大气中N2O 的气相源汇机制研究

Nitrous oxide (N2O) is a potent greenhouse molecule as well as the major source of stratospheric NOy which catalyzes the loss of ozone in the stratosphere. The primary sources of N2O based on current knowledge are mainly emitted from edaphic and oceanic microbial activity as a by-product of nitrification and denitrification reactions, and the major sink is in its stratospheric ultraviolet photolysis and reaction with electronically excited oxygen atoms, O(1D). Even though considerable progress has been made in recent years in the identification of new in-situ sources, the uncertainties associated with individual sources have not been reduced. In addition, the recent discovery of isotope anomalies in atmospheric N2O and the remarkably increase trend of NO2 in the northern hemisphere have also cast doubts on the current understanding of the global N2O budget. Several possible atmospheric N2O sources stemming from O3 have been proposed for explaining such anomalies of atmospheric N2O, however, most of proposed atmospheric formation sources have been ruled out by experimental evidences. Therefore, the production and consumption of N2O in various gas mixtures will be systemically investigated in this study under irradiation of filtered low pressure Hg lamps with different central wavelength.

氧化亚氮（N2O）是大气中重要的温室气体之一，也是参与平流层臭氧损耗的一种关键物质。大气中N2O主要源于土壤和水体，汇于平流层的光解和与激发态氧原子（O(1D)）的反应。但其源汇不平衡以及同位素异常现象等对N2O的现有大气化学认识提出质疑。此外，采用当前N2O 大气化学的已有认识，目前还无法解释北半球NO2的年增长率远远低于N2O的年增长率以及大气化学模型对低平流层活性氮物种（NOy）低估现象。长期以来，由于实验手段的缺乏或方法学不完善，对于N2O的大气化学反应机制以及源汇机制的研究受到很大的制约。为解释以上异常现象，人们提出了多种源自O3的光化学反应机制，但大多机制已被近年来的实验证据所否认。为此，本项目拟利用光化学反应器在不同波长的低压汞灯辐射下系统研究N2O在大气光化学反应体系中的产生与消耗，以期从大气气相反应过程认识大气中N2O的气相源汇机制，并解释其同位素异常现象。

氧化亚氮（N2O）是对流层大气中重要的温室气体之一，也是参与平流层臭氧损耗的一种关键物质。大气中N2O目前被认为主要来自地表微生物的硝化和反硝化过程，其在大气中的消耗主要发生在平流层的光解和与激发态氧原子（O(1D)）的反应。但同位素测定结果表明大气中N2O存在明显重同位素分馏现象，且氧同位素表现为质量无关异常现象，从而对N2O的现有大气化学认识提出质疑。为解释大气中N2O氧同位素质量无关异常现象，人们提出了多种源自O3的光化学反应机制，但大多机制已被实验证据所否认。为此，本文研究了N2O在氮气和氧气光化学反应体系中的产生与消耗，以期探索大气中N2O的光化学源汇机制，并取得了以下主要研究结果：（1）在室温（298 K）条件下氮气和氧气光化学反应体系中由（O(1D) + N2 + M）生成N2O速率比基于已有动力学数据估算值高两个数量级以上。通过温度系列实验研究得出该反应的三级速率常数与温度的阿伦尼乌斯表达为：k(T)=1.06×10-38 exp(3374/T) cm6 molecule-2 s-1；（2）推测激发态氧原子O(1D)与N2分子碰撞结合所形成的高激发态N2O*与O2分子的碰撞可能是形成稳态N2O的主要通道，且稳态N2O中的氧原子可能主要来自于O2，从而可解释N2O氧同位素质量无关的异常现象：N2O* + O2 → (N2O*···O2) → N2O* + O* + O；（3）提出振动激发态O2*与N2O反应可能会形成一种过渡复合物中间体，该中间体类似N2O3可以吸收紫外辐射光，从而可导致N2O的破坏，即：N2O* + O2 → (N2O*···O2) → products，评估出由此引起的N2O的消耗速率常数约为~2 × 10-16 cm3 molecule-1 s-1。