[Spatiotemporal Variations of PM_2.5 Pollution Evolution in China in Recent 20 Years]

We use measured aerosol fine particulate matter (PM_2.5) data, moderate resolution imaging spectroradiometer (MODIS) aerosol optical depth (AOD) data, and meteorological parameters (temperature, wind speed, wind direction, boundary layer height, and relative humidity) from the Chinese national control monitoring network, to consider seasonal and regional differences in the relationship between AOD and PM_2.5. We propose a two-stage combined estimation model of PM_2.5 concentrations based on the ε-support vector regression (ε-SVR/Epsilon-SVR) and the Mind Evolutionary Computation-BP neural network (MEC-BP) for analyzing spatiotemporal variations in PM_2.5 concentrations in China between 2000 and 2017. The results showed that the two-stage combined estimation model provided a reliable estimation of the monthly ground-level PM_2.5 concentrations at a spatial resolution of 1°×1° during 2000-2017 in China. This effectively offsets the time and space gaps in the current data sets of the ground monitoring network (R²=0.838, root mean square errors (RMSE)=11.512 μg·m^-3, mean absolute percentage error (MAPE)=14.905%, mean squared percentage error (MSPE)=0.243%, mean absolute error (MAE)=6.476 μg·m^-3, mean squared error (MSE)=132.519 μg·m^-3). The preliminary spatiotemporal analysis results showed that:① Over the period 2000-2017, 2014 represented an important demarcation point for the annual PM_2.5 concentration, as its trend changed from one of continuous increase to one of rapid decrease. The PM_2.5 concentration decreases more rapidly in areas with high concentrations of PM_2.5 in particular, including the northern coastal area, the eastern coastal area, and the middle reaches of the Changjiang River. ② During the studied period, the annual average PM_2.5 concentration exceeded the second level criterion of the Chinese national air quality standard (35 μg·m^-3) over more than 65% of China. Although the PM_2.5 pollution situation in China improved to a certain extent in the latter years of the studied period, the air pollution situation remained poor.