Fig. 1. Location of the study area and distribution of the stations in Xi'an
Fig. 2. Comparison of AQI averages in Xi'an during the corresponding study periods in different years
Fig. 3. Comparison of AQI averages for the two outbreak control periods in 2020 (a) and 2022 (b) compared to the same time period in other years
Fig. 4. Comparison of the number of days with different air quality level in Xi'an during the strict pandemic control period of 2020 and 2022
Fig. 5. Comparison of pollutant concentration distribution in Xi'an at different stages from 2019 to 2022
Fig. 6. Comparison of main pollutant concentrations in Xi'an during different pandemic prevention stages from 2019 to 2022
Fig. 7. Radar plot of pollutant characteristics during strict epidemic control period in Xi'an in 2020 (a) and 2022 (b)
Fig. 8. Comparison of diurnal variations of AQI, precipitation and average temperature (a) with wind speed and relative humidity (b) during the study period
Fig. 9. Rose chart of wind direction during the study period during the research period in Xi'an
Fig. 10. Cluster analysis of backward trajectory at 500 m height during strict epidemic control period in Xi'an in February 2020 (a) and January 2022 (b)
Fig. 11. Topographic map of Xi'an City
Fig. 12. Comparison of inner city travel intensity during Spring Festival in Xi'an during the study period
时期 | 2019—2020 | 2020—2021 | 2021—2022 | 疫情严控期 |
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12月 | 1月 | 2月 | 12月 | 1月 | 2月 | 12月 | 1月 | 2月 | 2020年 | 2022年 |
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AQI均值 | 132.80 | 166.87 | 106.20 | 110.68 | 123.87 | 108.64 | 96.68 | 146.26 | 109.61 | 114.31 | 137.21 | AQI峰值 | 323 | 275 | 183 | 201 | 275 | 328 | 188 | 230 | 179 | 275 | 252 | 优良天数/% | 45.16 | 19.35 | 51.72 | 35.48 | 41.94 | 67.86 | 64.52 | 19.35 | 39.29 | 54.28 | 33.33 | 重度污染及以上天数/% | 16.13 | 32.26 | 0 | 3.23 | 9.68 | 10.71 | 0 | 16.13 | 0 | 2.86 | 15.15 |
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Table 1. Comparison of air quality in different periods of Xi'an
评价指标 | 三门峡 | 洛阳 | 运城 | 渭南 |
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AQI均值 | 132.10 | 144.65 | 135.58 | 141.58 | AQI峰值 | 229 | 244 | 208 | 228 | 优良天数/% | 7 | 4 | 8 | 5 | 重度污染及以上天数/% | 2 | 3 | 1 | 3 |
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Table 2. Air quality in eastern cities of the Fenhe and Weihe plains in January 2022
时期 | 轨迹类型 | 出现/% | 途径区域 | PM2.5/(μg·m-3) |
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2020-02 | 1 | 15 | 西安市 | 97.03 | 2 | 39 | 咸阳市 | 49.80 | 3 | 20 | 西安, 宁夏东部, 甘肃东部 | 31.41 | 4 | 20 | 河南西部 | 69.75 | 5 | 7 | 内蒙古西部 | 14.53 | 2022-01 | 1 | 12 | 甘肃北部, 内蒙古西部, 宁夏 | 65.80 | 2 | 17 | 甘肃东部 | 88.65 | 3 | 69 | 西安市 | 124.07 | 4 | 2 | 甘肃, 内蒙古西部, 宁夏 | 35.85 |
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Table 3. Number of cluster tracks and corresponding PM2.5 concentration