Abstract

The complexity of ozone chemistry makes it difficult to quantify the extent to which different national ozone reduction scenarios affect the country that implemented the strategy, neighboring countries, and the world. To better understand ozone sensitivity to national/regional emission scenarios, the zero-out method was applied for the anthropogenic emission of ozone precursors, i.e., nitrogen oxides (NO_x), in nine regions and three countries worldwide. In this study, ozone simulations were performed at a high global resolution of 56 km via the use of a nonhydrostatic icosahedral atmosphere model coupled with chemistry (NICAM-Chem). The model performance in predicting surface ozone was evaluated with ground-based measurements worldwide. The correlation was moderate to high, but both bias and uncertainty were high but remained within the ranges of values obtained from other global chemistry models. Experiments based on zero-out methods for assessing anthropogenic NO_x emissions revealed that the domestic impacts of anthropogenic NO_x emission reduction on ozone reduction were the largest in India (14 ppbv, regional and annual average) and China (11 ppbv, regional and annual average). The influence of anthropogenic NO_x emission reduction in China on transboundary ozone concentrations was notable, with a reduction in ozone of 7 ppbv (regional and annual average) in Japan and even yielding ozone reductions in North America (1 ppbv, regional and annual average) and western Europe (1 ppbv, regional and annual average). Sensitivity experiments with a 20% reduction in anthropogenic NO_x emissions were also conducted, and the results were generally consistent with those of previous studies. However, the results for a 20% reduction in anthropogenic NO_x emissions partially differed from the results for a 100% reduction in anthropogenic NO_x emissions because of the high nonlinearity of ozone chemistry. For example, in Japan, a 20% reduction in domestic NO_x emissions resulted in a larger reduction in ozone levels than that obtained with a 20% reduction in Chinese NO_x emissions. This indicates the inherent uncertainty resulting from experimental settings that assume a substantial reduction, such as a 100% decrease in the NO_x flux. Understanding the impact of emission reduction in a given country on other countries is important for accelerating future international collaboration on ozone reduction for ensuring climate and human health.