The effect of filter storage conditions on degradation of organic Aerosols

Current practices involve wrapping organic aerosol (OA) filter samples with aluminum foil and storing in a freezer to avoid sample degradation prior to analysis. However, there is a lack of evidence supporting these practices. Here, we investigate the effect of thermal and photodegradation on toluene-combustion OA, pine-combustion OA, and ambient OA samples by storing them in petri dishes for one month at three different conditions: covered with aluminum foil and in a freezer (F), covered with aluminum foil at room temperature (RC), and uncovered at room temperature (RUC). We performed three types of analyses: thermal-optical measurements, electrospray ionization mass spectrometry, and UV-Vis spectroscopy. For all samples, we observed a mild reduction in the relatively volatile organic carbon fraction (OC1) in the RC and RUC conditions. We did not account for positive artifacts in this study, and therefore, this reduction was at least partially due to the evaporation of adsorbed vapors rather than particles. There was no evidence of significant systematic change in chemical composition relative to control for any of the storage conditions, except for loss of some small-molecular-size compounds in conditions RC and RUC for pine-combustion OA, likely due to evaporation. There was no evidence that photobleaching occurred in the RUC condition for any of the samples. On the contrary, we observed an increase in absorption in the RC and RUC conditions for pine-combustion OA, likely due to evaporation of small-molecular-size species with relatively weak absorption. Copyright © 2023 American Association for Aerosol Research

当前学界通用的样品保存流程为：将有机气溶胶（Organic Aerosol，缩写OA）滤膜样品用铝箔包裹后，置于冷冻柜中储存，以避免分析前发生样品降解。然而，目前尚无证据支持此类操作的有效性。本研究针对甲苯燃烧OA、松木燃烧OA及环境OA样品，将其置于培养皿中并在三种不同条件下储存一个月，以探究热降解（thermal degradation）与光降解（photodegradation）对样品的影响：1. 铝箔包覆并冷冻储存（F组）；2. 铝箔包覆且室温储存（RC组）；3. 无包覆室温储存（RUC组）。本研究采用三类分析手段：热光测量法（thermal-optical measurements）、电喷雾电离质谱法（electrospray ionization mass spectrometry）及紫外-可见分光光度法（UV-Vis spectroscopy）。在RC与RUC组的全部样品中，我们均观测到相对挥发性有机碳组分（OC1）出现小幅下降。本研究未考虑正偏差干扰，因此该下降至少部分源于吸附蒸汽的挥发，而非颗粒物本身的变化。与对照组相比，所有储存条件下的样品均未出现化学成分的显著系统性变化；仅松木燃烧OA在RC与RUC组中出现部分小分子化合物的损失，该现象大概率由挥发作用导致。所有样品在RUC组中均未观测到光漂白（photobleaching）现象。与之相反，松木燃烧OA在RC与RUC组中出现吸光度上升的情况，该现象大概率由吸收较弱的小分子物种挥发所致。© 2023 美国气溶胶研究协会（American Association for Aerosol Research）