Determination of uptake coefficients for heterogeneous loss of HO2 radicals to atmospheric aerosols

自由基OH、HO 2和RO 2介导低层大气中几乎所有的氧化化学反应，负责将一次排放转化为二次污染物，如NO2、O3和颗粒物。自由基控制着气候气体（如CH 4）的寿命、O3的收支和酸性物质的产生。了解自由基在大气中的行为对于了解污染物迁移的寿命和空间尺度至关重要。未来空气质量和气候变化的预测模型包含复杂的化学方案，与目前大气中自由基测量结果的比较构成了对这些方案的最佳验证。一些实地活动报告了OH和HO 2自由基（HOx）的模型和现场观测之间的重大差异，例如在海洋和城市环境中。气溶胶中HO 2的不均匀损失被用来解释这些差异。HO 2对云滴和卷云粒子的反应也被用来解释对流层上部HOx的模型差异。然而，HOx的非均相和多相化学是相对研究不足，有相当大的变化，在文献中的HO 2吸收系数，在实验室中确定的温度/相对湿度和气溶胶类型的有限条件下。此外，这些实验是在相对高浓度的HO 2下进行的，其中通过HO 2自反应的二阶损失在一阶非均相损失上占主导地位，使得精确值的提取复杂化。该项目的主要目的是使用FAGE（气体膨胀荧光分析）技术以极高的灵敏度（0.04 pptv检测限）测量HO 2，使其浓度保持极低，从而使气相自反应速率与气溶胶流管中测量的非均相损失速率相比可以忽略不计。将使用具有与大气相关的一系列直径和成分（H2SO 4、（NH 4）2SO 4、海盐、油酸和煤烟）的气溶胶测量HO 2吸收系数，并在不同温度、相对湿度和pH值下记录数据，以便获得参数化。研究结果将通过纳入一个箱型模型，重新评估在海洋和城市环境中举行的前两次活动的异质损失的作用。

The free-radicals OH, HO2 and RO2 mediate virtually all of the oxidative chemistry in the lower atmosphere, being responsible for the transformation of primary emissions into secondary pollutants such as NO2, O3 and particulates. Free-radicals control the lifetime of climate gases (e.g. CH4), the budget of O3, and the production of acidic species. Understanding the behaviour of free-radicals in the atmosphere is of paramount importance in understanding the lifetime and spatial scales of pollutant transport. Predictive models for future air quality and climate change contain complex chemical schemes, and comparison with measurements of free-radicals in the present atmosphere constitutes the best validation of these schemes. Some field campaigns report significant discrepancies between models and in situ observations of OH and HO2 radicals (HOx), for example in marine and urban environments. Heterogeneous losses of HO2 to aerosol have been invoked to explain these differences. Reactions of HO2 on cloud droplets and cirrus particles have also been invoked to explain model discrepancies for HOx in the upper troposphere. However, the heterogeneous and multiphase chemistry of HOx is relatively understudied, and there is considerable variation in the literature for HO2 uptake coefficients, determined in the laboratory under limited conditions of temperature/relative humidity and aerosol type. In addition, these experiments were performed at relatively high concentrations of HO2 where second order loss via the HO2 self-reaction was dominant over the first order heterogeneous loss, complicating the extraction of accurate values. The primary aim of this project is to use the FAGE (Fluorescence Assay by Gas Expansion) technique to measure HO2 with exquisite sensitivity (0.04 pptv detection limit) so that its concentration can be kept extremely low such that the rate of the gas phase self-reaction is negligible compared with the rate of heterogeneous loss, which will be measured in an aerosol flow-tube. Measurements of HO2 uptake coefficients using aerosols with a range of diameters and composition relevant to the atmosphere (H2SO4, (NH4)2SO4, sea salt, oleic acid and soot) will be made, and data recorded under different temperatures, relative humidities and pH will enable parameterisations to be obtained. The results will be exploited through incorporation into a box model to re-evaluate the role of heterogeneous loss for two previous campaigns held in marine and urban environments.