Compound specific concentration and stable carbon isotope ratio measurements of secondary organic aerosols

The formation, composition and processing of secondary organic aerosols (SOA) m the atmosphere is poorly understood and is of great interest to the scientific community. It has been proposed that the use of concentration and isotope ratios measurements from precursors and products can test the applicability of laboratory results to the atmosphere. A method has been developed for the determination of concentration and stable carbon isotope ratios of SOA in the atmosphere. Nitrophenols, formed specifically from the photooxidation of aromatic volatile organic compounds (VOC), are the target compounds in this study. These compounds are semi-volatile in nature, and therefore can exist in both the gas phase and in particulate matter (PM). Consequently, two types of filters, uncoated quartz filters and quartz filters coated with XAD-4 TM resin, were used for the collection of nitrophenols. Filters were extracted with acetonitrile with an HPLC and solid phase extraction (SPE) clean up step followed by derivatization with N,O-bis(trimethylsilyl) trifluoroacetamide (BSTF A). Samples were analyzed for concentrations by GC-MS at York University and isotope ratio analysis was conducted at Environment Canada using GC-IRMS.

The developed method gave a precision of 5 % for concentration measurements and 0 .3 ‰ for isotope ratio measurements. The detection limits achieved for concentration measurements were in the pg m-3 range. Through comparison of concentration measurements in each phase, the majority of nitrophenols was found to be in the gas phase, consistent with findings from Facca (2013). The isotope ratios of the nitrophenols were used to find their photochemical ages (PCA), during which they were formed by oxidation of the precursor, which is the time the precursor has been exposed to the HO radical in the atmosphere. With the inclusion of possible isotopic fractionation that could occur during the reaction mechanism of the precursor and possible loss reactions of the product, the PCA were found to be comparable to those found from precursor isotope ratios by Komilova (2012). Ambient yields of the nitrophenols were found to be orders of magnitude lower than predicted from laboratory studies, indicating that the quantitative extrapolation of laboratory studies to the atmosphere may not necessarily be acceptable.