氨对二次有机气溶胶光学特性的影响
Ammonia effect on optical properties of secondary organic aerosols
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收稿日期: 2019-11-15
Received: 2019-11-15
作者简介 About authors
王军明(1974—),男,硕士,从事燃煤污染控制研究.orcid.org/0000-0001-8972-8695.E-mail:
开展一系列甲苯/NOx光氧化烟雾箱模拟实验,研究大气环境中氨对二次有机气溶胶(SOA)光学特性的影响. 采用特氟龙膜作为烟雾箱的反应腔材料,实验过程中同时检测颗粒的粒径分布、光学参数和质谱信息. 讨论不同氨体积分数对甲苯光氧化生成SOA的消光截面、复折射率及粒径等参数的变化特征. 研究发现,上述实验中SOA的消光作用均以散射为主;随着粒径的增大,消光截面呈指数增大,复折射率则先增大后减小. 与低湿无氨环境相比,增湿或加氨均导致SOA中元素O和C物质的量之比增大、复折射率增大;高湿加氨条件下,SOA的粒径明显减小、颗粒数密度急剧增大、SOA中元素O和C的物质的量之比、元素N和C的物质的量之比均增大.
关键词:
A series of toluene/NOx photo-oxidation experiments were carried out in a smog chamber, and the effect of ammonia on the optical properties of secondary organic aerosols (SOA) evolved by toluene was investigated. The chamber was made of Teflon. The distribution of SOA particle size, extinction efficiency, and mass spectrometric patterns were measured simultaneously during the experimental process. The variation of SOA parameters was discussed, including extinction section, refractive index, and particle size by changing ammonia volume fraction. Results show that the extinction of SOA was dominated by scattering. With the increase of particle size, the extinction cross section exponentially increases, the refractive index firstly increases and then decreases. Compared with low humidity ammonia free environment, the increase of molar ratios of O to C and refractive index of SOA was found either increasing humidity or with ammonia. Under high humidity with ammonia, there is obvious decrease of particle size, sharp increase of number density of particles, and increase both molar ratios of O to C and N to C.
Keywords:
本文引用格式
王军明, 赵兴亚, 陈玲红, 韩黎霞, 高翔, 岑可法.
WANG Jun-ming, ZHAO Xing-ya, CHEN Ling-hong, HAN Li-xia, GAO Xiang, CEN Ke-fa.
本文以大气挥发性有机物(VOCs)典型成分甲苯为例,研究甲苯光氧化生成SOA的反应过程中氨对SOA光学特性的影响,重点研究不同湿度环境中氨对甲苯氧化生成的SOA的粒径、颗粒数密度、组分和复折射率的影响.
1. 实验部分
1.1. 烟雾箱实验装置
甲苯的光氧化实验在CAPS-ZJU烟雾箱[19]中进行. 烟雾箱反应腔由高透光性的特氟龙膜材料制成,容积为2.88 m3,长×宽×高为2.0 m×1.2 m×1.2 m. 反应腔体放置在一个由木板制成的外罩中,外罩内部两侧安装有黑光灯(GE F40BLB),用于提供紫外光照. 实验过程中黑光灯的光照强度用NO2的光解速率常数进行表征,J=0.38 min−1. 外罩放置在一个由空调控温的房间中,实验开始前腔体内温度为20 °C左右,黑光灯打开后反应腔体温度逐渐上升,2 h后稳定在37 °C左右. 实验用甲苯、NO、NO2和氨等标准气体体积分数均为1×10−4,平衡气均为N2,储存在钢瓶中. 将零空气通入盛装超纯水的容器,采用鼓泡加湿的方式控制反应腔内的相对湿度.
烟雾箱反应腔内的背景空气来自一套零空气发生装置(Acadco 737 series,USA). 反应过程中采用气体分析仪(Thermo 42i和49i,Thermo Scientific,USA)分别在线测量NOx和O3的体积分数;甲苯体积分数由一套预浓缩仪(Entech. 7100A)耦合气相色谱-质谱联用仪(GC-MS: Agilent model 7890B GC and Agilent model 5977A mass selective detector,USA)测得. 采用扫描电迁移率粒径谱仪(SPMS,由CPC 3776和DMA 3080组成,TSI,USA)测量反应过程中的颗粒物粒径和颗粒数密度. 采用高分辨率飞行时间气溶胶质谱(HR-TOF-AMS,Aerodyne Research Incorporated,USA)测量SOA的化学组分,具体测量设置和数据分析详见文献[4]和[20]. 采用腔衰减相移式单散射反照率监测仪(CAPS-ALB,Shoreline Science Research)检测实验过程中甲苯SOA的消光系数,检测波长为530 nm,该仪器的详细信息及性能详见文献[21].
每次实验开始前使用零空气冲洗反应腔体(≥12 h),确保反应腔内颗粒数密度小于10个/cm3、O3与NOx的体积分数均低于1×10−9. 将标准气体通入烟雾箱反应腔后通入零空气,流速约为60 L/min,持续通气时间为3~5 s,上述过程反复操作5~8次,以脉冲气流的方式将通入的气体混匀,静置约1 h后打开黑光灯进行光氧化实验.
1.2. 复折射率计算方法
对于单分散的球形颗粒物,其消光系数可用下式进行计算:
式中:C为颗粒数密度,σext为消光截面积,D为颗粒直径,Qext为消光效率.
对于具有一定粒径分布的多分散系颗粒,颗粒总表面积Stot=πCDsm2,其中,Dsm为平均粒径,则多分散系颗粒的消光系数可表示为
实验过程中,αext为腔衰减相移式单散射反照率监测仪检测数据,利用扫描电迁移率粒径谱仪测得各个粒径段的C和Dsm. 由式(2)可得不同Dsm对应的Qext. 采用最小二乘法确定复折射率:
式中:N为计算过程中选取的粒径段数,Qext(i)为第i个粒径段的消光效率测量值,Q'extt(i)为根据Mie理论计算不同折射率的消光效率.
2. 实验结果与分析
表 1 甲苯/NOx光氧化实验初始设定值
Tab.1
实验序号 | φNH3/10−9 | φC7H8/10−9 | φNO/10−9 | φNO2/10−9 | RH/% |
ZJU235 | 0 | 120.6 | 62 | 1.3 | ~7 |
ZJU239 | 0 | 146.8 | 63 | 1.4 | ~7 |
ZJU243 | 50 | 147.3 | 62 | 1.9 | ~7 |
ZJU246 | 50 | 154 | 62 | 2.6 | ~7 |
ZJU249 | 0 | 158.2 | 57 | 1.1 | 68 |
ZJU252 | 50 | 184 | 58 | 2.1 | 63 |
ZJU254 | 50 | − | 63 | 1.4 | 65 |
2.1. 粒径与颗粒数密度
分别选取上述4类的大气模拟环境条件下测量甲苯光氧化反应实验过程中SOA的粒径和颗粒数密度,如图1所示,其中t为反应持续时间,黑光灯打开时刻为t=0. 由图可知,实验过程中SOA粒径逐渐增大,大约经5 h后粒径值趋于稳定. 与低湿无氨的条件相比,低湿通氨时SOA粒径明显减小,颗粒数密度增加;高湿无氨条件下,SOA粒径减小,颗粒数密度明显增加,SOA形成所需时间变短;在高湿有氨条件下,SOA粒径最小,颗粒数密度最高.
图 1
图 1 实验过程中SOA平均粒径与颗粒数密度的时域图
Fig.1 Temporal profile of surface mean diameter and number concentration of secondary organic aerosol(SOA)during experimental process
2.2. 不同反应体系中SOA的光学特性
如图2所示为甲苯/NOx光氧化系列实验中测得的消光系数与散射系数. 可以看出,实验过程中的消光系数值与散射系数值相关性接近于1.0,SOA的消光作用均以散射为主.
图 2
图 2 实验过程中SOA消光系数与散射系数的关系
Fig.2 Relationship between efficiencies of extinction and scattering during experimental process
由Mie理论计算可知,粒径小于100 nm的颗粒物消光效率Qext值接近于0,为此,本文主要研究表面平均粒径大于100 nm的微粒. 如图3所示为上述4类实验过程中SOA的消光效率与粒径的关系,根据式(3)计算上述4类大气环境模拟条件甲苯氧化反应实验工况下的复折射率Ri,可以看出,在低湿无氨条件下,甲苯/NOx光氧化体系中SOA的复折射率为1.560,这与文献[7]、[10]的结果一致. 当保持该反应体系的相对湿度不变而仅通入氨时,SOA的复折射率略增大;当提高相对湿度但不通入氨时,SOA的复折射率增大至1.611;当提高相对湿度并通入氨时,SOA的复折射率为1.582. 随着光氧化反应的进行,SOA的组分发生变化:氧、碳元素物质的量之比(RO-C)逐渐增大,氮、碳元素物质的量之比(RN-C)逐渐减小,这些组分的变化可能影响SOA的复折射率[26].
图 3
图 3 实验过程中SOA的消光效率与粒径的关系
Fig.3 Relationship between extinction efficiency of SOA and particle diameter during experimental process
为了更准确地描述实验过程中SOA复折射率随时间的变化,对每一个数据点求其对应的复折射率,如图4所示. 不同实验体系下SOA复折射率的变化趋势有所不同:在低湿无氨条件下,SOA的复折射率随着粒径的增加基本保持不变;在低湿通氨条件下,SOA的复折射率随着粒径的增大而减小;当高湿无氨或高湿通氨时,随着的粒径增大,SOA的复折射率则呈增大的趋势.
图 4
图 4 不同实验条件下SOA的复折射率与粒径的关系
Fig.4 Relationship of refractive index of SOA and mean diameter under different experimental conditions
2.3. SOA组分比与复折射率的关系
如图5所示为上述4类甲苯/NOx光氧化反应体系中SOA的质谱分析图. 图中,f为有机组分离子碎片信号强度占SOA离子碎片总信号强度的比例. 从图中可以看出,不同反应体系下SOA的质谱信号出峰位置基本相似,信号强度最高的有机碎片主要集中在离子质荷比值m/z=28,29,43,44处. 其中,m/z=28和44对应的有机碎片主要为CO+和CO2+,含羧基官能团的物质在气溶胶质谱仪(AMS)中电离时主要产生这2种离子碎片[29];m/z=29对应的有机碎片主要为CHO+,含羰基官能团的物质在AMS中电离时主要产生这种碎片[30];m/z=43对应的有机碎片主要来自酮类和醛类物质[31]. 由此可见,甲苯的光氧化产物SOA主要组分为有机酸、酮和醛,在高温通氨条件下,SOA的组分发生变化,导致其消光特性随之发生改变.
图 5
图 5 SOA有机碎片离子组分占比与质荷比的关系
Fig.5 Relationship between proportion of ions in organic debris of SOA and ratio of mass to charge
不同实验条件下SOA的复折射率Ri随元素物质的量之比(RO-C、RN-C)的变化趋势如图6所示. 从图中可知,与低湿无氨的环境条件相比,低湿通氨时SOA中元素O和C物质的量之比(RO-C)、元素N和C物质的量之比(RN-C)、复折射率(Ri)等值均增大;与高湿无氨条件相比,高湿通氨时RO-C值变化不大,RN-C值则显著增大,Ri值减小,这说明在甲苯/NOx光氧化体系中通入氨或提高相对湿度均促进SOA的老化,引起SOA组分变化,从而改变SOA的光学特性. 此外,氨参与SOA生成反应,导致SOA中的RN-C值增大,在不同湿度条件下SOA的生成途径发生变化,导致SOA的组分存在差别.
图 6
图 6 SOA的复折射率分别与其中元素O和C的物质的量之比、N和C的物质的量之比的关系
Fig.6 Relationship of refractive index of organic aerosol and its molar ratios of O to C and N to C
3. 结 论
(1)在甲苯/NOx光氧化体系中通入氨或增加湿度均导致SOA的平均粒径减小、颗粒数密度增加. 此外,SOA生成所需时间随着湿度的增加而减小.
(2)与低湿无氨的环境条件相比,在低湿加氨以及高湿无氨条件下,甲苯/NOx光氧化体系中新生成的SOA的复折射率值均有所增大.
(3)甲苯/NOx光氧化反应过程中SOA的主要成分为有机酸、酮和醛,这些成分具有很强的消光作用. 通入氨或提高相对湿度后,SOA的平均粒径减小、颗粒数密度增加、颗粒物中元素O和C物质的量之比、复折射率增大. 在通氨的高湿环境中,SOA的复折射率变化不大,但SOA的粒径明显减小、颗粒数密度急剧增大、元素O和C物质的量之比以及元素N和C物质的量之比均增大.
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