Revisiting the effectiveness of HCHO/NO₂ ratios for inferring ozone sensitivity to its precursors using high resolution airborne remote sensing observations in a high ozone episode during the KORUS-AQ campaign
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Type of Work32 pages
Citation of Original PublicationSouri, Amir H. et al.; Revisiting the effectiveness of HCHO/NO2 ratios for inferring ozone sensitivity to its precursors using high resolution airborne remote sensing observations in a high ozone episode during the KORUS-AQ campaign; Atmospheric Environment, Volume 224, 117341, 12 February, 2020; https://doi.org/10.1016/j.atmosenv.2020.117341
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This work was written as part of one of the author's official duties as an Employee of the United States Government and is therefore a work of the United States Government. In accordance with 17 U.S.C. 105, no copyright protection is available for such works under U.S. Law.
The nonlinear chemical processes involved in ozone production (P(O3₃) have necessitated using proxy indicators to convey information about the primary dependence of P(O₃) on volatile organic compounds (VOCs) or nitrogen oxides (NOₓ). In particular, the ratio of remotely sensed columns of formaldehyde (HCHO) to nitrogen dioxide (NO₂) has been widely used for studying O₃ sensitivity. Previous studies found that the errors in retrievals and the incoherent relationship between the column and the near-surface concentrations are a barrier in applying the ratio in a robust way. In addition to these obstacles, we provide calculational-observational evidence, using an ensemble of 0-D photochemical box models constrained by DC-8 aircraft measurements on an ozone event during the Korea-United States Air Quality (KORUS-AQ) campaign over Seoul, to demonstrate the chemical feedback of NO₂ on the formation of HCHO is a controlling factor for the transition line between NOₓ-sensitive and NOₓ-saturated regimes. A fixed value (~2.7) of the ratio of the chemical loss of NOₓ (LNOₓ) to the chemical loss of HO₂+RO₂ (LROₓ) perceptibly differentiates the regimes. Following this value, data points with a ratio of HCHO/NO₂ less than 1 can be safely classified as NOₓ-saturated regime, whereas points with ratios between 1 and 4 fall into one or the other regime. We attribute this mainly to the HCHO-NO₂ chemical relationship causing the transition line to occur at larger (smaller) HCHO/NO₂ ratios in VOC-rich (VOC-poor) environments. We then redefine the transition line to LNOₓ/LROₓ~2.7 that accounts for the HCHO-NO₂ chemical relationship leading to HCHO = 3.7 × (NO₂ – 1.14 × 10¹⁶ molec.cm⁻²). Although the revised formula is locally calibrated (i.e., requires for readjustment for other regions), its mathematical format removes the need for having a wide range of thresholds used in HCHO/NO₂ ratios that is a result of the chemical feedback. Therefore, to be able to properly take the chemical feedback into consideration, the use of HCHO = a × (NO₂ – b) formula should be preferred to the ratio in future works. We then use the Geostationary Trace gas and Aerosol Sensor Optimization (GeoTASO) airborne instrument to study O₃ sensitivity in Seoul. The unprecedented spatial (250 × 250 m²) and temporal (~every 2 h) resolutions of HCHO and NO₂ observations form the sensor enhance our understanding of P(O₃) in Seoul; rather than providing a crude label for the entire city, more in-depth variabilities in chemical regimes are observed that should be able to inform mitigation strategies correspondingly.
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