保定市2013~2019年秋冬季污染物浓度变化特征

为深入了解保定市空气质量状况，揭示保定市空气污染变化趋势、多尺度变化特征和突变特性，综合利用Morlet小波分析和Mann-Kendall非参数检验方法，对保定市2013~2019年秋冬季PM₁₀、PM_2.5、SO₂、NO₂、CO和O₃-8h 6种常规大气污染物年均浓度和秋冬季污染特征逐日数据进行分析.结果表明：除O₃-8h外保定市其它各污染物年均浓度逐年下降，全年重度污染天数占全年天数的比例从31%下降到6.6%，整体呈好转趋势，但2013~2019年秋冬季重度污染天数占全年重度污染天数的81%~97%，秋冬季污染依然需要重视；小波分析结果显示，保定市秋冬季各污染物浓度存在显著的周期性变化，周期震荡主要在20d准双周、50~90d左右的季节内震荡和90~110d的季节震荡3个时间尺度范围，污染物浓度存在的低频震荡与大气中存在的低频震荡密切相关；历年各污染物污染最严重的月份多集中在12月、1月和2月，主要与污染源排放强度和相对静稳的大气条件有关；各污染物污染序列突变点多集中于10月和3月；2019~2020年秋冬季NO₂呈极显著的下降趋势，且突变点较往年提前1个月，这与疫情期间车辆排放大幅降低有直接关系；SO₂和CO在2013~2014年和2015~2016年秋冬季的突变点时间相近，这可能与冬季居民取暖散煤的不完全燃烧有关，2015年后保定实施了煤改气、煤改电，劣质散煤专项治理等措施后，2种污染物突变点时间存在差异，说明清洁取暖措施对降低SO₂浓度，改善空气质量的效果明显.

Variation characteristics of pollutant concentration in autumn and winter from 2013 to 2019 in Baoding City

In order to deeply understand the air quality of Baoding, Morlet wavelet analysis and Mann-Kendall non-parametric test were utilized for revealling the variation trends, multi-scale changes and mutation characteristics of six conventional air pollutions (PM₁₀, PM_2.5, SO₂, NO₂, CO and O₃-8h) annual average concentration and pollution characteristics. Except for O₃-8h, the annual average concentrations of pollutants decreased year by year, and the proportion of severe pollution days in the whole year decreased from 31% to 6.6%, showing an overall improvement trend in Baoding. However, the severe pollution days in autumn and winter of 2013~2019 accounted for 81%~97%, and the air pollution in autumn and winter still needs to be paid great attention. The wavelet analysis revealed the obviously periodic variations of pollutant concentrations during autumn and winter time in Baoding City. The periodic oscillations appeared at 20d (quasi-biweekly), 50~90d (intra-seasonal oscillations), and 90~110d (seasonal oscillations). In this time scale, the low-frequency oscillations for the pollutant concentrations were closely related to the low-frequency oscillations of atmospheric condition. The most severe pollution often occurred in December, January and February, which were mainly related to the source emission intensity and relatively stable atmospheric conditions. The mutation points of each pollutant pollution sequence were concentrated in October and March. In the autumn and winter of 2019~2020, NO₂ showed the obvious decreasing trend, and the mutation point was one month earlier than previous years, which was directly related to the significant reduction in vehicle emissions during the epidemic, while the times of mutation point for SO₂ and CO in the autumn and winter of 2013~2014 and 2015~2016 were similar, this may be related to the incomplete combustion of scattered coal for residential heating in winter. Since the implementation of control measures in Baoding in 2015, such as coal-to-gas, coal-to-electricity, and special treatment of low-quality scattered coal, the difference in the time of the mutation point between the two air pollutions indicated that clean heating measures had obvious effects on reducing the concentration of SO₂ and improving air quality.