Visibility-Based PM2.5 Concentrations in China: 1957-1964 and 1973-2014

China established ground PM2.5 monitoring network in late 2012 and hence the long-term and large-scale PM2.5 data were lacking before 2013. In this work, we developed a national-scale spatiotemporal linear mixed effects model to estimate the long-term PM2.5 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-2 is 0.72 and 0.71, suggesting that the model is not oveffitted. Validation beyond the model year (2014) indicated that the model could accurately estimate historical PM2.5 concentrations at the monthly (R-2 = 0.71) level. The historical PM2.5 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 PM2.5 concentrations have reached 60-80 mu g/m(3) in the north part of North China Plain during 1950s-1960s and increased to generally higher than 90 mu g/m(3) during 1970s. The results also show that the entire China experienced an overall increasing trend (0.19 mu g/m(3)/yr, P < 0.001) in PM2.5 concentrations from 1957 to 2014 with fluctuations among different periods. This paper demonstrated visibility data allow us to understand the spatiotemporal characteristics of PM2.5 pollution in China in a longterm.