Editorial: mass spectrometric approaches for chemical characterisation of atmospheric aerosols

Atmospheric aerosols impair human health, degrade visibility and modulate the Earth’s climate.^[1] Mass spectrometry (MS) has been increasingly applied to advance our understanding of atmospheric aerosols leading to the establishment of the field of ‘mass spectrometry of atmospheric aerosols’.^[2] This field has become the fastest growing area of aerosol research, evident from the rapid increase in publications and in citations. Since 1988 to date (June 2012), over 1900 papers have been published and over 43 300 citations have accumulated. In the year of 2011 alone, more than 230 papers were published in comparison with ~10 papers per year in the early 1990s.

This Special Issue features eleven contributions discussing mass spectrometric approaches for characterising atmospheric aerosols. It begins with a comprehensive and yet easy-to-read review by Laskin et al.^[2] of the literature between late 2010 and January 2012 on instrument developments and field and laboratory applications of MS methods in atmospheric aerosol science. The review provides just enough details for readers to determine if a particular paper will be of interest to them. Thoughtful views are also given about important developments and applications that we can expect in the coming years.

The ten research papers included in the Special Issue report a wide range of MS applications at the forefront of aerosol chemistry research. They employ either ‘online’ or ‘offline’ sample introduction methods, and the type of studies range from controlled laboratory and smog chamber experiments to field studies.

Online MS measurements provide chemical composition data of atmospheric particles or their precursors at high temporal resolution. They are ideally suited for studies designed to investigate evolution of physicochemical properties of atmospheric particles and mechanisms of particle formation. Yu and Lee^[3] describe a chemical ionisation mass spectrometer using protonated ethanol and acetone ions as ionisation reagents for online detection of gaseous alkyl amines to investigate the role of amines in atmospheric new particle formation. Huang et al.^[4] utilised an aerosol time-of-flight mass spectrometer in urban Shanghai, China, to investigate the distribution of amine-containing compounds in ambient aerosols at the single particle level. Their observations suggest that amines might account for a significant part of organic aerosol mass in atmospheric environments of high sulfur dioxide and nitrogen oxides. Kroll et al.^[5] and Ge et al.^[6] demonstrate the unique advantage of providing simultaneous measurements of particle size distributions and chemical composition by online MS methods in advancing our understanding of secondary organic aerosol (SOA) formation. Both studies utilised an Aerodyne aerosol mass spectrometer. Kroll et al.^[5] studied the aging of diesel exhaust particles in the laboratory in order to better understand how they may evolve in the atmosphere. Ge et al.^[6] sampled regional fog events in the Central Valley of California, USA. Their observations indicate that aqueous-phase reactions contribute to the production of secondary aerosol species and significantly affect submicron aerosol chemistry and microphysics.

Offline MS techniques are capable of providing more selective and sensitive quantification of individual organic markers for important aerosol sources through coupling with chromatography for separation. Identification of characteristic tracers is also greatly aided by coupling gas chromatography (GC) or liquid chromatography (LC) with MS detection. Yasmeen et al.^[7] used an LC/MS system to characterise major SOA products formed from ozone oxidation of α-pinene and subsequent aging through OH-initiated oxidation in a smog chamber experiment. Their efforts in detailed interpretation of LC/MS data have led to identification of tracers for α-pinene SOA aging and insights into the related reaction mechanisms. In the study by Zhang et al.,^[8] offline analysis by GC/MS and high-resolution quadrupole time-of-flight MS coupled with LC were used to examine SOA products from the photooxidation of methacrolein in an outdoor smog chamber under varied reaction conditions. Results inferred from the detailed product analysis highlight the importance of aqueous-aerosol phase in forming organosulfates from oxidation of methacrolein. Stone et al.^[9] and Claeys et al.^[10] applied offline MS methods to characterise ambient organic aerosols. Stone et al.^[9] assessed biogenic contributions to SOA in the Himalayas by quantifying tracers for SOA formed from isoprene, monoterpenes and sesiquiterpenes using GC/MS. They tackled the issue of measurement uncertainties associated with using surrogate compounds for quantification of tracers without authentic standards. Their results indicate biogenic SOA is a significant source of organic aerosol in the Himalayan region. Claeys et al.^[10] focus on characterisation of humic-like substances (HULIS, a ubiquitous and major fraction of continental organic aerosols) in a few locations in Europe and in a biomass burning-affected Amazon site using LC/MS. Chromophoric nitrocatecholic compounds were found to be among major species of HULIS, suggesting volatile aromatic hydrocarbons emitted during biomass burning are important gas-phase precursors for HULIS.

Another development in offline MS applications is to take advantage of the high mass resolving power and mass accuracy of ultra-high resolution mass spectrometers to resolve and unambiguously identify thousands of compounds in complex mixture samples such as ambient organic aerosols. Using ultrahigh resolution Fourier transform-ion cyclotron resonance mass spectrometry (FT-ICR MS), Mazzoleni et al.^[11] identified more than 4000 individual molecular formulas in nonurban aerosol water-soluble organic carbon fraction. Rincón et al.^[12] used an LTQ Orbitrap instrument to analyse the water-soluble organic fraction of urban aerosols collected in Cambridge, UK. Several thousand compounds could be assigned unambiguous formulas and a higher fraction of compounds containing oxidised nitrogen and sulfur functional groups in summer samples signifies a higher degree of oxidation in the summer.

The collection of papers in this special issue amply demonstrates that MS is a powerful tool providing molecular level data of key importance for revealing the relationship between chemical composition and physicochemical properties of atmospheric particles. It is clear that the field of mass spectrometry of atmospheric aerosols will continue to grow and to make invaluable contributions to our understanding of the role of atmospheric particles in the atmospheric environment and their effect on human health.

I thank all the authors contributing to this special issue – their dedication to aerosol research is clearly reflected in this collection of high quality papers which has been a pleasure to edit.

Jian Zhen Yu, Editor