Estimation and Spatiotemporal Analysis of NO₂ Pollution in East Asia During 2001–2016

Ambient nitrogen dioxide (NO₂) not only has adverse health effects on humans but also contributes to the production of two major secondary atmospheric pollutants, ozone (O₃) and fine particulate matter (PM_2.5). In this study, surface NO₂ concentrations in East Asia from 2001 to 2016 were estimated by combining an ensemble backpropagation neural network method, satellite NO₂ column data, and reanalysis data. The estimated monthly and annual mean NO₂ concentrations were well-correlated with the observations, with R (correlation coefficient) values of 0.89 and 0.91, respectively. Our results indicate that the NO₂ concentrations in most areas of East Asia peaked during 2011–2013. The NO₂ concentrations in autumn and winter, especially in the eastern and northern parts of China, were much higher than those in summer. In terms of population NO₂ exposure, over 25 million South Korea residents (∼45% of the population) were exposed to NO₂ concentrations higher than the 2005 World Health Organization's annual standard (40 μg/m³, ∼22 ppbv at 25°C) in 2015. In contrast, the entire populations of some developing countries, such as Vietnam, Cambodia, Myanmar, the Philippines, and Lao PDR, were exposed to NO₂ concentrations less than 14 ppbv. Based on the estimation, NO₂-related asthma cases in East Asia increased by 1.37% annually from 2001 to 2015, reaching 139,000 cases (95% confidence interval: 37,400–263,400) in 2015. NO_x emission inventories vary from country to country in East Asia; thus, more targeted NO_x emission-control policies are urgently required.