重污染过程中亚微米级颗粒物化学组分消光特性的变化规律研究

Ambient particulate matter (PM) is the dominant cause of heavy haze events and visibility degradation occurring in eastern urban China. Understanding the temporal variation of extinction properties for chemical species of submicron PM (PM1) during heavy pollution episodes is of great scientific and practical importance, providing insights into haze formation mechanism and scientific foundation for visibility improvement. The empirical PM species-extinction formulas developed from clean environment, such as US IMPROVE equations, have been shown to cause large errors when applied to pollution episode in urban China. Several studies on extinction properties of PM species in China were based on the statistical regression method and limited to a temporal resolution of 24 hours or more. In our previous studies, we developed methods to retrieve scattering extinction properties of PM species based on offline analysis of filter samples. In this study, we propose to retrieve extinction coefficient and mass extinction efficiencies for major PM1 species in urban China from online measurement in real time. This is achieved by simultaneous online measurement of size-segregated major PM composition using an Aerosol Mass Spectrometer (AMS), and aerosol extinction coefficient by Cavity Attenuated Phase Shift (CAPS) Spectroscopy over a one-year period to capture heavy pollution episodes in urban China. This study aims to develop an online method of retrieving the extinction coefficient and mass extinction efficiencies for PM1 species with 1-hour resolution, then investigate the extinction coefficient and mass extinction efficiencies variation during heavy pollution episodes. The goal of this study is to unravel the root cause of heavy haze episodes by investigating the relationship between particle size, composition, and light extinction, especially for the range of high mass concentration which is still unknown. This study will provide theoretical support for prioritizing strategies for visibility improvement.

大气颗粒物是我国东部地区霾重污染事件频发、能见度下降的主要因素，探究颗粒物化学组分消光特性在重污染过程中的变化规律，对弄清霾污染成因机制和改善能见度具有显著的科学意义和应用价值。多个城市的外场观测发现欧美经验公式并不适用于我国颗粒物组分消光特性的表征，已有组分消光特性研究报道多基于统计回归方法，尚不能高时间精度解析重污染过程中颗粒物组分的消光特性指标。本课题拟在我们前期基于离线采样方法对组分散射消光特性的研究基础上，通过先进的气溶胶质谱仪对亚微米级颗粒物的理化特征及光腔振荡消光仪对消光系数的同步高分辨率观测，实现基于米散射理论方法的组分消光系数和质量消光效率在线解析，揭示重污染过程中各组分消光系数和质量消光效率的变化规律及关联因素。研究成果将有助于阐明重污染过程中颗粒物的粒径、组分和消光特性之间的影响机制，特别是在目前尚不明了的高浓度区间，为科学制定能见度改善的控制策略提供理论支撑。

探究颗粒物化学组分消光特性在重污染过程中的变化规律，对弄清霾污染成因机制和改善能见度具有显著的科学意义和应用价值。针对估算颗粒物化学组分对消光系数贡献应用欧美经验公式误差较大而原因尚不十分明确，且目前缺少适用我国城市地区的估算方法现状，本课题基于高分辨率飞行时间气溶胶质谱仪，对上海城区亚微米级颗粒物主要化学组分各季节粒径分布及浓度水平进行在线测量，结合消光仪对颗粒物散射消光系数的同步高分辨率观测，深入探究了重污染过程中各组分消光系数和质量消光效率的变化规律及关联因素。研究结果显示，在清洁时期，硫酸铵对散射系数的贡献超过硝酸铵，但在污染时段，有机物和硝酸铵是散射系数的主要贡献者。有机物、硝酸铵和硫酸铵的平均质量散射效率为5.3±0.53、4.3±1.0和4.2±0.64 m2/g，分别高出旧版IMPROVE公式中对应质量散射效率的32.5%、43%和40%；颗粒物组分在不同浓度水平下的质量粒径分布变化（200~300nm向500~600nm偏移），是造成以IMPROVE为代表的欧美经验公式在我国城市地区应用误差的主要原因；在海量观测数据基础上归纳了颗粒物由低到高的整个质量浓度区间散射消光效率的变化规律，并建立了适合我国城市地区的亚微米级颗粒物散射系数贡献估算方法，验证结果显示其准确性显著高于其它经验公式。本课题研究成果有望为更加深入认识颗粒物致霾机制，为科学有效地制定能见度改善策略提供理论支撑和决策依据。