Ozone Continues to Increase in East Asia Despite Decreasing NO2: Causes and Abatements

1. Introduction

Surface ozone (O₃) is harmful to humans [1] and ecosystems [2] at elevated concentrations, whereas O₃ present in the upper troposphere acts as a greenhouse gas [1,3]. It is produced by the photochemical reaction of directly emitted nitrogen oxide precursors (NO_x = NO + NO₂) and volatile organic compounds (VOCs). Long-range transport of tropospheric O₃ is possible such that it can reach the United States (U.S.) from East Asia [4,5,6,7,8,9,10,11,12,13,14,15]. In the 2000s, major East Asian countries began actively regulating air pollutants. South Korea and Japan have been enforcing stringent regulations on NO_x emissions since the early 2000s [16,17,18]. In China, NO_x emissions have also been on a downward trend since mid-2010s due to tougher restrictions on automobile emissions.

The O₃ formation mechanism is based on the O₃–NO_x–VOC photochemical reaction chain, identified as either NO_x-limited or NO_x-saturated (VOC-limited) depending on the VOC/NO_x ratio, and is relevant to O₃ reduction policies focused on the NO_x or VOC emission control priority. At high VOC/NO_x ratios, O₃ production is NO_x-limited, and at low VOC/NO_x ratios, the production is VOC-limited [19,20,21]. On the global scale, O₃ production is largely dominated by the NO_x-limited regime, though heavily polluted urban/metropolitan areas with high NO_x emissions are frequently under the NO_x-saturated condition [21,22]. NO_x reductions could lead to decreasing ambient O₃ under the NO_x-limited regime as NO_x emission reductions reduce the NO₂ photolysis related to primary production of free oxygen atoms reacting with O₂ into O₃. Under the VOC-limited regime, a reduction in NO_x emissions has resulted in increased O₃ [20] due to weakened NO + O₃ titration. Thus, as NO_x emissions decline, O₃ levels will continue to increase until the regime shifts to NO_x-limited. Thus, surface O₃ does not respond to a linear reduction in NO_x.

Numerous O₃ studies have been carried out to identify the characteristics of its changing trends and precursors in East Asia, in which the focus has been on high-emission areas, including...