Climate variability or anthropogenic emissions: which caused Beijing Haze?

Beijing Haze has been phenomenal, especially for winter, and widely considered a result of the increasing anthropogenic emissions of atmospheric pollutants in the region. Since 2013, the pollutant emissions have been reduced with the help of a series of emission-control actions. However, severe haze events still occurred frequently in Beijing in recent winters, e.g., those of 2015 and 2016, implying that other factors such as meteorological conditions and interannual climate variability have also played an important role in forming the haze. Based on homogenized station observations, atmospheric circulation reanalysis and anthropogenic emissions data for the period 1980-2017, this paper attempts to quantify the relative importance of anthropogenic emissions and climatic conditions to the frequency and intensity of Beijing Haze in winter. It is found that the frequency (number) of hazy days exhibits large interannual variability and little trend, and its variations were mainly controlled by climate variability, with a correlation coefficient of 0.77. On the other hand, the intensity of haze displays strong interannual variability and a significant increasing trend during 1980-2012 and a notable decreasing trend during 2012-2017. The multiple linear regression model suggests that about half of the total variance of the haze intensity is explained by climate variability (mainly for interannual variations), and another half by the changing emissions (mainly for the trends).