This section examines the regional patterns of different meteorological variables corresponding to persistent pollution days in the four seasons. Persistent pollution events were determined separately for the four seasons as follows. First, we defined the polluted day as a day when the concentration of PM10 was > 150 μg m–3 (He et al., 2014). A persistent pollution event was recorded when there were three or more consecutive pollution days. For the period 2013–15 when data from 12 stations were available, we used an alternative definition of a persistent pollution event as when the PM10 concentration reached the level of a persistent event simultaneously at more than 8 of the 12 stations. The persistent pollution events determined based on 12 air quality monitoring stations all fell into the events defined by the mean PM10 data over the last two years. Therefore the mean PM10 data were a good representation of the regional pollution conditions in Beijing. Because the temporal coverage of the 12 air quality monitoring stations was shorter than that of the mean data, we used the mean data to analyze the relationship between the meteorological conditions and air pollution.
Table 1 presents the number of persistent pollution events and the mean PM10 values by season. The number of events in spring, summer, fall, and winter was 24, 7, 21, and 21, respectively. The highest mean PM10 concentration was observed in spring and the lowest mean PM10 concentration was observed in summer. The lowest number of events was observed in summer, representing the lowest probability of persistent pollution events for the year.
Number of events Mean PM10 value (μg m–3) 1st day 2nd day 3rd day Spring 24 211.36 273.48 282.49 Summer 7 179.55 199.23 203.18 Fall 21 198.08 261.89 254.60 Winter 21 217.19 256.97 266.60
Table 1. Number of persistent pollution events and mean PM10 values by season
Composite maps of the persistent pollution events were constructed for the surface air temperature, the sea-level pressure, the low-level relative humidity, and the surface winds in each of the four seasons by averaging the reconstructed high-frequency meteorological variations (referred to as anomalies to distinguish them from the original values) at each grid point. We then examined the temporal evolution of synoptic patterns in each of the four seasons, which may help us to understand the persistence of air pollution in Beijing. The composite anomalies in summer may not be as robust as in the other seasons because there were only seven pollution days during the period of analysis.
The synoptic patterns appeared to be fairly stable for persistent pollution events in spring. A higher surface air temperature was maintained over most of eastern China (Figs. 1a–c). A large region of temperature anomaly was located north of Beijing. Anomalous low sea-level pressure was observed north of Beijing during the pollution events (Figs. 1d–f). This was accompanied by an anomalous cyclone, the center of which was located south of Lake Baikal on the first polluted day, over Mongolia on the second polluted day, and north of Beijing on the third polluted day. During the first two days, the region around Beijing was under the continuous influence of anomalous southwesterly winds, but the anomalous winds weakened on the third day. Anomalous southwesterly winds controlled southeast China during the pollution events, which transported warmer and wetter air from lower latitudes, leading to higher surface air temperatures (Figs. 1a–c). The effect of an increase in temperature appeared to overcome the increase in specific humidity, leading to a decrease in relative humidity (Figs. 1g–i). The North China Plain was in a region with southerly winds or relatively weak winds and lower level anomalous convergence, which provided favorable conditions for the persistence of pollution.
Figure 1. Composite anomalies of (a–c) surface temperature (K), (d–f) sea-level pressure (shading; hPa) and winds at 10 m (vector with scale at top-right corner; m s–1), and (g–i) relative humidity (%) during persistent pollution events in spring. Polluted_1, Polluted_2, and Polluted_3 refer to the first, second, and third day, respectively. The dotted region shows where the composite anomalies are significant at the 95% confidence level according to the one-tailed Student’s t-test. Only winds that are significant at the 95% confidence level are plotted.
For the persistent pollution events in summer, a higher surface air temperature was recorded in Mongolia on the first polluted day (Fig. 2a). This higher temperature region moved slowly southwestward over time, reaching north of Beijing on the third polluted day (Figs. 2b, c). The distribution of anomalous sea-level pressure featured a northwest–southeast contrast, with lower pressure lying northwest and higher pressure southeast of Beijing (Figs. 2d–f). A similar pattern of anomalous pressure on pollution days has been reported previously by You et al. (2017). The associated southerly winds transported warmer air from the south, contributing to a higher surface air temperature (Chen et al., 2008; Wang et al., 2010). Under the influence of southerly winds, pollution particles south of Beijing were easily transported to Beijing, which favored persistent pollution events over Beijing. The lower relative humidity concurred with the higher surface air temperature during this period (Figs. 2g–i). It was obvious that the movement of the anomalous synoptic patterns was slow. The slow movement of synoptic patterns provided favorable conditions for the persistence of air pollution over Beijing. The composite features were relatively weak in summer due to the limited number of days with pollution.
Figure 2. As in Fig. 1, but in summer.
For persistent pollution events in fall, a zonal band of higher surface air temperature with a slow increase in magnitude was maintained north of Beijing (Figs. 3a–c). A region of anomalously low sea-level pressure was observed over Mongolia with the center moving slowly southeastward (Figs. 3d–f). Anomalous southerly winds therefore controlled most of eastern China, which accounted for the higher surface air temperature in the north. A band of high relative humidity covered the region southeast of Beijing (Figs. 3g–i), which may be attributed to the anomalous southerly winds bringing wetter air from lower latitudes. The North China Plain overall was under the influence of a higher air temperature, higher relative humidity, and anomalous southerly winds during this time period. These synoptic patterns facilitated the transport to, and storage of, pollutants near Beijing.
Figure 3. As in Fig. 1, but in fall.
For persistent pollution events in winter, a large region of higher surface air temperature moved eastward over the midlatitudes (Figs. 4a–c). During this process, a higher temperature was maintained over Beijing. An anomalous low sea-level pressure moved southeastward, with the center located southwest of Lake Baikal on the first day, over Mongolia on the second day, and over North China on the third day (Figs. 4d–f). The North China Plain was under the influence of large anomalous southerly winds on the first two days and the anomalous winds weakened on the third day. Positive relative humidity anomalies controlled the region around Beijing, with their magnitude increasing during the pollution events (Figs. 4g–i). The higher relative humidity and higher temperatures were attributed to anomalous southerly winds that brought warmer and wetter air from the south. The temperature, pressure, and wind patterns displayed clearer movement over time in winter than in the fall. However, the North China Plain was still under the influence of anomalous southerly winds, higher temperatures, and higher relative humidity in the winter, as in the fall, conditions that provided favorable conditions for the persistence of pollution.
Figure 4. As in Fig. 1, but in winter.
A higher surface air temperature was observed during the persistent pollution events in all four seasons, with the center located to the north of Beijing. Another common feature was that Beijing was under the influence of anomalous southerly winds. The regional synoptic patterns experienced relatively small changes during the pollution events in all four seasons, which was associated with the slow movement of the synoptic patterns. However, some notable differences were seen among the four seasons. An anomalous negative pressure covered the region around Beijing in the spring, fall, and winter, whereas a northwest low pressure–southeast high pressure pattern was observed in summer. The relative humidity in Beijing was lower in spring and summer, but higher in fall and winter, consistent with the results of You et al. (2017).
This section examines the regional patterns of different meteorological variables corresponding to non-persistent pollution days in the four seasons. After identifying a pollution day based on the method described in the preceding section, we examined the PM10 value in the two days after the pollution day. The day with the PM10 value < 50 μg m–3 in an individual season was identified as a clean day in that season. When two consecutive clean days were detected after the polluted day, a non-persistent pollutant event was recorded.
Table 2 shows the number of identified non-persistent pollution events and the corresponding mean PM10 values in the four seasons. There were 3, 5, 8, and 11 non-persistent pollution events in spring, summer, fall, and winter, respectively. The mean PM10 concentration varied from 26 to 43 μg m–3 on the clean days in the four seasons. As for the persistent pollution events, composite maps of the meteorological variables were constructed for non-persistent pollution events in the four seasons by averaging the reconstructed variations with periods < 90 days. The number of events was small, particularly in spring and summer. Thus the features based on the composite map may not be as robust as those for the persistent pollution events.
Number of events Mean PM10 value (μg m–3) 1st day 2nd day 3rd day Spring 3 284.95 33.45 42.99 Summer 5 210.46 26.18 32.05 Fall 8 197.39 31.85 30.10 Winter 11 204.25 28.94 28.79
Table 2. Number of non-persistent pollution events and mean particulate matter values in the four seasons
The regional meteorological patterns corresponding to the non-persistent pollution events in spring showed pronounced changes during the events. The surface air temperature around Beijing changed from positive to negative anomalies following the eastward movement of large regions of temperature anomaly along the midlatitudes (Figs. 5a–c). The North China Plain was dominated by an anomalous low pressure on the polluted day, accompanied by anomalous cyclonic winds with a convergence centered southwest of Beijing (Fig. 5d). On the following two days, the anomalous low and the cyclone moved southeastward quickly and Beijing was influenced by large anomalous northerly winds (Figs. 5e, f). On the pollution day, two large regions of positive relative humidity were observed, one lying on the border of Mongolia and the other over central China (Fig. 5g). The latter was associated with an anomalous lower level convergence of moisture. The northern region remained stationary, whereas the southern region moved quickly eastward, following the movement of the anomalous cyclone, and was replaced by a larger region of negative relative humidity in the following two days (Figs. 5h, i). Thus Beijing experienced a dramatic change from higher temperatures, lower pressures, and an anomalous cyclone to lower temperatures and anomalous northerly winds. The relative humidity near Beijing did not show a large change, however. It appears that the large region of anomalous northerly winds plays an important part in cleaning the air.
Figure 5. Composite anomalies of (a–c) surface temperature (K), (d–f) sea-level pressure (shading; hPa) and wind at 10 m (vector with scale at top-right corner; m s–1), and (g–i) relative humidity (%) during non-persistent pollution events in spring. Polluted_1, Clean_1, and Clean_2 refer to the polluted day and the first and second clean day, respectively. The dotted region shows where the composite anomalies are significant at the 95% confidence level according to the one-tailed Student’s t-test. Only winds that are significant at the 95% confidence level are plotted.
The surface air temperature anomalies around Beijing switched from positive to negative corresponding to the non-persistent pollution events in summer (Figs. 6a–c). An anomalous low and associated anomalous southerly wind controlled the North China Plain on the polluted day (Fig. 6d), which was favorable for the transportation of air pollutants from regions south of Beijing where more emissions were located. With the southeastward movement of the anomalous low and the cyclone, anomalous northerly winds dominated the region surrounding Beijing on the following two days (Figs. 6e, f). The relative humidity anomalies were small near Beijing on the pollution day (Fig. 6g). This was replaced by very large positive anomalies in the relative humidity (Fig. 6h), which may be due to the anomalous convergence of moisture at lower levels (Fig. 6e). This indicated that the cleaning of the air was a result of precipitation quickly washing out particles of pollution. The anomalies in relative humidity weakened on the following day (Fig. 6i). The distribution of the pressure anomaly featured a northwest high–southeast low pattern on the second clean day. This pattern was typical of clean days in summer (You et al., 2017). The composite anomalies were generally weak in relation to the small number of events.
Figure 6. As in Fig. 5, but in summer.
Pronounced changes were observed in temperature, pressure, and wind patterns corresponding to the non-persistent pollution events in fall. On the pollution day, Beijing and the surrounding regions were under the influence of higher temperatures (Fig. 7a) and lower pressures and the anomalous convergence of winds (Fig. 7d), which was favorable for the accumulation of pollutants. There were positive anomalies in relative humidity around Beijing (Fig. 7g). On the following clean days, Beijing was characterized by lower temperatures (Figs. 7b, c), higher pressure, and anomalous northerly winds (Figs. 7e, f). Such changes were associated with the quick southeastward movement of negative temperature anomalies and an anomalous anticyclone from southwest of Lake Baikal. The change in temperature may be explained by anomalous northerly winds carrying colder air from higher latitudes. However, the anomalies in relative humidity decreased in magnitude (Figs. 7h, i). The changes in wind after the rapid movement of the synoptic patterns appeared to be a major reason for the dispersion of pollutants.
Figure 7. As in Fig. 5, but in fall.
The changes in the meteorological patterns corresponding to non-persistent pollution events in winter were mostly similar to those in the fall. A switch in the temperature anomalies from positive to negative (Figs. 8a–c) and the replacement of lower pressure and the anomalous cyclone by high pressure and anomalous northerly winds (Figs. 8d–f) were observed around Beijing. The relative humidity anomalies were small around Beijing on clean days in winter (Figs. 8g–i). Another difference was that the movement of the synoptic pattern was faster in winter than in the fall.
Figure 8. As in Fig. 5, but in winter.
The switch from polluted to clean air was associated with rapid changes in the meteorological conditions. The temperature anomaly was positive on the polluted day and changed to negative on the clean days. The sea-level pressure was lower on the polluted day, but it became higher on the clean days, except in summer. An anomalous convergence of lower level winds was observed on the polluted day, which favored the accumulation of pollutants. Anomalous northerly winds were observed on clean days, which was beneficial for the dispersion of air pollutants. These large changes in local meteorological variables were associated with the rapid movement of synoptic patterns, which was distinct from the days with persistent pollution. The distinction between persistent and non-persistent pollution events will be discussed further in Section 5.