United States
Environmental Protection
Agency
Atmospheric Sciences
Research Laboratory
Research Triangle Park NC 27711
'I \
Research and Development
EPA/600/S3-86/039 Nov. 1986
v>EPA Project Summary
A Study of the Formation and
Transport of Acidic Species by
Non-Precipitating Cumulus
Clouds During VENTEX-84
A. J. Alkezweeny
RECEIVED
A field experiment was conducted by
Pacific Northwest Laboratory (PNL) in
Kentucky during the period July 8 to
August 18,1984 as part of the VENTEX-
84 field study to investigate the forma-
tion of sulfate and nitrate aerosols and
the vertical transport of pollutants by
non-precipitating cumulus clouds.
VENTEX is a research component of the
National Acid Precipitation Assessment
Program.
Analyses of data collected from DC-3
and Cessna 411 aircrafts and from
ground sampling show ratios of sulfate
concentration to the total sulfur con-
centration (the sum of sulfate and sul-
fur dioxide) to be larger at the top of
clouds than at their bases. In-cloud oxi-
dation rates were calculated to be in
excess of 100%/hr. The ratio of the total
nitrate concentration (the sum of nitric
acid and nitrate aerosols) to the total
sulfur concentration at cloud tops, was
higher than that at cloud bases on
many days. This result suggests that ni-
trate can form in the clouds but not as
frequently as does sulfate. Ground con-
centrations of ammonia declined
around midday followed by an increase
in the afternoon. Sulfur dioxide concen-
trations exhibit an opposite trend. A
case study of morning and afternoon
soundings of ozone indicated vertical
transport of pollutants from the mixed
layer to the cloud layer.
This Project Summary was devel-
oped by EPA's Atmospheric Sciences
Research Laboratory, Research Triangle
Park, NC, to announce key findings of
the research project that is fully docu-
mented in a separate report of the same
title (see Project Report ordering infor-
mation at back).
Introduction
Cumulus clouds form by convective
activity that originate near the surface.
These clouds ingest and chemically
process polluted air from various alti-
tudes within the mixed layer. Oxidation
is enhanced, so that transformation
rates over regional or continental scales
are increased significantly when clouds
are present even part of the time. Also,
pollutants transferred to the cloud layer
undergo horizontal transport and dis-
persion quite different from flow in the
mixed layer. These cloud processes
need to be investigated and parameter-
ized for use in regional models that sim-
ulate the transport, transformation, and
deposition of airborne acidifying
agents. Currently under development is
an Eulerian framework. Regional Scale
Acid Deposition Model (RADM). This
development work is a scientific re-
sponse to the requirements of the En-
ergy and Security Act of 1980, Title VII
for a means by which proposed acid de-
position mitigation strategies can be
tested. RADM is designed to incorpo-
rate all known important processes
dealing with emissions, transport and
transformation of acidic chemical
species and their deposition to the sur-
face of the earth.
In keeping with the comprehensive-
ness of the model development effort,
the modeling of the vertical exchanges
and transformations that occur as a re-
-------
suit of these convective clouds, so ubiq-
uitous during the warm season, and so
highly variable in space and time is an
important component of the RADM. The
processes associated with these clouds
are many and complicated. Presently,
knowledge about them is very limited.
The program, VENTEX, (Venting Experi-
ment), sponsored by the National Acid
Precipitation Assessment Program, and
administered by the United States Envi-
ronmental Protection Agency is de-
signed to provide an experimental basis
for the development of parametric
schemes and computer modules for
treating these processes in the RADM.
The principal objectives of VENTEX are
to determine the production of sulfate
and nitrate aerosol particles by non-
precipitating cumulus clouds and to in-
vestigate the vertical transport of pollu-
tants caused by cumulus convection.
The field study effort was conducted
near Lexington, Kentucky over a period
of three summers from 1983 to 1985.
This report covers the experiments con-
ducted by Pacific Northwest Laboratory
(PNL) during the summer period from
July 8 - August 18,1984, which is called
VENTEX-84.
Cloud Chemistry
Samples of aerosols and trace gases
were collected above the mixed layer in
cloud-free air (background samples),
generally above 3,000 m MSL, at the
cloud inflow region (cloud bases) and
cloud outflow region (cloud tops) using
the instrumented DC-3 aircraft. At each
level, the aircraft flight routes consisted
of horizontal bow-tie paths with one leg
of the flight path passing over the
VENTEX ground sampling site. Several
parameters were measured in real-
time; these are: O3, SO2, NOX, aerosol
light scattering, aerosol size distribu-
tion, temperature, dew point tempera-
ture, altitude, turbulence in the inertial
subrange, and aircraft position. The air-
craft was equipped with two high-
volume samplers. Each sampler used a
filter pack that exposed 21.2 cm2 of the
filter area. One filter pack consisted of a
Teflon filter for the collection of aero-
sols, followed by a cellulose filter im-
pregnated with sodium chloride for ni-
tric acid collections, followed by
another cellulose filter impregnated
with potassium carbonate and glycerine
for sulfur dioxide collections. The sec-
ond filter pack employed a Teflon filter,
backed by a cellulose filter treated with
oxalic acid for the collection of ammo-
nia gas. As part of the surface-level
sampling, two filter packs were used to
measure chemical composition of aero-
sols and to monitor the concentrations
of sulfur dioxide and ammonia gas.
Vertical Transport
The study of vertical transport utilized
two instrumented aircraft, the PNL DC-3
and Cessna 411. The Cessna 411 aircraft
was instrumented to measure tempera-
ture, dewpoint temperature, ozone,
light scattering, altitude, and aircraft po-
sition. The aircraft flew two missions
each experiment day and obtained at
least two to three vertical profiles of
these species per mission. The aircraft
also made several in-cloud passes near
the tops of the clouds between vertical
transects. Wind speed and direction,
temperature, and relative humidity
were measured at midday by means of
radiosonde.
Results and Discussion
Cloud Chemistry
The in-cloud formation of sulfate and
nitrate were examined by comparing
the concentrations at cloud tops against
the concentrations measured at cloud
bases. In Table 1 the concentrations of
sulfate and total nitrate at cloud bases
were divided by the total sulfur (the sum
of sulfate and sulfur dioxide), and the
values at cloud tops were first corrected
for background values and then divided
by the total sulfur at that altitude. The
background samples were needed be-
cause when clouds penetrate into the
free atmosphere above, they mix with
dry air and evaporate; thus, the samples
taken at the cloud outflow must be cor-
rected for the background concentra-
tions. The nitric acid and the nitrate
aerosols were combined because when
ammonium nitrate and sulfuric acid dis-
solved in the droplet evaporates, an ex-
change of ions takes place resulting in
the formation of nitric acid and ammo-
nium sulfate. The letters B and T refer to
the measurement location, at cloud
base and cloud top respectively.
The table shows that the ratio of sul-
fate to the total sulfur at cloud tops is
higher than that at cloud bases in all five
days, and the changes in the ratio is
much larger than the estimated com-
bined errors in the measurements. In
the case of total nitrate, three days (Au-
gust 4, 8, and 12) show in-cloud nitrate
formation. On the other two days, no
production was detected. This is the
first time that nitrate formation in natu-
ral clouds has been observed. The oxi-
dation rate of sulfur dioxide in the cloud
system was estimated using the follow-
ing procedures. Let the concentrations
of sulfur dioxide and sulfate be repre-
sented by X and Y, respectively. There-
fore, the time rate of changes in X and Y
are:
dX/dt=-kX-KX (1)
dY/dt = kX - KY (2)
where k is the oxidation rate of S02' and
K is the reduction rate due to dilution
and diffusion. Define a descriptor R
such that:
R = X/(X+Y)
(3)
TABLE 1. Ratios N/S and SO4/S at cloud bases, B, and cloud tops, T, where N is the concen-
tration sum of nitric acid and nitrate aerosols, and S is the sum of sulfate and
sulfur dioxide concentrations. K is the first order oxidation rate of SO2 in clouds
in %/minute. The ammonia and ozone concentrations are in nanomoles/m3 and
ppb respectively, and the temperature, T is in °C.
Date Height N/S S04/S K NH3 O3 T
7/30
7/31
8/04
8/08
8/12
B
T
B
T
B
T
B
T
B
T
0.53 ±
0.27 ±
0.46 ±
1.77 ±
0.52 +
2.39 ±
0.62 ±
0.90 ±
0.27 +
0.24 ±
.07
.04
.01
.12
.07
.31
.08
.11
.03
.30
0.80
0.99
0.72
0.92
0.005
1.00
0.24
0.71
0.18
0.71
±.02
±.01
+ .03
±.01
±.00
±.00
+ .03
±.03
+ .02
±.03
15.0
1.0
9.9
6.5
10.9
3.3
6.7
20.4
8.0
4.6
11.7
15.2
52.3
5.0
—
60
56
40
27
57
48
81
66
78
68
19.1
16.3
20.3
14.3
22.6
14.4
24.9
19.6
20.2
16.8
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Differentiating both sides of equation
(3) with respect to t, and substituting for
the time derivatives from equations (1)
and (2) results in the following equa-
tion:
dR/dt = -kR
(4)
k is influenced by many processes. It
includes the reaction rate between S02
and the oxidizers, functions that take
into account the transfer of the gases
from the air to the cloud droplets, and
the concentrations of the oxidizers.
Temperature changes as droplets rise
from cloud bases to cloud tops will af-
fect the value of k. For diagnostic analy-
sis, it is assumed that k is constant with
respect to t, X, and Y. Integrating equa-
tion (4) yields the following:
kt = In R0 - In R
(5)
where R0 is the descriptor value at t = 0,
at cloud bases.
In order to calculate k, the transport
time t between cloud bases and cloud*
tops is needed. Assuming an average
updraft velocity, w, of 1.0 m/s for the
cloud t is determined by dividing the
thickness of the cloud field by w. The
calculated values of k are shown in
Table 1. The result shows very fast con-
version rates. It should be noted that
even if the updraft velocity is off by a
factor of two or three, the rate is still
very high. Furthermore, the calculated
rate varies from one day to the next and
does not seem to correlate with any of
the parameters listed in the table. Of
course what is missing from the data is
a measurement of H2O2. This measure-
ment was not taken during VENTEX-84
but was made during VENTEX-85. Al-
though the VENTEX-85 data have not
been thoroughly processed and ana-
lyzed, preliminary examination show
that the measured concentration of
H2O2 is in the range of 1.0 to 6.0 ppb,
certainly large enough to oxidize dis-
solved S02 in the cloud droplets. Fur-
thermore, there is a strong indication
that H2O2 formed in the clouds. This
was evident from vertical profiles which
show peaks in the concentrations within
the cloud layer.
Results of the ground measurements
of ammonia concentration show a de-
cline around midday followed by an in-
crease in the afternoon. On the other
hand, the sulfur dioxide concentration
peaks around noontime. At night and
early morning the ammonia is emitted
at the surface into and confined within a
shallow layer that is capped by the noc-
turnal temperature inversion. By mid-
morning the inversion height rises and
the ammonia is mixed and distributed
over a deeper layer. In the late after-
noon, mixing is suppressed and there-
fore the concentration rises. The sulfur
dioxide data suggests that the main
sources of S02 are elevated plumes.
The weak (or absent) vertical mixing
during the late afternoon, nighttime,
and early morning keep the concentra-
tion at the surface low. However, at mid-
day, when vertical mixing is most vigor-
ous, the plumes are mixed to the
ground and a rise in the S02 concentra-
tion is expected.
Vertical Transport
Vertical sounding and horizontal tran-
sect data collected by the instrumented
Cessna-411 aircraft were analyzed to in-
vestigate the pollutant transport into
the cloud layer. The data indicated
ozone concentrations and aerosol light
scattering inside the clouds are higher
than those outside at the same eleva-
tion. The difference in the ozone con-
centration between the cloud and its en-
vironment is in the range of 5 to 15 ppb,
which is comparable to the difference
between concentration of the mixed
layer and the layer above the mixed
layer. This shows that the in-cloud con-
centrations are of mixed layer origin.
During the morning, the baseline ozone
concentration values above the mixed
layer varied from about 49 to 53 ppb
while in the afternoon the range was 52
to 60 ppb. Since these measurements
where taken above the mixed layer, the
results suggest an accumulation of
ozone, and by inference other pollu-
tants, in the cloud layer (including the
entrainment layer), due to cloud activity
during the intervening period.
Summary
Preliminary results show that the ra-
tio of sulfate concentration to the total
sulfur concentration (the sum of sulfate
and sulfur dioxide) at top of clouds is
much higher than the measured at their
bases. As a result, cloud oxidation rates
were calculated to be in excess of 100%/
hr. The calculation assumed an average
updraft velocity of 1.0 m/sec. The ratio
of the total nitrate concentration (the
sum of nitric acid and nitrate aerosols)
to the total sulfur concentration at cloud
top, was higher than that at cloud base
on many days. This result suggests that
nitrate can form in the clouds but not as
frequently as sulfate. The ammonia
concentrations measured on the
ground show a decline around midday
followed by recovery in the afternoon.
On the other hand, the sulfur dioxide
concentrations show an opposite trend
for the one day that was examined. The
vertical profiles of ozone measured be-
tween the morning and afternoon
sounding indicated that pollutants from
the mixed layer have been transported
vertically to the cloud layer, and accu-
mulation occurred.
A. J. Alkezweeny is with Pacific Northwest Laboratory. Rich/and, WA 99352.
Jason K. S. Ching is the EPA Project Officer (see below).
The complete report, entitled "A Study of the Formation and Transport of Acidic
Species by Non-Precipitating Cumulus Clouds During VENTEX-84. "(Order No.
PB 86-220 357/AS; Cost: $9.95, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Atmospheric Sciences Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
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