青岛不同强度霾天气溶胶中二次无机离子的生成及粒径分布

Formation and Size Distribution of the Secondary Aerosol Inorganic Ions in Different Intensity of Haze in Qingdao,China

于2015年9月至2016年2月在青岛近海连续收集了大气气溶胶分级样品,用离子色谱法分析了其中的水溶性无机离子组分,并讨论了不同强度霾天下气溶胶中二次无机组分的粒径分布,初步探索了霾天SNA的形成过程和影响因素.结果表明,气溶胶中NO_3^-、SO_4^(2-)、NH_4^+、NO_2^-和Cl^-的质量浓度变化范围分别是10.32~193.46、4.42~74.05、2.21~57.75、0.05~2.22和1.35~17.39μg·m^(-3),且SNA的质量浓度随霾污染程度的加剧明显增加.与非霾天相比,轻微、轻度、中度和重度霾天与非霾天相比,NO_3^-的质量浓度分别增加了55%、77%、240%和537%;SO_4^(2-)的质量浓度分别增加了4.7%、35%、77%和262%;NH_4^+的质量浓度分别增加了72%、83%、201%和526%.细粒径上的NO_3^-、SO_4^(2-)与其气态前体物NO_2、SO_2均有显著相关性,且与相对湿度、能见度、风速等气象条件相关性较好,说明细粒径SNA的生成是造成霾天能见度下降,形成空气污染的主要原因之一,同时,高浓度前体物、较大相对湿度、低风速都是影响霾天形成的重要因素.除轻微霾天外,其他不同强度霾天的SOR(硫氧化率)、NOR(氮氧化率)均大于非霾天,且随着霾程度的加剧,SOR、NOR都有明显的升高,尤其是0.43~0.65μm和0.65~1.1μm粒径段;在重度霾天,氮和硫的转化率平均为非霾天的1.5倍,说明细粒径上的硫酸盐和硝酸盐大部分是气-粒转化而来.NO_3^-、NH_4^+、NO_2^-和SO_4^(2-)主要存在于细粒径段,霾天下在细粒径上的比例都显著增大,NO_3^-和SO_4^(2-)在严重霾天所占比例最高,分别达到79.4%和74.4%.NO_3^-在非霾、轻微、轻度霾天时均呈双峰分布,峰值出现在0.43~0.65μm和3.3~4.7μm处,中度霾天时细粒子峰值移动到0.65~1.1μm,在重度霾天粒径分布变为0.65~1.1μm的单峰分布.SO_4^(2-)只在非霾条件下呈双峰分布,峰值出现在0.43~0.65μm和2.1~3.3μm粒径段,霾天下均是单峰分布,轻微和轻度霾天下峰值出现在0.43~0.65μm,中度和重度霾天下峰值在0.65~1.1μm处.NH_4^+呈单峰分布,在非霾和轻微霾天下峰值出现在0.43~0.65μm粒径段,轻度、中度和重度霾天下峰值均出现在0.65~1.1μm粒径段.

Size-segregated atmospheric aerosol samples were collected from September 2015 to February 2016 at a coastal site in Qingdao, and the concentrations of major water-soluble inorganic ions were analyzed by ion chromatography. Characteristics and variation of size distribution of secondary inorganic components in aerosol were discussed, as well as the formation process and influencing factors of SNA（ sulfate, nitrate and ammonium）. The results indicated that the concentrations of NO3^-, SO4^2-, NH4^＋, NO2^- , Cl^- in the aerosols were in the range of 10.32-193.46, 4.42-74.05, 2.21-57.75, 0.05-2.22 and 1.35-17.39 μg·m^-3 respectively. And the mass concentration of SNA increased with the intensity of haze pollution. The concentrations of NO3- on the slight, mild, moderate and severe haze days were 55% , 77% , 240% and 537% higher than that on non-haze days respectively, while concentrations of SO]- increased by 4.7% , 35% , 77% and 262% respectively, and concentrations of NHf increased by 72% , 83% , 201% and 526% respectively. The contribution of these water-soluble ions to PM25 showed that the proportion of sum of NO3- , SO24- , NH4＋ , NO2- and C1 - to PM25 was in range of 62.03% -80. 93%. The proportion of ion to PM25 decreased in the order of NO3 〉 SO4^2- 〉 NH4^＋ 〉 Cl^- 〉 NO^2-. With the enhancement of haze pollution, the proportion of NO3^- in PM25 increased from 29.53% to 45.54%. The correlation analysis showed that NO3^- and SO4^2- in the fine particle were significantly correlated with gaseous precursors NO2 and SO2, and also showed good correlations with relative humidity, visibility, wind speed and other weather conditions. These results indicated that the formation of SNA in fine particles was one of the main reasons for visibility decrease and the formation of air pollution in haze days. Meanwhile, high concentration of gaseous precursors, high relative humidity and low wind speed were the important influencing factors of haze formation. Except for slight haze days, SOR and NOR in the haze days were higher than those on the non-haze days, and increased significantly with the intensifying of haze, especially for 0.43-0. 65 and 0.65-1. 1 μm particle size. Conversion rates of nitrogen and sulfur in severe haze days were 1.5 times that in non-haze days, which showed nitrate and sulfate in these fine mode were mainly from gas-to-particle conversion. NO3^- , SO4^2- , NH4^＋ and NO2- increased in haze significantly, which mainly existed in the fine particles. The cloud process played a more important role on haze days. While on non-haze day, cloud process and the heterogeneous reaction were both the main factors. And the highest proportion of fine mode concentration to total one was observed for NO3^- （79.4%） and SO4^2- （74.4%） on severe haze days respectively. NO3^- showed a bimodal distribution with peaks in the size-bin of 0.43 -0.65 μm and 3.3-4. 7 μm on non-haze, slight, mild haze days, and the fine peak moved to 0.65-1. 1μm on moderate haze days, however the bimodal distribution changed to unimodal distribution with peak at 0.65-1.1 μm on severe hazy days. SO4^2- showed a bimodal distribution with peaks at 0.43-0.65 μm and 2. 1-3.3 μm in the non-haze weather, while the size distribution changed to unimodal distribution on hazy days. But the peak sizes were different in different intensity of haze, with peak at 0.43-0. 65 μm on mild and slight haze and 0.65-1. 1 μm in moderate and severe haze days. NH4 showed a single peak distribution in the fine mode, with the peak in the particle size of 0.43-0.65 μm on slight and non-hazy days, and 0. 65-1. 1 μm on mild, moderate and severe hazy days. Therefore, haze has a great influence on the size distribution of SNA.