As the third-largest SO₂ emitter in the world, China is facing mounting domestic and external pressure to tackle the increasingly serious SO₂ pollution. Figuring out the convergence and persistence of sulfur dioxide (SO₂) emissions matters much for environmental policymakers in China. This study mainly utilizes the Fourier quantile unit root test to survey the convergence of the SO₂ emissions per capita in 74 cities of China during the period of December 2014 to June 2019, by conducting five traditional unit root tests and a quantile root unit test as a comparative analysis. The empirical results indicate that the SO₂ emissions per capita in 72 out of 74 cities in China are convergent in the sample period. The results also suggest that the unit root behavior of the SO₂ emissions per capita in these cities is asymmetrically persistent at different quantiles. For the cities with the convergent SO₂ emissions, the government should consider the asymmetric mean-reverting pattern of SO₂ emissions when implementing environmental protection policies at different stages. For Hefei and Nanjing, the local governments need to enact stricter environmental protection policies to control the emission of sulfur dioxide.

Appropriate social insurance contribution rates are crucial for the green development of firms. While the existing literature lacks an exploration of the relationship between social insurance policy and corporate environmental performance, this paper empirically examines the impact of social insurance contributions on corporate environmental performance using unbalanced panel data of 2947 A-share listed firms in China from 2008 to 2019. Our study shows that there is an inverted-U-shaped relationship between the social insurance burden and firms’ environmental performance, and the result remains robust after changing the measurements of core variables, replacing estimation method, and controlling endogenous problems. The inverted-U-shaped relationship is more pronounced in non-heavily polluting industries, non-labor-intensive industries, and non-state-owned enterprises (non-SOEs). In addition, corporate innovation and digital transformation can positively moderate the inverted-U-shaped effect of social insurance burden on corporate environmental performance, and firms should grasp the “double-edged sword” effect of innovation and digital transformation in different periods of social insurance burden. Further analysis reveals that a reasonable social insurance burden can enhance firm value and risk taking through improving firms’ environmental performance, whereas an excessive social insurance burden is not conducive to the improvement of firms’ environmental performance, internal value creation, and risk taking.