Concentration
and dispersion modeling of the Kilauea plume
Annette
Baerman, Steven Businger, John Porter and Duane Stevens, University of
Hawaii
Roland Draxler, NOAA/ARL
Introduction
Active since January 1983, the Kilauea
volcano on the island of Hawaii is the longest actively erupting volcano
in the world. During the last 14
years Kilauea has remained in a quiescent outgassing stage.
Emissions from the volcano chemically react with the ambient air resulting
in a perpetual plume of volcanic smog, also known as vog.
The vog plume is composed mainly of water vapor and sulfate particulates.
Consequently, sulfate particulates are an effective size (0.1-0.6 µm)
to reach down into the human lung (Morrow 1991) and in  the
presence of high relative humidity may readily expand up to three times
its original volume to further obstruct airways (Porter & Clarke 1997).  The
presence of vog has been linked to numerous health problems (Worth 1995). The
particulates may also dissolve in liquid water (i.e. in cloud) resulting
in acid rain, which has the potential to destroy crops and leach lead from
roofs and plum-bing. During episodes
of increased sulfate production and atmo-spheric stability, vog may be
thick enough to create a major visibility hazard to aircraft (Fig.
1) . In summary, vog is a significant
threat to the island community and a need for a prognostic tool is evident.
Hawaii Volcanoes National Park measurements of SO2
often reveal average concentrations exceeding 290 ppb in a single hour,
far exceeding EPA health standards. At
present, SO2 emission rates average approximately 2,000 tons/day and have
reached a maximum of 32,000 tons/day (Elias et al. 1998).
Vog dispersion is primarily a function of synoptic and local wind patterns
as well as stability of the environment.
As SO2 converts to sulfate particulates near the main vent, the prevailing
NE trade wind flow, along with island blocking effects and daily sea-breeze
regimes, advect the vog past South Point and into Kona (Figs. 2a
, 2b
and 3). The majority of the vog pollutants
are
 
trapped within the boundary layer due to the strong trade wind inversion
(Fig. 3a
and 3b
). Only during occurrences of weak
inversions, strong trade winds or southerly winds associated with fronts,
shearlines or Kona lows will the pollution be sufficiently advected away.
The primary goal of this research was to create
a prognostic tool to aid in the prediction of vog plume concentration and
dispersion. The windfields and
 
thermal data from the Meso-scale Spectral Model (MSM) were used as input
into the Hybrid Single Particle Lagrangian Integrated Trajectory Model
(HY-SPLIT) in order to produce vog simulations (Juang 2000; Draxler &
Hess 1998). Validation of model results
was con-ducted using aerosol concentrations derived from aircraft and ground-based
data, as well as satellite imagery.
Satellite imagery was also used to validate plume size, shape, and location.
 Results
and Discussion
Sun photometer measurements (Fig. 4a
and 4b
) reveal low background concentrations in the Hilo and Puna areas.
Modeled values in this area were several orders of magnitude smaller, consistent
with their upwind placement. Small
observed values down-wind of the vent are likely due to residual drainage
flow off the mountain during late morning advecting the aerosol plume off-shore.
Modeled values at these locations experience a strong spike up to 34 µg
m-3. It appears the model plume is
slightly misplaced, perhaps a reflection of a lack of representation of
the diurnal land-breeze in the MSM wind field.
A decreasing aerosol content is observed from South Kona to North Kona.
Concentrations stay relatively constant for the duration of the journey,
reducing slowly northward and inland.
Leeward model output shows similar trends.
The simulation results (Fig.
5) show skill in reproducing general plume evolution based upon synoptic
conditions. 
The satellite image (Fig.
6a) shows a narrow plume off the SE coast.
Modeled plume (Fig.
6b) orientation and concentration gradients are in good agreement with
satellite observations.
A significant diurnal cycle in the wind field was
observed during the field experiment due to light synoptic-scale trade
winds and enhanced surface heating of the island.The
thermally driven circulation that results is one of the key components
enhancing aerosol build-up along the Kona coast, the MSM surface wind fields
on the leeward coast tended to remain weak and offshore inhibiting this
effect.
Aircraft data suggest that even in the area of aerosol
pooling in the lee of Mauna Loa, model concentrations were under-forecast.
In this study, background concentrations were not included.
A reasonable explanation for the lower model concentrations near the leeward
coast is the fact that the vog event in this research was unusually persistent.
An extended period of SE winds caused aerosol to accumulate for a considerable
amount of time, raising background concentrations prior to the field experiment.
Conclusions
Aircraft data confirm the importance of the trade
wind inversion in trapping aerosols in the boundary layer, documented by
the large drop in aerosol optical depth from 0.274 to below 0.023 (no units)
when climbing from 151 meters to 2452 meters (Fig. 3).
Comparisons between model and ground-based data
indicate that the model replicates observed trends reasonably well from
North Kona to South Kona.Sun photometer
measurements show an average concentration in Hilo of 4.55 µg m-3.
Since Hilo remained upwind of the vent during the field experiment, these
values reflect background concentration.
The model produced plume characteristics including
size, shape, orientation, and concentration gradients consistent with those
observed in satellite imagery.An
interesting comparative result is the success of the model to reproduce
the narrower-type plume and clear slots over Keauhou and South Point as
seen in the satellite imagery (Fig. 6).Figures
5 and 6 suggest that the model performed best over the southern portion
of the island and downwind over the ocean.As
the MSM had difficulty resolving sea-breeze/land-breeze circulations and
complex terrain effects on the leeward side, prognoses degraded with time
and distance northward along the Kona coast.
References
Draxler, R., and G.D. Hess (1998): An overview of the HY-SPLIT_4 modelling
system for trajectories, dispersion and deposition. Aust. Meteoral. Mag.,
47. 295-308.
Elias, T., A.J.Sutton, J.B. Stokes, and T.J. Casadevall (1998): Sulfur
dioxide emission rates of Kilauea Volcano, Hawaii, 1979-1997. USGS Open-File
Report 98-462.
Juang, Hann- Ming Henry (2000): The NCEP mesoscale spectral model: A revised
version of the nonhydrostatic regional spectral model. MWR, 128. 2329-2362.
Morrow, J.W. (1991c): Volcanic effects on the elemental composition of
inhalable particulates in Hilo and Captain Cook. Vog and Laze Seminar,
Hilo, Hawaii.
Nash, A.J., M.S. thesis, Dept. of Meteorology, School of Ocean and Earth
Science and Technology, Univ. of Hawaii.
Porter, John N. and Antony D. Clarke (1997): Aerosol size distribution
models based on in situ measurements. J. Geophys. Res. 102, D5. 6035-6045.
Worth, Robert M. (1995): Respiratory impacts associated with chronic vog
exposure on the Island of Hawaii. Hawaii Department of Health Vog Symposium.
Acknowledgements
This research was sponsored by NASA Solid Earth
and Natural Hazards, Research and Application Programs: NRA 98-OES-13.
We would like to thank Roland Draxler and NOAA/ARL for access and use of
HY-SPLIT_4 code, Duane Stevens, Derek Funayama and Bruce Anderson for their
work on the MSM data, MHPCC for computer time and John Porter for coordinating
the aircraft observations.
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