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近年来,PM2.5对我国城市空气质量有着严重的影响[1-3]。碳组分是PM2.5的关键组分,主要包含有机碳(organiccarbon,OC)和元素碳(Elementalcarbon, EC)[4]。OC包括直接排放的一次有机碳(Primaryorganiccarbon, POC)和经光化学反应生成的二次有机碳(Secondaryorganiccarbon, SOC)[5];EC主要通过各种燃料的不完全燃烧产生[6]。研究表明,OC具有“三致”风险[7],并且对光具有散射作用[8];而EC的吸附活性较强,易成为活性中心和化学反应床,从而威胁人体健康[9],同时EC还具有很强的吸光性[10]。综上所述,碳组分对大气环境及人体健康的危害不容忽视。
天津市是“2+26”城市之一,同时是人口过千万的发达城市。公报数据显示,2018年天津大气PM2.5浓度年均值达到52 μg·m−3,超过国家标准(35 μg·m−3)的0.49倍,大气污染较为严重。同时天津市是北方工业中心之一,化石燃料的燃烧导致大量碳气溶胶排放到大气中,严重影响京津冀地区环境。因此有关PM2.5中碳组分污染己成为该区域研究热点[11-14],但相关研究大多仅关注其季节变化特征,关于天津市城郊区PM2.5中碳组分的研究较少,霍静等[14]研究了天津市区和东丽区PM2.5中的碳组分,徐伟等[15]针对天津市武清区PM2.5中的碳组分进行了研究。在兰州[16]、北京[17]、西安[18]等地的研究中,颗粒物中OC浓度均表现为白天高于夜间;EC浓度的昼夜差异在各城市间有所不同,兰州、西安EC浓度表现为白天低于夜间,主要是由于当地夜间大气层结稳定,扩散条件较差;而北京EC浓度表现为昼夜相近,主要与当地昼夜间机动车贡献不同有关。可见,受多种因素影响,大气中碳组分浓度表现出昼夜变化规律具有地区差异性。
为研究天津市PM2.5中碳组分的昼夜变化特征及影响因素,本研究于2019年7—8月对天津市夏季大气PM2.5进行采样,针对PM2.5中碳组分浓度的时空变化特征及其来源进行分析,旨在为天津市制定相关防治政策提供参考。
天津市夏季PM2.5中碳组分时空变化特征及来源解析
Spatial and temporal variation characteristics and source apportionment of carbonaceous components in PM2.5 during summer of Tianjin
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摘要: 为研究天津市夏季PM2.5中碳组分的时空变化特征及来源,于2019年7—8月设立2个点位分昼夜采集天津市PM2.5样品,并测定了其中有机碳(OC)和元素碳(EC)的含量。结果表明,城区PM2.5、OC和EC浓度日均值分别为(53.4±20.8) μg·m−3、(8.72±2.56) μg·m−3和(1.67±0.90) μg·m−3,郊区PM2.5、OC和EC浓度日均值分别为(54.2±24.5) μg·m−3、(7.54±2.50) μg·m−3和(1.82±1.06) μg·m−3;白天PM2.5、OC、EC的平均浓度分别为(47.3±16.1) μg·m−3、(8.7±2.1) μg·m−3和(1.5±0.6) μg·m−3,夜间PM2.5、OC、EC的平均浓度分别为(60.2±26.2) μg·m−3、(7.5±2.9) μg·m−3和(2.0±1.2) μg·m−3。OC浓度表现为城区高于郊区,白天高于夜间;EC及PM2.5浓度表现为郊区高于城区,夜间高于白天。OC/EC比值分析得,城区(6.04)高于郊区(5.08);白天(6.58)高于夜间(4.54)。城区OC与EC相关性弱于郊区,白天OC与EC相关性弱于夜间。采用EC示踪法与MRS模型对SOC含量进行估算,得到白天与夜间SOC浓度分别为(5.71±1.35) μg·m−3和(3.81±1.20) μg·m−3,白天SOC污染比夜间严重。丰度分析与主成分分析的结果表明,天津市夏季城郊区PM2.5中碳组分均主要来源于燃煤和机动车尾气排放。Abstract: In order to study the characteristics and sources of carbonaceous components in PM2.5 in summer in Tianjin, two sites were set up in Tianjin from July to August 2019 to collect PM2.5 samples during day and night, and the contents of organic carbon (OC) and elemental carbon (EC) were measured. The results showed that the daily mean values of PM2.5, OC and EC concentrations in urban areas were (53.4±20.8) μg·m−3, (8.72±2.56) μg·m−3, and (1.67±0.90) μg·m−3, and those in the suburbs were (54.2±24.5) μg·m−3, (7.54±2.50) μg·m−3 and (1.82±1.06) μg·m−3. The concentration of OC was higher in urban areas than in suburb, and higher in daytime than at night. The concentration of EC and PM2.5was higher in suburb than in urban and higher in night than in day. OC/EC value was higher in urban area(6.04) than in suburb areas(5.08), and that was higher in day(6.58) than in night(4.54). The correlation between OC and EC in urban areas was weaker than that in suburbs, and that between OC and EC in daytime was weaker than that at night. The EC tracer method and MRS model were used to estimate the SOC content, and the SOC concentration were (5.71±1.35) μg·m−3 during the day and (3.81±1.20) μg·m−3 during the night, indicating that the SOC pollution was more serious during the day than at night. The results of abundance analysis and PCA(principal component analysis) showed that the carbonaceous components in PM2.5 in summer in urban and suburban of Tianjin mainly came from coal combustion and vehicle exhaust.
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Key words:
- PM2.5 /
- carbonaceous component /
- diurnal variation /
- secondary organic carbon (SOC) /
- source apportionment /
- Tianjin
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表 1 采样期间气象参数
Table 1. Meteorological parameters during sampling
站点
Site采样时段
Sampling period温度/℃
Temperature相对湿度/%
Relative humidity风速/mph
Wind speed市区 白天 30.78 55.47 2.65 夜间 26.36 70.44 2.28 津南 白天 31.03 51.53 2.68 夜间 27.19 65.94 2.39 表 2 昼夜间碳组分两独立样本非参数检验结果
Table 2. The results of nonparametric tests for independent samples
OC1 OC2 OC3 OC4 EC1 EC2 EC3 OPC OC EC P值 0.421 0.208 0.027 0.040 0.821 0.463 0.153 0.087 0.076 0.248 表 3 与其他城市PM2.5中碳组分的比较
Table 3. Comparison of the carbon component in PM2.5 with other cities
表 4 主成分分析结果
Table 4. The results of principal component analysis
组分
Component市区Urban 郊区Suburb 因子1 Factor 1 因子2 Factor 2 因子1 Factor 1 因子2 Factor 2 OC1 0.654 0.577 0.793 0.129 OC2 0.860 0.413 0.883 0.313 OC3 0.906 0.030 0.941 0.093 OC4 0.967 0.114 0.908 0.175 EC1 0.900 −0.033 0.705 0.608 EC2 −0.011 0.943 −.567 0.004 EC3 −0.285 0.193 0.010 0.980 OPC 0.845 0.022 0.823 0.063 解释方差/% 58.989 15.651 62.121 14.156 特征值 4.719 1.252 4.970 1.133 -
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