Simulating and Simplifying the Physicochemical Complexity of Gas-Aerosol Systems to Promote Development of the Next Generation of Atmospheric 3-D Models

气溶胶粒子是大气的重要组成部分，影响云层、空气质量和地球气候。这些微小颗粒在大气表层的数量和分布是空气质量的重要指标，通常以颗粒物质量浓度（PM10、PM2.5）以及臭氧和一氧化二氮浓度来报道。细颗粒和超细颗粒很容易进入肺部，可能对人体健康产生不利影响。气溶胶粒子在云的形成中起着至关重要的作用，并影响液态水和冰云的微物理特性。最近，政府间气候变化专门委员会（IPCC）的第五次综合评估报告（AR5）强调，气溶胶仍然是制约不佳的、相当重要的气候因子：“高度可信的是，气溶胶及其与云的相互作用抵消了相当一部分来自混合良好的温室气体的全球平均强迫。它们继续对总辐射强迫（RF）估算贡献最大的不确定性。”观测到的全球气溶胶水平表明，有机化合物通常占对流层气溶胶质量的30%至80%。这个有机气溶胶部分的主要部分是所谓的二次有机气溶胶（SOA），由挥发性有机化合物的氧化和随后的气体-颗粒分裂形成。了解挥发性有机和无机物质的化学反应和分配如何影响大气气溶胶的质量浓度、化学成分和大小分布，对于旨在改善城市和区域空气质量的行动以及气候敏感性的关键评估至关重要。目前的大气三维模型仅通过高度简化的方案来实现这种物理化学过程。大多数三维模式都大大低估了观测到的气溶胶水平，这构成了当前空气质量和全球气候评估的主要不确定性之一。我们研究计划的长期目标是开发和利用方法，将实验室气溶胶实验、实地研究、理论和盒模型中的过程级知识转化为实际和合理的过程参数化，用于3-D化学输送和化学-气候模型。因此，我们未来五年的短期目标包括开发一种新的物理化学建模框架，简化气溶胶形成和化学演变的模拟，以及评估和设计烟雾室实验。从包含有机气溶胶形成和化学老化的关键物理化学过程的箱形模型中获得的基本见解将为评估不同程度简化的可行性提供坚实的基础，例如在3-D模型中过程参数化所需的有机替代化合物的数量和类别。相对湿度对气粒分配的影响是我们提出的研究各种二次有机气溶胶类型的几个问题之一。反过来，模型模拟将允许我们建立一系列约束和建议，以开发简化但改进的3-D模型应用的参数化，这在很大程度上取决于计算效率的方案。新的参数化将在大气化学输运模式中实施和测试。拟议的研究为麦吉尔大学培养研究生提供了极好的机会。我们的研究和培训工作将得到与欧洲和北美领先研究小组的互利科学合作的支持。

Aerosol particles are important constituents of the atmosphere affecting clouds, air quality and Earth's climate. The amounts and distribution of these tiny particles in the surface layers of the atmosphere are important indicators of air quality, often reported in terms of particulate matter mass concentrations (PM10, PM2.5) alongside with ozone and nitrous oxide concentrations. Fine and ultrafine particles can easily enter the lungs and may affect adversely the health of humans. Aerosol particles play a crucial role in the formation of clouds and influence the microphysical properties of liquid water and ice clouds. The recent, comprehensive fifth assessment report (AR5) of the Intergovernmental Panel on Climate Change (IPCC) highlights that aerosols remain poorly constrained climate agents of considerable importance: “There is high confidence that aerosols and their interactions with clouds have offset a substantial portion of global mean forcing from well-mixed greenhouse gases. They continue to contribute the largest uncertainty to the total radiative forcing (RF) estimate.”Observed levels of aerosols globally show that organic compounds typically contribute 30% to 80% of the aerosol mass in the troposphere. The major part of this organic aerosol fraction is so-called secondary organic aerosol (SOA) formed from the oxidation of volatile organic compounds and subsequent gas-particle partitioning. It is of central importance for actions targeting the improvement of urban and regional air quality, as well as the critical assessment of climate sensitivity, to understand how chemical reactions and partitioning of volatile organic and inorganic species influences mass concentrations, chemical composition, and size distribution of atmospheric aerosols. Current atmospheric 3-D models implement such physicochemical processes by means of highly simplified schemes only. Most of the 3-D models substantially underpredict observed aerosol levels, constituting one of the main uncertainties in current assessments of air quality and global climate.It is a long-term goal of our research program to develop and utilize methods to translate process-level knowledge from laboratory aerosol experiments, field studies, theory, and box models into practical and justified process parameterizations for use in 3-D chemical transport and chemistry-climate models. Accordingly, our short-term objectives for the next five years include the development of a novel physicochemical modeling framework enabling simplified simulations of aerosol formation and chemical evolution and the evaluation and design of smog chamber experiments. The fundamental insights gained from a box model comprising key physicochemical processes of organic aerosol formation and chemical aging will provide a sound basis from which to assess the feasibility of different levels of simplifications, such as the number and classes of organic surrogate compounds required for process parameterizations in 3-D models. The influence of relative humidity on gas-particle partitioning is one of several questions we propose to study for a wide variety of secondary organic aerosol types. Model simulations in turn will allow us to establish a series of constraints and recommendations for the development of simplified yet improved parameterizations for applications in 3-D models, which largely depend on computationally efficient schemes. New parameterizations will be implemented and tested in an atmospheric chemical transport model.The proposed research offers excellent opportunities for the training of graduate students at McGill University. Our research and training efforts will be supported by mutually beneficial scientific collaborations with leading research groups from Europe and North America.