Reactions of Stabilised Criegee Intermediates in the Atmosphere: Implications for Tropospheric Composition & Climate

化学反应控制着许多排放到大气中的主要物质的去除率，并控制次生物质的产生。大气中主要的氧化剂是 OH 自由基；与 OH 的反应会引发许多有机化合物、氮氧化物和其他物质（如二氧化硫 (SO2)）的去除。对于 SO2，OH 的气相氧化会产生硫酸，从而增加气溶胶质量，并且还可能充当成核剂，在大气中形成新的颗粒 - 通过直接散射太阳辐射影响气候，并通过影响液滴形成间接影响气候，从而做出非常大的冷却贡献。了解氧化率对于准确预测这些因素对大气成分和气候的影响至关重要。该项目将确定另一种潜在大气氧化剂的重要性：与烯烃臭氧分解形成的稳定 Criegee 中间体 (SCI) 发生反应。臭氧可以充当直接氧化剂，与烯烃（具有一个或多个双键的物质）发生反应。此类化合物包括大多数生物活性碳排放，它们在释放到大气中的有机化合物中占主导地位。气相臭氧-烯烃反应产生反应中间体 SCI，其在大气中的寿命为几秒（或更短 - 这是一个关键的不确定性）。一段时间以来，人们就知道 SCI 可以与其他物质发生反应，特别是 SO2；然而，目前普遍接受的观点是，与水蒸气的反应或分解在对流层中 SCI 的去除中占主导地位，因此它们不被认为是重要的氧化剂。最近的一些证据正在改变这一情况——模型研究指出缺少 SO2 氧化机制；现场和室内研究表明，在烯烃含量升高的情况下，SO2 氧化会增强，最近的实验室研究也表明这一点。研究发现，至少一种 SCI 物质与 SO2 和 NO2 的反应非常快，而与 H2O 的反应非常慢（至少在所考虑的特定实验条件下）。如果这一结论得到推广，简单的计算表明，对于边界层中 SO2 的气相氧化，SCI 反应与 OH 的反应相当。相关的硫酸盐气溶胶增加意味着辐射强迫计算将发生重大变化。同样，NO2 氧化的增强会导致硝酸盐产量增加。然而，重要的是，最近的结果与之前对 SCI 反应系统的实验室研究并不一致，可能是由于方法和条件（试剂丰度、压力、时间尺度等）的差异，与大气相关的结果有很大差异。在这个项目中，我们将采用一种新方法来解决这个关键而及时的问题：应用大气模拟室来直接评估 SCI 作为氧化剂的重要性。我们将使用 EUPHORE（欧洲光反应器）室，它使我们能够复制环境条件（使用人造和真实空气样本），以与在大气中形成相同的方式产生 SCI（即通过烯烃臭氧分解），并直接监测它们对 SO2 和 NO2 的影响。这种方法将避免影响先前研究解释的（大）外推的不确定性。我们的实验将通过复制真实大气并可通过直接检查进行分析的实验来确认（或以其他方式）SCI 反应的重要性；此外，我们将确定一系列 SCI 物质反应的动力学参数，这将用于修正大气化学模型中 SCI 形成的机制。然后，我们将应用此类模型（MCM 和 GEOS-Chem）来量化 SCI 反应对局部和全球尺度大气氧化的贡献。

Chemical reactions govern the rate of removal of many primary species emitted into the atmosphere, and control the production of secondary species. The dominant atmospheric oxidant is the OH radical; reaction with OH initiates the removal of many organic compounds, nitrogen oxides and other species such as sulphur dioxide (SO2). In the case of SO2, gas-phase oxidation by OH produces sulphuric acid, which increases aerosol mass, and may also act as a nucleating agent, forming new particles in the atmosphere - affecting climate by directly scattering solar radiation, and indirectly by affecting could droplet formation, making very substantial cooling contributions. Understanding oxidation rates is critical to accurate prediction of the impacts of these factors upon atmospheric composition and climate. This project will determine the importance of an additional potential atmospheric oxidant: reactions with stabilised criegee intermediates (SCIs), formed from the ozonolysis of alkenes.Ozone can act as a direct oxidising agent, reacting with alkenes (species with one or more double bonds). This class of compounds includes most biogenic reactive carbon emissions, which dominate the organic compounds released to the atmosphere. Gas-phase ozone-alkene reactions produce reactive intermediates, SCIs, which have lifetimes of a few seconds (or less - this is a critical uncertainty) in the atmosphere. It has been known for some time that SCIs can react with other species, notably including SO2; however the current generally accepted wisdom is that reaction with water vapour, or decomposition, dominates the removal of SCIs in the troposphere, and so they are not considered to be important oxidants.A number of recent pieces of evidence are changing this picture - model studies pointing to missing SO2 oxidation mechanisms; field and chamber studies pointing to enhanced SO2 oxidation in the presence of elevated levels of alkenes, and recent lab. studies which found that reactions of at least one SCI species with SO2 and NO2 are very fast, and with H2O very slow (at least under the specific experimental conditions considered). If this conclusion is generalised, simple calculations indicate that SCI reactions would be comparable to those of OH for the gas-phase oxidation of SO2 in the boundary layer. The associated sulphate aerosol increase would imply a significant change to radiative forcing calculations. Similarly, enhanced oxidation of NO2 would lead to increased nitrate production. Critically however, the recent results are not consistent with previous laboratory studies of the SCI reaction system, potentially as a consequence of differences in approach and conditions (reagent abundance, pressure, timescales etc.) which diverge substantially from those of relevance to the atmosphere.In this project, we will apply a new approach to this critical and timely issue: application of an atmospheric simulation chamber to directly assess the importance of SCIs as oxidants. We will use the EUPHORE (European Photoreactor) chamber, which will allow us to replicate ambient conditions (using both artificial and real air samples), produce SCIs in a manner identical to their formation in the atmosphere (i.e. through alkene ozonolysis) and directly monitor their impacts upon SO2 and NO2. This approach will avoid the uncertainties of (large) extrapolation which affect interpretation of previous studies. Our experiments will confirm (or otherwise) the importance of SCI reactions through experiments which replicate the real atmosphere and may be analysed by direct inspection; in addition we will determine kinetic parameters for the reactions of a range of SCI species, which will be used to revise the mechanism for SCI formation in atmospheric chemical models. We will then apply to such models (the MCM and GEOS-Chem) to quantify the contribution of SCI reactions to atmospheric oxidation on both local and global scales.