FEATURE pubs.acs.org/ac
Improved Understanding of Atmospheric Organic Aerosols via Innovations in Soft Ionization Aerosol Mass Spectrometry Organic molecules are a significant and highly varied component of atmospheric aerosols. Measurement of aerosol composition and improvements in our understanding of the complex chemistry involved in their formation and aging are being aided by innovations in soft ionization aerosol MS. (To listen to a podcast about this feature, please go to the Analytical Chemistry multimedia page at pubs.acs.org/page/ancham/audio/index.html.) James Zahardis, Scott Geddes, and Giuseppe A. Petrucci University of Vermont
Jane Petrillo
A
n atmospheric aerosol is technically defined as a suspension of particles in the air; however, the common usage in the atmospheric community refers to the aerosol as the particulate component only.1 These particles range in size from a few nanometers to tens of micrometers in diameter and exert an important influence on visibility,2 climate,3,4 and health5 (Figure 1). Organic aerosols (OAs) make a significant contribution to the total mass of fine aerosols (aerodynamic diameter e2.5 μm), contributing ∼20-50% of the total mass at continental midlatitudes and up to 90% in tropical forested areas.3 Primary OA (POA) particles are directly emitted into the atmosphere from sources including biomass burning and incomplete fossil fuel combustion. Secondary OA (SOA) is formed by oxidation of gasphase volatile organic compounds (VOCs) from both biogenic and anthropogenic sources and subsequent condensation of low volatility product molecules. Most OA is oxygenated (i.e., OOA, characterized by O:C atomic ratios of ∼0.25-1), and the degree r 2011 American Chemical Society
of oxygenation increases with atmospheric aging.6,7 SOA is a main contributor to OOA,6 with most SOA derived from biogenic precursors (BSOA).8 All atmospheric aerosols, including OAs, influence climate by scattering and absorbing radiation (i.e., the direct aerosol effects),3 the magnitude of which depends on the size and composition of the particles. The Intergovernmental Panel on Climate Change has also identified the chemical composition of aerosols as one of the key parameters for determining the cloud condensation nuclei (CCN) activity of aerosols.4 CCN activity describes the ability of aerosol particles to grow to larger than a critical threshold diameter under conditions of supersaturated relative humidity (i.e., >100%) to form droplets. Increased CCN concentrations influence climate through the modification of cloud properties (i.e., indirect aerosol effects9) that include increased cloud albedo and longer cloud lifetime. Furthermore, molecular level speciation is important for predicting physical properties of aerosols, such as surface tension, that may impact CCN properties.10 Quantitatively addressing the molecular composition of OA is a formidable challenge because ambient OA in a typical air parcel is often comprised of hundreds or more compounds from diverse chemical classes.11 To achieve this goal, a number of aerosol MS techniques have been developed.12-15 Here we present recent advances to aerosol mass spectrometers that use soft ionization sources. These high time resolution instruments are particularly suited for the online molecular analysis of OA because they produce minimal fragmentation of organic compounds. A brief overview and current applications of these instruments are presented, as well as recent insights into the atmospheric chemistry of OA afforded by their development. Further developments required for soft ionization aerosol MS and the potential application of such systems in both laboratory and field settings to clarify some of the current uncertainties associated with OA are also discussed.
’ SOFT IONIZATION AEROSOL MASS SPECTROMETRY Aerosol mass spectrometers are a class of instruments specifically designed to enable direct chemical analysis of aerosol particles. An overview of common components of these instruments, Published: January 24, 2011 2409
dx.doi.org/10.1021/ac102737k | Anal. Chem. 2011, 83, 2409–2415
Analytical Chemistry
FEATURE
referred to earlier reviews for accounts of previous progress from which the current work has developed.14,15 As discussed below, a number of soft ionization techniques that are based on photonic or chemical ionization (CI) have recently been applied to aerosol MS.
Figure 1. Atmospheric aerosols, ranging in sizes from a few nanometers to tens of micrometers, are either emitted directly to the atmosphere (for example from combustion sources) or are produced in situ by (photo)oxidation of volatile and semivolatile organic compounds. Aerosols scatter and absorb radiation; impact cloud formation, longevity, and precipitation; and exert detrimental health impacts on humans.
along with detailed technical descriptions, is provided elsewhere.12-15 The principal difference from conventional mass spectrometers is the method of sample introduction, which is typically achieved by an aerodynamic lens system or particle inlet.16 This accelerates a tightly collimated beam of particles through a differentially pumped flight chamber into the high vacuum region of the ionization source and mass analyzer. Once a particle has entered the ionization source, it can either be ionized directly as in single particle MS techniques13 or directed onto a collection/vaporization surface, where a number of particles can be collected, increasing sample size but sacrificing mixing state information.12 Ionization mechanism differentiates aerosol mass spectrometers. Methods that employ high energy ionization processes, which typically provide high ionization efficiencies at the expense of molecular information, are referred to herein as “hard ionization aerosol MS”, whereas techniques specifically designed to prevent extensive fragmentation of molecules are referred to as “soft ionization aerosol MS”. Instruments that employ hard ionization sources are the most widespread and have been used in both laboratory and field measurements throughout the world, resulting in many important advances in knowledge of atmospheric aerosols and their processing.6,17-21 Although instruments employing soft ionization sources may be considered a niche within the aerosol MS field, their mass spectral simplicity continues to provide unprecedented molecular level detail of the chemistry of OAs. For example, Figure 2a and b clearly demonstrate the large amount of fragmentation from hard ionization sources; low m/z ions dominate. Figure 2c, however, illustrates a soft ionization technique in which peaks correspond directly to the molecular weight of analyte molecules. The orchestrated simplicity of the mass spectrum allows identification of not only different classes of products but also specific molecular products, which then improves understanding of the atmospheric chemistry of OA. This article focuses on advances made in the field of soft ionization aerosol MS published in recent years. The reader is
’ PHOTONIC SOFT IONIZATION SOURCES Though the atmospheric chemistry community has thus far mainly used hard ionization sources, adaptation of soft ionization sources is becoming a focal point of next generation instruments. For example, Aerodyne aerosol mass spectrometers (denoted Aerodyne AMS herein though they are often referred to simply as “AMS instruments” in recent literature) have been very useful in both laboratory and field measurements of atmospheric aerosol.6,10,12,24 These instruments typically couple (hard) electron impact ionization (EI) to thermal vaporization.12 However, these instruments have also used vacuum UV (VUV) photoionization (PI)24 with a low pressure rare gas discharge lamp, but the low photon flux resulted in limits of detection (LODs) several orders of magnitude poorer than for EI. In an effort to enhance sensitivity, aerosol mass spectrometers, including the Aerodyne AMS24 and aerosol TOFMS instruments,25,26 have been coupled to a synchrotron radiation (SR) source, a tunable quasi-continuous VUV source with increased photon fluxes (∼1016 photons s-1).24-26 The SR source generates photons of 7-25 eV, which is close to the molecular ionization energy of many organic compounds,25 producing fragment-free mass spectra. This method has been applied to studies of reaction kinetics and product formation, including those for ozoneexposed, size-selected, NaCl particles coated with anthracene.26 However, the SR source, though versatile because its tunability makes it useful for controlled laboratory experiments, is not field deployable. An alternative approach to using low energy photonic radiation is to direct the photons onto a metallic surface to generate very low energy electrons that can accomplish ionization. The Photoelectron Resonance Capture Ionization Aerosol Mass Spectrometry (PERCI-AMS) instrument employs this technique to generate low energy (ca.
