Taiwan ozone trend in response to reduced domestic precursors and perennial transboundary influence

Abstract

Two decades of nitrogen oxides (NOx) and volatile organic compounds (VOCs) in ambient concentrations have been continuously reduced by nearly 60% since 1994. However, the annual mean ozone (O3) concentrations in Taiwan had leveled off for more than a decade. Furthermore, the significant cuts in precursors did lead to reduction in episodic days (O3≥ 100 ppbv) over time, implying the weakened photochemical production of O3. Simultaneously, the number of low O3 days (O3 < 20 ppbv) also decreased due to weakened NO titration. Nevertheless, the reduction of both high and low O3 days resulted in an increase in the number of medium O3 days (O3 from 60–80 ppbv).While the nighttime mean O3 continued to increase, the daytime mean O3 leveled off and coincided with the marine background concentration of approximately 40 ppbv, revealing a gradual decrease in the daytime-nighttime mean O3 difference. In addition to multiple chemical reasons, transboundary O3 from Asian continental outflow is thought to be another major contributor to the prolonged leveling of domestic O3. To quantitatively illustrate the role of the transboundary effect, model simulations were conducted in two ways: one assessed the time percentage of a year affected by the transboundary influence; and the other involved decoupling the transboundary O3 from the domestically produced O3 to quantify the amount of domestic O3 that can be mitigated by prescribing scenarios; namely, the so-called sensitivity test.The sensitivity test suggests that although controlling domestic emissions is still vital to contain O3 under weak transboundary conditions, the overall O3 averages and the long-term trend at the island scale are closely associated with the transboundary influence. The gradual increased medium O3 concentrations to approximate the marine background level could imply that future O3 concentration on the island will be more influenced more by the transboundary conditions and less relevant to the domestic containment measures.