (FTS) observations

“…Human health, terrestrial ecosystems, and material degradation are impacted by poor air quality resulting from high photochemical ozone (O 3 ) levels (Wennberg and Dabdub, 2008;Edwards et al, 2013;Schroeder et al, 2017). In polluted areas, tropospheric O 3 is generated from a series of complex reactions in the presence of sunlight involving carbon monoxide (CO), nitrogen oxides (NO x ≡ NO (nitric oxide) + NO 2 (nitrogen dioxide)), and volatile organic compounds (VOCs) (Oltmans et al, 2006;Schroeder et al, 2017). Briefly, VOCs first react with the hydroxyl radical (OH) to form a peroxy radical (HO 2 + RO 2 ), which increases the rate of catalytic cycling of NO to NO 2 .…”

“…Subsequent reactions between HO 2 or RO 2 and NO lead to radical propagation (via subsequent reformation of OH). Radical termination proceeds via the reaction of OH with NO x to form nitric acid (HNO 3 ) (Reaction R1, referred to as LNO x ) or by radical-radical reactions resulting in stable peroxide formation (Reactions R2-R4, referred to as LRO x , where RO x ≡ RO 2 + HO 2 ) (Schroeder et al, 2017):…”

An Inversion of NO<sub><i>x</i></sub> and NMVOC Emissions using Satellite Observations during the KORUS-AQ Campaign and Implications for Surface Ozone over East Asia

“…NOx-sensitive regimes [Schroeder et al, 2017;Souri et al, 2020]. The resultant changes in the LNOx/LROx ratios shed some light on ozone sensitivity with respect to its major precursors, but P(O3) is also dependent on the absolute values of emissions, the degree of reactivity of VOCs, and the abundance of radicals.…”