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1. Introduction
China’s economy has developed at an incredibly fast pace for decades and received broad attention. By 2010, China’s gross domestic product (GDP) reached 5.8 trillion dollars, making China the world’s second largest economy (NBSC 2011b). The country is turning from an agricultural one to a modernized one [1], with more than half of the population living in urban areas (NBSC 2011b). In the meantime, the rapid economic growth and urbanization came with a rocketing consumption of resources and a soaring emission of air pollutants, with nitrogen oxides (NOx) being the fastest accelerating air pollutant in China in the last two decades [2,3]. NOx is an important air pollutant because it contributes to the formation of photochemical smog, which can have significant impacts on human health. The main adverse effects of NOx to public health is that it causes respiratory diseases. Chronic exposure to NOx under ultraviolet radiation can cause respiration symptoms in people with asthma, and bronchial symptoms (especially in children) and airway inflammation in healthy people. In addition, NOx is the main source of nitrate aerosol (the important component of particulate matter (PM) 2.5) in China [3,4,5].
The literature on the relationship between economic development and environmental quality is extensive in the field of environmental economics. The present empirical study relies on the Environmental Kuznets Curve (EKC) hypothesis carried out by Grossman and Krueger [6] because its expanded form has the potential to be a policy tool for sustainable development [7]. Some researchers have examined NOx EKC through cross-sectional data [8,9] and panel data [10,11]. Most researchers applied country-level data to explore sulfur dioxide (SO2) EKC, while only a few studies used prefecture-level data in some specific countries.
So far, few studies have explored the relationship between nitrogen oxides emissions and socioeconomic factors through quantitative empirical approaches in the context of China, even though such empirical analyses are sorely urgent, because they can shed some new light on the driving forces and precise regularities of pollutants emission levels, and the estimated parameters and functions could be very helpful for policymakers to implement suitable policies for emission reduction. Brajer, et al. [12] carried out the solely related study through panel data. Although panel data...