我国城市大气PM_(2.5)与O_(3)浓度相关性的时空特征分析

The temporal and spatial distribution of the correlation between PM_(2.5) and O_(3) contractions in the urban atmosphere of China

近些年,我国大气PM_(2.5)的质量浓度显著降低,O_(3)污染加剧.为了厘清PM_(2.5)和O_(3)浓度相关性的时空差异,本研究分析了我国城市大气PM_(2.5)浓度与O_(3)日最大8 h滑动平均值(MDA8 O_(3))和大气总氧化剂(O_(x)=O_(3)+NO_(2))浓度之间的相关性.2015年以来,PM_(2.5)与O_(3)双超标天数大幅下降,现阶段双超标情况主要发生在京津冀地区的4~5月.在40°N以南的地区,PM_(2.5)与O_(3)浓度的相关性呈现显著的“南高北低、夏高冬低”的时空分布规律.较强的正相关关系出现在中纬度京津冀地区的夏季以及低纬度的珠江三角洲地区,表明这些区域O_(3)和PM_(2.5)的浓度具有相同的变化趋势;而在京津冀地区的冬季,由于PM_(2.5)中一次组分占比增高以及较弱的O_(3)光化学生成,O_(3)和PM_(2.5)浓度具有相反的变化趋势.不同于PM_(2.5)与MDA8 O_(3)之间的关系,PM_(2.5)与O_(x)在不同季节均表现出较好的正相关性,表明我国大气的强氧化性驱动了PM_(2.5)中二次组分的生成.在O_(3)超标的情况下(MDA8 O_(3)>160μg/m^(3)),PM_(2.5)和O_(3)浓度的相关性与PM_(2.5)浓度有关,当PM_(2.5)浓度低于50μg/m^(3)时,O_(3)与PM_(2.5)浓度具有较好的正相关性,呈现同步变化趋势;当PM_(2.5)浓度进一步增加时,二者浓度具有相反的变化趋势,说明更高浓度的PM_(2.5)可能会抑制O_(3)生成.

The mass concentrations of fine particles(PM_(2.5))have decreased significantly in China in recent years,while surface ozone pollution shows an opposite trend.To better understand the combined PM_(2.5) and O_(3) pollution in China’s urban atmosphere,the Spearman correlation between PM_(2.5) mass concentration and daily maximum 8-hour average ozone concentration(MDA8 O_(3))was analyzed in this study,as well as the correlation between the concentrations of PM_(2.5) and O_(x)(O_(x)=O_(3)+NO_(2)).Since 2015,the number of days when both O_(3) and PM_(2.5) concentrations exceed the national ambient air quality standards has decreased significantly with the decrease in PM_(2.5) concentrations.The pollution combined O_(3) and PM_(2.5) usually occurs in April and May in the Beijing-Tianjin-Hebei(BTH)area.It is worth noting that the correlations between PM_(2.5) and MDA8 O_(3) concentrations depend on regions and seasons on south of 40°N in China.A stronger positive correlation between the concentrations of PM_(2.5) and MDA8 O_(3) in the Pearl River Delta(PRD)area was obtained throughout the year(R>0.6).In the BTH area,this type of relationship occurs only in summer(R~0.5)whereas in winter,a weak negative correlation between PM_(2.5) and MDA8 O_(3) concentrations was observed(R<–0.2).This may be due to a higher primary contribution of PM_(2.5) and a low concentration of O_(3) due to low photochemical production compared to summer in wintertime.For the case of O_(3) concentration exceeds national ambient air quality standards in Beijing and Xuzhou,MDA8 O_(3) and PM_(2.5) concentrations correlate positively when PM_(2.5)≤50μg/m^(3).In contrast,when PM_(2.5)>50μg/m^(3),a weak negative correlation was observed(R~–0.1),suggesting that high concentrations of particulate matter may inhibit O_(3) formation on days when is polluted.However,the mechanism of such phenomenon remains confusing due to the complex relationship between the production of O_(3) and PM_(2.5).Meanwhile,PM_(2.5) and MDA8 O_(3) concentrations show a stronger positive correlation during daytime when O_(3) concentration exceeds the national ambient air quality standards,which is due to the strong photochemical formation of O_(3) as well as secondary aerosols.Furthermore,the positive correlation coefficients between O_(x) and PM_(2.5) are significantly higher than those between PM_(2.5) and MDA8 O_(3) in most cities.This result is consistent with the predominant contribution of secondary aerosol to PM_(2.5) mass concentrations,both in wintertime and in summertime,after the stringent control of primary source emissions.In addition,the chemical concentration of water-soluble inorganic composition in PM_(2.5) samples collected in urban Beijing was analyzed in this work using ion chromatography in 2020.Comparing the correlation coefficients between PM_(2.5) and MDA8 O_(3) concentrations,there are stronger positive correlations in the concentrations between MDA8 O_(3) and the ratio between PM_(2.5) mass concentration and secondary inorganic aerosols(SNA=sulfate+nitrate+ammonium).This result may be due to the fact that a high concentration of O_(3) can promote the formation of secondary aerosols.