Visibility-Based PM Concentrations in China: 1957-1964 and 1973-2014.

China established ground PM monitoring network in late 2012 and hence the long-term and large-scale PM data were lacking before 2013. In this work, we developed a national-scale spatiotemporal linear mixed effects model to estimate the long-term PM concentrations in China from 1957 to 1964 and from 1973 to 2014 using ground visibility monitoring data as the primary predictor. The overall model-fitting and cross-validation R is 0.72 and 0.71, suggesting that the model is not overfitted. Validation beyond the model year (2014) indicated that the model could accurately estimate historical PM concentrations at the monthly (R = 0.71) level. The historical PM estimates suggest that air pollution is not a new environmental issue that occurs in the recent decades but a problem existing in a longer time before 1980. The PM concentrations have reached 60-80 μg/m in the north part of North China Plain during 1950s-1960s and increased to generally higher than 90 μg/m during 1970s. The results also show that the entire China experienced an overall increasing trend (0.19 μg/m/yr, P < 0.001) in PM concentrations from 1957 to 2014 with fluctuations among different periods. This paper demonstrated visibility data allow us to understand the spatiotemporal characteristics of PM pollution in China in a long-term.