Electron microbeam analyses of aerosol particles from the ... · Electron microbeam analyses of aerosol particles from the plume of Poa´s Volcano, Costa Rica and comparison with

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Journal of Volcanology and Geotherm

Electron microbeam analyses of aerosol particles from the

plume of Poas Volcano, Costa Rica and comparison with

equilibrium plume chemistry modeling

M.A. Pfeffer *, F.J.M. Rietmeijer, A.J. Brearley, T.P. Fischer

Department of Earth and Planetary Sciences, MSC03-2040, 1-University of New Mexico, Albuquerque, NM 87131-0001, USA

Received 5 December 2004; received in revised form 13 July 2005; accepted 27 October 2005

Available online 4 January 2006

Abstract

Aerosol particles have been collected passively at four locations in and around the Poas Volcano crater. The compositions and

morphologies of the collected particles were examined using transmission electron microscopy and atomic force microscopy. This

study demonstrates that submicron volcanic aerosol particles including volatile species can be analyzed at high resolutions to

observe compositional and morphological variations at close distances to fumaroles. Collected particles are both solids and

condensed liquids. Liquid particles exhibit three distinct morphologies: aggregate, vesicular, and dendritic. In order to gain insight

into the collected particles’ formation mechanisms and conditions within the plume, the original composition of the volcanic

emissions as defined by gas and vapor condensate samples was introduced to an equilibrium chemistry model. A Gaussian plume

dispersion model was used to calculate the rate of air entrainment into the plume for use in the plume composition model. Of the

collected particles, only the aggregate condensed-liquid particles have compositions predicted by the model. These particles display

morphologies consistent with formation by homogeneous nucleation—the only particle formation mechanism included in the

model. The other collected particles exhibit traits that suggest they were formed by additional mechanisms including heterogeneous

nucleation, liquid particle coalescence, and the inclusion of background dust particles. The model demonstrates that observable

SO2 gas within the plume is in a state of disequilibrium with the surrounding air. We conclude that assumptions of instantaneous

equilibration between low temperature volcanic gases and surrounding ambient air and homogeneous nucleation as the sole source

for volcanic aerosol formation are insufficient to explain our observations of particles within a near-source volcanic plume.

D 2005 Elsevier B.V. All rights reserved.

Keywords: Poas; volcanic plume sampling; analytical transmission electron microscopy; atomic force microscopy; volcanic particles; volcanic

plume chemistry; equilibrium plume modeling

1. Introduction

Volcanoes release gases and particles that can have

influences on the atmosphere and on the earth’s radi-

0377-0273/$ - see front matter D 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.jvolgeores.2005.10.009

* Corresponding author. Present address: Max Planck Institute for

Meteorology, Bundesstrahe 53, 20146 Hamburg, Germany. Tel.: +49

40 41173 147; fax: +49 40 41173 298.

E-mail address: [email protected] (M.A. Pfeffer).

ative budget depending on many factors, including

release height, composition, and size. Most volcanic

emissions are released via quiescent degassing (e.g.

Berresheim and Jaeschke, 1983; Andres and Kasgnoc,

1998; Halmer et al., 2002). Quiescent emissions are

rapidly rained out and/or deposited locally and region-

ally compared with eruptive emissions, but they are

continually replenished over long periods of time. Non-

eruptive volcanic emissions that are not released with

al Research 152 (2006) 174–188

M.A. Pfeffer et al. / Journal of Volcanology and Geothermal Research 152 (2006) 174–188 175

sufficient force to carry them into the stratosphere are

still able to have a significant atmospheric impact

because volcanoes are often at a sufficient elevation

to be above the planetary boundary layer, allowing

those emissions to remain in the troposphere longer

than, for example, most anthropogenic emissions. An

example of the relative significance of quiescent vol-

canic degassing is that such sources may be responsi-

ble for 24% of the total annual mean direct radiative

top-of-atmosphere forcing (Graf et al., 1997).

The environmental impacts of volcanic emissions

are largely due to the formation of volcanic aerosols

when volcanic gases interact with ambient air. Aero-

sols are collections of solid and/or liquid particles

suspended within a gas; in the case of volcanic aero-

sols, the suspending gas is air. Microscopic studies of

aerosol particles can provide information about parti-

cle sizes—the most important trait for characterizing

aerosol behavior (Hinds, 1999). Aerosol particle dia-

meters can range from 10�9 m to more than 10�5 m.

Particles are generally separated into different size

classes: nucleation, or Aitken, mode: 10�9–10�7 m

radius, accumulation mode: 10�7–10�6 m radius, and

coarse mode: N10�6 m radius. Mather et al. (2003)

have summarized the research performed on volcanic

particles and they describe that volcanic aerosol par-

ticle characterization has thus far been limited to

particles larger than ~100 nm and has not yet extend-

ed down to the nucleation size mode.

Particle formation mechanisms are homogeneous

and heterogeneous nucleation, which generate so-called

bsecondaryQ particles. Homogeneous nucleation, or self-

nucleation, is the bformation of particles from a super-

saturated vapor without the assistance of condensation

nuclei or ionsQ (Hinds, 1999). The more common het-

erogeneous nucleation is ba process of particle forma-

tion and growth that is promoted by the presence of

condensation nuclei or ionsQ (Hinds, 1999). Homoge-

neous nucleation is relatively rare in ambient air, but

may occur in a volcanic plume, when relatively hot

gases are cooled extremely rapidly upon contact with

air, producing a supersaturated vapor. Homogeneous

nucleation within a volcanic crater environment, how-

ever, is suppressed due to the presence of many parti-

cles in the air that the gases can heterogeneously

nucleate on, for example on particles carried out from

the fumarolic conduit by exiting gases. Heterogeneous

nucleation can occur in unsaturated conditions, and

even in relatively bcleanQ ambient air there are generally

particles or ions in the air that can serve as nucleation

sites, while the bdirtyQ air around a volcano is particu-

larly conducive to this particle formation mechanism.

Homogeneously and heterogeneously nucleated aerosol

particles are considered bsecondaryQ particles because

there was a precursor gas present in the atmosphere

prior to the particle formation, whereas solid particles

lofted into the air are considered bprimaryQ particles.

Particle growth mechanisms can include condensation:

whereby a gas becomes a liquid, coagulation: whereby

individual particles join together into a larger particle,

and coalescence: whereby multiple liquid particles col-

lide and merge to form a larger liquid particle.

In this study, we were interested in examining aero-

sol particles from a low temperature, persistently degas-

sing volcanic plume to learn about the aerosol

formation and modification processes occurring in

such a plume. To this end, aerosol particles were col-

lected passively from the plume of Poas Volcano, Costa

Rica and examined at the submicron scale so as to be

able to observe nucleation mode-sized particles. Poas

Volcano is a composite basaltic-andesite cone on the

western edge of the Central Volcanic Range of Costa

Rica. Poas has continually degassed at highly variable

rates (8–755 tonnes/day SO2) since at least 1828 (Casa-

devall et al., 1984; Rowe et al., 1992; Andres and

Kasgnoc, 1998; Zimmer et al., 2004). Poas’ active

crater is approximately 1500 m in diameter and con-

tains a crater lake.

2. Sample collection and analyses

Fieldwork was performed on March 12, 2001 when

most emissions were observed to be released by a

cluster of fumaroles on the edge of the crater lake,

with one fumarole degassing much stronger than the

others. This one fumarole appeared to be the dominant

source for the plume observed on this day. Within the

last 20 yr, other gas sources have included a 30-m-high

dome of altered lava and pyroclastic material inside the

active crater, the crater lake, and other fumaroles

(Rymer et al., 2000). The volcanic emissions appeared

to follow the generally westerly prevailing winds, as

evidenced by a downwind swath of dead vegetation.

Airborne particles were passively collected at four

locations: (A) 1 m above the fumarole degassing the

strongest on the day of collection, (B) on the crater

rim—280 m downwind from the lakeside cluster of

fumaroles, (C) 1 m above the dome, and (D) on the

crater rim—250 m downwind from the dome (Fig. 1).

Locations bAQ and bBQ are within the dominant plume

on the day of sampling; bCQ and bDQ are related to the

path of diffuse emissions from the dome. The particles

were collected using a device designed and built for this

study, consisting of a sealed box connected to a motor

Fig. 2. Schematic of the particle-sampling device.

Fig. 1. Photograph of Poas’ active crater facing north indicating

sampling locations. (A) Vapor condensates collected and (2 m alti-

tude) particles collected. (B) Crater rim, downwind from the fumarole

field. (C) Dome. (D) Crater rim, downwind from the dome. (E) Crater

rim, where COSPEC was employed.

M.A. Pfeffer et al. / Journal of Volcanology and Geothermal Research 152 (2006) 174–188176

that opened and closed the box for the pre-programmed

sampling period of 30 s (Fig. 2). The sampling device

was held by hand about 2 m above the ground during

collection of airborne aerosols that impacted on the

exposed surface. In the opened box, 5 standard Cu

Transmission Electron Microscope (TEM) grids (SPI

Supplies, 2 mm square 200 mesh), each supporting

a continuous, C thin film, were exposed to the plume.

Boxes were prepared inside a laminar flow hood at

UNM to prevent contamination prior to sampling. A

prepared box was mounted on the collector prior to

sampling. Immediately following collection, each ex-

posed sample box was placed in cold storage to help

prevent further reactions, such as devolatilization, be-

fore TEM analysis.

Collected particles were studied using a JEOL 2010

high-resolution TEM (HRTEM) operating at 200 keV

with a point-to-point resolution of 0.193 nm. In situ

individual chemical analyses were obtained using an

Oxford LINK ISIS 200 energy dispersive X-ray spec-

trometer (EDS) with probe size smaller than or similar

to the individual particles. The EDS system has an

ultra-thin window that allows light element detection,

with a relative error of 3% for non-volatile particles.

Only semi-quantitative analyses could be obtained for

volatile particles due to volatilization when exposed to

the incident electron beam in the high vacuum in the

TEM. The composition of the TEM grids prohibits

analysis of Cu and C in the particles. Seven different

TEM grids were examined; TEM images were obtained

for 20 randomly selected individual particles; EDS

analyses were obtained for 52 individual particles. Se-

lected area electron diffraction (SAED) analysis was

performed on individual particles to determine their

crystallographic properties. Direct lattice fringe imag-

ing was performed on crystalline particles. For two

non-volatile, crystalline particles the unit cell para-

meters were measured (relative error ~10%). Particle

size was measured directly on calibrated TEM images;

relative error is expected to be the same as that mea-

sured by Posfai et al. (1998) (~10% for comparatively

non-volatile particles and ~35–55% for volatile parti-

cles). Three-dimensional morphologies were obtained

using a NanoScope IIIa (Digital Instruments) atomic

force microscope (AFM) in tapping mode for particles

collected on two grids different from those used in the

TEM analyses. The AFM analyses were conducted at

room temperature and atmospheric pressure. Particle

size measurements using the AFM’s integrated software

generally have less than 5% relative error.

One vapor condensate sample was collected at loca-

tion bAQ by inserting a Ti tube into the fumarole. The

tube was connected to a glass condenser in a water bath

that collected the fumarolic gases as a condensed liquid.

The collected condensed liquid was stored in a sealed

glass bottle until analysis. The composition of the vapor

condensate sample was established using ion chroma-

tography (IC), atomic absorption spectroscopy (AAS),

and Inductively Coupled Plasma Mass Spectrometry

(ICP-MS). The collection and analysis of a gas sample

collected simultaneously with a bGiggenbachQ bottle is

described in Zimmer et al. (2004) (Sample P5).

Correlation Spectroscopy (COSPEC) measurements

of the plume SO2 flux were made at Poas on the same

day as the fieldwork described here by Zimmer et al.

(2004).

3. Analytical results

The sizes and compositional major elements of the

collected aerosol particles and the composition of the

vapor condensate sample are presented.

Table 1

EDS-determined particle compositions (at.%)a

(a) Particles collected above fumarole (location A)

Particle ID O Mg Al Si S K Fe Au % Error

NPO 1 87.9 12.1 4

NPO 2 75.0 19.5 5.5 2

16 8.0 2.4 86.8 2.8 1

17 100.0 1

18 81.0 19.0 1

19 71.1 28.9 1

20 78.1 21.9 4

21 7.3 0.5 91.7 0.6 1

22 29.9 4.2 63.8 1.5 0.7 1

23 58.7 39.2 0.9 1.0 0.2 3

24 62.2 1.9 35.7 0.1 0.1 3

25 85.0 15.0 9

26a 78.4 2.5 19.1 3

26b 83.6 16.4 3

27a 64.2 13.6 19.2 2.2 0.4 0.5 1

27b 58.2 18.9 22.6 0.4 3

28 95.1 4.9 1

29 55.7 21.8 21.7 0.8 4

30 65.0 16.2 18.8 3

Summary of compositions

[Fe+Au]

[SFOF (Si, Fe, Au)]

[O+Mg+Si+(S)]

[OFSiFAlF (S, Fe, K)]

(b) Particles collected on crater rim originating from fumarole (location B)

Particle ID O Na Mg Al Si P S Cl K Ca Fe Ni Au % Error

NPO 76.8 23.2 4

31a 56.6 20.3 22.6 0.1 0.2 0.1 0.1

31b 50.5 5.7 12.8 1.3 10.6 0.3 0.3 18.3 0.1 1

32 51.7 10.0 15.7 0.5 0.5 0.3 20.8 0.3 0.2 1

33 57.5 19.6 21.6 0.3 0.3 0.2 0.5 0.1 1

34 60.2 38.1 0.7 0.2 0.6 0.2 2

35a 66.8 3.4 12.9 2.8 12.9 1.1

35b 62.3 8.6 2.6 1.9 10.1 2.2 1.4 9.8 0.9 2

36 71.7 26.5 1.1 0.4 0.3 1

37 65.5 13.2 8.1 1.4 8.1 3.9 2

38 50.7 33.8 3.8 3.4 3.2 2.4 2.4 0.4 4

39 68.4 19.4 7.4 2.0 2.8 5

40 30.2 2.0 3.4 40.6 18.2 3.0 1.6 1.1 1

41 63.0 2.1 19.9 11.6 1.5 1.2 0.8 1


[Si+OF (S, Fe, Au, Cl, Ca, Ni)]

[Si+O+AlFNiFFeFClF (S, Ca, Au)]

[S+Ca+Mg+O+(Si, P, Cl, K, Fe)]

[S+Cl+OF (Si, Ca, Fe, Au, Al)]

[Na+Si+OF (Si, Ca, Fe, Au, Al)]

(continued on next page)


(c) Particles collected above dome (location C)

Particle ID O Al Si S Cl Ca Cr Fe Sn Au Pb % Error

1a 41.1 12.8 15.1 8.5 13.4 9.1 1

1b 46.7 10.0 10.9 6.5 16.1 9.8

2 2.3 58.3 4.4 14.5 6.9 6.3 3.2 2.3

3 0.7 0.6 1.6 1.0 93.6 1.1 0.5 1.1

4 45.7 50.1 0.6 0.9 0.5 0.4 1.0 0.2 0.6 1

5 71.8 22.6 3.6 2.1 1

6 68.9 21.1 8.0 2.1 2


[Si+Cl+Fe+S+Au+CrF (Pb, Al, Ca)]

[O+Si+S+FeF (Sn, Cl, Pb, Cr, Au)]

(d) Particles collected on crater rim originating from dome (location D)

Particle ID O F Na Mg Si S Cl K Ca Fe % Error

NPO 1 43.3 50.1 6.6

NPO 2 39.2 47.8 13.0 1

NPO 3 21.8 60.1 3.6 3.7 10.8 1

7 37.5 43.7 18.8 3

8 28.5 16.7 1.3 0.7 52.8 1

9a 27.1 24.6 22.4 1.3 2.4 4.2 10.0 7.4 0.7 1

9b 36.7 24.8 16.2 1.2 1.3 5.1 8.5 5.8 0.6 1

11a 51.9 28.8 18.5 0.9 2

11b 32.7 39.6 26.0 0.4 1.3 1

12a 69.2 25.8 5.0

12b 69.1 19.5 11.4

13 77.3 22.7 1

14 69.5 30.5 1

15 86.6 13.4 2


[S+Cl+O]

[Fe+F+O]

[Na+Mg+K+Ca+OF (Cl, S, Si, Fe)]

[Mg+Si+O+(Fe, S)]

[Si+S+Fe]

[Si+O]

a The nomenclature described here applies to Table 1a–d. baQ and bbQ indicate that two analyses were performed on the same particle. Blank

regions indicate element not detected. bNPOQ designates analyses of regions without observable particle deposits. b+Q is used to indicate that

elements are always together in the combination listed; bFQ is used to indicate that an element is not found in all particles of each specific grouping;

elements in parenthesis are observed in trace amounts.

Table 1 (continued)


3.1. Aerosol particles

Aerosol particle compositions and a summary for

each of the four sampling locations are presented in

Table 1a–d. Presented error estimates were calculated

from the number of counts in each measurement. Only

particles collected at sampling site bAQ contained nei-

ther Cl nor Ca (Table 1a). Particles containing Ni and

P were only collected at bBQ (Table 1b). Particles

containing Cr, Pb, and Sn were only collected at

bCQ, the only location where K was not collected

(Table 1c). Only particles collected at location bDQwere Al-free and may contain F (Table 1d). Sodium

was only detected in particles collected at the crater

rim locations (bBQ and bDQ). Areas on the carbon thin-

film exposed to the plume without visible particle

deposits when viewed in the TEM imaging mode

were found to contain Si+O and lesser Fe (identified

as bNPOQ for no particle observed; Table 1a–d). One

such area also contained small amounts of S and Cl.

We assume due to our sample preparation in a laminar

flow hood and observations performed under vacuum

Fig. 3. Bright-field TEM image of a solid particle (Particle ID 8, see Table 1d) and its corresponding EDS spectrum.


that these signals are not due to contaminants, but that

the condensable fraction of the plume vapor was

dominated by [Si+O].

The collected particles demonstrate two distinct

morphologies: (a) non-spherical with well-defined

straight edges (6 particles; Fig. 3) and (b) (semi-) spher-

ical (12 particles). Two particles have intermediate

morphologies. The non-spherical particles include crys-

talline particles and will be referred to as solid particles.

The semi-spherical morphology exhibited by the second

group indicates these particles are likely condensed

liquid droplets. The condensed liquid particles are sub-

divided by morphology into: (a) aggregate (3 particles),

(b) vesicular (6 particles; Fig. 4), and (c) dendritic (3

particles; Fig. 5) particles. Samples from location bAQincluded 1 solid, 2 aggregate, 2 vesicular, and 2 den-

dritic particles. At location bBQ, 1 solid, 1 aggregate, and2 vesicular particles were collected. Only 1 solid particle

was collected at location bCQ. At location bDQ, 3 solid, 2vesicular, and 1 dendritic particles were collected.

The sizes measured by AFM of particles collected

at the fumarole-related sampling locations (bAQ and

bBQ) show a bimodal distribution for the particles

collected at bAQ (Fig. 6). These particles have a

large concentration in the 0–50 nm diameter range

and a secondary cluster in the 401–450 nm diameter

range. Particles collected at bBQ demonstrate a peak at

301–350 nm.

3.1.1. Solid particles

The solid particles (#29, 31, 1, 8, 11, 12) have an

average length of 150 nm (11–500 nm range). Two

particles have been tentatively identified (Table 2) as

(a) a high-temperature SiO2 phase (cristobalite, h-quartz, or tridymite) and (b) FeF2*4H2O. The latter

identification is subject to considerable uncertainty as

the interplanar values are also consistent with Fe-oxide

minerals. Its EDS spectrum shows a peak at 0.6 keV

that could be either the F Ka1 and Ka

2 peaks or the Fe Lk

peak. We note that F was present in the emitted vapor

Fig. 4. Bright-field TEM image of a vesicular condensed liquid particle (Particle ID 39, see Table 1d) and its corresponding EDS spectrum.


(Table 2) making the identification of this mineral

plausible but not definitive.

3.1.2. Aggregate condensed liquid aerosol particles

The three aggregate condensed liquid aerosol parti-

cles (#22, 24, 40) have an average diameter of 35 nm

(28–65 nm range) and appear to be aggregates of many

semi-spherical ellipsoids. These particles are primarily

[Si+O] (Table 1a and b).

3.1.3. Vesicular condensed liquid aerosol particles

The six vesicular condensed liquid aerosol particles

(# 25, 27, 36, 39, 13, 14) have an average diameter of

470 nm (200 nm–1 Am range). These particles appear

as homogeneous deposits with irregular shading in

the TEM viewing mode likely due to variable thickness

of the deposited particles. The vesicular nature of these

particles is attributed to devolatilization, which we

assume occurred during HRTEM analysis. While the

morphological distinctions of the solid and aggregate

condensed liquid particles were defined by primary

characteristics, the distinguishing feature of the vesicu-

lar condensed liquid aerosol particles is a secondary

trait due to volatile materials being exposed to the

incident electron beam and the vacuum during

HRTEM analysis. The degree of vesiculation varies

among the particles from concentrated zones of very

small vesicles within a relatively intact sphere to highly

vesicular particles that appear to be only a thin residue,

as would be consistent with a large degree of devola-

tilization (Fig. 4). The particles exhibiting high vesicu-

lation (#13, 14, 25, 36; Table 1a, b, and d) have almost

complete [Si+O] compositions and are very similar in

appearance to the aggregate particles. The vesicular

particles with the least vesiculation (#27, 39; Table 1a

and b) contained 15% Al and 20% Na, respectively.

3.1.4. Dendritic condensed liquid aerosol particles

The three dendritic particles (#16, 18, 9) have an

average diameter of 2500 nm and vary in the size and

Fig. 5. Bright-field TEM image of a dendritic condensed liquid particle (Particle ID 9, see Table 1a) and its corresponding EDS spectrum.

Fig. 6. Distribution of particle sizes from sampling locations A

(bfumaroleQ, white) and B (bcrater rimQ, black) as determined by AFM.


density of dendrites, each with a unique composition

without a common chemical trend (Table 1a and d). One

dendritic particle has dendrites that are 25–100 nm wide

Table 2

Mineral identification of solid particlesa

Particle IDb Interplanar values (nm) Mineral identification

Cristobalitec

h Quartzd

1 0.2 Tridymitee

8 0.42, 0.34, 0.21 FeF2*4H2O

a Interplanar (d-) values were obtained by SAED analyses or lattice

spacing value measurements directly from TEM images. d-values

were not unique to the identified minerals; based on the measured

major elements of the particles, the best fitting identification was

chosen. The data does not allow unambiguous identification of the

silica polymorphs.b Particle identification numbers from Table 1c and d.c JCPDS ref. # 11-693: [202]; 2.019 A.d JCPDS ref. # 11-232: [201]; 2.01 A.e JCPDS ref. # 18-1169: [004]; 2.039 A.

Table 3

Vapor condensate composition

Element Concentration

(ppm)

SO4a 160.0F1

Nab 16.5

Fa 15.4F0.3

Cla 11.8F0.1

Sib 0.90

Alc 0.25

Mgb 0.07F0.01

Feb 0.065F0.01

Nic 0.00752

Cab 0.005F0.001

Auc 0.00121

Coc 0.00024

a Determined using ion chromatography.b Determined using atomic absorption spectroscopy.c Determined using inductively coupled plasma-mass spectrometry.


and contains embedded regular grains averaging 70 nm

across (50–75 nm range) (Fig. 5).

3.2. Vapor condensate

The vapor condensate sample contained primarily

SO42�, Na+, F�, and Cl� (Table 3).

4. Theoretical modeling

We were interested in determining if the collected

particles possessed compositions consistent with equi-

librium formation conditions. To this end, field observa-

Fig. 7. Schematic of the

tions and theoretical modeling were integrated, whereby

the initial composition of the volcanic emissions, emis-

sion flux rate, and the calculated volume of ambient air

entrained within the plume were used as input informa-

tion for HSC Chemistry (produced by the Outokumpu

Corporation) to calculate theoretically stable particle

compositions (Fig. 7). The theoretical particle composi-

tions were then compared with the measured major

element ratios in collected aerosol particles. We initially

used GASWORKS (Symonds and Reed, 1993) to cal-

culate the particle compositions, but found the model to

be unsuitable for the cool temperatures in the plume of

Poas and subsequently used HSC Chemistry for these

calculations as it runs stably at lower temperatures. We

used the model in a stepwise fashion at progressive

distances away from the volcanic crater. A Gaussian

dispersion model was used to calculate the volume of

ambient air that was entrained within the plume for use

in the equilibrium chemistry modeling.

4.1. Dispersion

Plume dispersion was calculated to provide the vol-

ume of air entrained within the plume at progressive

distances away from the volcanic crater. The Gaussian

dispersion equation (USEPA, 1995; Eq. (1)) was used to

calculate the concentration of volcanic gases in the

plume as it moved downwind. Gaussian dispersion

was selected because it is simple and the Gaussian

function is commonly used to describe the dispersion

of plumes from factories (USEPA, 1995), which are

point sources, a designation also applicable to volca-

presented work.

Table 4

Fumarolic gas compositiona

Element Concentration

(mol%)

H2O 98.9

CO2 0.94

St 0.16

SO2 0.15

HCl 0.02

H2S 0.014

N2 2.3�10�3


noes. The volume of air necessary to be entrained

within the plume to achieve that concentration was

then calculated (Eq. (2)).

C x; y; zð Þ ¼ Q

2plryrz

e�y22ry e

� z�Hð Þ2

2r2z þ e� zþHð Þ2

2r2z

� �ð1Þ

where:

C(x,y,z) Gas concentration at the specified Coordinate

HF 6.8�10�4
[ML�3]
H2 2.6�10�4

O2 1.7�10�4

Ar 2.9�10�5

He 8.6�10�6

CO 1.4�10�6

NH3 2.1�10�8

CH4 0.00

aFrom Zimmer et al. (2004); sample ID P5.

x Downwind distance [L]

y Crosswind distance [L]

z Vertical distance above ground [L]

Q Gas emission rate [MT�1]

ry Lateral dispersion coefficient function [L]

rz Vertical dispersion coefficient function [L]

l Wind velocity in downwind direction [LT�1]

H Effective stack height [L]

Vair ¼Q4t

C� Vgas ð2Þ

where:

Vair Volume of air necessary to achieve calculated

concentration [L3]

Vgas Volume of gas prior to dilution with air (set

here to 0.125 s) [L3]

C Gas concentration at the specified coordinate

[ML�3]

Q Gas emission rate [MT�1]

t Time required to reach given distance [T]

The flux rate of the emissions (Q; Eq. (1)),

3�106 kg gas/day, was obtained by multiplying the

flux of SO2 from the volcano measured by Zimmer et

al. (2004) by the ratio of S to total gas in the gas sample

collected by Zimmer et al. (2004) (Table 4). The wind

velocity (l, Eq. (1)) was initially set to the observed

vertical 1.2 m/s bpuffQ rise-rate, and the x direction was

vertically upwards. At the height of the crater rim

(270 m), x was then changed to be horizontal and lchanged to the measured 2 m/s prevailing wind. The

slow wind speeds (1.2 and 2 m/s) and bright sunlight on

the day of sampling indicate bvery stableQ environmen-

tal conditions, or stability class bAQ (Pasquill stabilityclass designations from USEPA, 1995), dictating our

choice of the lateral and vertical dispersion coefficients

(ry and rz; Eq. (1)). We began applying Gaussian

dispersion at 100 m above the crater floor as the Gauss-

ian equation drives concentrations very high very near

to the source. Gas concentrations from the bottom of the

crater to 100 m elevation were calculated by extrapolat-

ing backwards logarithmically. Logarithmic extrapola-

tion was chosen in order to replicate the rapid diffusion

of the plume when the gases initially mix with the air.

On the day of our field observations, one fumarole

was visually dominating the emissions contributing to

the plume. This observation justifies our assumptions of

treating the fumarole field as a point source for the

volcanic plume and using the measured temperature

and composition of the dominant fumarole to represent

the entire fumarole field.

The modeled plume width was calculated for vali-

dation of the Gaussian plume dispersion model by

comparison with COSPEC observations (Eq. (3)). The

plume width at 50 m above the crater rim was calcu-

lated to be 350 m (assuming that the plume boundary

includes 95% of the gas’s mass).

W ¼ f ry ð3Þ

where:

W Width of the plume at the given distance

downwind [L]

f Constant dependent on the % of gas used to

define the edge of the plume; f =3 corresponds

to 95.3% of the gas’s mass included within the

plume boundary

ry Lateral dispersion coefficient function [L]

4.2. Particle composition

After the Gaussian plume dispersion model was used

to calculate the volume of air entrained within the

ig. 8. Predicted temperature (broken gray) and concentration (solid

lack) profiles of the plume as a function of logarithmic distance from

ource.


plume at consecutive distances away from source, the

HSC Chemistry model was used to calculate the com-

position of particles stable in each given gas/air mix-

ture. The original composition fumarolic emissions

were defined by the collected vapor condensate sample

(Table 3) and from the composition of a gas sample

collected at the same fumarole simultaneously (Zimmer

et al., 2004; Table 4). The composition of air included

in the model was modified from the dry air composition

described in the CRC Handbook (Lide and Frederikse,

1993) to be 0.0002 mol% specific humidity (Table 5).

The temperature of the plume at each consecutive

distance was calculated assuming cooling is solely a

function of the entrainment of air. The temperature of

the plume decreases until it reaches the ambient tem-

perature of 25 8C after 300 m of travel (Fig. 8).

Instantaneous thermodynamic equilibration and ho-

mogeneous nucleation of particles are inherent assump-

tions of the HSC Chemistry model. Fractional

condensation was not considered. Thermal equilibrium

at the calculated temperatures of the predicted volcanic

gas/air mixtures was modeled in a step-wise fashion.

The model includes over 1000 species that can be

generated by combining the initial volcanic gas with

air. In the original volcanic gases at 92 8C, the temper-

ature of the dominant fumarole, more than 200 species

are predicted to be stable. After air is introduced to the

system 60 species are stable in the volcanic gas/air

atmosphere. Stable species were grouped by similar

composition to facilitate plotting (Fig. 9).

Significant changes are predicted to occur at 0.15 m

from the source as a result of the initial introduction of

air (0.125 s of plume travel). For example, Al and Ca

are initially stable as sulfates in the emitted volcanic

gas. Following the introduction of air, they are predom-

inantly stable as CaAl2Si3O10(OH)2. The reaction pre-

dicted by the HSC Chemistry model for the Al and Ca

sulfates reacting to form CaAl2Si3O10(OH)2 is: Al2(SO4)3 + CaSO4+H2O+3SiO(g)=CaAl2Si3O10(OH)2 +

4SO2(g). Sulfur dioxide, H2S, and S rapidly become

Table 5

Moderately humid air

Component mol%

N2 0.781

O2 0.209

Ar 9.3�10�3

CO2 2.3�10�4

He 2.0�10�5

CH4 1.0�10�6

H2 5.0�10�7

H2O 2.0�10�4

F

b

s

unstable upon the introduction of air while NaCl and

FeSO4 lose stability at a slightly slower rate. Na2SO4,

MgSO4, NiSO4, and H2SO4 remain stable in the entire

region. NOx compounds including NaNOx vary contin-

ually and do not achieve a steady concentration in the

entire region of the modeled plume. The stability of

FeO*OH fluctuates until achieving a constant concen-

tration around 100 m from source.

5. Discussion

Plume width was predicted by the Gaussian plume

dispersion model to be 350 m at 50 m above the crater

rim and COSPEC measured a width of 500 m. This is

less than 30% disagreement between the modeled

plume expansion and observations. Better agreement

would be expected if a more complex model of air

entrainment were applied, however given the uncertain-

ties associated with wind speed and with COSPEC

measurements, we consider the agreement between

modeled plume expansion and observations to be

quite reasonable and supportive of applying this simple

Gaussian plume dispersion model. The collected parti-

cles have been paired, based on their major elements,

with the compositions of particles predicted to be stable

by the HSC Chemistry model (Table 6). We find a very

poor correlation, at best. The major element ratios in the

compounds predicted by the model are typically not

those observed in the collected particles. Further, there

are elements present in trace amounts that are not

accounted for by the model in almost all of the ob-

served particles. We will now consider how the major

element compositions of the collected aerosol particles

are able to be matched with the model’s predicted

particle compositions, and how the morphologies and

Fig. 9. Predicted compositional profile of the plume. Depicted compounds were chosen because they are the most abundant species bearing a

particular ion or of potential atmospheric influence. Compositionally similar compounds are shown as summations of the grouped compounds’

concentrations. 1All summed species have non-zero concentrations, but the marked species is dominating the total concentration.


compositions of the collected particles provide clues as

to why they were (were not) predicted by this model

and what this suggests about the particles’ formation

and growth mechanisms.

Table 6

Tentative comparison of observed and predicted particle major elements

Particle type Observed Predicted

Aggregate S+O (Si, Fe, Au) H2SO4

O+Si (Al, S, Fe) SiO2

O+Na (Si, S, Cl, K, Ca, Fe) NaNOx, Na2SO4

Vesicular O+Si SiO2

O+Si+Al (S, K, Fe) Al2(SO4)3*6H2O; S

O+Si (S, Fe, Au) SiO2

O+Na+Si (Cl, S) NaNOx, Na2HSO4,

Solid O+Mg+Si (S) MgSO4; SiO2

O+Si+Ni+Al+Cl (S, Fe, Ca, Au) NiSO4; SiO2; N4H4

Si+Cl+Fe+S+Au+Cr SiO2; HCl; FeSO4;

Fe+O+F (Si, S) FeO*OH; SF5Cl

O+Mg+Si (Fe, S) MgSO4; SiO2

Si+S+Fe FeSO4; SiO2

Dendritic S+O (Fe, Si) H2SO4

Fe+Au Au(OH)x; FeO*OH

O+Na+Mg+K+Ca (Cl, S, Si, Fe) Na2HSO4; MgSO4

5.1. Particle formation and growth mechanisms

In this section, we will attempt to deduce what

particle formation and growth mechanisms were re-

Prediction

reasonable

Prediction reasonable

(multiple species)

Yes

Yes

Yes

Yes

iO2 No Yes

Yes

SiO2 No Yes

No Yes

Cl8; HCl; CaAl2Si3O10(OH)2 No No

Au(OH)x No No

No No

No Yes

No No

Yes

No No

No No


sponsible for each morphological type of collected

aerosol particle.

5.1.1. Homogeneous nucleation

The condensed-liquid aggregate aerosol particles

have major elemental ratios that agree well with the

model-predicted particle compositions (Table 6). These

particles are the typical size of aerosol particles formed

by homogeneous nucleation (Heintzenberg et al.,

2003)—the only particle formation mechanism consid-

ered by the HSC Chemistry model. We interpret that

H2SO4, a Si-oxide, and Na2SO4 or NaNOx may have

nucleated homogeneously in Poas’ plume.

The aggregate particles represent only ~12% of the

particles examined. The other morphological types of

aerosols do not havemajor element ratios that matchwell

with the predicted stable compositions of the HSC

Chemistry model (Table 6). These other particles are

also considerably larger than homogeneously nucleated

aerosol particles (Heintzenberg et al., 2003). These

observations suggest that other mechanisms of particle

formation or modification, or both, additional to homo-

geneous nucleation are occurring in the plume. It is likely

that a second formation mechanism or growth mecha-

nism is active in the plume at an early stage, as two size

modes of particles are observed in samples collected just

above the dominant fumarole (Fig. 6). The lower peak is

the original size of the collected aggregate particles, and

the larger peak correlates with the sizes of the least

vesiculated of the vesicular particles.

5.1.2. Coalescence

The vesicular aerosol particles exhibit a trend where-

by the most heavily vesiculated particles exhibit simple

element combinations and the least vesiculated particles

have more complex chemistries (Table 6). The degree

of vesiculation in the vesicular particles could be due to

a large number of variables, including: initial variable

abundances of volatile materials, differing volatility of

various chemical species, temperature or pressure var-

iations during analysis, and heating and/or cooling rates

of the particles. The trend of increased chemical com-

plexity concurrent with decreased vesiculation suggests

that it is likely due to initial variable abundances of

volatile materials and/or differing volatility of various

chemical species. The most vesiculated of the vesicular

particles are very similar in appearance and in compo-

sition to the aggregate particles, and these heavily

vesiculated particles have major element combinations

that agree with the HSC Chemistry model’s predictions

of particle compositions. While the least vesiculated of

the vesicular particles do not have element combina-

tions that agree with the model’s predictions, when

multiple predicted particle compositions are grouped

together, quite good agreement between observed ele-

ment combinations and the model can be achieved.

These traits suggest that the vesicular particles were

formed by the coalescence of the previously generated,

homogeneously nucleated liquid aggregate particles.

The material that bound together the aggregate particles

volatilized during HRTEM analysis, creating the mor-

phological features that distinguish the vesicular class

of the collected aerosol particles. This possible forma-

tion mechanism for the observed vesicular aerosol par-

ticles is consistent with the increased size and increased

chemical complexity of the vesicular particles com-

pared with the aggregate particles (Table 6).

5.1.3. Heterogeneous nucleation

In addition to coalescence, bheterocoagulationQ of

liquid particles with background solid particles, as

well as the heterogeneous nucleation of vapor on solid

particles can also occur in the plume (Jacobson and

Seinfeld, 2004). Solid particles could be transported

into the crater by wind or could be generated in situ,

including particles produced by the weathering and

erosion of indigenous rocks. Background particles are

not considered by the model we employed or by any

currently existing model for volcanic gas/air interac-

tions. bPrimaryQ particles are usually 100 nm to 1 Amsize (Heintzenberg et al., 2003), similar to the measured

sizes of our collected solid particles. Most of the solid

particles were collected at the dome-related sampling

sites (four of six). The Poas dome is composed of poorly

consolidated debris and could provide plentiful material

for aolian dispersion. In order to test the idea that the

solid particles originated from the dome, we converted

the average composition of the collected solid particles

to the corresponding oxide mineralogy using the "Min-

eralogy Iterations" function of the HSC Chemistry

model and compared the oxide mineralogy with those

of rocks collected at the dome by Rowe et al. (1995).

The solid particles were found to contain less SiO2 and

Al2O3 and more Fe2O3 and MgO than the rocks collect-

ed inside the Poas crater from the dome, and contained

no CaO, Na2O, TiO2, K2O, MnO, or P2O5. We do not

observe a correlation between the compositions of the

dome rocks and the collected solid particles. This is not

surprising given the small number of collected solid

particles, rendering it unlikely that the average solid

particle composition would indeed match that of the

dome rocks. Aged, altered dome rock can also have

complex, heterogeneous compositions, rendering this

comparison even more difficult. Despite the incongruity


between the solid particle compositions and the dome

rock, we consider it reasonable that the dome could be

one source for solid dust particles within the crater.

Other sources that may contribute include the inner

crater walls and the altered crater floor.

Dendritic solidification can occur when a seed crystal

is nucleated or when a foreign solid particle is placed

within an under cooled liquid. The particles we collected

with dendritic morphologies have compositions much

more complex and enriched in metals compared with the

other condensed liquid particles. We do not, unfortu-

nately, have separate analyses for the embedded solid

crystals and the engulfing material observed in these

particles. If small particle inclusions in the non-equilib-

rium vapor did serve as heterogeneous nucleation sites

for the dendritic particles, these inclusions could also

perturb the particles’ growth, leading to the formation of

irregular dendritic morphologies (Granasy et al., 2003).

The crystal inclusions within our collected dendritic

particles do appear to deflect the path of the advancing

dendrites, suggesting that the crystalline inclusions were

solid and present before the dendritic growth. We infer

that the collected dendritic particles were formed due to

heterogeneous nucleation onto earlier existing crystal-

line particles. The observed dendritic morphology could

have developed upon impact with the collector if this

resulted in immediate solidification of the particle or by

devolatilization when exposed to the incident electron

beam during HRTEM analysis. The distinction between

the collected solid and dendritic particles could be due to

the relative proportion of solid to condensable liquid

fractions within the particles or the relatively large

dendritic particles could be the result of the coagulation

of multiple condensed-upon solid particles.

5.1.4. Condensation

Most of the collected particles contain trace elements

that are not accounted for by the predictions of the

model (Table 6). While the crystalline particles are

identified as simple minerals, their EDS spectra show

much more complex chemical compositions (#1, 8;

Table 1c and d). This suggests that these solids scav-

enged the vapor for pre-existing condensed liquid par-

ticles or served as substrates for vapor condensation.

The areas of O+Si and lesser Fe found on the sample

grids without associated observable particles suggests a

high concentration of this non-equilibrium vapor inside

and around the crater that condensed upon our sampling

grids, and likely upon particles as well.

Coalescence, condensation, and heterogeneous nu-

cleation are energetically favorable processes to contin-

ued homogeneous nucleation (Heintzenberg et al.,

2003) and would explain why we collected relatively

few examples of homogeneously nucleated particles (3

of 18). As with the inclusion of mineral dust, hetero-

geneous nucleation mechanisms, coalescence, and con-

densation are not considered by the model we

employed or by any of the other currently existing

models for volcanic gas/air interactions. We would

therefore not expect the compositions of particles pro-

duced by these processes to match the predicted particle

compositions. The failure of the model to predict these

aerosol types is a strong indication that they were not

formed by homogeneous nucleation, while the accuracy

of the aggregate aerosols indicates that they were in-

deed formed via this mechanism, demonstrating that

this model was an appropriate tool to learn about

aerosol particle formation mechanisms.

5.2. Disequilibria

A major assumption of the model is instantaneous

thermodynamic equilibrium. Reaction rates (i.e. reaction

kinetics) are not considered. The significance of this

assumption can be illustrated by the presence of SO2 in

the plume downwind, while the model predicts it to drop

in concentration from 1�10�4 to 1�10�7 mol as soon

as air mixes with the plume (Fig. 9). SO2 is detected with

COSPEC in this study on the crater rim, and is routinely

measured kilometers away from source at other volca-

noes (e.g. Sasaki et al., 1988; Oppenheimer et al., 1998;

Fischer et al., 2002). The oxidation of SO2 in a volcanic

plume does not result in instantaneous equilibration, and

it logically follows that other chemical reactions in the

plume may be similarly limited by kinetics. Discrepan-

cies between the model’s predictions and the observed

particle compositions can be explained by the inaccuracy

of an assumption of instantaneous equilibration, as well

as the inaccuracy of solely homogeneous particle nucle-

ation, which appear to be invalid assumptions in this

low-temperature volcanic plume.

6. Conclusions

The study presented here has demonstrated that

HRTEM and AFM are both powerful techniques to

characterize the compositions and morphologies of in

situ collected volcanic aerosol particles. Information

from these microscopy techniques can be combined

with theoretical chemical modeling to learn about the

processes occurring in a volcanic plume. In order to

describe the complex reactions that occur upon volcanic

gas/air interaction, we suggest that models of volcanic

particle evolution should evolve to incorporate homoge-


neous as well as heterogeneous nucleation, background

dust particles (possibly indigenous), condensation, coa-

lescence, and reaction kinetics. As these factors are

incorporated into improved volcanic plume models,

our understanding of the atmospheric effects of volcanic

particles will increase dramatically.

Acknowledgements

We thank two anonymous reviewers for their helpful

comments and ideas. Fieldwork at Poas was funded by

NSF grants EAR 0079402 MARGINS and EAR

0003668 and was carried out in collaboration with Car-

los Ramirez and Guillermo Alvarado I and with the

support of the staff of the Poas National Park. Transmis-

sion electron microscopy was carried out in the Electron

Microbeam Analysis Facility in the Department of Earth

and Planetary Sciences of the University of New Mex-

ico. Atomic force microscopy was carried out in the

laboratory of David Keller in the Department of Chem-

istry of the University of NewMexicowith the assistance

of SolomonBasame. TPF andMAPwere funded byNSF

grants EAR 0079402 MARGINS and EAR 0003668.

FJMR was supported by NASA grant NAG5-11762. We

gratefully acknowledge field assistance and laboratory

support from numerous people at UNM especially John

Husler, Yemane Asmerom, Bob Macy, Mindy Zimmer,

David Vinson, and Lynne Elkins. MAP thanks Philipp

Weis and Angelika Heil.

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