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262 Analytical Techniques for Atmospheric Measurement

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Chapter 6

Mass Spectrometric Methods for Aerosol

Composition Measurements

Hugh Coe and James D. Allan

6.1 Introduction to atmospheric aerosols

Aerosols are ubiquitous and are an important feature of the earth’s atmosphere that are

produced and modified by many anthropogenic and natural processes. There are many

ways that aerosols impact directly upon the quality of life in polluted environments, for

example through unsightly smogs and their effect on human respiratory and cardiovas-

cular health (Dockery et al., 1993; Pope et al., 2002; Schwartz, 1994; White & Roberts,

1977). On a wider scale, atmospheric aerosols are also known to play a major part in many

global issues, such as the hydrological cycle, climate forcing and the general chemistry

of the atmosphere (Penner et al., 2001; Ramanathan et al., 2001; Ravishankara, 1997).

In addition, aerosol particles control the formation of polar stratospheric clouds (PSCs),

which are responsible for converting halogen species locked in longer-lived reservoirs into

more labile forms that have the ability to catalytically remove ozone in the springtime

lower polar stratosphere in both hemispheres.

There are many identified sources for atmospheric particulate matter. The transport

sector is known to be a major source, through the direct emission of particles into

the atmosphere in the form of exhaust emissions (Colvile et al., 2001; Mayer, 1999).

Other combustion sources such as industry, forest fires and coal burning are also known

to be important. Particles may also be mechanically suspended into the atmosphere;

mechanisms include the action of wind on loose soil or dust, volcanic eruptions and sea

spray.

Another major source of the mass of particulate matter in the atmosphere is the

conversion of gas-phase chemical species to the condensed phase. These include not

only lower volatility gas-phase oxidation products such as sulphuric acid and partly

oxidised organic chemicals, but also semivolatile species that will partition into the particle

phase under suitable conditions, such as ammonia, nitric acid and water. Under certain

conditions, particles are known to spontaneously form from their gas-phase precursors if

the vapour pressures become high enough. These are observed in the atmosphere in the

form of discrete, localised bursts of very small particles that quickly grow to larger sizes

through coagulation or further condensation (Kulmala, 2003; Kulmala et al., 2004).

Atmospheric aerosol particles exist over a wide range of shapes and sizes. Those that

are created mechanically, such as sea salt or dust particles, tend to be of around a

micron upwards in diameter, a size definition normally referred to as the coarse fraction.

266 Analytical Techniques for Atmospheric Measurement

These particles, while relatively low in number, tend to make up the majority of the mass

of particulate matter in the atmosphere. They also tend to be the most important when

considering the optical properties of the atmosphere, due to their size. The most persistent

fraction of particles in the atmosphere is that of around 01–1 m in diameter. This

fraction is often referred to as the accumulation mode and consists mainly of matter that

has condensed from the gas phase through various processes. These particles are generally

the most important when considering atmospheric chemistry, as they tend to collectively

posses the largest amount of chemically active surface area. They also tend to be the

particles that most readily form the nuclei for cloud droplets. There are also particles of

smaller sizes in the atmosphere, and these are often believed to be very important when

considering human health, as they can penetrate deeper into the lungs when inhaled

(Harrison & Yin, 2000; Seaton et al., 1995). These exist in high numbers, especially in

urban environments, but do not represent a high fraction of the total particulate mass.

Despite significant research to study aerosol processes and effects, much of our under-

standing has consistently been limited by the small size of aerosols, making their study

intrinsically difficult. To understand many of the important properties of aerosol particles

it is necessary to know both their composition and size. For example, these properties

dictate (a) the toxicity of the particles and their transmission through the human respi-

ratory system; (b) the hygroscopicity and refractive index of the particles and hence

their radiative properties; (c) their ability to act as cloud condensation nuclei and so

affect cloud albedo; and (d) their transport in the atmosphere and subsequent deposition

rate of pollutants. For many years, the study of this field utilised filter or impaction

technology and subsequent offline analysis. Progress in the physical characterisation of

aerosol particles significantly improved during the 1970s and 1980s but online chemical

analysis of aerosol particles has only taken place in the 1990s or so and much progress

is still being made. This chapter provides a brief overview of traditional techniques and

physical measurements of aerosol particles, but concentrates largely on the developments

in online mass spectrometry of aerosol particles since 2000.

6.2 Physical aerosol measurements and sizing

deﬁnitions

The simplest measure of the amount of particulate matter in an air sample is the

gravimetric mass; particles are collected on a filter or impactor substrate over a period and

are subsequently weighed. As larger particles are not thought to be significant to human

health, these are often removed using impactors or cyclones before sampling. This work

has formed the basis for official environmental monitoring and compliance standards

such as PM

and PM

25

, where the total mass of particles with aerodynamic diameters

less than 10 and 25 m respectively is measured. The technology has become available

to measure the mass of particulate matter in a sample online and with a high time

resolution. Examples include the beta attenuation monitor (e.g. Met One Instruments Inc.

BAM-1020, Grants Pass, OR, USA), which works by continuously passing beta radiation

through a collected filter sample and measuring the reduction in measured radiation on

the other side, or the tapered element oscillating microbalance (e.g. Thermo Electron

Corporation Series 1400a TEOM, East Greenbush, NY, USA), which collects the sample

Mass Spectrometric Methods for Aerosol Composition Measurements 267

on a filter mounted on the end of an oscillating element and records the changes to the

resonant frequency (Patashnick & Rupprecht, 1991).

The most common method of counting the volumetric number concentration of

particles present in a sample is the condensation particle counter (CPC, e.g. GRIMM

Aerosol Technik Model 5.403, Airnring, Germany; TSI Inc. Model 3025a, Shoreview,

MN, USA). These use a thermal gradient to supersaturate the air with respect to a

working fluid (usually butan-1-ol, as this condenses readily on both polar and non-polar

surfaces, although some recent instruments such as the TSI model 3785 use water) and

grow particles to micron sizes, before counting their number using light scattering. The

smallest size of particles observable with a commercial instrument of this kind is 3 nm.

The size of aerosol particles in the atmosphere is an important property to measure.

This dictates many properties such as their mechanical behaviour (e.g. where in the

human respiratory system it will be likely to deposit), optical properties, surface areas

and curvatures. The latter two properties are important in how the particle will interact

with the gas phase, and so knowing the sizes and numbers of particles is vital when

considering condensation, heterogeneous chemistry and water uptake.

The technology to accurately measure the size of particles is similarly well established

and many commercial instruments are available for these purposes. However, at this

stage it is important to define what is meant by the ‘size’ of an aerosol particle. If

all particles were spherical, it would be sufficient to measure particle size in terms of

their geometric diameter but in the atmosphere this is not the case; electron microscopy

studies have consistently shown that ambient particles can exist in a wide variety of

other morphologies, such as amorphous, crystalline, fractal, agglomerate and aggregate,

depending on their sources and compositions (e.g. Dye et al., 2000; Li et al., 2003;

McMurry et al., 1996).

The simplest empirical measure of a particle’s size is the ‘volume equivalent’ diameter

or D

. This is defined as the diameter of a sphere with the same volume as the subject

particle’s condensed-phase constituents. However, there is no straightforward and univer-

sally reliable way of probing this quantity directly. Aerosol sizing instrumentation uses

a variety of methods to determine the size of particles and these report many different

metrics, which must be considered as being physically different properties of the subject

particles (DeCarlo et al., 2004).

Particles are frequently counted and sized optically in situ and in real time using

instruments referred to as optical particle counters or OPCs (e.g. GRIMM Model 1.108;

Met One Model 9012; Droplet Measurement Technologies Cloud Droplet Probe, Boulder,

CO, USA). A particle is illuminated with a light source, either monochromatic (normally

from a laser) or polychromatic (from an incandescent source), and the scattered light

is measured at a fixed angle. The ‘optical’ diameter or D

can be derived from the

peak scattering intensity. This measurement can be based on the responses to aerosols

generated using manufactured spheres of known sizes but can also be predicted from

first principles using Mie theory (Mie, 1908).

As a quantity of a given particle, D

depends upon its physical diameter and refractive

index relative to the material used for calibration. The advantages of this technique are

that it is fast, relatively cheap and easy to implement. Depending upon the light source

used and the scatting angle monitored, it can be used to study particles of around 01 m

or larger. However, the method is not generally very well suited for the study of irregularly

shaped particles, as these can give unpredictable scattering intensities.

268 Analytical Techniques for Atmospheric Measurement

Aerodynamic particle sizers, or APSs (e.g. TSI Model 3321), work by accelerating the

sampled aerosol through a nozzle into a partly evacuated chamber, where they pass

through two laser fringes. As well as being able to derive the optical diameter from the

scattering intensity, the particle velocities can be calculated based on the difference in

detection times by the two lasers. This velocity can then be used to calculate another

property of the particles, the aerodynamic diameter or D

. This is an important property

to measure, as it influences particle behaviours in many real-world scenarios, such as

dry deposition. Indeed, most environmental monitoring standards are based on selecting

particles according to their aerodynamic diameter (e.g. PM

 PM

25

), as it dictates where

in the human respiratory system a particle is likely to become deposited and potentially

harm health.

The aerodynamic behaviour of a particle is influenced by both its inertia and the

drag forces it experiences, so it is dependent upon the particle’s physical size, shape and

density. Therefore, the D

of a particle is defined as the diameter of a sphere of unit

density 1gcm

−3

 that would attain the same velocity in the instrument. The APS is

again limited by the wavelength of the light used but is also limited by the fact that the

speeds of the smaller particles are very similar to that of the gas, so the lower limit of this

instrument is around 05 m (Chen et al., 1985). As with the OPCs, the measurements

are in real time so the time resolution of the data generated is very high.

Smaller particles cannot be reliably detected optically but those of around 1m and

smaller can be charged and then filtered by electrostatic classifiers, such as differential

mobility analysers or DMAs (Flagan, 1998, 2004; Knutson & Whitby, 1975; Winklmayr

et al., 1991). These use carefully controlled gas flows and variable electrical fields to select

charged aerosol particles according to the speed at which they migrate through a region

of flowing, particle-free air. Terminal velocity is reached when the electrostatic forces

they experience are balanced by their drag forces and is dictated by the electric field

strength and the charge, size and shape of the particle. A different size definition, the

electro-mobility diameter D

, is defined as the diameter of a sphere charged to −1e

that would have the same velocity of the subject particle under the same conditions. Note

that this quantity is completely independent of the particle’s density. Also note that the

effect of a particle’s shape on D

is qualitatively the opposite to its effect on D

;an

irregularly-shaped particle will have an increased D

but a decreased D

relative to a

spherical particle of the same volume. It is important to remember details such as these

when comparing data from different instruments.

By stepping or scanning the DMA voltage and taking multiple measurements of the

number concentration in the output using a CPC, a complete size-distributed number

concentration can be built up. This forms the basis of instruments such as the scanning

mobility particle sizer (e.g. TSI model 3034) (Wang & Flagan, 1990). The sizing limits

of these instruments are dictated by the geometry and flow rates of the DMA and the

capabilities of the CPC used.

Prior to size selection, the aerosol particles must be charged. A bipolar neutraliser

(e.g. TSI model 3077) is usually used for this purpose. A radioactive source (such as

Kr

Sr

210

Po or

241

Am) charges the gas molecules present in the aerosol and the

particles with positive or negative charges are neutralised by successive collisions with

the oppositely charged gas molecules. When equilibrium is reached, most particles are

neutralised, while a fraction will have a small positive or negative charge of one elementary

Mass Spectrometric Methods for Aerosol Composition Measurements 269

charge. As a certain fraction inevitably become charged with two or more elementary

charges, an amount of the particles of a given D

will be of larger physical sizes than those

intended. However, as this charging behaviour is predictable, it can be taken account of

at the analysis stage through weighted subtractions of the appropriate higher size bins

(Adachi et al., 1985; Fuchs, 1963; Liu and Pui, 1974; Wiedensohler, 1988).

A disadvantage of coupling a DMA to a CPC is that the scanning procedure normally

takes minutes, which makes it undesirable for the study of rapidly changing systems. To

address this problem, fast electrostatic particle spectrometers such as the TSI model 3090

and Cambustion Ltd. model DMS500 (Cambridge, UK) have been developed to electro-

statically count and size particles in real time. These use an array of electrometers within

the classifier chamber to count the charged particles of multiple mobilities simultane-

ously, allowing complete size distributions to be delivered at a rate of up to 10 Hz. The

overall sensitivity of this design is lower, making it best suited to environments with high

particle numbers, such as when sampling emissions at their source. Unipolar diffusion

chargers are normally used to charge the aerosol particles with these instruments, which

use ionised gas molecules of a single polarity produced from a radioactive source or

corona discharge to charge the particles. These yield higher numbers of charged particles

but as they work under complex, non-equilibrium conditions, can add extra ambigu-

ities to the sizing and counting abilities of the instrument (Biskos et al., 2004; Tammet

et al., 2002).

6.3 Particle composition

In addition to the number, size and mass, the structure and compositions of particles

in an aerosol sample are very important quantitative factors to consider in atmospheric

science. These can be a fundamental factor in determining the important properties of

particles, such as how they interact with incident light, what physiological damage they

may cause and whether they are likely to form suitable nuclei for cloud droplets. There

are many instruments that are designed to probe these changes in behaviour directly,

such as integrating nephelometers (e.g. TSI model 3563) and cloud condensation nucleus

(CCN) counters (e.g. Droplet Measurement Technologies CCN). However, in order to

better understand the underlying processes it is desirable to explicitly know the chemical

compositions of the aerosol particles.

It is also desirable to know how the component chemical species are distributed among

the particles, or their mixing state, as this is again important in how they will interact

with the gas phase. Two particulate constituents are said to be internally mixed if they

coexist in the individual particles in a population, whereas if they are externally mixed,

the two components are present in two distinct particle populations mixed within the

aerosol. In the real world, more complex mixing states are often observed.

6.3.1 Ofﬂine methods

The chemical analysis of aerosols is not as straightforward as the counting and sizing,

due to the small amount of mass present. The most direct method is to pass a sample

270 Analytical Techniques for Atmospheric Measurement

flow through a filter and analyse the accumulated material in the laboratory using

standard analytical procedures. A common method of analysing the collected material

is to dissolve the material in water and analyse the solution using ion chromatography

(IC). This well-established analytical technique quantitatively measures the amounts of

aqueous inorganic ions present. However, other aqueous-phase analysis techniques are

possible; for example, proton nuclear magnetic resonance (HNMR), which is useful for

the identification of functional groups in the water soluble organic fraction (Decesari et al.,

2000). Analysis using other solvents and extraction techniques can also be performed.

Other techniques include evolved gas analysis (EGA), where the material is heated

and the resultant gasses analysed using any one of a number of gas-phase techniques.

This is particularly useful for analysing insoluble organic components and the evolved

gasses are frequently analysed by gas chromatography coupled with mass spectrometry,

another common analytical procedure, which is described in detail in Chapter 8. While

this is useful for identifying specific compounds, much of the matter within particles

typically remains unresolved. The thermo-optical organic carbon (OC)/elemental carbon

(EC) analyser (Sunset Laboratories, Forest Grove, OR) (Birch & Cary, 1996) is another

common example of EGA; the sample is first heated in an inert atmosphere (e.g. helium)

and the amount of evolved carbon measured to give a measure of the organic carbon

(OC), after which it is exposed to oxygen and the subsequent amount of gas-phase carbon

that is evolved gives a measure of the amount of elemental carbon present (EC). The

advantage of this method is that while it does not speciate the organic matter present, it

does account for all of the mass.

There are also common methods that can analyse the sample in place, without trans-

ferring it to the mobile phase, such as X-ray fluorescence, which is useful for the detection

of specific elements, metals in particular, and Fourier transform infrared spectroscopy,

which can be used to identify organic functional groups and structure.

The material used for the filter (the substrate) can be tailored to the intended analytical

technique, and types in common use include cellulose (paper), PTFE and quartz fibre.

A degree of size-selection is possible with this technique. For instance, upstream cyclones

or impactors can be used to remove larger particles and filters with larger pores can be

chosen to so that smaller particles are not collected. However, it is not generally possible

to obtain any detailed size-resolved information with these methods.

An alternative method of sample collection is the cascade impactor (e.g. MSP Corp.

MOUDI, Shoreview, MN, USA). The principle behind an impactor is to direct the

sample flow at high speed towards the substrate, mounted on a solid surface. The

rapid change in direction as the air passes around the surface causes particles over a

certain D

(dictated by the flow rate, pressure and impactor geometry) to leave the gas

streamlines and impact on the substrate. By arranging several impactor stages in series

with successively decreasing cutpoints, particles can be separated and collected according

to their sizes. Because the substrates do not need to be porous, a wider choice of materials

(e.g. aluminium) is available. The Dekati Measurements Ltd. Electrical Low Pressure

Impactor (Tampere, Finland) is a variation on this technique, where the particles are

charged prior to collection. By measuring the current being passed to each impactor stage

using electrometers, a real-time aerodynamic size distribution can be generated during

sample collection.

A problem with impactors in general is that they rely on the particles sticking to the

substrates when they impact; they are frequently known to ‘bounce’ or otherwise become

SCIAMACHY tropospheric NO

: 08.2002

VC NO

[molec cm

–2

]

–20 –15 –10 –5 0 5 10 15

Longitude

Latitude

5.0 10

4.0 10

3.0 10

2.0 10

1.0 10

0.0 10

–1.0 10

20 25 30 35 40 45 50

UNIVERSITÄT

BREMEN

Plate 1 Tropospheric vertical column for NO

over Europe measured by the SCIAMACHY instrument

aboard the ENVISAT satellite. The pixel size is 30 × 60 km, and the retrieval used the reference sector

method. (Reproduced with permission from Richter et al., 2004, 2005, University of Bremen.)

(See Figure 1.8.)

1970

1975 1980 1990 1995 2000 20051985

Year

320

340

360

380

400

420

(ppm)

1900

1800

1700

1600

1500

1400

(ppb)

Barrow CO

Mauna Loa CO

Samoa CO

South Pole CO

Barrow CH

Mauna Loa CH

Plate 2 Concentrations of the greenhouse gases methane and CO

measured by the NOAA CMDL

monitoring network since 1973. Methane is measured using a gas chromatograph with a flame ionisa-

tion detector and CO

is measured using a non-dispersive infrared (NDIR) analyser. (Data courtesy of

Pieter Tans, NOAA.) (See Figure 1.10.)

ALT-Colorplate 1/19/06 12:53 PM Page 1