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

Abstract

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.