Improved ozone sensitivity regime classification over Seoul using high-resolution GeoTASO HCHO and NO2 observations

Abstract:

An accurate estimation of the non-linear ozone formation requires a precise measurement of several compounds including the concentrations of reactive VOCs and NO_x. Owing to the fact that satellite-based HCHO and NO₂ columns are readily accessible, many studies have attempted to leverage remotely sensed HCHO/NO₂ ratios to pinpoint the underlying chemical regimes. While a qualitative analysis of the ratio with respect to time and space can provide relevant information, its quantitative use has some limitations steming from the fact that both biases associated with satellites and the relationship between columns and near-surface concentrations can be highly variable. Here, we introduce an additional layer of complexity when it comes to using the ratio that is the chemical feedback of NO₂ on the formation of HCHO; under VOC-rich (VOC-poor) environments, NO_x tends to increase (decrease) the formation of HCHO, complicating the thresholds used in the ratio for labeling the regimes. We redesigned this indicator in a different mathematical format in a way that is capable of accounting for the feedback. This opens up an opportunity to remove the chemical dependency, thereby providing a more robust way to carry out the labeling; Ultimately, we classified the chemical regimes over Seoul using HCHO and NO₂ columns from the GeoTASO sensor with unprecedented high temporal and spatial resolutions on a degraded air quality day during the KORUS-AQ campaign. We observe a much more comprehensive (and complex) picture of ozone formation regimes suggesting that future geostationary satellites should be a great addition to observing monitoring systems.