Molecular Characterization of Atmospheric Organic Aerosol by Mass Spectrometry

Abstract

Atmospheric aerosol, particulate matter suspended in the air we breathe, exerts a strong impact on our health and the environment. Controlling the amount of particulate matter in air is difficult, as there are many ways particles can form by both natural and anthropogenic processes. We gain insight into the sources of particulate matter through chemical composition measurements. A substantial portion of atmospheric aerosol is organic, and this organic matter is exceedingly complex on a molecular scale, encompassing hundreds to thousands of individual compounds that distribute between the gas and particle phases. Because of this complexity, no single analytical technique is sufficient. However, mass spectrometry plays a crucial role owing to its combination of high sensitivity and molecular specificity. This review surveys the various ways mass spectrometry is used to characterize atmospheric organic aerosol at a molecular level, tracing these methods from inception to current practice, with emphasis on current and emerging areas of research. Both offline and online approaches are covered, and molecular measurements with them are discussed in the context of identifying sources and elucidating the underlying chemical mechanisms of particle formation. There is an ongoing need to improve existing techniques and develop new ones if we are to further advance our knowledge of how to mitigate the unwanted health and environmental impacts of particles.

Open questions on the chemical composition of airborne particles

“…Historically, and continuing to this day, particles have been collected using filters or impactors and the bulk samples collected in this manner have typically been extracted into a solvent and analyzed using chromatographic, spectroscopic, and mass spectrometric methods 11,12 . While very useful, the results are dependent on a number of factors such as the nature of the extraction solvent.…”

“…This led to the development of a number of on-the-fly mass spectrometric approaches for analysis of bulk composition of airborne particles or single particle analysis without the need for prior collection 7,8,12 . Mass spectrometry (MS) is a very powerful technique for providing both elemental information and in the case of high resolution MS, molecular formulae of complex, higher molecular mass organics.…”

“…With the complex mixtures found in SOA there are so many different contributions to the mass spectrum that identifying parent compounds (those from the sampled particles that are ionized but have not fragmented) can be very challenging. In this regard, chemical ionization approaches have proven powerful for sensitive and molecular detection of specific classes of compounds 7,12 . Commonly used ionization reagents include protons in the positive ion mode and O 2 − , iodide, nitrate, and CF 3 O − ions in the negative ion mode.…”

“…A relatively new approach uses ambient ionization, which combines the transfer to the gas phase and ionization by intersecting a particle flow with a stream of charged droplets or a metastable plasma 12,14 . The former can be generated using different techniques such as electrospray or sonic spray.…”

Implications of Matrix Effects in Quantitative HPLC/ESI-ToF-MS Analyses of Atmospheric Organic Aerosols

“…In LC/MS techniques with electrospray ionization (ESI), achieving optimal LC/MS conditions represents a considerable challenge [8,9]. However, even under such conditions important contributors to atmospheric SOA, such as organosulfates, terpenoic acids (e.g., terebic, pinic, and pinonic acids), di-or tricarboxylic acids, nitrophenols and their related compounds, contributing to brown carbon or acting as tracers for biomass burning, are measured by LC/MS techniques in several laboratories [10].…”