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 R2 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 PM2.5 concentrations at the monthly (R2 = 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 μg/m3 in the north part of North China Plain during 1950s-1960s and increased to generally higher than 90 μg/m3 during 1970s. The results also show that the entire China experienced an overall increasing trend (0.19 μg/m3/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 long-term.