United States Department of Agriculture - Agricultural Research Service San Joaquin Valley Agricultural Center, Parlier, California 93648, United States.
Environ Sci Technol. 2013 Mar 5;47(5):2230-7. doi: 10.1021/es303891q. Epub 2013 Feb 15.
The aging of fresh secondary organic aerosol (SOA), formed in a flow tube reactor by α-pinene ozonolysis, was studied by passing the fresh SOA into a second chamber for reaction with high levels of the hydroxyl radical. Two types of experiments were performed: (1) injection of a short plug of fresh SOA into the second chamber, where the particle mass and average O/C mole ratio were measured as a function of time after injection, and (2) injection of a continuous stream of fresh SOA into the second chamber, where particles were collected on a filter over a period of time for off line analysis by high performance mass spectrometry. These setups allowed the chemistry of SOA aging to be elucidated. The particle mass decreased and average O/C ratio increased with increasing aging time. Aged SOA showed an oligomer distribution shifted to lower molecular weight (fragmentation) and molecular formulas with higher O/C and lower H/C ratios (functionalization). Carbon oxidation states of individual molecules were higher for aged SOA, 0 to +2, than fresh SOA, -1 to 0. Tandem mass spectrometry of oligomers from fresh SOA showed small neutral losses associated with less oxidized functional groups such as aldehydes and ketones, while oligomers from aged SOA showed losses associated with more highly oxidized groups such as acids and peroxyacids. Product ion spectra of fresh SOA showed monomer building blocks with formulas corresponding to primary ozonolysis products such as pinic and pinonic acids, whereas aged SOA monomer building blocks corresponded to extremely oxidized products such as dimethyltricarballylic acid.
新鲜二次有机气溶胶（SOA）的老化研究，通过α-蒎烯臭氧氧化在流管反应器中形成，通过将新鲜 SOA 引入第二腔室与高浓度羟基自由基反应来进行。进行了两种类型的实验：（1）将新鲜 SOA 的短段注入第二腔室，测量注入后随时间推移的颗粒质量和平均 O/C 摩尔比；（2）将新鲜 SOA 的连续流注入第二腔室，在一段时间内将颗粒收集在过滤器上，然后离线进行高效质谱分析。这些设置允许阐明 SOA 老化的化学性质。随着老化时间的增加，颗粒质量减小，平均 O/C 比增加。老化 SOA 表现出向低分子量（碎片化）和高 O/C、低 H/C 比（官能化）的分子公式分布的转变。与新鲜 SOA 相比，老化 SOA 中单个分子的碳氧化态更高，为 0 到+2，而新鲜 SOA 为-1 到 0。新鲜 SOA 中低聚物的串联质谱显示与醛酮等低氧化官能团相关的小中性损失，而老化 SOA 中低聚物的损失与更高度氧化的基团（如酸和过氧酸）相关。新鲜 SOA 的产物离子光谱显示单体构建块的分子式对应于初级臭氧氧化产物，如 pinic 和 pinonic 酸，而老化 SOA 的单体构建块对应于极其氧化的产物，如二甲基三羧基丁酸。
