A quantitative assessment of the air pollution purification effect of a super strong cold-air outbreak in January 2016 in China

Although numerous studies have been conducted around the world to investigate the meteorological causes of and disasters due to cold-air outbreaks, the effects of these events on air pollution have received little attention. This study quantitatively investigated the purification of air pollution by a super strong cold-air outbreak along with cold front movement from the north to the south of the Chinese mainland in January 2016 using routinely observed meteorological data, air pollution monitoring data, and NCEP/NCAR and ERA-Interim reanalysis data. Some of the main results are as follows: (1) There were strong decreases in the concentrations of the five studied air pollutants in most parts of the Chinese mainland during the cold frontal passage. Spatially, the regions with the largest decreases in air pollutant concentrations were consistent with those with negative anomalous centers of 24-h surface air temperature (SAT) changes and positive anomalous centers of 24-h sea level pressure (SLP) changes. These findings provide a new reference for air quality forecasts in the Chinese mainland. (2) During the cold frontal passage, near-ground wind speed increased extensively due to downward momentum transportation and isallobaric wind, the atmospheric stratification became unstable, the atmospheric boundary layer (ABL) height was significantly uplifted, and the mean maximum mixing depth (MMD) greatly increased. These changes generated a wide-range improvement in air quality for a large area of the Chinese mainland. (3) Wind speed was identified as the most important meteorological parameter affecting the diffusion of pollutants in the absence of precipitation and snow. Variations of air pollutant concentrations (y) with wind speed (x) were fitted with a negative exponential function of y = a x e (-bx) . (4) The clearance ratios (CRs) of the five air pollutants by the cold front differed during the cold-air outbreak. Of these, the CR of PM2.5 was the highest, reaching 85%. Overall, the cold-air outbreak greatly contributed to improving air quality in most parts of the Chinese mainland. This shows that cold front activity is one of the most important meteorological factors to be considered to improve air quality forecasts.