大气环境科学实验室知识库

      大气中挥发性有机污染物（VOCs ）在光氧化降解过程中可导致臭氧 O 3和二次有机气溶胶 ( 的形成，对区域以及全球大气环境具有显著影响。异戊二烯 ( 是大气中含量最高的自然源 VOC ，其 氧化对 SOA 的贡献已成为当今大气环境化学研究领域关注的热点课题之一，但已有研究所获 SOA 的产率仍存在很大不确定性，仍需进一步研究揭示其根本原因，从而为科学评估异戊二烯的环境影响提供科学依据。为此，本文建立了一套烟雾箱模拟研究平台，并利用该平台对比模拟研究了异戊二烯在不同条件下臭氧化反应以及光氧化反应生成SOA 的产率，取得了以下研究成果：
（1) 阐明了异戊二烯臭氧化生成 SOA 的产率与初始反应物浓度之间的关系：在控制初始反应速率相同的条件下，反应体系中异戊二烯浓度大大过量时 SOA的产率远高于对应的 O 3 大大过量的反应体系；
（2) 证实了异戊二烯臭氧化 SOA 的形成主要源自稳定 Criegee 中间体的反应：在相同异戊二烯和 O 3 初始反应浓度条件下，反应体系中引入 Criegee 清除剂H 2 O 和 CO ）可导致 SOA 产率由 2.96% 分别降低到 1.47% 和 2.08% 。虽然 SO2对 Criegee 具有明显抑制作用，但 Criegee 中间体对 SO 2 的氧化可产生硫酸盐和有机硫酸酯，从而导致颗粒物产率由 2.96 上升到 57.6%
（3) 发现稳定 Criegee 中间体很容易光解，导致异戊二烯臭氧化的 SOA 产率显著下降：对比暗反应条件，异戊二烯臭氧化所有反应体系在 365nm 光照条件下 SOA 产率下降比例范围为 27% 50% 。对暗反应和 365nm 光照下异戊二烯臭氧化生成的 SOA 成分进行对比分析，发现在光照条件下消失的物种具有低 O:C比、低 H:C 比、高不饱和度以及较大的分子量（大于 800 ），推测稳定不饱和 C 4Criegee 中间体自身的聚合在 SOA 的形成中起 着重要作用；
（4) 发现异戊二烯光氧化生成 SOA 的产率与 OH 自由基的浓度密切相关：反应体系随着 OH 自由基浓度增加可检出 SOA 所需要消耗异戊二烯的量越少，SOA 产率越大。因此，提出了异戊二烯光氧化反应产生 SOA 存在一种竞争机制，该机制可合理解释 SOA 产率对 OH 自由基浓度的依赖关系，即：异戊二烯与 OH自由基反应首先产生过氧化物，而过氧化物可进一步与OH 自由基反应产生 SOA
同时过氧化物在紫外辐射下又可发生光解，从而导致 SOA 产率降低。以往研究由于所采用反应体系中 OH 自由基的浓度较低，有可能极大低估了中午时分异 戊二烯光氧化生成 SOA 的产率。
       此外，本文还对比研究了丙烯和三甲苯等人为源VOCs 的光氧化体系，发现三甲苯和丙烯两个光氧化体系在相同等效丙烯浓度和 NO x 浓度条件下以及相同反应时间内，三甲苯比丙烯的消耗量高约 50%50%，且三甲苯光氧化体系具有明显HONO 生成现象，提出大气中三甲苯光氧化可能为大气 HONO 的一种新的来源。

      The oxidation of Volatile Organic Compounds (VOCs) could lead to the formation of Secondary Organic Aerosol (SOA) and O 3 , which had significant impact on regional and global atmospheric environment. Isoprene is the most abundant natural source VOC, and its contribution to atmospheric SOA has become a hot topic in Atmospheric chemistry area. Up to now, the SOA yields obtained still had many uncertainties, which needs more resea rches to reveal the root cause, and then to provide scientific basis for evaluating the environmental effects of isoprene. In order to do the work, a smog chamber similation platform was constructed, and based on this platform, SOA formation from isoprene ozonolysis and isoprene photooxidation was investigated under different experimental conditions. The main points are as
following:
      (1) The relationship between SOA formation from isoprene ozonolysis and the initial reactant concentrations was illuminated: the initial reaction rates between the pair reaction systems (one with excessive isoprene, the other with excessive O 3 ) were approximately the same, the SOA yield of 0% for 1ppm O 3  71 ppb isoprene and of 1.05% for 1 ppm O 3  329 ppb isoprene decreased 10 0% and 64.5% with respect to the corresponding reaction systems with excessive isoprene, respectively 
        (2) Stable Criegee Intermediates (SCIs) played key roles in SOA formation isoprene ozonolysis: with almost the same experimental conditions of isoprene a nd O 3 concentrations, SOA yields decreased from 2.96% to 1.47% and 2.08% with presence of H 2 O and CO (SCIs scavengers), respectively; although SO 2 could greatly suppress SCI, its oxidation by SCIs could generate large amount of organic and inorganic sulfat es, which the particle yield remarkably increased from 2.96to 57.6%.
        (3) SCIs were found to be easily photolyzed under 365nm irradiation, which made the SOA yields greatly decreased: SOA yields deceased for 27% to 50% in all the reaction systems under ligh t irradiation with respect to those of dark condition; Base on the analysis of SOA composition both under light and dark conditions, it was found that many species disappeared under light irradiation and the disappeared species had the following characteri stics: relatively low O:C ratio and H:C ratio; high Double Bond Equivalency (DBE) values and large molecule weight ( 500), and thus it is speculated that the polymerization of C 4 Criegee intermediates played important role in SOA formation.
       (4) SOA formati on from isoprene photooxidation was found to be strongly related to the OH concentrations in the reaction system: the higher the OH concentration was, the earlier the detectable SOA mass concentration and the higher peak SOA mass concentration appeared. SO A yield was also found to increase with increasing OH concentration for a given aerosol loading (M 0 ) at atmospherically relevant conditions. And thus, a competitive mechanism was proposed in SOA formation from isoprene photooxidation, which could well expl ain the dependence of SOA formation on OH concentration. The mechanism was as following: the reaction between isoprene and OH firstly produced hydroperoxides. The hydroperoxides could either further react with OH to produce SOA or be photolyzed to produce gas products. Thus, when OH concentrations increased in the reaction system, more hydroperoxides would react with OH to produce SOA. Because OH concentrations used in the previous studies were less than that in the summer noon time, SOA yields might be remarkably underestimated in the noon time.
      Besides, the photooxidation reaction systems of trimethylbenzene (TMB) and propene were also investigated with presence of NO x . Although the concentrations of TMB and propene were propylene equivalent concentrations and the NO xconcentration were the same in the two photochemical reaction systems, the consumption percentage of TMB was 50% higher than propene consumption percentage. Furthermore, HONO formation was obviously observed in the TMB photochemical reaction s ystem. Thus, it is speculated that TMB photooxidation might  be an important HONO source in the atmosphere.