HONO在大气NO光氧化过程中的形成和农田土壤排放

HONO has been confirmed as one important source for OH radicals in the troposphere, and plays pivotal roles for removing atmospheric pollutants and producing of secondary pollutants. Although several possible sources for atmospheric HONO have been recognized, it is still hard to explain the relatively high daytime HONO concentrations based on these known sources for model simulation, implying unknown daytime sources for atmospheric HONO. According to the knowledge about HONO production via the reaction of H2O with excited NO2 induced by visible light, we rationally speculate that the excited NO2 formed by atmospheric oxidation of NO through several pathways may be an important unknown source of atmospheric HONO. Our preliminary results indeed suggest that HONO can be evidently produced through the oxidation of NO by O3 and NO3 radical and the yield of HONO due to NO oxidation by NO3 radical is about one magnitude greater than that by O3. Therefore, HONO formation though various oxidation processes of NO (including the oxidation of O3, NO3 radicals, and peroxy radicals) in the atmosphere will be extensively investigated by using smog chamber technique and the contribution of these unknown sources to atmospheric HONO will be explored. In addition, although agricultural soils have been confirmed as a source for atmospheric HONO, it is still difficult to quantify the contribution of this source based on the current results observed by soil incubation and simulation in laboratories. Therefore, HONO emission fluxes from a typical agricultural field in the North China plain (NCP) will be measured by using twin dynamic chambers, and hence new scientific evidence will be provided for quantifying the regional contribution of HONO emission from the agricultural field in the NCP. The new knowledge about HONO formation in the atmosphere and quantification of HONO emission from the agricultural field will also be helpful to disclose the reasons for the two serious atmospheric environmental problems of haze and photo-oxidants which are currently being faced in China.

HONO 是对流层大气中OH自由基的一种主要来源，在大气污染物去除以及二次污染物形成方面发挥着至关重要作用。尽管人们已经认识到大气中HONO存在多种可能来源，但这些来源仍无法解释白天观测到的高HONO浓度，表明仍存在HONO未认知的重要来源。基于可见光诱发激发态NO2产生HONO的新认识，申请者推测NO在大气中氧化过程产生的激发态NO2可能为大气中HONO一种尚未认知的重要来源，并发现O3和NO3自由基氧化NO有明显HONO的形成。土壤排放HONO的新认识目前仅基于实验室模拟研究，仍需田间实验去量化其贡献。本申请项目拟采用烟雾箱模拟手段，研究NO在大气氧化过程中（包括O3，NO3，过氧自由基）HONO的形成产率；拟采用孪生动态箱系统，开展华北农田HONO的排放通量测定，获得大气HONO来源的新认识及农田土壤对HONO的实际贡献，为认识我国灰霾和大气氧化剂污染问题的成因提供新的科学依据。

本项目针对日间大气HONO存在强未知来源这一重要科学问题，系统开展了华北地区农田ＨＯＮＯ的排放通量观测研究以及苯系物在氮氧化物存在情况下光氧化ＨＯＮＯ的生成研究。为了获得农田ＨＯＮＯ排放通量的可靠观测数据，我们开发了一套孪生动态通量箱技术，巧妙解决了箱体器壁效应对活性ＨＯＮＯ通量测定的严重干扰问题；搭建了多套螺旋管气－液高效捕集大气ＨＯＮＯ的自动采样装置，实现了两个通道以上的大气ＨＯＮＯ同步监测，满足了本项目孪生动态通量箱农田ＨＯＮＯ排放通量以及农田ＨＯＮＯ垂直分布测定的需求。为了揭示苯系物在氮氧化物存在情况下光氧化ＨＯＮＯ的生成机制，我们采用３０立方米大型室内烟雾箱进一步开展了四种苯系物在不同初始反应物浓度条件下ＨＯＮＯ生成情况的研究，并获得了一系列新的认识。项目取得的主要研究结果如下:　1)、华北农田夏玉米施肥期间存在强ＨＯＮＯ排放，并呈现日高、夜低的排放特征，中午排放峰值可高达数百纳克每平方米每秒，较圆满地解释了该区域在夏玉米施肥期间白天ＨＯＮＯ的强未知来源；２）采用流动管土壤模拟研究发现，华北农田ＨＯＮＯ的排放主要归咎于铵态氮的硝化过程以及土壤水分的挥发，并获得了ＨＯＮＯ排放通量与土壤温度和大气相对湿度之间的阿伦尼乌斯定量关系；３）基于ＨＯＮＯ排放通量的定量关系结合区域空气质量模型分析发现，华北农田在夏玉米施肥期间ＨＯＮＯ的排放可促进区域臭氧抬升约１０个ｐｐｂ；4）苯系物相对烷烃、烯烃以及异戊二烯在光氧化过程中呈现显著ＨＯＮＯ生成，每个苯系物降解最大可产生０.７分子的ＨＯＮＯ，并且最大可实现８分子ＮＯ向ＮＯ２的转化，且苯系物降解过程伴随显著ＯＨ自由基增殖过程等。基于实验结果分析，提出了苯系物在降解过程中ＨＯＮＯ产生的新机制，包括增殖ＯＨ自由基与ＮＯ的均相反应以及苯系物降解产生的有机硝酸酯的光解等，这些ＨＯＮＯ产生新机制仍有待于盒子模型模拟进行验证。