The Studies Of Secondary Organic Aerosols Formed From The Atmospheric Oxidation And Photooxidation Of Volatile Organic Compounds

Atmospheric aerosols, consisting of liquid or solid particles suspended in air, play a key role in the atmospheric chemistry and physics, climate system, public health, and many environmental processes. The airborne solid and liquid particles in the nanometer to micrometer size range influence Earth’s energy budget by scattering or absorbing radiation, can modify the characteristics of clouds and enhance or suppress precipitation. Aerosol particles can act as cloud condensation nuclei (CCN) and also affect human health by degrading the cardiovascular and respiratory system. Such aerosol can be primary (emitted directly in the particle phase as solids or liquids) or secondary (formed in situ as condensable vapors via oxidation or photooxidation reaction) in nature. Secondary organic aerosol (SOA) is formed when atmospheric oxidants such as the hydroxyl radical (OH), nitrate radical (NO3), and ozone (O3) react with volatile organic compounds (VOCs) from both anthropogenic and biogenic sources to form numerous lower volatility organics. SOA formed by the oxidation and photooxidation of biogenic and anthropogenic volatile organic compounds (BVOC and AVOC) is a major global contributor to atmospheric aerosol mass.In the chemistry of urban atmosphere,1,3-butadiene and aromatic compounds are of great interest. Aromatic hydrocarbons are estimated to be the most significant anthropogenic SOA precursors and the dominant component of SOA in urban areas. The most representative aromatic hydrocarbons emitted to the atmosphere from anthropogenic are Toluene, Benzene,1,3,5-Trimethylbenzene etc. The major atmospheric sink for aromatics such as Toluene is reaction with the hydroxyl radical. On a global scale, Monoterpene (e.g. α-Pinene, β-Pinene,△3-Carene, and d-Limonene) and Isoprene oxidation products are estimated to be the major global contributors to atmospheric aerosol mass. Isoprene SOA has been estimated to be the single largest source of atmospheric organic aerosol and contribute about50%to the global SOA budget, with OH being the primary oxidant. The photoionization and photo-induced fragmentation studies of these important SOA precursors play a role in the photophysics and photochemistry of the atmosphere.Because of the importance and large intensity of SOA, it has become a focus of intense laboratory, field, model, and theoretical research. A detailed knowledge on the formation and properties of SOA is therefore essential to characterize the chemical composition. However, accurate determination of the chemical composition of organic aerosol (OA) is a formidable analytical challenge. The detailed chemical mechanisms involved in SOA formation remain highly uncertain and not fully understood or characterized due to the chemical and physical processes associated SOA formation are complex and include a large number of difficult-to-measure compounds from diverse chemical classes. The chemical complexity and labile nature of OA strongly favors real-time instrumental analysis techniques that characterize pertinent chemical properties without several intermediate steps of sample collection, storage, transport, and preparation before chemical analysis. To achieve this goal, a new thermal desorption/tunable vacuum-ultraviolet photoionization time-of-flight aerosol mass spectrometer and a photochemical environmental chamber have designed and constructed at the Atomic and Molecular Physics Beamline (U14-A) at the National Synchrotron Radiation Laboratory (NSRL) for the real time analysis of SOA in Chamber experiments. The use of this combination is the first time in the international arena. The overall experimental apparatus consists of a sampling system, an environmental chamber system and the detection system. Photooxidation of VOCs (e.g. Isoprene, Toluene, α-Pinene, β-Pinene,△3-Carene, d-Limonene,. carvone, menthone, and1,3,5-trimethylbenzene) was performed using UV (320-400nm, R-UVA) irradiation of VOC/CH3ONO/air mixtures in a1.5m3flexible Teflon bag suspended in the chamber. Oxidation of VOC (α-Pinene, β-Pinene,△3-Carene, and d-Limonene) by O3was performed in the chamber in the dark. This is the first application of a thermal desorption/tunable VUV TOF photoionization aerosol mass spectrometry technique combined with photochemical environmental chamber for real time analysis of laboratory generated SOA. The temperature controlled evaporation and tunable photon energy for ionization are advantageous for conducting Mass Spectra experiments for characterizing constituents of these complex particles. Aided by the multidimensional mass peak analysis and the ionization energies from the ab initio calculations, experimental results, or the literatures, a wealth of molecular information can be extracted for molecular identification.The results suggest the potential use of thermal desorption/tunable vacuum-ultraviolet photoionization time-of-flight aerosol mass spectrometry (TD-VUV-TOF-PIAMS) as tools for the direct and more accurate analysis of compounds of chamber-based SOA. Measurements of these chemical products and the evolution of SOA provide substantial insights into these SOA formation processes. Detailed molecular information provided by TD-VUV-TOF-PIAMS helps us better understand and predict the chemistry and physics of SOA and SOA formation eventually. Many chemical reaction pathways and related physical processes are proposed, presented, and discussed in this work. There are many open questions, for example, how VOCs react with atmospheric OH radicals and O3, how the products in SOA particles react with each other in the bulk or react with the gas-phase radicals in the surface of particle. Eventually, it has impacted on the elucidation of the chemical mechanism and SOA formation pathways of atmospheric VOCs. The detection and analysis of SOA and further studies of the formation mechanism and reaction kinetics are among the central topics in the Atmospheric and Environmental Sciences. They play important roles in investigations of regional and global atmospheric chemistry, climate change, and environmental effects.