A review of atmospheric aerosol measurements☆
Introduction
Atmospheric aerosol particles range in size over more than four orders of magnitude, from freshly nucleated clusters containing a few molecules to cloud droplets and crustal dust particles up to tens of microns in size. Average particle compositions vary with size, time, and location, and the bulk compositions of individual particles of a given size also vary significantly, reflecting the particles’ diverse origins and atmospheric processing. Particle surface composition is also an important characteristic since it affects interfacial mass transfer and surface reactions, which play a role in atmospheric chemical transformations. Such transformations can be significant both for their effects on gas-phase composition, as in stratospheric ozone depletion, and for their effects on particle composition. The production of fine (sub 2.5 μm) sulfates by liquid transformations in clouds is an example of a process that involves gas-to-particle mass transfer of species including water, sulfur dioxide, and oxidants.
An aerosol is defined as a suspension of liquid or solid particles in a gas. In reviewing aerosol measurement it is important to remember the gas. While atmospheric particles contain nonvolatile species such as salt, soot, metals, and crustal oxides, they also contain semivolatile compounds such as nitrates and many organic compounds. The distribution of such semivolatile compounds between the gas and particle phases varies with the amount of available particulate matter on which they can accumulate, the thermodynamic properties of the semivolatile compounds, and the gas and particle composition. Furthermore, fine (<2.5 μm) atmospheric particles are mostly hygroscopic and the water mass fraction in the condensed phase increases with relative humidity. Water typically constitutes more than half of the atmospheric fine particle mass at relative humidities exceeding roughly 80%. Thus, particle composition is inextricably linked with the composition of the gas phase, adding to the challenge of adequately characterizing the aerosol. Furthermore, sampling and/or measurement can change the thermodynamic environment or gas-phase composition thereby causing changes in particle composition before measurements are carried out.
In his visionary articles Friedlander, 1970, Friedlander, 1971 introduced a conceptual framework for characterizing instruments used for aerosol measurement. In these articles, he defined the aerosol size-composition probability density function g(v,n1,…,nk−1) for an aerosol containing k chemical species. This function is defined such that the fraction of the total number concentration N∞ having particle volume between v and v+dv, and molar composition of species i between ni and ni+dni at time t isOnly k−1 species are specified as independent variables because particle volume depends on the species’ molar composition:where is the partial molar volume of species i. This formulation does not explicitly account for particle charge states, surface composition, morphologies, phase composition, etc., but it could in principle be generalized to include such information. Gas-phase compositions are implicitly coupled through the dependence of particle composition ni on the gas phase.
Knowledge of N∞g(t,v,n1,…nk−1) would provide a comprehensive characterization of the size-resolved aerosol composition, including variations in composition among particles of a given size. Advances in single-particle mass spectrometry during the past several years have moved us closer to making such information a reality. Most aerosol measurements, however, provide integrals over time, size, and/or composition.
Fig. 1, adapted from Friedlander (1971), illustrates the type of information provided by various aerosol instruments in terms of N∞g(t,v,n1,…nk−1). The following notation is used to indicate integrations over size, time, and composition:The weighting factor, W(v), for continuous integral measurements depends on the integral aerosol property being measured. Examples of weighting factors include:where Dp is the particle diameter, Ksp the single-particle scattering efficiency, and ρp the particle density. Additional information on integral measurements is available from various sources (e.g., Friedlander, 1977; Hinds, 1982; Seinfeld, 1986).
Because the available instruments use a variety of approaches to measure particle size, different sizes can be reported for the same particle. For example, the “aerodynamic size” obtained with impactors and aerodynamic particle sizers depends on particle shape, density, and size, while the “electrical mobility size” obtained by electrostatic classification depends on particle shape and size but not on density. “Optical sizes”, which are determined from the amount of light scattered by individual particles, depend on particle refractive index, shape, and size. These sizes can be quite different from the “geometric” or “Stokes” sizes that would be observed in a microscope. Converting from one measure of size to another typically involves significant uncertainty. Such conversions, however, are often essential in utilizing aerosol measurements. These observations underline the importance of understanding the means used to measure sizes and of developing techniques to measure such properties including shape, density, and refractive index.
Laboratory calibrations can provide a misleading impression of accuracies that can be achieved when an instrument is used to measure atmospheric aerosols. Similar instruments that have been carefully calibrated in the laboratory may disagree when used for ambient aerosol measurements due to subtle difference in size cuts, or different sensitivities to aerosol hygroscopic properties, particle density or hygroscopicity. Therefore, rather than provide a misleading table of measurement precision and accuracy, I have discussed factors that affect measurement accuracy when discussing individual measurement techniques.
This review of aerosol instrumentation is organized according to the categories suggested by Friedlander with the order of presentation following Fig. 1. We first discuss measurements that provide a single piece of information integrated over size and composition and progress towards instruments that provide more detailed resolution with respect to size and time. We then follow a similar progression for instruments that measure aerosol chemical composition.
A previous comprehensive review on ambient particulate measurements was written by Chow (1995). Chow's paper focuses on fixed-site sampling and includes comprehensive discussions of size-selective inlets, flow measurement, filter media, methods and sensitivities of analytical methodologies, etc. Much of the material that was discussed in Chow's article is pertinent to the NARSTO review, and the reader is referred to her paper for an in-depth critical review of measurements used for compliance monitoring. The present paper complements this earlier review.
Section snippets
Aerosol sampling inlets
The ideal aerosol sampling inlet would draw in 100% of the particles in a specified size range and would transport them all without modification to the detector or collector. Unfortunately, obtaining representative samples of aerosols can be difficult. The efficiency with which particles enter the inlet can be more or less than 100% and varies with particle size, wind speed, and direction. Particles can be lost en route from the inlet to the measurement device, and thermodynamic changes in the
Integral measurements
Instruments that provide totals (integrals) of specified variables over a given size range are often used for aerosol measurement. For example, condensation nucleus counters provide the total number concentration of particles larger than a minimum size, and cloud condensation nuclei counters measure the subset of particles that can form cloud droplets when exposed to water vapor at a specified supersaturation. Filter samplers are often used to measure total mass concentrations, integrated with
Off-line measurements
Measurements of particle composition typically involve the chemical analysis of deposited particles in a laboratory some time after sample collection. Filters are the most commonly used collection substrates, but a variety of films and foils have been used with impactors to collect size-resolved samples. Sampling times vary with ambient loadings, sampling rates, substrate blanks, and analytical sensitivities but typically vary from several hours in urban areas to a day or more under clean
Calibration of atmospheric aerosol instrumentation
A review of techniques used to produce calibration aerosols is given by Chen (1993). In this section the techniques that are most commonly used to calibrate atmospheric aerosol instrumentation are discussed. Significant progress has been made since 1970 in the development of techniques for generating calibration aerosols, but the measurement of atmospheric aerosols introduces challenges that are not all resolved by these tools.
As was mentioned in the introduction, measured particle “sizes”
Federal reference method
The original National Ambient Air Quality Standard (NAAQS) for particulate matter was for “total suspended particulate matter” (TSP) and was in force from 1970 to 1987, when it was replaced by a standard for particles smaller than 10 μm aerodynamic diameter (PM10) (Register, 1987). More recently, an additional fine particle (PM2.5) has been proposed (Register, 1997). These methods are briefly reviewed in light of the preceding discussion on measurement methodologies.
The PM10 standard defines
Summary of significant advances
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Instrumentation for producing laboratory calibration aerosols of known size, composition and concentration became available about 25 years ago. This instrumentation is now widely used to characterize the response of aerosol instrumentation to known aerosols. These calibration techniques have facilitated a steady advance in the quality of atmospheric aerosol measurements.
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Mass spectrometers that can measure the composition of individual atmospheric particles in real time are now available. These
Summary of future aerosol measurement needs
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Gravimetric techniques that are used for regulatory compliance purposes involve filtration. While such methods are relatively simple and inexpensive to implement, they require manual operation, provide only rough time and spatial resolution, and are subject to sampling errors that cannot be quantified. Real-time techniques for accurate measurement of mass that avoid such sampling errors are needed.
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The response of aerosol instruments depends on particle properties including density, complex
Acknowledgements
Preparation of this review was supported in part by the Electric Power Research Institute through Grant No. EPRI W09116-08/W04105-01 and in part by the Department of Energy through Grant No. DE-FG02-91ER61205. Colleagues too numerous to mention have readily responded to my requests for information. Thank you all.
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Prepared for the NARSTO assessment.