摘要/Abstract

由于二次有机气溶胶（SOA）对气候变化、区域污染和人体健康具有明显的影响，因此受到了广泛的关注.基于实验室的方法可以在稳定可控的条件下探讨SOA的生成机制，其中环境烟雾箱和气溶胶生成潜势反应器是最常用的两种模拟工具.本文综述了基于这两种模拟工具对SOA产率的影响因素、SOA生成机制和SOA老化过程中性质演变特征的研究.影响SOA产率的因素主要包括OH暴露量，NO_x水平（VOCs/NO_x），种子颗粒物的浓度及种子颗粒物的化学组成.SOA产率随着OH暴露量、VOCs/NO_x比值的增加均先增后减；种子颗粒物的存在会通过提高气态中间产物的凝结汇，从而促进物质从气相到颗粒相的转化；酸性种子颗粒物可以通过提高摄取系数并提供酸催化条件促进SOA的生成；种子颗粒物中的金属离子和矿物质也会通过催化作用或者影响氧化剂的产生等过程对SOA的生成和老化产生作用.本综述还总结了不同源排放气态前体物SOA的生成潜势以及生成SOA的特征.等效光氧化龄约为2~3 d时，汽油发动机排放生成SOA量达到最高值，增长倍数（SOA/POA，POA即为一次有机气溶胶）约为10~14，SOA生成潜势约为400~500 mg/kg fuel；生物质燃烧排放，在等效光氧化龄约为3~4 d时，SOA增长倍数最大，平均约为1.42~7.6；而其他源如天然气燃烧、餐饮等排放也具有很高的SOA生成潜势，天然气燃烧排放后SOA的增长倍数高达268倍，餐饮源排放SOA的增长倍数约为3~8倍.不同地区的实际大气中气态前体物氧化生成SOA的最高值出现在等效光化学龄为2~4 d时.综合不同研究中源排放和实际大气中前体物生成SOA演化特征发现，随着OH暴露量的增加，SOA的氧化态逐渐增加，O/C比约从0.2增长到1.3，O/C与H/C变化斜率均在-1到0之间，说明氧化机制可能包括生成羟基、过氧羟基以及羧酸基团的物质；氧化过程中SOA的挥发性逐渐降低，吸湿性逐渐增加.SOA生成过程中间态物种的测量技术开发、复杂体系下SOA生成机制的研究和SOA演化过程中特征的表征是未来SOA研究的重要方面.
关键词: 二次有机气溶胶, 产率, 生成潜势, 烟雾箱, 氧化流动管
Secondary organic aerosol (SOA) is a major component of aerosols in the atmosphere, which plays a crucial role in climate change, regional pollution and human health. Laboratory simulations are usually used to mimic SOA formation. The most commonly used simulation facilities are environmental chambers and potential aerosol mass (PAM) reactors. Here in this work, we review the studies about influencing factors and mechanisms of SOA formation, as well as the evolution of SOA aging. We summarize the influencing factors on SOA yields, i.e. OH exposure, NO_x level, and the loading and chemical composition of seed particles. The effects of NO_x level (i.e. VOCs/NO_x) and OH exposure are nonmonotonic. The NO_x level influences the fate of RO₂ radicals, so SOA yields will increase and then decrease with the addition of NO_x. Similarly, the increase of OH exposure affects the major oxidation mechanism from functionalization to fragmentation, leading to the up and down trend of SOA yields. The higher seed particle loading provides more surface area for condensable products and then increases the SOA yields. The particle acidity favors the uptake process for gas-phase products and promote the SOA formation via further reactions in the condense phase. Trace components e.g. transition metals and minerals can be involved in the SOA formation and aging by catalysis or affecting the uptake of oxidants and their products. Chambers and PAM reactors are usually used to explore SOA formation potential of different sources. SOA formation potential from vehicles will be influenced by engine types, engine loading and composition of fuel. The highest SOA enhancement ratio (SOA/POA) from gasoline engines is about 4~14, when the equivalent photochemical days are 2~3 d. The SOA production mass from gasoline vehicles is from about 10~40 to 400~500 mg/kg fuel. The SOA formation potential is about 400~500 mg/kg fuel. The largest SOA enhancement ratio for biomass burning is 1.4~7.6, which occurs at 3~4 photochemical days. The SOA enhancement ratio from ambient air differs from region to region. However, the highest ratios all occur at the photochemical age of about 2~4 d. We summarize the SOA characteristics evolution with aging. Oxidation state of particles will increase with OH exposure. Changes of H/C and O/C with increasing OH exposure can be plotted in the Van Krevelen diagrams. The slopes of fitted curve range from -1 to 0, indicating OA evolution chemistry involving addition of carboxylic acids or addition of alcohols/peroxides. In addition, the volatility and hygroscopicity of oxidized OA will be higher than primary organic aerosols. In the future, more studies should be focused on developing new technologies to measuring the oxidized intermediate products at a molecular level. Also the researches on the mechanism of SOA formation from complex precursors are also crucial to understand the SOA formation at real atmosphere.
Key words: secondary organic aerosols, yield, formation potential, chambers, oxidation flow tube


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