United States
Environmental Protection
Agency
Atmospheric Research and Exposure
Assessment Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S3-90/044 June 1990
&EPA Project Summary
Impact of NADP/NTN Sampling
Protocols on Winter Storm
Estimates of Wet Deposition in
Central Pennsylvania
James A. Lynch, David R. DeWalle, and Kevin Horner
This report describes the field
efforts over two snow seasons in
central Pennsylvania. Due to a
prolonged drought for the past two
years, only a limited number of
snow/rain events were sampled. The
authors are careful to limit the
conclusions to the specific site
under study. However, the dynamics
of the situation are such that
conditions found at the central
Pennsylvania site are typical for
many of the sites in the wet
deposition network.
The goal of the project was to
determine if snow chemistry from
samples collected in the Aerochem
Metrics sampler (used in the
NADP/NTN Network) was repre-
sentative of the entire event or was it
biased due to field sampling
difficulties. To evaluate this question,
the investigators set up three wet
deposition samplers; one that
operated in the normal manner
(sensor operated), one that operated
in the normal manner except that it
was manually activated at the start
and end of an event, and one that had
the collection buckets changed three
times during an event. Three
precipitation volume collection
gauges were also located at the site;
one regular gauge, one gauge that
had an Alter shield attached, and one
that had a Nipher shield attached.
Meteorology instrumentation was
also installed.
This Project Summary was
developed by EPA's Atmospheric
Research and Exposure Assessment
Laboratory, Research Triangle Park,
NC, to announce key findings of the
research project that is fully
documented in a separate report of
the same title (see Project Report
ordering information at back).
Introduction
The National Atmospheric Deposition
Program (NADP) and the National Trends
Network (NTN) utilize Aerochem Metrics
wet/dry samplers to collect precipitation
for chemical analysis. This instrument
consists of a two-container system with a
movable lid that ts designed to expose a
wet-side container and cover a dry-side
container during periods of precipitation,
and vice versa. A sensor mounted on the
instrument reacts electrically to the onset
of precipitation causing the lid to move.
Heaters mounted below the sensor serve
to both melt snow and ice as well as
evaporate moisture from the sensor. This
instrument is used at all NADP and NTN
monitoring sites and is used to sample
both liquid and solid forms of
precipitation. The standard sampling
period is one week.
Co-located with each Aerochem
Metrics precipitation sampler is an
unshielded Belfort weighing-type
recording rain gauge. Each rain gauge is
equipped with a timer chart recorder
keyed to the sampling interval of one
week and an event recorder. The weekly
interpretation of the rain gauge chart by
site operators provides daily and weekly
amounts of precipitation. The coincidence
of Aerochem Metrics collector operation
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with precipitation events is determined
through the tracing of the event recorder.
The volume of weekly precipitation from
the rain gauge should correspond to the
volume of liquid present in the wet-side
container of the Aerochem Metrics
collector. Precipitation measurements
from the Belfort rain gauge are used in all
calculations of wet deposition and in
determining precipitation-weighted mean
annual and seasonal concentrations of
solutes found in precipitation.
Calculations of wet depositions and
precipitation-weighted mean concen-
trations are influenced by the accuracy of
precipitation measurements and the
catch efficiency of the Aerochem Metrics
collector. Errors in precipitation
measurements can be attributed to
instrumental and operational/observa-
tional problems, as well as errors
associated with siting and climatic
fluctuations. Instrumental errors include
the inability of the rain gauge to record
small precipitation events (response
time), damage to the rain gauge orifice,
and problems associated with the clock
drive mechanism. Operational/observa-
tional problems include mistakes in
interpreting the rain gauge chart and
errors resulting from improper calibration
of the weighing mechanism. Siting and
climatic problems include: evaporation,
splash, adhesion, geometry of gauge,
form of precipitation, and wind speed.
In general, the greatest single source
of error in precipitation measurements is
associated with wind. The effects of wind
are due primarily to an increase in
pressure on the windward side of the
gauge, a decrease in pressure and a
marked acceleration of wind over the
gauge orifice and the formation of eddy
currents over and within the gauge
orifice. In general, precipitation
measurement errors increase as wind
velocity increases, are greatest for light
versus heavy rainfall, and are greatest for
snow versus rain, particularly light, dry
snow. Another potential source of error in
obtaining accurate precipitation
measurements is associated with gauge
response time and mistakes in
interpreting the rain gauge chart. The
frequency of these errors is greatest for
light intensity precipitation events,
particularly snowfall.
Inaccuracies in precipitation measure-
ments associated with wind can be
reduced by installing a wind shield
around the rain gauge. Such shields are
designed to divert the flow of air down
and around the rain gauge and to
eliminate updrafts. There are basically
two types of shields. The Alter shield is a
ring approximately six feet in diameter
that is placed around the gauge from
which hangs swinging metal slats. The
shield extends one-half inch above the
rain gauge orifice. The Nipher shield is a
rigid, inverted bell-shaped solid device
constructed of fiberglass and made
specifically for the Belfort rain gauge.
The device consists of a 28-inch cylinder
that is used to extend 20 inches vertically
the 8-inch orifice of the Belfort gauge.
The inverted bell-shaped device is
attached to the housing of the Belfort
gauge and positioned at the same height
as the extension from the rain gauge
orifice. Comparisons of the Alter and
Nipher shields have shown that the
Nipher shield is superior to the Alter
shield, especially when precipitation falls
as snow.
A slow response of the Aerochem
Metrics collector to the onset of
precipitation or the frequent opening and
closing of the movable roof during low
intensity storms can result in a loss of
precipitation from which solute concentra-
tions are determined. This loss in turn
can result in an over- or under-estimation
of the solute concentrations, depending
upon the temporal variability of solute
concentrations during a storm and the
relative solute concentrations of sampled
and unsampled precipitation. The
response time of the Aerochem Metrics
varies with the form of precipitation and
appears to be longer for light, dry snow
than for low intensity rainfall.
Another problem associated with the
Aerochem Metrics precipitation collector
is its aerodynamics and the effect wind
might have on the collector's ability to
capture precipitation. In the above
discussion, it was pointed out that the
catch efficiency of the Belfort rain gauge
is affected by wind and that better
estimates of the true depth of
precipitation can be obtained using wind
shields around the rain gauge. The same
wind-related problems apply to the
Aerochem Metrics collector. These
problems may be even more severe
given the shape and aerodynamics of the
sampler. Consequently, the depth of
precipitation caught in the wet-side
container may be considerably less than
the actual depth of precipitation. In fact,
comparison of wet-side bucket volumes
with unshielded rain gauge measure-
ments at four NADP/NTN sites in
Pennsylvania in 1987 demonstrates these
problems. The ratio of bucket volume to
rain gauge volume ranged from 0.91 to
0.98 during the warm season and from
0.68 to 0.94 during the cold period at the
four sites. The lower ratios during the
cold period reflect difficulties associated
with sampling snow. The lowest cold
period ratio was associated with the site
that received the largest percentage of its
annual precipitation as snowfall.
Recognizing potential problems and
limitations of Aerochem Metrics samplers
and unshielded Belfort rain gauges and
realizing that these problems might affect
wet deposition estimates and the
calculation of precipitation-weighted
mean concentrations from the
NADP/NTN, a study was undertaken to
compare cold season precipitation
measurements from unshielded and
Nipher- and Alter-shielded rain gauges
located in an area subject to high wind
speeds. These precipitation
measurements were compared to volume
measurements from sensor-activated and
manually-activated Aerochem Metrics
collectors. The chemical concentrations
found in these composite samples were
compared to precipitation-weighted mean
concentrations obtained from sequential
samples collected from an Aerochem
Metrics sampler. The sensor-activated,
manually collected sequential samples
were evaluated to determine temporal
variability in solute concentrations during
winter storms and how this variability
might affect deposition estimates.
Differences in precipitation estimates
from shielded and unshielded rain
gauges were evaluated to determine what
impact these differences might have on
total estimates of wet deposition during
selected precipitation events.
Site Location and Description
This study was conducted at the
University Park campus of The
Pennsylvania State University located in
the southern portion of Centre County
near State College, Pennsylvania. Instru-
ments were installed within the oval of
the University's outdoor Running Track
located at latitude 40 48' North and
longitude 77 51' West. This site was
selected because it is level and subject
to windy conditions due to a long, open
fetch upwind of the track. Although the
site was located on the Penn State
Campus, storms approaching the re-
search site were not affected directly by
neighboring buildings and vegetation.
The track is located in the center of a
wide east-west oriented valley within the
Ridge and Valley Province of central
Pennsylvania.
Instrumentation and Installation
Equipment used in this study consisted
of three Aerochem Metrics wet/dry
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precipitation collectors (Model 301), three
Belfort weighing bucket-type recording
rain gauges, an Alter wind shield, a
Nipher wind shield, and a R.M. Young
wind run anemometer (Model #6106) with
wind direction capabilities (Model #6301).
The Nipher shield was specifically
designed for the Belfort gauge and
purchased from the Canadian
Meteorological Service. During the winter
of 1988, only two Belfort recording
gauges were utilized as the Nipher shield
was not received until April, 1988.
Two adjacent Aerochem Metrics
precipitation collectors were hooked up to
a common precipitation sensor. One of
the samplers was used to collect
sequential samples that were manually
removed from the samplers. The other
sampler remained exposed to
precipitation during the entire storm. This
composite, event sample was removed
for chemical analysis immediately after
cessation of precipitation. Essentially, this
sampler functioned the same as samplers
located at NADP/NTN monitoring sites,
although it was operated on an event
basis instead of the normal weekly
sampling interval.
A third Aerochem Metrics sampler was
also installed at the site to obtain another
composite event sample. Unlike the
sensor activated samplers, this sampler
was manually opened at the beginning of
a storm and remained open during the
entire storm. Since this sampler was not
activated by a sensor and thus not
subject to periodic opening and closing
of the lid, a problem that is frequently
encountered at NADP/NTN sites, its
sample volume and chemistry was
considered to be representative of the
entire event. However, it should be noted
that samples from the manually-operated
sampler might also contain dry
deposition that may have fallen in the
sample container during intervals when
precipitation temporarily ceased. During
such temporary interruptions, sensor-
activated samplers would (should) cover
the wetside bucket to protect against
dryfall inputs into the wet-only sample.
The manually-operated composite
sample was removed for analyses
immediately upon cessation of
precipitation.
During the winter of 1988, two Belfort
weighing-type rain gauges were installed
at the site to measure precipitation. Both
gauges were equipped with event
recorders. The event recorder and rain
gauge pen traces provided a record of
precipitation and the coincidence of
operation of the Aerochem Metrics
samplers that were activated by the
precipitation sensor. One of the rain
gauges was equipped with an Alter-type
wind shield, the other was exposed
directly to prevailing winds. In 1989, an
additional Belfort rain gauge was installed
at the site. This gauge was equipped with
a Nipher-type wind shield. Prior to
installation, each rain gauge was
calibrated following procedures
recommended by the manufacturer.
Co-located with the above equipment
was a R. M. Young wind run anemometer
(Model #6106) with wind vane (Model
#6301). These instruments were installed
to provide a measure of wind direction
and speed to aid in the interpretation of
the results.
All instruments were positioned along a
north-south transit with each piece of
equipment being approximately 12 to 15
feet from the nearest instrument. The
wet-side container of the Aerochem
Metrics samplers were oriented to the
west per NADP/NTN installation proto-
cols. The recording rain gauges and wind
shields were installed according to manu-
facturer specifications. The Aerochem
Metric samplers were operated with
alternating current. The anemometer was
powered by a 12-volt battery.
Sampling Procedures
Prior to each storm, clean buckets
were placed in the Aerochem Metrics
collectors. Upon initiation of precipitation,
the manually-operated collector was
opened and left open during the entire
storm. Essentially, analyses from this
sample were used to describe the true
chemical composition of each event
sampled. The other collectors were
activated by a standard precipitation
sensor mounted on one of the samplers.
Precipitation samples from one of the
sensor-activated collectors were collected
sequentially. An attempt was made to
collect threes samples per storm with
sufficient volume to permit complete
chemical analysis. Essentially these
samples were to represent the chemical
composition of precipitation at the
beginning, middle, and ending portions of
each storm. Since it is extremely difficult
to predict the amount of precipitation
expected from a given storm, the
sequential samples were changed after
every 0.20 to 0.25 inch of liquid
precipitation Samples from the other
sensor-activated collector were used to
represent "normal" NADP/NTN samples.
All precipitation samples were taken to
the Water Quality Lab of the
Environmental Resources Research
Institute at the Pennsylvania State
University and analyzed for the following
parameters: pH, specific conductance,
ammonium-nitrogen, nitrate-nitrogen,
chloride, sulfate, calcium, magnesium,
sodium and potassium. In addition, the
liquid volume of each sample was also
measured. Standard laboratory quality
control and quality assurance procedures
were followed. Split sample analyses
were included in these procedures
whenever sufficient sample volume was
available. In addition, simulated rainwater
samples from the U.S. Department of
Commerce, National Bureau of
Standards, were submitted to the lab
during sample analysis. Whenever
available, the means of the split sample
analyses were used in evaluating the
results of this study. In addition to these
quality assurance samples, the water
quality lab also participates in a number
of EPA-sponsored QA/QC programs and
interlab comparisons, including EPA's
Acid Rain Performance Survey. The
Belfort weighing-bucket type recording
rain gauges were used to determine both
measured and recorded precipitation
amounts. Recorded precipitation volumes
from the unshielded gauge were used to
represent the "normal" NADP/NTN
estimate of precipitation. Eight-hundred
and twenty-three ml of polyethylene
glycol antifreeze were added to each rain
gauge bucket prior to an event. This
volume is equivalent to one inch of
precipitation. Following a storm, the
antifreeze-precipitation mixture was
measured and the amount of precipitation
determined by subtracting the one inch
of antifreeze. This measured volume was
compared to the recorded precipitation
amount obtained by interpretation of the
rain gauge chart. These data were used
to evaluate the catch efficiency of the
Aerochem Metrics collector and in
calculating wet deposition. Volumes from
the sequential samples were used to
determine precipitation-weighted mean
concentrations from the sequential
samples.
In addition to the precipitation
measurements, wind direction, wind run,
snow particle size and type, and air
temperature were measured at the site.
Meteorological data from a weather
station located at the Rock Springs
Research Farms located approximately
nine miles to the west of the track were
used as a back-up to the instruments
located at the site.
Results and Discussion
Because of unusual climatic patterns
during the past two winters, precipitation
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in general and snowfall in particular in
central Pennsylvania were well below
normal. Consequently, only five
precipitation events were sampled for
chemical analyses during the winter
months of 1988 and 1989. Four of these
storms produced sufficient volume to
permit the collection of sequential
samples, although two of the storms did
not produce sufficient volume in each
sequential sample to permit complete
chemical analysis. The fifth storm
stopped after the collection of the first
sequential sample and thus represents a
composite, event sample.
Chemical Analyses
pH
Hydrogen ion concentrations
(expressed as pH) were quite variable
both within and between storms. This
variability was sufficient to preclude
detection of any definite trend or pattern
in precipitation pH, recognizing of course
that the number of available data points
are quite limited. The differences in pH of
sequential samples collected during the
individual storms varied from 0.17 to 0.67
pH unit.
There is some indication that the
amount of variability within a storm may
have been related to changes in wind
direction during the storm. For example,
pH values for the February 11-12, 1988
storm ranged from 4.68 at the beginning
of the storm when the wind was out of the
East-Northeast to 4.37 at the end of the
storm when the wind was predominately
from the Northwest or West. A similar
pattern was evident during the January
25-26, 1988 and the March 3-4, 1988
storms, although wind direction remained
constant during the later storm and
shifted from the Northeast to the
Northwest during the January 25-26,
1988 storm. Storms that track from West
to East in Pennsylvania generally have
much lower pH values than those that
move into the state from the East Coast.
These differences are related to relatively
higher sulfur and nitrogen oxide
emissions in western than eastern
Pennsylvania.
In comparison to the composite
samples, the volume-weighted mean pH
of the sequential samples were generally
lower (more acidic) than the sensor-
activated composite samples and
generally higher (less acidic) than the
manually-activated composite samples.
However, the differences between the
sequential mean pH values and e ther the
sensor-activated composite samples or
the manually-activated composite
samples were quite small ranging from
0.01 to 0.10 pH unit and from 0.02 to
0.14 pH unit, respectively. As expected,
the pH values from the manually-
activated composite samples were more
variable (because of inputs from dryfall)
when compared to the sequential
samples than the sensor-activated
composite sample pH values. However,
the pH of samples from the manually-
activated sampler were generally (4 out
of 5 storms) more acidic than the sensor-
activated samples. This might suggest
that unsampied precipitation (due to
sampler response time) during the early
and later stages of a storm was
influencing the pH of the sensor-activated
samples. This is supported by the fact
that, for the most part, precipitation was
more acidic at the beginning and ending
of the storm and that the manually-
operated sampler consistently caught
more precipitation than the sensor-
activated sampler. Why the sequentially
sampled means differ from the sensor-
activated composite pH values is
uncertain.
Ammonium-Nitrogen
No consistent pattern in ammonium
concentrations in sequential samples was
evident for all storms sampled, although
ammonium concentrations varied
considerably within storms ranging from
0.03 to 0.44 mg/L. Nevertheless, there is
some indication that ammonium
concentrations were lower at the
beginning and middle portions of the
storms than at the end of the storms.
When snowfall dominated the form of
precipitation, ammonium concentrations
were consistently lower during the middle
portion of the storm and highest at the
end of the storm. Changing wind
direction (East, Northeast to West,
Southwest) and generally higher
ammonium concentrations in western
Pennsylvania may have had some
influence on this pattern. No consistent
pattern was evident for the storms
comprised of both rain and snow.
In comparison to composite storm
samples, the volume-weighted mean
ammonium concentrations of the
sequential samples were consistently
lower. The differences in concentration
between the manually-activated
composite samples and the sequentially
sampled means ranged from 19% to
62% (0.02 to 0.09 mg/L). For the most
part, ammonium concentrations from the
sensor-activated and manually-activated
composite samplers were fairly similar;
although some differences were evident,
these differences were quite small
(generally 0.01 mg/L or less) and did not
exhibit any consistent pattern.
Consequently, it is difficult to ascertain,
given the limited data available, whether
unsampied precipitation (due to sensor
response time) is affecting ammonium
concentrations in sensor-activated
collectors. However, given the observed
variations in ammonium concentrations
during sequentially sampled storms, the
possibility does exist that unsampied
precipitation could influence the
concentration of precipitation collected
with sensor-activated Aerochem Metrics
collectors.
Nitrate
Differences in nitrate concentrations
between sequential samples varied from
0.40 mg/L to 2.32 mg/L. Between storm
nitrate concentrations ranged from 0.77
mg/L to 2.93 mg/L based on the volume-
weighted means of the sequential
samples. Because of this variability,
specific temporal patterns were difficult to
ascertain, although there is some
indication that nitrate concentrations were
generally lower in the beginning and
middle portions of the storms and highest
at the end of the storms. Part of this
suggested pattern might be explained by
variations in wind direction within and
between storms. Two of the four storms
sequentially sampled experienced their
highest nitrate concentrations at the end
of the storm. Wind direction for both
storms changed from the East or
Northeast to the Northwest or West
during the course of the storm and were
comprised entirely of snow. As previously
stated, air masses in the western portion
of Pennsylvania tend to be more polluted
than in the eastern portion of the state.
The only storm that demonstrated
decreasing nitrate concentrations
throughout the storm maintained a
consistent Northeasterly wind direction.
In contrast, nitrate concentrations during
the March 5-6, 1989 storm increased
throughout the storm as the winds shifted
from the East to the Northeast. Such
variability might suggest local pollution
sources were influencing nitrate
concentrations at the site.
Nitrate concentrations from the
manually-activated composite samples
were consistently higher than either the
sensor-activated composite samples or
the volume-weighted mean concentra-
tions from the sequential samples. This
suggests that dryfall during periods of
low intensity precipitation or when
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precipitation ceased temporarily (which
should have caused the sensor-activated
collectors to close) might be a
contributing factor. The differences in
nitrate concentrations between the
manually-activated samples and the
sensor-activated composite samples and
the volume-weighted means from the
sequential samples ranged from 0.10
mg/L to 0.39 mg/L and from 0.02 mg/L to
0.26 mg/L, respectively. Although no
consistent patterns existed between the
concentration data for both composite
samples and the sequential samples,
there is a suggestion that unsampled
precipitation due to sensor sensitivity,
particularly during snowfall, might
influence the nitrate concentrations in
samples collected with the standard
sensor-activated Aerochem Metrics
sampler. This is supported by the fact
that precipitation volumes from sensor-
activated samplers were generally
smaller than those measured in the
manually-activated sampler.
Sulfate
Differences in sulfate concentrations
during the four sequentially sampled
winter storms varied from 0.13 mg/L to
1.00 mg/L. Temporal variability within and
between storms was so great that no
temporal pattern was evident. Within
storm sulfate concentration variability did
not appear to be related to wind direction,
or any other climatic parameter
measured.
The potential impact of within storm
temporal variability is evident when the
mean volume-weighted sulfate concentra-
tions from the sequential samples are
compared to the composite event
samples. The sequential mean sulfate
concentrations were from 0.02 mg/L to
1.22 mg/L lower than the manually-
activated composite samples. The largest
difference might be atypical based on the
fact that the manually-activated sampler
was exposed for more than 5.5 hours
longer than the sensor-activated sampler,
because of light flurry activity. The
sequentially sampled mean
concentrations were lower (0.11 and 0.36
mg/L) than the means from the sensor-
activated sampler for two storms, but
higher (0.21 and 0.28 mg/L) during the
other storms. Sulfate concentrations from
the manually-activated samples were
consistently greater (0.01 mg/L to 1.13
mg/L) than the sensor-activated
composite samples. These differences
might reflect the relative length of time
the various samples were exposed to dry
deposition. Although unsampled
precipitation due to poor sensor response
time could possibly affect sulfate
concentrations in Aerochem Metrics
sampled winter storms, the significance
or direction of such impacts could not be
determined from the data collected
during this study.
Chloride
Chloride concentrations also varied
during individual storms. These variations
were fairly consistent, with the
differences ranging from 0.14 mg/L to
0.20 mg/L. Actual chloride concentrations
ranged from 0.10 to 0.30 mg/L. Although
the concentration data themselves do not
indicate any specific pattern, analysis of
these data with respect to wind direction
and the location of potential sources of
chloride does reveal a fairly consistent
pattern.
Wind direction during the early stages
of both the January 25-26, 1988 and
February 11-12. 1988 storms was
primarily from the North or Northeast
bringing with it sea salt and consequently
relatively high chloride concentrations. As
the wind shifted during the middle portion
of the storm to the relatively chloride-free
air mass located to the North of
Pennsylvania, chloride concentrations
dropped substantially. The higher
chloride concentrations during the latter
stages of the storms when winds were
from the Northwest or West reflect the
generally higher chloride concentrations
found in western Pennsylvania as a result
of the combustion of coal and the release
of HCI. The generally decreasing pattern
of chloride concentrations during the
March 3-4, 1988 storm could reflect, to a
great extent, the movement of the storm
away from the state and a subsequent
decrease in the influence of sea salt in
central Pennsylvania. However, this
explanation does not explain the
increasing pattern evident during the
March 5-6, 1989 storm, although subtle
differences in the storm track may
influence these differences.
Unfortunately, complete sodium
concentration data were not available for
all sequentially sampled storms.
Nevertheless, the available sodium data
do tend to support the significance of
storm track and sea salt influence on
chloride concentrations.
No consistent pattern was evident
between the volume-weighted mean
chloride concentrations from the
sequential samples and either the
manually-activated or sensor-activated
composite samples. Differences in
chloride concentrations between the
manually-activated samples and the
sequential means ranged from 0.02 to
0.04 mg/L with half of the composite
samples being greater than the
sequential means and vice versa. The
sensor-activated composite sample
concentrations followed an identical
pattern, but had generally higher
concentrations. Differences in
concentrations between the sequential
means and the sensor-activated
composite samples ranged from 0.02 to
0.11 mg/L. The reasons for this variability
and why sequentially sampled means
would be greater than manually-activated
or sensor-activated composite samples is
unclear.
Calcium
Of the four sequentially sampled
storms, only two produced sufficient
volumes to permit the determination of
calcium concentrations for all sequential
samples. However, partial analyses are
available from two additional storms for at
least two of the three sequential samples.
During one storm (March 3-4, 1988)
calcium concentrations decreased from
0.47 mg/L at the beginning of the storm
to 0.03 mg/L at the end of the storm.
During another storm (March 5-6, 1988)
calcium concentrations increased from
0.10 mg/L to 0.32 mg/L as the storm
progressed. Wind direction during both
storms was from the East or Northeast
and did not change very much. A similar
pattern was evident for the two storms
where only two sequential samples
contained sufficient volume for calcium
analysis. Although limited in number, the
results do indicate that calcium
concentrations vary during winter storms.
Calcium concentrations were measured
in five sensor-activated and five
manually-activated composite samples of
which only two had comparable
sequential sample means. Although the
calcium concentrations from the sensor-
activated composite samples agreed
fairly well with the two sequential sample
means, the calcium concentration in one
of the manually-activated composite
samples was 0.23 mg/L greater. This
higher concentration (March 5-6, 1989)
may be a result of analytical error as
indicated by the low cation/anion ratio. It
may also reflect the influence of dryfall in
that the sampler was exposed 5.5 hours
longer than either sensor-activated
sampler due to light flurry activity.
However, the potential influence of dryfall
was not evident in the other samples as
indicated by the fact that the calcium
concentrations from the sensor-activated
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samples were greater than the manually-
activated samples for three of the
remaining four storms. The reason for the
higher calcium concentrations in sensor-
activated samples is unclear.
Magnesium
Magnesium concentrations were
measured in five sensor-activated and
five manually-activated composite events
of which only two had comparable
sequentially sampled means. In all cases,
magnesium concentrations were quite
low ranging from less than 0.01 rng/L to
0.02 mg/L. Although magnesium
concentrations did vary somewhat, the
low concentrations, which are typical for
central Pennsylvania, and the limited data
makes it very difficult to detect any
temporal patterns. Unlike some of the
other analytes, magnesium concentra-
tions in both composite samples and the
sequential means agreed fairly well,
partly due to their low concentrations.
Sodium
Sodium concentrations were measured
in five composite sensor-activated and
manually-activated events of which only
two had comparable sequentially
sampled means. Sodium concentrations
in the sequential samples ranged from
0.03 mg/L to 0.14 mg/L and exhibited a
temporal pattern similar to chloride
concentrations. Sodium concentrations in
the composite samples ranged from 0.02
mg/L to 0.155 mg/L. Sequentially
sampled event means were higher than
both sensor-activated composite sodium
concentrations but not for both manually-
activated composite samples. The
sodium concentration in the March 5-6,
1989 manually-activated composite
sample was substantially higher than
either the sequential sample mean or the
sensor-activated mean. This difference
might reflect analytical error as indicated
by the previously mentioned low
cation/anion ratio.
Potassium
Like magnesium, potassium
concentrations were generally very low,
seldom exceeding 0.01 mg/L. This was
true for sequentially sampled means as
well as the composite event samples.
The only exception was a 0.14 mg/L
concentration measurement in the
second sequential sample of the March
3-4, 1988 storm. Since potassium
concentrations in both composite
samples from this storm were 0.02 mg/L,
it is likely that the second sequential
sample concentration is in error. A
relatively high cation/anion ratio (1.225)
supports this suspicion.
Water Quantity
Sampler Comparisons
The liquid volume trapped in the
sequential sampler and the composite,
sensor-activated and manually-activated
samplers were compared along with
recorded precipitation estimates from the
shielded and unshielded rain gauges. In
all cases, the manually-activated sampler
trapped more precipitation than the
sensor-activated sampler. The increased
volumes varied from 0.02 inch (7%) to
0.07 inch (35%). When compared to the
sequential sampler volumes, the
manually-activated composite sample
volumes were greater (from 0.01 inch,
4% to 0.04 inch, 4.6%) for all events
except the March 5-6, 1989 storm. During
this storm the sequential sampler trapped
0.10 inch (15%) more precipitation than
the manually-activated sampler. During
this and other storms that had fairly
strong winds predominately from one
direction, snow accumulated in the
bucket unevenly with the greatest depth
being downwind. When such conditions
occurred, snow trapped in the bucket was
more likely to be blown out of the
composite bucket or was less likely to be
trapped in the composite bucket than in
the sequential sampler. This was also the
reason why the sensor-activated event
sampler volume during the March 5-6,
1989 storm was substantially lower (0.15
inch, 19%) than the total from the
sequential sampler and most likely the
reason for the discrepancy between
these samplers during the January 25-26,
1988 storm. The larger manually-
activated sample volumes reflect, for the
most part, precipitation that was not
sampled by the sensor-activated
collectors due to poor sensor response
time and the frequent opening and
closing of the sampler during low
intensity storms. With the exception of
the January 25-26, 1988 and March 5-6,
1989 storms (which were discussed
above), the sensor-activated composite
sample volumes were very comparable to
the total volume of all sequential
samples. These results suggest that
sample volumes from sensor-activated
Aerochem Metrics samplers have a
tendency to under-sample precipitation
(when compared to the manually-
activated sampler) and that the
magnitude of the loss will be dependent
upon the amount and form of
precipitation and wind speed and
direction. When compared to recorded
precipitation volumes from the
unshielded Belfort rain gauge, the total
volume of the sequential samples
equaled or exceeded the unshielded rain
gauge measurements for four of the five
storms. The maximum deviation was 0.06
inch. During the March 20-21, 1989
storm, recorded precipitation in the
unshielded gauge was 0.08 inch higher
than the composited sequential samples.
In contrast, the sensor-activated
composite volumes were smaller than the
unshielded gauge for all but one storm;
the maximum difference was 0.09 inch.
Manually-operated sampler volumes were
greater (by as much as 0.08 inch) than
the unshielded rain gauge during three
storms and less (by as much as 0.05
inch) during two storms. The
underestimation by the manually-
operated gauge was most likely due to
snow being blown out of the bucket for
reasons previously discussed.
With exception of the March 3-4, 1988
storm, the Alter-shielded gauge recorded
more precipitation than any of the
Aerochem Metrics samplers, as well as
the unshielded rain gauge. The volume
differences varied from as little as 0.01
inch to as much as 0.26 inch. If the Alter-
shielded rain gauge precipitation
measurement is assumed to be the most
accurate measurement of precipitation for
these storms and that the chemistry of
unsampled precipitation is the same as
that sampled, the average wet deposition
estimates for the five storms based on
the unshielded precipitation gauge
measurement would be in error by an
average of 14%. If the volume from the
sensor-activated Aerochem Metrics
sampler is used in place of the
precipitation measurement from the
unshielded gauge, wet deposition would
be underestimated by approximately
20%. Wet deposition estimates from the
sequential sampler and the manually-
activated composite sampler would be
approximately 12% lower.
Shielded and Unshielded Rain
Gauge Comparison
Precipitation data from 11 winter
storms were evaluated to determine the
influence of wind shields on the volume
of precipitation recorded by the Belfort
weighing-type rain gauge. In addition to
the recorded values, direct
measurements of precipitation volumes
were also determined to evaluate the
accuracy of the recording device. The
data show clearly that the catch
efficiency of the Belfort rain gauge is
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influenced by wind and wind shield
design.
On average, the Alter-shielded gauge
caught 0.05 inch (11.5%) more
precipitation (based on direct
measurement) than the unshielded
gauge, while the Nipher-shielded gauge
caught 0.09 inch (18%) more than the
unshielded gauge. Maximum differences
in measured precipitation between the
unshielded and Alter-shielded and
Nipher-shielded gauges were as rruch as
29% (0.20 inch) and 53% (0.36 inch),
respectively, for individual storms. It is
obvious from these results that the
unshielded Belfort rain gauge
underestimates precipitation amounts and
that estimates of wet deposition based on
unshielded rain gauge measurements
could be m error by as much 18% on
average and as high as 53% during
individual events.
Co in pounding these errors are
additional errors associated with the
accuracy of the rain gauge measure-
ments and the correct interpretation of
the ram gauge chart. When compared to
the various recording rain gauge
estimates, the volumes of precipitation
actually measured in the ram gauge
buckets were generally less, with the
differences ranging from zero to 0.09
inch and averaging 0.024 inch over the
sampled storms. Although these
differences might be viewed as being
relatively small, it should be noted that
the Belfort recording rain gauges used in
this study were calibrated before and
during the study to assure the highest
possible accuracy. Belfort gauges used
at most NADP/NTN sites are generally
calibrated annually or biannually and are
thus prone to greater potential error in
estimating precipitation amounts.
Summary and Conclusions
A comparison of sequentially sampled
precipitation volumes and chemistry with
sensor-activated and manually-activated
samplers using the NADP'NTN-approved
Aerochem Metrics collector was
conducted at the Pennsylvania State
University in central Pennsylvania.
Precipitation measurements from
unshielded, Alter-shielded, and Nipher-
shielded Belfort recording rain gauges
were also compared. The results indicate
that the chemistry of winter precipitation
events can vary substantially
within storms and that the variations may
be related to changes in wind direction
and emission sources as a storm moves
through the area. Precipitation volume
estimates based on sensor-activated
sequential and composite samplers and
manually-operated samplers indicate that
the sensor-activated composite sampler,
which was considered to be
representative of normal NADP/NTN
sampling protocols, undersampled
precipitation because of slow response
time upon initiation of precipitation and
the frequent opening and closing of the
roof during periods of low intensity
precipitation. Precipitation may also go
unsampled as a result of being blown out
of the sampling container. This problem
was observed to occur during several
snow storms and can significantly reduce
sample volume. The volume of
unsampled precipitation was highly
variable and appeared to be related to
the form of precipitation and possibly
other climatic parameters, such as wind
speed and direction.
The net effect of unsampled
precipitation on storm chemistry and wet
deposition estimates is uncertain, difficult
to quantify, and highly dependent upon
the relative chemistry of unsampled
versus sarnpied precipitation. It is
obvious from the results of this limited
study that measurement errors related to
the collection efficiency of the Aerochem
Metrics sampler and variations in
precipitation chemistry during a storm are
possible and that these errors can have
substantial impact on the estimates of
seasonal wet deposition. The magnitude
of the errors will also be ion specific.
Underestimation of wet deposition can
also occur as a result of inaccurate
measurement of precipitation using an
unshielded Belfort recording rain gauge.
The results from this study indicated that
the unshielded rain gauge under-
estimated precipitation volumes when
compared to Alter- and Nipher-shielded
gauges and that the differences were
greater when compared to the Nipher
wind shield. Assuming that precipitation
measurements from the Nipher-shielded
rain gauge are the most accurate and that
the chemistry of unsampled and sampled
precipitation is the same, wet deposition
from the storms sampled would be
underestimated by 18% on average and
by as much as 53% during individual
storms using current NADP/NTN
sampling protocols.
Compounding these errors are
additional sources of error associated
with the accuracy of the Belfort rain
gauge. A comparison of recorded
precipitation with measured precipitation
in the Belfort bucket indicated that the
Belfort rain gauge generally
overestimated precipitation amounts,
although some storms were
underestimated. The errors associated
with rain gauge accuracy approached
25% on an individual storm basis.
Although the results of this study
indicate that intra-storm variability in
precipitation chemistry and the sampling
efficiency of the Aerochem Metrics
collector can affect wet deposition
estimates, the limited number of sampled
storms and the high degree of variability
between and within these storms make it
very difficult to quantify the magnitude,
direction, and significance of these
problems. Nevertheless, the results do
suggest that substantial error is possible.
Consequently, additional studies need to
be undertaken that would provide a
greater number of observations.
The results from the shielded and
unshielded rain gauge comparisons do
indicate that wet deposition based on
precipitation measurements from the
unshielded Belfort rain gauge are likely to
be substantially underestimated.
Although these results are similar to other
published wind shield comparison
studies, additional investigations into the
importance of these impacts on wet
deposition estimates in various climatic
regions need to be undertaken. In
addition, a study should be conducted
where precipitation chemistry is
determined from sequential and
composite samples from shielded and
unshielded Aerochem Metrics samplers
and compared to samples collected with
standard NADP/NTN sampling protocols.
Such a study should be conducted under
a variety of climatic conditions. The
accuracy of the NADP/NTN Belfort rain
gauge and the effects of aging on the
spring balance mechanism also need to
be addressed. All of the above studies
are essential to the correct interpretation
of spatial and temporal variability of wet
atmospheric deposition as determined
from precipitation samples collected at
NADP/NTN monitoring sites.
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James A. Lynch, David R. DeWalle and Kevin Homer are with the Pennsylvania
State University, University Park, Pennsylvania 16802.
Steven M. Bromberg is the EPA Project Officer (see below).
The complete report, entitled "Impact of NADP/NTN Sampling Protocols on Winter
Storm Estimates of Wet Deposition in Central Pennsylvania," (Order No. PB90
219 411/AS; Cost: $15.00, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, V'A 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Atmospheric Research and Exposure Assessment Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States Center for Environmental Research
Environmental Protection Information
Agency Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S3-90/044
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