摘要
大气二次有机气溶胶是PM2.5的主要组分,具有重要的环境和气候效应.在其主要生成途径中,气溶胶液态水中的液相生成是当前大气化学研究领域的前沿科学问题之一.液相二次有机气溶胶(aqueous secondary organic aerosol, aqSOA)前体物通过摄取进入湿气溶胶后,参与气溶胶内部的液相反应,生成有机硫、有机氮等aqSOA.本文对aqSOA前体物的摄取过程、生成的化学机制以及当前主要的研究手段进行详细总结,并对该领域未来研究进行展望. aqSOA前体物的摄取受到相对湿度、气溶胶水溶性等因素的影响,从而影响其进入气溶胶的后续反应.aqSOA生成的化学机制可分为自由基反应和非自由基反应,其中,自由基反应以OH自由基液相化学为主;非自由基反应则多为羰基化合物的液相反应.近年来,先进的离线和在线质谱技术的发展推动了对实际环境大气中aqSOA生成机制的认识,并从分子层面识别出aqSOA产物,但aqSOA生成的大气化学过程仍未明晰.未来研究中,开发新的分析技术,扩充aqSOA反应动力学数据库,进一步完善模型模拟,为aqSOA生成研究提供新思路.
Secondary organic aerosol(SOA) is the main component of PM2.5, with great impact on regional air quality and global climate. The traditional view that SOA forms through the partitioning of photochemical processing involving volatile organic compounds(VOCs) cannot fully explain measured SOA concentrations. It has been increasing recognized that SOA can form through aqueous reactions in recent years. Besides cloud/fog aqueous chemistry, aqueous SOA(aqSOA)formation in aerosol liquid water has become one of the frontier scientific problems of atmospheric chemistry. AqSOA precursors enter into aqueous phase through uptake to wet aerosol particles, participate in reactions inside aerosol particles,and then form aqSOA such as organic sulfur compounds and organic nitrogen compounds. This paper provides an overview of the uptake of aqSOA precursors, aqSOA formation mechanism and current research methods of aqSOA.AqSOA precursors include atmospheric oxidants(OH, HO2, O3, etc.), anthropogenic and biogenic VOCs and related gasphase oxidation products. Aerosol liquid water can influence the uptake of aqSOA precursors on wet aerosols, but related researches are limited. OH uptake coefficient(γOH) on different kinds of aerosols varies from 0.02 to 2.41, depending on chemical composition of aerosols and relative humidity. For the uptake of VOCs, take methylglyoxal as an example,measured and theoretical methylglyoxal uptake coefficient(γMGLY) differ by 4 to 5 orders of magnitude. Aerosol liquid water may change ionic strength, diffusion limitation and viscosity of wet aerosols, but how these affect the uptake process of aqSOA precursors remain poorly understood. Based on previous analyses of aqueous chemistry, aqueous-phase reactions can be divided into radical reactions and non-radical reactions. Aqueous-phase radical reactions resemble gasphase reactions in general. However, there are also OH radical reactions unique to the aqueous phase: efficient conversion of aldehydes to carboxylic acids, rapid OH oxidation of carboxylate, and radical induced oligomerization. Recent studies also pay increasing attention to the role of other oxidants in the aqueous radical chemistry, like singlet oxygen, peroxyl radicals, peroxides, molecular oxygen(1O2*), and triplet excited states of organic compounds(3C*). Non-radical reactions include hemiacetal formation, aldol condensation, imine formation and other types of reactions(anhydride formation,organosulfate formation, etc.). Most non-radical reactions lead to the formation of high molecular weight compounds.Although a lot of investigations have been taken to explore aqSOA formation mechanism, the majority are laboratory studies, because of the limit of technology in field measurements. Laboratory simulation includes bulk solution simulation and reaction chamber experiments. Bulk solution cannot simulate typical ambient wet aerosols well, so nowadays reaction chamber is used more widely. There are two types of reaction chambers: smog chamber and flow tube, differing in the volume of reaction chamber and simulated atmospheric oxidation timescale. However, the appropriate application of these laboratory results into field observations and model framework needs further efforts. Moreover, one key factor that has enabled great progress in aqSOA chemistry studies is the development of mass-spectrometric methods, mainly including electrospray ionization-mass spectrometry(ESI-MS), Fourier transform ion cyclotron resonance electrospray ionization mass spectrometry(FTICR-MS), chemical ionization mass spectrometry(CIMS) and extractive electrospray ionizationmass spectrometry(EESI-MS). These techniques can realize accurate molecular level identification of complex compounds. But quantification remains a thorny issue, owing to the absence of available authentic standards. Finally,possible future directions regarding aqSOA chemistry studies are discussed.
作者
肖瑶
吴志军
郭松
何凌燕
黄晓锋
胡敏
Yao Xiao;Zhijun Wu;Song Guo;Lingyan He;Xiaofeng Huang;Min Hu(State Key Joint Laboratory of Environmental Simulation and Pollution Control,College of Environmental Sciences and Engineering,Peking University,Beijing 100871,China;Key Laboratory of Urban Habitat Environmental Science and Technology,School of Environment and Energy,Peking University Shenzhen Graduate School,Shenzhen 518055,China)
出处
《科学通报》
EI
CAS
CSCD
北大核心
2020年第28期3118-3133,共17页
Chinese Science Bulletin
基金
国家自然科学基金(91844301,41977179,41875149)资助。
关键词
液相二次有机气溶胶
前体物
摄取
液相反应
生成机制
分析技术
aqueous secondary organic aerosol
precursor
uptake
aqueous-phase reaction
formation mechanism,measurement technique
