Reconnaissance gas measurements on the

East Rift Zone of Kīlauea volcano, Hawai‘i by

Fourier transform infrared spectroscopy

By Kenneth A. McGee1, Tamar Elias2, A. Jefferson Sutton2, Michael P.

Doukas

1

, Peter G. Zemek3 and Terrence M. Gerlach1

Open File Report 2005-1062

2005

Use of any trade, firm or product name is for descriptive purposes only and does not constitute

endorsement by the U.S. Government

U.S. GEOLOGICAL SURVEY

U.S. DEPARTMENT OF INTERIOR

1

U.S. Geological Survey, Cascades Volcano Observatory, Vancouver, WA, 98685

2

U.S. Geological Survey, Hawaiian Volcano Observatory, Hawaii National Park,

HI 96718

3

MIDAC Corporation, Costa Mesa, CA 92626
2

Abstract. We report the results of a set of measurements of volcanic gases on two small

ground level plumes in the vicinity of Pu‘u ‘Ō‘ō cone on the middle East Rift Zone

(ERZ) of Kīlauea volcano, Hawai‘i on 15 June 2001 using open-path Fourier transform

infrared (FTIR) spectroscopy. The work was carried out as a reconnaissance survey to 

assess the monitoring and research value of FTIR measurements at this volcano. Despite

representing emissions of residual volatiles from lava that has undergone prior degassing,

the plumes contained detectable amounts of CO2, CO, SO2, HCl, HF and SiF4. Various

processes, including subsurface cooling, condensation of water in the atmospheric plume,

oxidation, dissolution in water, and reactions with wall rocks at plume vents affect the

abundance of these gases. Low concentrations of volcanic CO2 measured against a high 

ambient background are not well constrained by FTIR spectroscopy. Although there

appear to be some differences between these gases and Pu‘u ‘Ō‘ō source gases, ratios of

HCl/SO2, HF/SO2 and CO/SO2 determined by FTIR measurements of these two small

plumes compare reasonably well with earlier published analyses of ERZ vent samples.

The measurements yielded emission rate estimates of 4, 11 and 4 t d-1 for CO, HCl and 

HF respectively for the ERZ.

Keywords: volcanic gases · Fourier transform infrared spectroscopy · degassing · plume ·

East Rift Zone · Kilauea volcano · Hawaii · FTIR
3

Introduction

Kīlauea volcano, one of five volcanoes located on the Big Island of Hawai‘i, is

one of the most active volcanoes in the world and has been in nearly continuous eruption 

since 3 January 1983 (Tilling and Dvorak 1993; Heliker and Mattox 2003). Kīlauea lies

on the southeastern flank of Mauna Loa volcano and consists of a summit caldera and 

two major zones, the east rift zone (ERZ) and the southwest rift zone (Fig. 1). The middle

portion of the ERZ is the site of the current activity. The ongoing Pu‘u ‘Ō‘ō –

Kūpaianaha eruption has been one of the most voluminous eruptions on the ERZ in the

recent past, producing more than 2 km3 of lava by the beginning of 2002 (Heliker and

Mattox 2003). The current phase of the eruption, termed episode 55, began in February 

1997 and continues to this day, characterized by multiple vents on and around the flanks

of Pu‘u ‘Ō‘ō. Before injection into the rift system, basaltic magma supplied from the

mantle first passes through a reservoir complex beneath the summit caldera where the

majority of its CO2 is degassed (Gerlach and Graeber 1985; Gerlach et al. 2002). Then,

magma with its remaining stored volatiles, such as SO2, travels down rift and exsolves

under lower pressure as it approaches the ground surface at the eruption site. Pu‘u ‘Ō‘ō

cone and several nearby point sources are presently the main degassing sites of the

current mid-ERZ eruption at Kīlauea.

Open-path Fourier transform infrared (FTIR) spectroscopy has been employed in 

recent years with increasing frequency at a variety of volcanoes throughout the world

using natural as well as artificial sources of infrared energy (Mori et al. 1993; Francis et

al. 1995; Oppenheimer et al. 1998; Burton et al. 2000; Edmonds et al. 2002). The
4

technique can be useful in many topographic settings for in-situ measurement of several

volcanic gas species at once in volcanic plumes. In some cases, only the ratios of

volcanic gases can be determined, but when the FTIR path length is accurately known,

the average concentration of each of the detectable gas species in the plume can be

computed. FTIR spectroscopy can be especially useful in characterizing the gas

chemistry of volcanoes where direct sampling of fumaroles is either not practicable or too 

dangerous.

In this paper we demonstrate the feasibility of using open-path FTIR spectroscopy 

to measure a suite of volcanic gases in the plumes emanating from two vents near Pu‘u

‘Ō‘ō cone on the middle ERZ of Kīlauea volcano. We then compare the results with 

analyses of earlier gas samples collected from eruptive vent areas by means of evacuated 

bottles containing alkaline solution (Giggenbach, 1975). We then combine our plume

measurements with independent measurements of SO2 emission rates made by correlation

spectroscopy (COSPEC) to estimate emission rates for other gases of interest.

Procedure and Instrumentation

The reconnaissance measurements reported here were made over several hours on 

15 June 2001, with meteorological conditions on that day consisting of clear skies, dry

weather, and northeasterly trade winds of about 4 ms

-1

. A total of 130 infrared spectra

were collected through two small ground level plumes. The measurements were made on

the upper lava flow field southeast and down rift of the Pu‘u ‘Ō‘ō cone. At the time of the

measurements, a complex lava tube system existed, with multiple tubes and degassing 
5

sites extending from Pu‘u ‘Ō‘ō to below 550-m elevation and one central tube branch

reaching the ocean. Small breakouts from the tube system were active along the east and 

west branches roughly 2 km inland of the coast. Small sparse surface lava flows were

also present at 600- and 520 to 550-m elevation.

The first measurement plume (HORNITO) emanated from a 9-m-tall spatter cone

located approximately 160 m southeast of Pu‘u ‘Ō‘ō at an elevation of about 800 m (Fig.

2). The actively degassing cone formed in September 1999 on the edge of a deep

skylight. The fume from this cone on the day the measurements were taken was blue and 

mostly translucent. The second measurement plume (PITER) was adjacent to a large

collapse pit located ~60 m west of the spatter cone. The voluminous plume emanating 

from the west wall of this pit was white and opaque and contained a significant amount of

condensed water. A slight odor of SO3 was suspected, but any SO3 present would react

rapidly with water to form H2SO4. Measurements at this site had varying amounts of the

plume in the FTIR measurement path due to some meander of the PITER plume edge.

Some of the earlier measurements at HORNITO also had occasional fume from PITER in 

the measurement path. Both plumes remained at ground level during the course of the

measurements. At some distance downwind, the fume from these and other nearby 

sources were incorporated into the larger Pu‘u ‘Ō‘ō plume. Since both plumes were

derived from degassing lava of the upper lava flow field southeast of Pu‘u ‘Ō‘ō, our

measurements are likely to represent emissions of residual volatiles from lava that

underwent prior degassing at Pu‘u ‘Ō‘ō cone.
6

For this study, we utilized a leased commercial open-path FTIR spectrometer

from MIDAC Corporation. The system includes a plane mirror Michelson-type scanning 

interferometer with a spectral resolution of 0.5 cm-1. The spectrometer, operating between 

5000-600 cm-1, is optically coupled to a 25-cm diameter Newtonian telescope with a 3

mrad field-of-view to extend the path length for open-path measurements. Liquid

nitrogen provides cooling for a 1-mm

2

mercury-cadmium-telluride (MCT) detector. Each 

scan of the interferometer produces an interferogram that is converted to a single beam

spectrum using a Fast Fourier Transform (FFT). In this study, 32 consecutive scans were

co-added to improve the signal-to-noise ratio. Instrument control and data collection were

performed with a notebook computer; a 12-volt DC deep-cycle battery powered the entire

system.

All of the measurements reported here were made in bistatic configuration using a

remote silicon carbide glower with a 50-cm aperture located either in the plume or on the

opposite side of the plume under study. The path lengths in this study were determined by

measuring tape and ranged from 32 to 73 m. Because the path lengths are precisely 

known, plume concentrations as well as gas ratios, could be determined. Sample

absorbance spectra were generated by first computing the ratio between each sample

single-beam spectrum and a background single-beam spectrum of clean ambient air

collected well away from these and other vents.

The data were reduced using AutoquantPro software from MIDAC Corporation.

The software uses MIDAC’s Classical Least Squares K-Matrix calculations coupled with 

an interpolative piece-wise calibration algorithm and multiple reference spectra to

generate individual target species concentrations. All reference spectra are scaled and 
7

subtracted simultaneously according to a classical least squares fitting routine based on 

the spectral regions chosen in the method. Interferences are visually identified and 

included in the analysis. The software then automatically accounts for any interfering 

species during calculations. In order to minimize residual spectra and concentration 

uncertainties, the piece-wise concentration algorithm utilizes the concentrations of two

reference spectra, the results of two independent classical least squares analyses, each of

which uses a single reference spectrum, and a concentration-based interpolation 

involving an empirically determined parameter.

For CO2, the available laboratory reference spectra proved inadequate so we

instead used a set of CO2 reference spectra made from a series of open-path FTIR

atmospheric CO2 measurements over several distances. The path lengths between the

spectrometer and the remote glower light source were precisely measured by tape

measure and ranged from 25 m to 300 m. Average CO2 concentrations along each light

path were then measured using a LICOR nondispersive infrared spectrometer (LI-6252).

Temperatures and pressures were also recorded and used to correct all concentrations

reported here. The result is a set of CO2 reference spectra that ranges from column

amounts of 9,073 ppm-m to 110,040 ppm-m that could be used in place of laboratory 

standards in the AutoquantPro software to reduce the ERZ FTIR measurements for CO2.

A plot of the field-based CO2 reference data is shown in Fig. 3. For gases other than CO2,

laboratory spectra were used in the retrieval.

The CO2 exhibited non-linearity in the calibration curve but there were enough 

reference spectra to cover the range of CO2 detected in the samples. One CO2 point did

not fit the curve as well as expected but was determined to be acceptable based upon a 2nd
8

order fit of the model. The curve was based upon a piece wise linear fitting routine as

described above and in the caption for Fig. 3. The background spectra were manually 

adjusted to remove CO2 so that all calculations were performed with CO2 absorbance

bands manually removed from the background transmission spectrum (single beam).

Results

A total of 130 spectra were collected and they clearly reveal the presence of SO2,

CO2, CO, HCl, HF and SiF4 in the plumes under study. Sulfur dioxide and CO2 were

detected in all of the collected spectra, while HCl, HF and SiF4 were present in 65%, 33%

and 53% of the measurements, respectively. Carbon monoxide was found in 46% of the

measurements but was confined only to the HORNITO plume. The results from all of the

FTIR measurements are shown in Fig. 4. All concentrations and ratios are presented here

and elsewhere in this paper on a molar basis. Most of the spectra from the HORNITO

plume were collected using a path length of 64 m and these are designated as

HORNITO1 in Fig. 4. A subset of the HORNITO measurements was collected after

increasing the path length to 73 m by moving the infrared light source back toward the

PITER plume; these are designated as HORNITO2 in Fig. 4. Because of the dense nature

of the PITER plume, a path length of only 32 m was used to insure that adequate infrared 

energy from the glower source was available for the FTIR detector.

Plume concentrations of SO2 can be clearly subdivided into different populations

(Fig. 4a). The PITER plume, which had a mean concentration of 24.5 ppm at the
9

sampling site, contained more than double the amount of SO2 than the HORNITO plume.

In-situ SO2 measurements by a hand-held chemical detection tube (Dräger) in both the

PITER and HORNITO plumes were similar and also showed SO2 levels in PITER to be

about twice those of the HORNITO plume. The FTIR data show that the mean 

concentration of SO2 in the HORNITO1 measurements was 6.9 ppm while that of the

HORNITO2 measurements was slightly higher at 10.5 ppm, possibly explained by a

small portion of the PITER plume having been incorporated into the extended 

HORNITO2 measurement path.

Carbon dioxide was detected in all 130 spectra, however plume concentrations of

volcanic CO2 computed by subtracting ambient atmospheric CO2 (367 ppm), measured 

by a LICOR LI-6252 nondispersive infrared spectrometer, from total CO2 were highly

variable and 37% of the values fell into the negative range. In general, plume CO2 is

similar to SO2 in that the HORNITO2 measurements have a higher average concentration

of CO2 than the HORNITO1 measurements (Fig. 4b). Carbon dioxide concentrations in

the PITER plume were more widely scattered. We cannot rule out another CO2 source

being intermittently in the measurement path as the source of some of this variability.

Likewise, some plume meander due to changes in wind speed and direction was noted

during the measurements resulting in variable amounts of the plume in the measurement

path. Meteorology, however, cannot account for lower than ambient CO2 levels so it is

likely that much of the difficulty in quantifying CO2 is inherent in the current FTIR

methodology for CO2.

Carbon monoxide was detected in 59 of 60 spectra from HORNITO1, with an

average concentration of 44 ppb, but not detected at all in the HORNITO2 or PITER
10

measurements (Fig. 4c). The halogen acid gases, HCl and HF, were detected in a number

of spectra throughout both plumes, with the PITER plume containing a distinctly higher

concentration of HF (Figs. 4d and 4e). With its relatively strong absorption band centered 

around 1032 cm-1, SiF4 was easily detected in many of the spectra from both plumes;

concentrations of SiF4 averaged 4 ppb in the HORNITO plume and 20 ppb in the PITER

plume (Fig. 4f).

Discussion

Figure 5 illustrates the concentrations of CO2, CO, HCl, and HF plotted against

SO2 for all published analyses of high-temperature gas samples collected during the Pu’u 

‘Ō‘ō eruption. The samples—collected from 1983 to 1985 when high-temperature

sampling was feasible by evacuated bottles containing alkaline solution (Giggenbach,

1975)—represent gas emissions from vents and small openings in spatter cones or spatter

deposits bridging vigorously fuming fissures in vent areas. We utilize these samples from

Pu‘u ‘Ō‘ō source lava for comparisons with our open path FTIR results for emissions

from what are likely to be relatively degassed Pu‘u ‘Ō‘ō lava. Graphs for CO2 vs. SO2

(Fig. 5a) and CO vs. SO2 (Fig. 5b) are based on 63 samples (Greenland 1986, 1988;

Gerlach 1993a, 1993b); a subset of 39 samples with HCl and HF analytical data

(Greenland 1988; Gerlach, 1993a 1993b) provide graphs of HCl vs. SO2 (Fig. 5c) and HF

vs. SO2 (Fig. 5d). The mean CO2/SO2 of these gas samples (0.19) agrees well with the

ratio of 0.20 obtained by airborne measurements on the Pu’u ‘Ō‘ō plume in 1995 by CO2

analyzer and closed path FTIR (Gerlach et al. 1998; McGee and Gerlach 1998). It also 
11

agrees with the mean ratio of 0.19 obtained for 204 samples that included, in addition to 

the 63 high-temperature samples above, low-temperature samples collected in evacuated 

bottles containing alkaline solution and all samples collected in pumped flow-through 

bottles and evacuated bottles without alkaline solution (Gerlach et al. 1998). Numerical

mean values of Xi/SO2 for other gases (Xi) are also shown in Fig. 5 (CO/SO2, 0.0064;

HCl/SO2, 0.015; HF/SO2, 0.018). The maximum and minimum Xi/SO2 values vary by a

factor of 4-6 except for CO/SO2, which varies by a factor of 19, suggesting that CO in 

several samples is depleted owing to oxidation. Seven samples shown in Figs. 5c and 5d 

have notably lower HCl and HF concentrations, but typical SO2 concentrations. On a

graph of HF vs. HCl (Fig. 6a), these samples form a trend between near-zero values and a

cluster of higher values. The SO2 concentrations of these samples are typical of the other

samples, suggesting to us that they have been depleted in HCl and HF by condensate

formation in the lead-in tube during collection, a known source of error in the sampling 

technique (Giggenbach and Matsuo, 1991), rather than simply being the residual

emissions of relatively more degassed lava.

The results of the ERZ FTIR measurements are shown in Fig. 7. Maximum,

minimum and mean lines for Xi/SO2 from the published analyses shown earlier in Fig. 5 

are plotted again in Fig. 7 for comparison. Note that the plume gas concentrations, as

measured by FTIR, are about 104 times more dilute than the vent samples. As with the

raw FTIR data shown in Fig. 4, the Xi/SO2 ratios fall clearly into different populations;

those containing less than 12 ppm SO2 are from the HORNITO plume and those with 

more than 20 ppm SO2 are from the PITER plume. This may simply be due to less

mixing of the PITER plume with air by the time it is measured, a likely result of a higher
12

mass flux rate from the PITER source. Alternatively, the different SO2 concentrations in 

the two plumes could reflect the extent of prior source magma degassing (i.e. high SO2 in 

the PITER plume represents the least degassed source, low SO2 in the HORNITO plume

represents the most degassed source), although this does not explain the presence of CO

only in the HORNITO plume (see below). More than likely, the differences between the

two plumes are due mostly to differences in mixing with air. In addition, the removal of

water vapor by condensation of liquid water droplets in the PITER plume may also play a

minor role in causing the differences in the plume gas concentrations.

It is evident that the CO2 results from the FTIR measurements (Fig. 7a) are not

well constrained. The CO2/SO2 ratios scatter considerably and nearly all are significantly 

above the mean line for the high temperature gas samples. We suspect much of the

discrepancy for CO2 is inherent in the FTIR methodology rather than due to meteorology

(discussed earlier) or variations in fractional degassing or some other natural process

unique to these vents. It may be that CO2 levels are simply too low and variable on the

ERZ of Kīlauea to resolve by existing FTIR technology. As noted earlier, it is known 

from earlier published analyses and airborne measurements that CO2/SO2 for ERZ gases

is about 0.2 (Gerlach et al. 1998). Using this ideal ratio and plume averages for SO2 from

the FTIR measurements, we can calculate expected average plume CO2 concentrations;

for the HORNITO plume, CO2 should be 1.4 ppm and for the PITER plume, CO2 should 

be 4.9 ppm. If ambient air is about 370 ppm, these plume CO2 concentrations represent

only 0.4% and 1.3% respectively of the total CO2. Considering the scatter of the CO2

measurements here, accurately resolving differences that close to ambient CO2 is

probably asking too much of the method.
13

FTIR spectroscopy was used in 1998 and 1999 to measure the CO2 emission rate

from Masaya volcano, Nicaragua (Burton et al. 2000). There, CO2/SO2 averaged about

2.4, 12 times greater than that for the ERZ of Kīlauea, and implies a plume CO2

concentration that is a significantly higher percentage of total CO2 than on the ERZ of

Kilauea. Using data in Burton et al. (2000, Fig. 2a), we estimate plume CO2

concentrations at Masaya to have averaged 30-50 ppm above the elevated ambient

background they report for the 9 March 1999 measurements. The scatter in Xi/SO2 at

Masaya was greater for CO2 than for other species and was attributed to the natural

variability in ambient air (Burton et al. 2000). The summit plume of Kīlauea, degassing 

hundreds of ppm CO2 with a CO2/SO2 that ranges from 53 to 183 (Gerlach et al. 2002),

may provide a better opportunity to resolve CO2 by FTIR than the ERZ.

The CO/SO2 ratios from the hotter HORNITO plume form a tight cluster close to

the arithmetic mean of the published vent samples (Fig 7b). The lack of detectable CO in

the PITER plume suggests that PITER source gases may have cooled subsurface to a

greater extent before emission to the air than the HORNITO source gases, and that

enough CO was reacted with water or SO2 to CO2 at depth to prevent later detection by

FTIR spectroscopy. The average CO/CO2 (0.0056) from the HORNITO FTIR

measurements is only one-sixth that of the earlier vent samples from Pu‘u ‘Ō‘ō (0.0323).

All of this difference is likely due to the reaction of CO to CO2 in the vent feeder

although, because of the small amount of CO, it is unlikely to be a major factor because

the mean CO/SO2 from the FTIR measurements is essentially the same as that of the

published analyses.
14

The HCl/SO2 ratios scatter throughout (and outside) the range for the published 

analyses, although a significant number of the PITER points are below the average for

the vent samples (Fig. 7c), suggesting that some HCl may have been lost to dissolution in

the abundant water droplets in the PITER plume or the product of prior degassing.

Though the HF/SO2 ratios from the PITER plume are distinctly higher than those from

the HORNITO plume, essentially all of the points from the FTIR measurements are

below the average for the vent samples (Fig. 7d). Figure 6b shows a graph of HF versus

HCl from the FTIR measurements. Note that a majority of points fall below the solid line

representing a 1:1 ratio. The mean value of HF/HCl from the FTIR measurements is 0.6 

as compared to 1.3 for the published analyses. A recent study of gases from the four

summit craters of Mount Etna as well as from a nearby lava flow showed a fairly constant

HCl/HF throughout but distinctly higher HCl/SO2 and HF/SO2 in the gases from what

was interpreted to be fractionally degassed magma and from the residual gases of the lava

flow (Burton et al. 2003).

Owing to different solubilities, HCl may be dissolving to a slightly greater degree

in the water droplets in the PITER plume than HF. However, some HF is probably also 

lost in the formation of SiF4 (Fig. 8). Symonds et al. (1992) showed that volcanic gases

typically contain trace amounts of SiF4 and that the gas may be present in greater

abundance at lower temperatures due to reactions of HF with various silicates. As HF is

present in Kīlauea gases, our FTIR results suggest that SiF4 may be forming by reaction 

of HF with the wall rocks at the ERZ plume vents. This is consistent with the results of

Francis et al. (1996) who found SiF4 in the plumes from both Mt. Etna and Vulcano using 

FTIR. For the HORNITO plume, the SO2/SiF4 ratio is 1.8 x 103while that for the PITER
15

plume is lower at 1.2 x 103. Both of these ratios lie between those for Mt. Etna and 

Vulcano reported by Francis et al. (1996). Although SiF4 was not reported, a more recent

study of fumaroles on Vulcano Island utilizing three different methods, FTIR

measurements, filter pack measurements and direct fumarole sampling, revealed 

generally consistent agreement in HCl/HF between the methods (Aiuppa et al. 2004).

However, the same study showed a significant increase in the amount of total sulfur in 

the direct fumarole samples over the amount of SO2 detected by FTIR and filter pack 

samples, a difference the authors attribute to the presence of H2S which would not be

picked up by the two plume measurement methods (Aiuppa et al. 2004).

If the plume concentrations determined by FTIR measurement can be considered 

representative of ERZ gases, an indirect method for estimating the emission rates of some

of these gases is to compare the average Xi/SO2 ratio determined by FTIR to the ERZ

emission rate of SO2 determined independently by standard COSPEC measurements. For

example, the mass emission rate of a gas (Exi) can be determined from

Exi =k(Xi/SO2)ESO2 (1)

where Xi/SO2 is the average molar concentration ratio from the FTIR measurements, k is

the ratio of the molecular weight of Xi to SO2, and ESO2 is the mass emission rate of SO2

from the ERZ (for CO, HCl, and HF, Xi/SO2 is 0.0065, 0.0138 and 0.0082; and k is 0.44,

0.57 and 0.31, respectively). Although there was no SO2 emission rate measurement on 

the same day as the FTIR campaign, we can use a monthly average of COSPEC data to 

estimate an emission rate for the day. Combining the four ERZ COSPEC measurements
16

for June 2001 gives an average ESO2 of 1440 ±380 t d-1; the daily average for all of the

ERZ COSPEC measurements in 2001 is similar at 1438 t d-1 (Elias and Sutton 2002).

Thus using Eq. (1), we can estimate the emission rates for CO, HCl and HF to be 4, 11 

and 4 t d-1, respectively. Because of the poor quality of the FTIR CO2 measurements, we

are not able to compute a mass emission rate for CO2, but from earlier studies the ERZ

emission rate of CO2 was determined to be 240-300 t d-1 (Gerlach et al. 1998). The caveat

is that Eq. (1) may not accurately predict emission rates for the other gas species if the

Xi/SO2 of the relatively small plumes of HORNITO and PITER do not reflect that of the

total ERZ gas output that is largely dominated by the plume from Pu’u ‘Ō‘ō.

It is clear from this reconnaissance study that the two degassing sources studied 

here are very different from one another and that, in fact, any small degassing source has

the potential to differ from its neighbor due to a variety of processes. This has significant

implications for the routine use of FTIR on the ERZ of Kīlauea. Future FTIR

measurements aimed at determining gas emission rates should be made such that as much 

of the main ERZ plume as possible is measured so that the overall signature is as close as

possible to the total plume. Conversely, this study also shows that FTIR spectroscopy has

the potential to look at variations between small degassing sources in a quantitative

manner, enabling researchers to better understand very localized changes in the gas phase

on the ERZ.
17

Conclusions

Plumes emanating from two point sources on Kīlauea’s ERZ a short

distance down rift from Pu‘u ‘Ō‘ō are derived from magma that has previously lost most

of its CO2 at the summit and much of its SO2 at Pu‘u ‘Ō‘ō. Open-path FTIR

measurements of these plumes conclusively reveal the presence of CO2, CO, SO2, HCl,

HF and SiF4. Distinct differences in the physical character of the two plumes are reflected 

in plume chemistry. The water-rich PITER plume contains more than double the

concentration of SO2 than the HORNITO plume due largely to higher vent gas output.

HCl dissolves to a greater extent than HF in the wet PITER plume, although significant

HF from both plumes may be lost in the formation of SiF4. CO, while present in the

HORNITO plume, is not detectable by FTIR in the PITER plume, a likely result of some

oxidation to CO2 during subsurface cooling. Although there appear to be some

differences between these gases and Pu‘u ‘Ō‘ō source gases, ratios of HCl/SO2, HF/SO2

and CO/SO2 determined by FTIR measurements of these two small plumes compare

reasonably well with earlier published analyses of ERZ vent samples and suggest that

further studies of ERZ gases by FTIR are warranted.

The procedures for measuring CO2 in volcanic plumes against a high and variable

ambient air background are not yet well constrained by open-path FTIR, but can be

improved possibly with the production of more applicable laboratory standards or

optimization of CO2 field standards that better define the concentration-pathlength

conditions likely be encountered in volcanic plumes. However, the prospect of

successfully measuring very low concentrations of plume CO2 that may represent only a
18

small fraction of total CO2, as on the ERZ of Kīlauea, remains uncertain. Further FTIR

measurements directed at determining total gas emission rates should be made with a

configuration such that the entire plume is imaged.

Acknowledgments

This work was supported by the U.S. Geological Survey through the Volcano 

Hazards Program. We are grateful to Marie Edmonds, Larry Mastin and Clive

Oppenheimer for insightful reviews and comments.
19

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21

Figures

Figure 1. Location map for Kīlauea volcano, Hawai‘i, showing the East Rift Zone and 

Pu‘u ‘Ō‘ō cone.
22

Figure 2. Photograph taken 15 June 2001 of the HORNITO and PITER plumes in the

foreground with Pu‘u ‘Ō‘ō cone and plume in the background. View is to the northwest.

HORNITO is located about 160 m southeast of Pu‘u ‘Ō‘ō and PITER is about 60 m west

of HORNITO. The FTIR measurements were taken across the mid-point of each plume

perpendicular to the axis of plume travel in this field of view.
23

Figure 3. Plot showing CO2 calibration points. The absorbance area is a unitless relative

function of the background over the wavelength range 2223-2284 cm-1. The areas are

calculated using an integral function and plotted against column amounts to determine

any points that are outliers to the data set. The curve demonstrates a non-linear compound

over a large differential optical density range; not uncommon for MCT detectors. The

processing software uses each calibration point in its unknown sample calculation by 

fitting a piece-wise calibration curve to these data and adding in an interpolative function 

analogous to a spline curve. The actual samples and reference spectra were calculated 

using the AutoquantPro interpolative Classical Least Squares function.
24

Figure 4. Volcanic gas concentrations in ppm (molar) from FTIR measurements of two 

plumes on the East Rift Zone of Kīlauea volcano, Hawai‘i on June 15, 2001 for (a) SO2,

(b) CO2, (c) CO, (d) HCl, (e) HF, and (f) SiF4. Absorbance spectra for SO2, CO2, CO,

HCl, HF and SiF4 were collected in the following spectral windows: 1071-1234, 2223-

2284, 2033-2269, 2655-3095, 3963-4293 and 979-1092 cm-1. A total of 130 spectra were

collected, but not every gas was detected in every spectrum. Errors (±1σ) for SO2, CO2,

CO, HCl, HF and SiF4 from these measurements were 0.28, 11.73, 0.04, 0.13, 0.24, 0.002 

ppm respectively. Solid horizontal lines represent the temporal extent of measurements

for each plume, HORNITO or PITER. The HORNITO measurements are further

subdivided into HORNITO1 and HORNITO2 with HORNITO1 representing those

measurements taken with a path length of 64 m and HORNITO2 a path length of 73 m.

The path length used for the PITER plume was 32 m.
25

Figure 4 (cont.) CO was only detected in the shorter path length HORNITO1 

measurements and was not found at all in the cooler PITER plume. As can be seen in (c),

the HORNITO1 CO concentrations were very low. By increasing the path length for the

HORNITO2 measurements to incorporate either fresh air devoid of CO or a small slice of

the adjacent PITER plume also devoid of CO, the signal-to-noise ratio of the

measurements was reduced to a level where the low CO signal became obscured by the

increased noise.

Figure 5. Concentrations of CO2, CO, HCl, and HF versus SO2 in high-temperature gas

samples of the Pu‘u ‘Ō‘ō eruption, 1983 to 1985. Graphs (a) and (b) are based on 63

samples (Greenland 1986, 1988; Gerlach 1993a, 1993b); (c) and (d) are based on 39

samples (Greenland 1988; Gerlach 1993a, 1993b). Lines show ranges and means of

CO2/SO2, CO/SO2, HCl/SO2, and HF/SO2. Open circles represent samples depleted in 

HCl and HF likely due to condensate formation in the lead-in tube during sample

collection.
26

Figure 6. Concentrations of HCl and HF (a) in high-temperature gas samples of the Pu‘u

‘Ō‘ō eruption, 1983 to 1985 and (b) in East Rift Zone gases from both degassing sources

measured by FTIR spectroscopy on June 15, 2001. Solid lines represent an HF/HCl ratio 

of 1:1. Samples in (a) are the same as in Figs. 5c and 5d except for omission of an outlier

at 0.55 mol % HF in Fig. 5d. As in Figs. 5c and 5d, open circles represent samples

depleted in HCl and HF where condensate formation in the lead-in tube during sample

collection is suspected.
27

Figure 7. Concentrations of CO2, CO, HCl, and HF versus SO2 derived from FTIR

measurements. Maximum, minimum and mean lines for Xi/SO2 for the published 

analyses shown earlier in Fig. 5 are plotted again for comparison. Samples with SO2

concentrations >20 ppm are from the PITER plume; those with lower concentrations are

from the HORNITO plume. Negative CO2 values produced after subtracting ambient CO2

from total CO2 are not shown in 7a.
28

Figure 8. Graph of SiF4 versus SO2 concentrations derived from open-path FTIR

measurements. As in Fig. 7, samples with SO2 concentrations >20 ppm are from the

PITER plume; those with lower concentrations are from the HORNITO plume.