Analysis of the Causes of Atmospheric Fine Particle Pollution in Winter in the Hohhot–Baotou–Ordos Area of Inner Mongolia

Using the Weather Research and Forecasting model and the Aerosol and Atmospheric Chemistry Model developed by the Institute of Atmospheric Physics, Chinese Academy of Sciences, several typical pollution episodes of fine particulate matter (PM_2.5) over the Hohhot–Baotou–Ordos area of Inner Mongolia in the winter of 2016 were analyzed. The results indicated that the air quality changes in the Hohhot–Baotou–Ordos area were mainly affected by the large-scale synoptic pattern. At the stage of pollution accumulation, at 500 hPa, this area was controlled by the flat westerly airflow in front of the blocking high pressure or weak high-pressure ridge. At the ground level, this area was located in the weak high-pressure or uniform-pressure field. The low wind speed and the low height of the boundary layer were unfavorable to pollutant dispersion. Meanwhile, the air temperature and relative humidity were high, which were conducive to the formation of secondary particles. At the stage of pollution dissipation, the synoptic patterns had significantly changed. Below 550 hPa, strong cold advection occurred, causing gale weather on the ground, which was beneficial to pollutant elimination. Cold air moved southward, and pollutants over downstream areas were removed. Local emission was the main source of PM_2.5 over the Hohhot–Baotou–Ordos area. Local emission accounted for over 60%, 80%, and 90% of the air pollution in Ordos, Hohhot, and Baotou, respectively. The change in the air quality of these regions can reflect the change in regional air pollution meteorological conditions. Cross-correlation analysis showed that the PM_2.5 concentrations in Shanxi, Hebei, and Henan regions were correlated with those in Hohhot–Baotou–Ordos , with a phase difference of 6–24 hours. The improvement of PM_2.5 pollution in Hohhot–Baotou–Ordos depended on the control of local source emissions. In winter, the air quality change in this region can serve as an indicator of the air quality change in the downstream region, which is helpful for the prediction and early warning of air quality in the downstream region.