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Determination of Elemental Composition of Secondary Organic Aerosols using Ultrahigh Resolution Mass Spectrometry: A Combined Laboratory and Field Investigation

Final Report Summary - SOAULTRAMASS (Determination of Elemental Composition of Secondary Organic Aerosols using Ultrahigh Resolution Mass Spectrometry: A Combined Laboratory and Field Investigation)


Executive Summary:

Biogenic volatile organic compounds (BVOCs) play an important role in atmospheric chemistry and give rise to secondary organic aerosols (SOA), which have effects on climate and human health. SOA is formed within the atmosphere from gaseous precursors and gas-to-particle conversion processes. Laboratory chamber experiments have been performed for decades in an attempt to mimic atmospheric SOA formation. However, it is still unclear how close the aerosol particles generated in laboratory experiments resemble atmospheric SOA with respect to their detailed chemical composition. One of the major challenges is the identification of the organic composition of the SOA, which is composed of thousands of organic compounds. These compounds generally cover a wide range of polarities, volatilities and masses and therefore it is difficult to find a single analytical technique for their detailed chemical analysis at the molecular level. Conventional chromatographic methods (gas chromatography (GC) and liquid chromatography (LC)) are not capable of resolving the highly complex mixtures with a wide variety of physico-chemical properties.

To date, most laboratory experiments reproducing atmospheric SOA formation have been performed using a single organic precursor (e.g. alpha- or beta-pinene or isoprene) while in the atmosphere a wide range of precursors contribute to SOA, which results in a more complex SOA composition compared to the one-precursor laboratory systems. Although, there are a few studies where SOA formation from the oxidation of volatile organic compound (VOC) mixtures was investigated, the main goal of these previous studies was to investigate SOA yields and specific products rather than to characterize the overall detailed molecular composition.

The overall objective of this project was to apply for the first time ultra-high resolution mass spectrometry (UHR MS) to determine the elemental composition of laboratory-generated organic aerosol (OA) components and compare the results with a range of ambient aerosol samples collected at different locations. Comparing the elemental composition of hundreds of compounds in samples generated under various laboratory conditions with those obtained from the field will likely reveal which of the laboratory OA experiments (type of precursor and reaction conditions) most closely reproduce the composition found in ambient aerosol. Currently no standard methods are available to determine the chemical composition of complex environmental samples with UHR MS. Therefore, it is essential to develop and optimise analytical methods for aerosol analysis using UHR MS.

In this project analytical methods for the analysis of organic aerosols based on direct infusion nano Electrospray Ionisation (nanoESI) and LC UHR MS have been developed. Despite high analytical throughput of the existing direct infusion MS methods, they are known to be prone to matrix artefacts such as changes in the ionisation efficiency of an analyte due to the presence of ‘matrix’ compounds in the complex organic mixtures. NanoESI-MS substantially reduces interference effects from inorganic salts, provides better sensitivity towards a variety of analytes in samples containing high levels of salts and decreases source contamination compared to conventional ESI sources. Chromatographic separation on the LC column is another method to reduce ion suppression caused by co-eluting compounds; moreover, it allows separation of chemical isomers. Therefore, both techniques when coupled with UHR MS have a great potential in resolving the organic composition of complex atmospheric aerosol.

The developed methods have been applied for the analysis of the organic fraction of aerosol samples from a boreal forest site. 460-730 elemental formulae have been identified with nanoESI UHR MS in negative ionisation mode and were attributed to organic compounds with molecular weights below 400 Da. Kendrick Mass Defect and Van Krevelen approaches have been used to identify compound classes and mass distributions of the detected species. The molecular composition of the aerosols strongly varied between samples with different air mass histories. An increased number of nitrogen, sulfur and highly oxygenated organic compounds was observed during days associated with continental air masses. On the other hand, samples with an Atlantic air mass history were marked by the presence of homologous series of unsaturated and saturated C12-C20 fatty acids, suggesting their marine origin. To our knowledge, we show for the first time that the highly detailed chemical composition obtained from UHR-MS analyses can be clearly linked to meteorological parameters and trace gases concentrations that are relevant to atmospheric oxidation processes. The additional LC/ESI-UHR-MS analysis revealed 29 species, which were mainly attributed to oxidation products of biogenic volatile compounds (BVOCs) (i.e. alpha-, beta-pinene, delta-3-carene, limonene and isoprene) supporting the results from the direct infusion analysis. The obtained information was further used to design the laboratory chamber experiments.

OA was generated in a simulation chamber from the ozonolysis of α-pinene and a BVOC mixture containing alpha- and beta-pinene, delta-3-carene, and isoprene. The detailed molecular composition of laboratory-generated SOA has been compared with that of a background ambient aerosol collected at a boreal forest site and an urban location (Cork, Ireland) using direct infusion nanoESI and LC UHRMS. Statistical methods (linkage distance cluster analysis and symmetric differences approach) were used to compare the very large mass spectral data sets. The laboratory-generated OA contained a distinguishable group of dimers that was not observed in the ambient samples. The presence of dimers has been found to be less pronounced in the SOA from the VOC mixtures when compared to the one component SOA system. The elemental composition of the compounds identified in the monomeric region from the ozonolysis of both α-pinene and VOC mixtures represented, fairly well, the ambient organic composition of particles collected at the boreal forest site with about 70% of common molecular formulae. In contrast, the molecular composition of laboratory-generated SOA was substantially different from that of the anthropogenically affected urban aerosol. The fraction of common molecular formulae from alpha-pinene and the VOC mixture relative to the total number of ions from the urban sample was only 16.1±1.7% and 16.9±1.2%, respectively, indicating the very different sources of organic compounds in these samples. To our knowledge this is the first direct comparison of detailed molecular composition of laboratory-generated SOA from VOC mixtures and ambient samples.

This work provides unique information on the composition of organic aerosols which is valuable for the interpretation of field data and guiding regulatory development. Effective air pollution policies can only be achieved with greater knowledge of aerosol sources, which themselves, can only be identified through a better understanding of the detailed aerosol chemical composition. Thus this study will enhance our understanding of aerosol formation and composition in the atmosphere. An improved understanding of organic aerosols will ultimately lead to a better understanding of aerosol-climate interactions and how anthropogenic activities perturb these natural processes.