United States
Environmental Protection
Agency
Environmental Monitoring
Systems Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S4-87/010 June 1987
x°/EPA Project Summary
Development and Evaluation
of a Real-Time pH and
Conductivity Rain Monitor
Richard J. Paur
Acidic wet deposition (acid rain) is
thought to be responsible for a variety
of deleterious effects on ecosystems
and on natural and man-made materi-
als. Determining and quantitating these
effects is complicated by the fact that
rain is a low ionic strength solution of
many different salts and organic com-
pounds. Measurements of parameters
such as pH are difficult and the rain
samples are subject to modification by
chemical and biological mechanisms.
Detailed understanding of a rain event
requires measurement of concentration
of chemical species as a function of
amount of precipitation during a rain
event because rain is more concen-
trated at the beginning of an event and
becomes more dilute as the various
salts are washed out of the atmos-
phere.
This report describes the features
and performance of a monitor that was
designed to fractionate a rain event into
samples corresponding to 0.3 mm of
rain, determine the pH and conductivity
of the samples within approximately
one minute of collection, and store the
remainder of the sample for more de-
tailed analysis.
This Project Summary was devel-
oped by EPA's Environmental Monitor-
ing Systems Laboratory, Research Tri-
angle 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 or-
dering information at back).
Introduction
The main objectives of this project
have been to design, construct and eval-
uate an automated wet precipitation
monitor that can measure the pH and
conductivity of rain in near real-time,
collect discrete fractions of rain events,
and store the samples for later labora-
tory analysis. The design of the monitor
called for unattended operation for peri-
ods of up to one week. Evaluation of the
system included studies of the stability
of rain samples under the refrigerated
storage conditions provided by the
sampler.
To achieve the primary objectives a
monitor (see Figure 1 for schematic of
unit) with the following key features
was designed, fabricated and tested:
The monitor utilized a Teflon-coated
inlet. The rain was collected in dual ac-
cumulators so that one sample could be
undergoing analysis while the succeed-
ing sample was being collected in the
other accumulator. This arrangement
allowed near real-time measurements,
since a sample corresponds to 0.3 mm
of rain and is analyzed as soon as that
increment of rain is accumulated (about
45 seconds during a 1-inch per hour
rainfall).
A microprocessor (Z80) was used to
control the pH and conductivity meas-
urements and the storage carousel. The
microprocessor also controlled a QC
program and produced a printed record
of the QC results (See Figure 2 for exam-
ple of 6 weeks of QC data). The system
allowed operator specification of the
frequency of QC analysis and a number
of measurement parameters1, such as
the tolerance band for successive pH
readings, before accepting a series of
readings as a valid pH measurement.
An additional option that was not imple-
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Rain Sensor
Cover
(Teflon Lined)
Insulated
Enclosure
Teflon Sample
Accumulators (2) i
I Microprocessor
Power Supply
Cover
(Open Position)
Insulated
Enclosure
Compressed Air to
Operate Cover
Figure 1. EPA automated rain sampler.
mented in the original sampler was
using the microprocessor to telemeter
data back to the laboratory.
Thirty-five milliliter aliquots of the
samples were stored in a refrigerated
carousel for more extensive analysis in
a laboratory.
Thus, the output from the sampler
consists of a printed record of analyses
of QC samples, of the time, date, vial
number, pH and conductivity of rain
samples, and up to fifty 35 ml aliquots
of rain.
This report contains some examples
(Figures 3 and 4) of typical rain events
described in terms of ionic concentra-
tions vs. vial number (equivalent to
amount of rain) and presents the results
of sample stability studies in addition to
providing conclusions and recommen-
dations regarding the performance of
the sampler. Appendix A of the full re-
port is a copy of the "Acid Precipitation
Monitor Manual" which was written by
Northrop Services, Inc. to describe the
monitor and its operation.
Conclusions
The objectives of the program have
been accomplished. The sampler can
reliably measure the pH and conductiv-
ity of rain aliquots representing small
fractions of a rain event and provides
systematic QC information that can be
used to support the validity of the meas-
urements or to alert the operator to
problems with the monitor.
A detailed manual (describing the
hardware and software in sufficient de-
tail to allow duplication by knowledge-
able readers) has been produced.
Sample stability does not appear to
pose serious problems, although these
results are quite limited.
Hardware problems, such as sticking
solenoid valves, have been overcome in
a systematic fashion and the monitor
appears to be reliable in terms of gen-
eral mechanical and electrical function.
Laboratory evaluations and a field
evaluation program during which ap-
proximately 40 rain events (approxi-
mately 900 vials of rain) were measured
led to the following conclusions:
1. The automatic QC program consists
of determining the pH of a QC solu-
tion at preset intervals (typically
twice a day). During normal opera-
tion the pH determinations are stable
to within ±0.05 pH units of the ex-
pected value.
2. The pH determinations during rain
events agree with laboratory pH de-
terminations to within about 0.05 pH
units for pH values of less than 4.5.
Higher pH samples, with corre-
spondingly lower ionic strengths, are
more difficult to measure and differ-
ences between the automated moni-
tor and the laboratory determina-
tions may increase to as much as
0.2-0.3 pH units.
3. The estimates of conductivity of rain
samples typically agree within ±15
percent with laboratory measure-
ments. Larger deviations are occa-
sionally observed early in rain
events. These deviations are consis-
tent with the hypothesis that the first
few samples may contain incom-
pletely dissolved paniculate which
has been washed out of the atmos-
phere. Such paniculate matter may
not have time to dissolve before the
conductivity measurements are
made by the automated sampler;
however, the paniculate may be
more completely dissolved before
the laboratory measurements are
carried out.
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V.J
4.2
%
V'1
1
3; 4.0
C4.
3.9
o a
\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \
.*: -S -S .c .c .5 -S c .c .c
I
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150 -
.1
too -
Figure 4.
14
Vial No.
Chemical analysis of typical rain event.
18
22
26
these materials in rain could be
measured.
2. As noted in the Conclusions, the inlet
opening was too large for many ap-
plications. It is estimated that an
opening that is a factor of 2 to 3
smaller than the current one would
be a better choice. However, the
large inlet does provide a fine frac-
tionation of a rain event and can pro-
duce detailed information on
washout phenomena.
3. A conductivity bridge with higher
conductivity ranges should be incor-
porated into future units. The current
model had an upper limit of 100 mi-
crosiemens, but that range was often
exceeded during the early portions
of a rain event.
4. The current monitor is limited to a
lower operating temperature of ap-
proximately 15°F. For routine field
operations the monitor should have
heaters and insulation added to per-
mit cycling to lower temperatures.
The EPA author Richard J. Paur is with the Environmental Monitoring Systems
Laboratory, Research Triangle Park, NC 27711.
The complete report, entitled "Development and Evaluation of a Real- Time pH
and Conductivity Rain Monitor," (Order No. PB 87-180 550/AS; Cost: $18.95,
subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield. VA 22161
Telephone: 703-487-4650
The EPA author can be contacted at:
Environmental Monitoring Systems Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection
Agency
Onter for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S4-87/010
0000529 PS
s
CHICAGO
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