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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