Enhanced secondary haze by emission reduction during COVID-19 lockdown in China
As one of the first epicenters of this disease, China announced the most efficient prevention measures to slow down the spread of COVID-19 by restricting the movement of populations nationwide after the Chinese New Year 2020, which caused tremendous economic losses and also a substantial reduction in emissions of air pollutants from vehicles and factories. Surprisingly, despite such large decreases in primary pollution, air quality in China was not as clean as expected. Heavy multi-day haze pollution still covered northern and eastern China several times, raising questions about the well-established relationship between human activities and air quality, and even China's efforts on air pollution control that have been extensively implemented in the past few years.