vxEPA
United States
Environmental Protection
Agency
Environmental Sciences Research ~ '. • \
Laboratory ' f .
Research Triangle Park NC 27711
Research and Development
EPA-600/S3-82-006 May 1982
Project Summary
Dependence of Nephelometer
Scattering Coefficients on
Relative Humidity: Fronts,
Nocturnal Disturbance and
Wood Smoke
George W. Griffing
The dependence of the nephel-
ometer scattering coefficient of atmos-
pheric air on the relative humidity at
Research Triangle Park is discussed
for four different meteorological ex-
amples. These examples feature (1)
the passage of a low-pressure system
with thunderstorms, (2) the passage
of a cold, dry front, (3) a nocturnal
weather disturbance due to an un-
known source, and (4) wood-smoke
aerosols from burning tree piles. Neph-
elometer scattering coefficient data
were obtained using two nephelom-
eters. One was operated at the ambient
outside relative humidity and the other
at a relative humidity inside an air
conditioned building. Using this opera-
tional mode of data acquisition, quali-
tative temporal information can be
deduced on the variations of aerosol
size and number density as the various
meteorological parameters vary. In
addition to the variations of the aerosol
physiochemical parameters, the tem-
poral trend of the visibility for each
example is discussed.
This Project Summary was develop-
ed by EPA's Environmental Sciences
Research Laboratory, Research Trian-
gle 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
In general, the nephelometer scatter-
ing coefficient measured for atmos-
pheric air varies with time. Temporal
variations of the atmospheric optical
properties of the ambient aerosols are
the source of the temporal variations of
the nephelometer scattering coefficient.
Aerosol parameters, which characterize
the optical properties, are the number
density, size, shape, and chemical com-
position.
The studies presented are concerned
with the dependence of the scattering
coefficient on the relative humidity. For
a given aerosol, if moisture condenses
onto or evaporates from an aerosol, it is
conceivable that the size, shape, and
chemical composition of the aerosol
changes. Whether moisture condenses
onto or evaporates from an aerosol
depends upon the magnitude of the
relative humidity.
In situ observations of the scattering
coefficient and the relative humidity
were taken for December 1978 until
September 1981. Except for occasional
downtime for equipment maintenance,
the measurements were taken continu-
ally. To determine the dependence of
the scattering coefficient on the relative
humidity, measurements of the scatter-
ing coefficient are required at two
different relative humidities.
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Procedure
To measure scattering coefficients at
two different relative humidities, one
nephelometer was operated in an out-
of-doors environment and another neph-
elometer was operated simultaneously
inside a building with air conditioning.
The relative humidity was determined
by measuring the temperature and dew-
point of the air flowing through the
nephelometer. Orifices for sampling the
outside air for both nephelometers were
located about 6 m above ground level
and separated by about 50 m horizontal-
ly. Before passing the outside air through
the inside nephelometer, the air was
heated. By heating the air, the relative
humidity of the air flowing through the
inside nephelometer was usually less
than the relative humidity of the air
flowing through the outside nephelom-
eter. Duplicate scattering coefficient and
relative humidity data were taken for
quality assurance of the data.
Various other ancillary data were also
observed. The wind speed and direction
were measured at about 10 m above
ground level. Solar radiation data were
observed. At various times, tempera-
tures were measured at 1 m and at 6 m
above ground level. Facsimile record-
ings of an acoustic sounder were also
taken. Except for the acoustic data, the
data were stored on a floppy disk of an
electronic recorder. The rate of data
acquisition was usually about 500 obser-
vations for each parameter in 24 hours.
Conclusions
Should the scattering coefficient in-
crease, the reason for the increase
should be determined; e.g., an increase
of anthropogenic emissions might ac-
count for the scattering coefficient
increase, or it might be caused by an
increase of the relative humidity due to
meteorological conditions. Operating
nephelometers at two different relative
humidities permits increased scattering
coefficient to be explained by an in-
crease in the relative humidity.
The EPA author George W. Gritting (also the EPA Project Officer, see below) is
with the Environmental Sciences Research Laboratory, Research Triangle
Park, NC 27711.
The complete report, entitled "Dependence of Nephelometer Scattering Coef-
ficients on Relative Humidity: Fronts, Nocturnal Disturbance and Wood
Smoke," (Order No. PB 82-188 913; Cost: $7.50, subject to change) will be
available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Environmental Sciences Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
U S GOVERNMENT PRINTING OFFICE, 1982 — 559-017/071 1
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Postage and
Fees Paid
Environmental
Protection
Agency
EPA 335
Official Business
Penalty for Private Use $300
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