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DISCLAIMER
This report has been reviewed by the Environmental Sciences Research
Laboratory, U.S. Environmental Protection Agency, and approved for publica-
tion. Mention of trade names or commercial products does not constitute
endorsement or recommendation for use.
AFFILIATION
The author is on assignment with the U.S. Environmental Protection
Agency from the National Oceanic and Atmospheric Administration, U.S.
Department of Commerce.
ii
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ABSTRACT
Observations on the temporal dependence of the nephelometer scatter-
ing coefficient on relative humidity are presented and discussed for four
different cases. For each case, the weather at the Research Triangle Park,
North Carolina was dominated by an anticyclonic weather system. By taking
simultaneous nephelometer scattering coefficient observation at two differ-
ent relative humidities, it was possible to conclude that with nocturnal
stable atmospheric conditions:
o In general, the scattering coefficient increases from sundown to
sunup due to aerosol growth and an increasing trend of the aerosol
number density;
o In general, the relatively rapid increase and subsequent decrease of
the scattering coefficient during a 2 to 3 hour period after sunup
is due to a relatively rapid aerosol growth and shrinkage, and a
relatively rapid increase and decrease of the aerosol number dens-
ity.
The latter behavior of the scattering coefficient was called an aerosol
burst. The relationship between an aerosol burst, fumigation, and early
morning visibility deterioration is also discussed.
111
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CONTENTS
Abstract iii
Figures vi
Tables vii
Acknowledgement viii
1. Introduction 1
2. Instrumentation 4
3. Scattering Coefficient Variations 7
4. Summary of Conclusions 26
References 28
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FIGURES
Number Page
1 Twenty four hour variation of the outside scattering coeffici-
ent, temperature, dew point and outside relative humidity.
The scale for the scattering coefficient is given on the left
hand side. Scales for the remaining parameters are given on
the right hand side. For temperature and dew point, degrees
fahrenheit is to be substituted for percent. A scale is also
given for the temperature and dew point in degrees Celsius.
The arrows indicate time of sundown and sunup 8
2 Twenty four hour variation of the outside and inside scattering
coefficient and the outside and inside relative humidity. The
arrows indicate the time of sundown and sunup 12
3 Twenty four hour variation of the outside scattering coeffici-
ent, temperature, dew point and relative humidity. The scale
for the scattering coefficient is given on the left hand side.
Scales for the remaining parameters are given on the right
hand side. For temperature and dew point, degrees fahrenheit
is to be substituted for percent. A scale is also given for
the temperature and dew point in degrees Celsius. Arrows are
shown for sundown (unmarked), sunup (S), diffuse solar radia-
tion (DF), and direct solar radiation (OR) 14
4 Twenty four hour variation of the outside and inside scattering
coefficient and the outside and inside relative humidity. The
arrows indicate the time of sundown and sunup 16
5 Twenty four hour variation of the inside and outside scattering
coefficient and the corresponding inside and outside relative
humidity. The scale for the scattering coefficient is on the
left hand side. The scale for the relative humidities is on
the right hand side. The scale for the portion of the direct
solar flux is arbitrary. The arrows indicate time of sundown
and sunup 18
6 Twenty four hour variation of the outside and inside scattering
coefficients and the outside and inside relative humidity.
The arrows indicate the time of sundown and sunup 23
VI
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TABLE
Number Page
1 Hourly Visibility During an Aerosol Burst 25
vn
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ACKNOWLEDGEMENTS
The author acknowledges the technical assistance of Mr. R. S. Seller
with the instrumentation. Various discussions on the experimental data with
Dr. F. Binkowski are gratefully acknowledged.
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SECTION 1
INTRODUCTION
Various experimental and theoretical studies have been concerned with
the dependence of aerosol size on the relative humidity.1-10 Since the
size of an aerosol will affect its light scattering properties, light
scattering by aerosols can depend on the relative humidity. In our studies,
an integrating nephelometer was used to measure the light scattering proper-
ties of atmospheric aerosols with respect to the relative humidity. The
nephelometer readings will be referred to as the scattering coefficient.
The scattering coefficient is a sum of contributions from atmospheric
aerosols and gases.
At the Research Triangle Park, North Carolina (RTP), the smallest
diurnal scattering coefficient usually occurs during the afternoon. A
typical afternoon value for the scattering coefficient is 0.1 km~l.
Afternoon values of 0.03 km~l and 0.3 km~l would be characteristic values of
a "clean" and "dirty" afternoon, respectively. During the night, there is
usually a general increasing trend of the scattering coefficient which is
typically 3 or 4 times larger at sunup than its afternoon value.
Beginning at sunup, the scattering coefficient is often observed to
increase relatively rapidly and subsequently decrease relatively rapidly.
The time span, from sunup until the scattering coefficient is changing
relatively slowly, is between 2 to 3 hours. As a descriptive name for this
phenomenon, the term aerosol burst will be used. Aerosol bursts are common-
ly associated with anticyclonic weather systems, and have been observed each
month of the year. With passage of warm fronts, there have been a few
occasions for which the scattering coefficient has been observed to vary
somewhat like the variation observed during an aerosol burst. Only aerosol
1
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bursts associated with anticyclonic weather systems will be discussed in
this paper.
A process involved in the aerosol burst phenomenon is the growth and
shrinking of the aerosols due to the increase and decrease of the relative
humidity, respectively. The increase of the relative humidity is due to the
evaporation of moisture by solar radiation from grass or anything else on
which dew is deposited during the night. Of course, after the moisture has
evaporated,the relative humidity will decrease. It is also possible that
the growth and shrinking of aerosols could be affected through the absorp-
tion of solar radiation by the aerosol which would result in moisture
evaporation from the aerosol. In addition, it is conceivable that solar
radiation absorption by an aerosol could induce aerosol spallation which
would affect the growth and shrinking of aerosols. However, our observa-
tions do not indicate that such a mechanism occurs.
In general, variations of the aerosol number density are also occurring
during an aerosol burst. After sunup, the aerosol number density increases
to a maximum in a period of about 2 or 3 hours and subsequently decreases.
A plausible explanation for the variations is that, as the mixing layer
grows under the influence of solar radiation heating, aerosols from aloft
can be diffused toward the ground. Assuming the aerosols aloft have a
larger number density than the aerosols at ground level, an increase of the
aerosol number density would be expected. As the nocturnal temperature
inversion is dissipated, the aerosols will be mixed through greater depths
and the aerosol number density will decrease.
In studies^ on sulfate aerosol formation, "bursts" of the aerosol
number density have been observed to occur by processes associated with
solar radiation. The increase and decrease of the aerosol number density
was attributed to aerosol formation and aerosol agglomeration, respectively.
There is no evidence that these processes are of importance for the aerosol
bursts discussed in this paper.
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In Section 2, a discussion of the instrumentation for the observations
is presented. The diurnal variation of the scattering coefficient is
discussed for three different examples in Section 3.1. In Section 3.2., the
relation between fumigation and visibility deterioration during the morning
and an aerosol burst is discussed. A fourth example of the observed data is
also presented. In Section 4, a summary of the conclusions are enumerated.
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SECTION 2
INSTRUMENTATION
For these observations, two nephelometers were operated in an air
conditioned room (inside nephelometers) and two nephelometers were operated
outdoors at ambient atmospheric conditions (outside nephelometers). The
intakes, for bringing outside air to the inlet orifices of the inside and
outside nephelometers, were about 50 m apart and 6 m above ground level.
All of the nephelometers were Model 1550 Meteorology Research Incorporated
(MRI) integrating nephelometers. The redundant inside and outside nephelo-
meters were used to perform various experiments as well as to monitor the
reliability of the other corresponding inside or outside nephelometer.
Since the scattering coefficient is determined by the amount of light
scattered from the air which is flowing through the nephelometer scattering
chamber, the relative humidity of the air flowing through the scattering
chamber must be known in order to determine the dependence of the scattering
coefficient on the relative humidity. At any given instant, the relative
humidity of the air in the scattering chamber can be determined provided the
temperature and dew point of the air are determined in the scattering
chamber.
Dew points were measured by placing humidity probes in the air flowing
through the inlet and/or outlet orifices of the nephelometer scattering
chamber. Initially, it was our intention to use a cooled mirror humidity
sensor. However, our tests showed that this type sensor was not reliable
because of sporadic contamination of the mirror. On the other hand, our
tests showed that the saturated salt (lithium chloride) humidity sensor was
very reliable and relatively unaffected by contamination. Measurements of
the dew point were taken for the inside nephelometers and for the outside
4
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nephelometers with the saturated salt humidity sensor. Separate measure-
ments were necessary because differences as large as 1 to 2°C in the dew
point of air flowing through the inside nephelometers and of air flowing
through the outside nephelometers were observed at various times. This
difference in the dew points is due to differences in the moisture content
of air entering the intakes, which are separated by 50 m, to the inside and
outside nephelometers.
Temperatures were also measured by placing temperature probes in the
air flowing through the inlet and/or outlet orifices of the scattering
chamber. If the temperature, at any given instant, of the air flowing
through the inlet and outlet orifices are the same, this temperature would
be the same as the temperature of the air flowing through the scattering
chamber.
For the outside nephelometers, the temperatures of the air flowing
through the inlet and outlet orifices of the nephelometer scattering chamber
were approximately the same (generally less than 0.5°C). In order to
obtain this near equality of the temperatures, it was necessary to keep the
temperature of the air in the enclosure housing the outside nephelometers
equal to the ambient atmospheric temperature. This was done by drawing the
ambient air into the enclosure and exhausting air in the enclosure to the
outside with a fan.
For the inside nephelometers, particular attention must be given to the
temperature measurements. For one inside nephelometer, ambient outside air
enters the nephelometer scattering volume after a relatively short journey
of about 2 m through tubing in the room. In general, the temperature of air
entering the scattering chamber will be different than the ambient outside
temperature. In addition, the temperature of air leaving the scattering
chamber will be different than the temperature of air entering the scatter-
ing chamber. Under certain conditions, the temperature difference of air
flowing through the inlet and outlet orifices of the nephelometer scattering
chamber was measured to be as large as 5°C. Thus, in general, there would
be a relative humidity of, say, less than 45 percent, the relative humidity
-------
gradient is probably not important since the scattering coefficient is not
much affected by the relative humidity. Consequently, for sufficiently small
relative humidities, the dependence of the scattering coefficient on rela-
tive humidity is relatively small.
Before air enters the other inside nephelometer, the system has been
designed so that the temperature and/or dew point can be increased or
decreased before the air enters the nephelometer scattering chamber. Since
the particular details of this system is not pertinent to the discussions of
this paper, the system will not be described. For air entering the scatter-
ing chamber of this nephelometer, the journey of the air through tubing in
the room has been sufficient for the air to be in equilibrium with the room
temperature. Consequently, the relative humidity gradient of the scattering
chamber is small. Usually, the relative humidity is small enough for the
inside nephelometers so that the scattering coefficients of the two inside
nephelometers do not vary significantly because of differences of the
relative humidity.
Scattering coefficient data taken with the inside and outside nephelo-
meters will be referred to as the inside and outside scattering coefficients
respectively. Likewise, the relative humidity of the air flowing through
the inside and outside nephelometers will be referred to as the inside and
outside relative humidity.
A Bascom-Turner electronic recorder was used for data collecting,
processing and storing. The rate of data acquisition was 500 readings in 24
hours for each observed parameter. The 24-hour period was arbitrarily
chosen to begin at 1400 EST. Except for periods of equipment maintenance
and repairs, data have been recorded continuously since December 1978.
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SECTION 3
SCATTERING COEFFICIENT VARIATIONS
3.1 DIURNAL
The RTP is located in a non-urban area. On rare occasions, the scat-
tering coefficient data have obviously been affected by local sources such
as the emissions from burning tree piles located 3 or 4 km away. However,
in general, the presence of possible emissions from nearby sources can not
be identified. For each of the four examples to be discussed, the RTP was
dominated by anticyclonic pressure systems.
(a) October 15-16, 1979
In addition to the outside scattering coefficient, the 24-hour varia-
tion of the atmospheric temperature, dew point, and outside relative humid-
ity are shown in Figure 1. The arrows indicate the time of sunup and
sundowns given in tables prepared by the U.S. Naval Observatory. For
discussion of the temporal behavior of the scattering coefficient, it is
convenient to consider two time periods. The first time period is from 1400
EST until sunup while the second time period is for a period of approximate-
ly 3 hours after sunup.
Between 1400 EST and sunup, the relative humidity and scattering
profiles began increasing at sundown and continued throughout the night.
Impressed on the increasing trend of both profiles is an undulating struc-
ture which became most prominent after about 2000 EST. At this same time,
the wind speed was calm (<2 ms~l) and remained calm until about 0800 EST
the next day. In addition, the weather observations at the Raleigh-Durham
Airport (RDU) indicated that there were no clouds.
7
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Figure 1. Twenty four hour variation of the outside scattering coeffici-
ent, temperature, dew point and outside relative humidity. The
scale for the scattering coefficient is given on the left hand
side. Scales for the remaining parameters are given on the right
hand side. For temperature and dew point, degrees fahrenheit is
to be substituted for percent. A scale is also given for the
temperature and dew point in degrees Celsius. The arrows indic-
ate time of sundown and sunup.
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
SCATTERING COEFFICIENT
111 I_L
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1400 1600 1800 2000 2200 2400 0200 0400 0600 0800 1000 1200 1400
OCT. 15, 1979 OCT. 16, 1979
TIME (EST), hr
32
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Nephelometer observations taken in New York and Ohio-'-2 indicated a
similar increasing trend of the scattering coefficient between sundown and
sunup. Evidently a corresponding undulating structure of the scattering
coefficient was not observed. The most plausible explanation of the undu-
lating structure is that, as calm conditions set in at about 2000 EST, there
is little turbulence to produce a homogeneous mixture of atmospheric mois-
ture. Thus, if there are undulations of the relative humidity, undulations
of the scattering coefficient are to be expected.
During the subsequent time period, which begins at sunup, an aerosol
burst occurs. Initially, the scattering coefficient increases relatively
rapidly. This increase is most likely to be due to aerosol growth resulting
from the increase of the relative humidity. This assertion will be examined
more critically in the discussion of the data shown in Figure 2.
Atmospheric processes similar to the formation of evaporation fogl3,14
may be responsible for the increase of relative humidity. In any case,
there must be an adequate source of moisture to account for an increase of
the relative humidity. The moisture source or, perhaps more correctly,
interim moisture source is formed during the night by the deposit of mois-
ture on the grass or other surfaces. After sunup, the ground is heated by
solar radiation. Part of the solar energy is used in moisture evaporation
which increased the dew point of the overlying air. Since the dew point
increased more rapidly than the air temperature during the initial period
after sunup, the relative humidity increases.
Solar radiation sensors were located near and at the same level as the
nephelometer inlet orifice. Solar radiation data are useful to examine the
energetics of increases of the temperature and dew point in addition to
information on the cloud cover.
In the first 30 minutes after the diffuse solar radiation was first
detected, the increases of the various parameters were: (1) air temperature
from 4.6 to 5.2 C; (2) dew point from 3.2 to 4.4 C; relative humidity from
91.2 to 95.2 percent; (4) scattering coefficient from 0.54 km~l to 0.75
-------
knfl; and (5) total solar radiation from 0.0 to 0.5 cal cm~2.
Assuming the temperature and dew point increases, in the first 30
minutes after the diffuse solar radiation was detected, are due to solar
energy, it is possible to answer the question of whether sufficient solar
energy was received to account for the increases. Our calcuations showed
that 0.1 cal cm-2 and 0.2 cal cm~2 were required to produce the tempera-
ture and dew point increases respectively. Since 0.5 cal cm~2 of solar
energy was received, it was concluded that the solar energy received was
sufficient to account for the temperature and dew point increases.
As will be noted in Figure 1, the maximum of the aerosol burst is
nearly coincident with the maximum of the relative humidity. There have
been observations for which the aerosol burst maximum occurs before the
maximum of the relative humidity. If the relative humidity maximum occurs
before the aerosol burst maximum, it must be rare at the RTP since it has
not been observed since the observations began in December 1978. Transition
between calm (<2 ms~l) and the customary daytime fluctuating winds oc-
curred at about 0800 EST. This transition is always observed to occur after
the maximum of the aerosol burst. This transition can also be noted by an
examination of the dew point profile since, as the winds start fluctuating,
the dew point will be observed to fluctuate.
During the discussion of the data presented in Figure 1, the growth of
the aerosols as a consequence of an increase of the outside relative humid-
ity, was asserted to be mainly responsible for the increase of the outside
scattering coefficient during an aerosol burst. Obviously, an increase of
the aerosol number density could also be responsible for part of the in-
crease of the outside scattering coefficient during an aerosol burst. By
analyzing the data presented in Figure 2 or the inside and outside nephelo-
meters, the relative importance of aerosol growth and aerosol number density
increase can be assessed.
From sundown until sunup, the inside scattering coefficient increased,
roughly, from 0.1 to 0.2 km~l. During this period, the inside relative
10
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humidity was less than 30 percent. In addition, there was a relatively
small decreasing trend of the inside relative humidity. Consequently, the
inside relative humidity can not be assumed to be responsible for the
increasing trend of the inside scattering coefficient. The most plausible
explanation is that there is an increasing trend of the aerosol number
density. During this period, the outside scattering coefficient increased,
roughly, from 0.1 to 0.5 km~l. Thus, from sundown until sunup, the growth
of aerosols and the increase of the aerosol number density, contributed
about 0.3 and 0.1 km~l respectively to the increasing trend of the outside
scattering coefficient.
By a similar analysis for the increase of the outside scattering
coefficient during the aerosol burst, the contribution by aerosol growth was
roughly 0.3 km~l and roughly 0.05 km~'l by an increase of the aerosol
number density. Thus, as asserted previously, the growth of the aerosols,
as a consequence of an increase of the outside relative humidity, appears to
be the dominate mechanism for the relatively rapid increase of the outside
scattering coefficient during the aerosol burst.
The mechanism, which is responsible for the increasing trend of the
aerosol number density during the night, is not known. There are several
possibilities. Among these possibilities are thermophoresis and or sedi-
mentation processes. To elucidate the nature of the processes responsible
for the increasing trend of the aerosol number density during the night,
additional observations would be needed.
In Figure 2, it will be noted that the maxima of the inside and outside
scattering coefficient do not occur at the same time. Also, the peak of the
inside scattering coefficient appears to be closely associated with the
relatively small protuberance on the aerosol burst profile. The signific-
ance of the protuberance will be discussed in Section 3.3.
Another feature to be noted in Figure 2 is that the outside scattering
coefficient at 1400 EST on October 15, 1979 is nearly equal to the outside
scattering coefficient 24 hours later. Thus, there was apparently no net
11
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change of the aerosol number density during the 24 hour period of the
observations even though there were changes of the aerosol number density at
certain times during the period.
(b) February 10-11, 1980
During the observing period of February 10-11, 1980, the ground was
snow-covered at the RTP. These conditions are rare at the RTP. The obser-
vational data are shown in Figure 3. It is also of interest to note that
throughout the period the dew point was below freezing and the temperature
was below freezing most of the period. Thus, particularly for the tempera-
ture and dew point, meteorological conditions were quite different from
those for Figure 1.
In comparison to the variations of the outside scattering coefficient
during the night shown in Figure 1, the outside scattering coefficient is
relatively smooth during the night in Figure 3. This feature of the pro-
files suggests that the scattering coefficient shown in Figure 3 was influ-
enced relatively little by variations of the relative humidity. Generally,
our observations show that there is a minimum relative humidity for which
the scattering coefficient is relatively more sensitive to variations of the
relative humidity as the relative humidity increases to larger relative
humidities. This sensitivity of the scattering coefficient to variations of
the relative humidity appears to be in harmony with the data shown in
Figures 1 and 3. Presumably, in Figure 3, the relative humidity would have
had to be greater than the relative humidity in Figure 1. The gradual
increase of the scattering coefficient from sunset to sunup is primarily due
to an increase of the aerosol number density rather than the growth of the
aerosols. This assertion will be discussed in relation to the data pre-
sented in Figure 4.
The behavior of the aerosol burst depicted in Figure 3 is quite similar
to that shown in Figure 1. In the first 30 minutes after the detection of
the diffuse solar radiation, the increase of the various parameters were:
(1) temperature from -8.0 to -6.2 C: (2) dewpoint from -11.7 to -7.0 C: (3)
13
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Figure 3. Twenty four hour variation of the outside scattering coeffici-
ent, temperature, dew point and relative humidity. The scale
for the scattering coefficient is given on the left hand side.
Scales for the remaining parameters are given on the right
hand side. For temperature and dew point, degrees fahrenheit
is to be substituted for percent. A scale is also given for
the temperature and dew point in degrees Celsius. Arrows are
shown for sundown (unmarked), sunup (S), diffuse solar radia-
tion (DF), and direct solar radiation (DR).
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
SCATTERING COEFFICIENT
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relative humidity from 75.3 to 93.9 percent; (4) scattering coefficient from
0.16 to 0.41 km-1; and (5) total solar radiation from 0.0 to 4 cal cm~2.
Comparing the total solar radiation received with the solar energy
received October 16, 1979, it will be noted that it was much larger on
February 1, 1980. This would be expected since the scattering coefficient
was much smaller on February 11, 1980. Our calculations showed that 0.3 cal
cm-2 was necessary to account for the temperature rise and 0.2 cal cm-2
for the increase of the dew point. Thus, solar radiation could provide the
necessary energy to account for the increases of the temperature and the dew
point.
At about 1000 EST, there is a protuberance on the scattering coeffi-
cient profile which, as remarked previously, appears to be related to
fumigation. It will be noted that the scattering coefficient at 1400 EST on
February 10, 1980 is nearly equal to the scattering coefficient 24 hours
later and suggests that there had been no net change of the aerosol number
density.
In analyzing the data presented in Figure 3, it was asserted that the
increasing trend of the outside scattering coefficient during the night was
primarily due to an increasing trend of the aerosol number density. Similar
to the previous analysis, the relative importance of aerosol growth and of
an increase of aerosol number density to the increasing trend of the outside
scattering coefficient during the night can be assessed by an analysis of
the data on the inside and outside nephelometers shown in Figure 4.
Referring to Figure 4, it will be noted that, except for the period
from about 1800 EST until midnight, there are no essential differences
between the inside and outside scattering coefficient profiles from sundown
until sunup. During this period, the inside relative humidity was less than
20 percent. Consequently, in contrast to the previous example, the increas-
ing trend of the outside scattering coefficient must have been almost
entirely due to an increase of the aerosol number density. On the other
hand, the increase of the scattering coefficient during the aerosol burst
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was almost entirely due to a growth of the aerosols which resulted from the
increase of the outside relative humidity.
Perhaps, it is worth mentioning again that the outside scattering
coefficient was observed to be larger than the inside scattering coefficient
from 1800 EST until midnight. This difference in the magnitudes of the
scattering coefficients suggests that aerosol growth, resulting from an
increase of the outside relative humidity, was responsible. As a consequ-
ence of this difference in the magnitude of the scattering coefficients, it
appears that the growth of an aerosol depends not only on the relative
humidity but also on the time rate of change of the relative humidity. It
is likely that this dependence is also responsible, at least in part, for an
aerosol burst.
Similar to the features shown in Figure 2, the maximum of the inside
scattering coefficient occurs after the maximum of the outside scattering
coefficient in Figure 4. The protuberance on the aerosol burst is clearly
associated with the maximum of the inside scattering coefficient. As
mentioned previously, the significance of the relatively small protuberance
will be discussed in Section 3.3.
As for the previous example, there were little net change of the
outside scattering coefficient from 1400 EST on February 10, 1980 to 1400
EST on February 11, 1980. Consequently, there was little net change of the
aerosol number density during this period.
(c) November 22-23, 1979
For the previous two examples, it was concluded that there was no net
change in the aerosol number density from the beginning of the observatinal
period at 1400 EST until the end of the observational period at 1400 EST the
next day. In contrast, the data presented in Figure 5 suggests that there
was a decrease of the aerosol number density during the observational
period. This conclusion is based on the observation that the inside and
outside scattering coefficients decreased from about 0.1 knrl at 1400 EST
17
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5. Twenty four hour variation of the inside and outside scattering
coefficient and the corresponding inside and outside relative
humidity. The scale for the scattering coefficient is on the
left hand side. The scale for the relative humidities is on
the right hand side. The scale for the portion of the direct
solar flux is arbitrary. The arrows indicate time of sundown
and sunup.
OUTSIDE
RELATIVE HUMIDITY
INSIDE
OUTSIDE
SCATTERING COEFFICIENT
INSIDE
1400 1600 1800 2000 2200 2400 0200 0400 0600 0800 1000 1200 1400
NOV. 22. 1979 TIME (EST) „, NOV. 23. 1979
18
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on November 22, 1979 to about 0.03 knrl at 1400 EST on November 23, 1979.
Examining the profile of the inside scattering coefficient in Figure 5, it
will be observed that the inside scattering coefficient was roughly constant
until the aerosol burst occurred. Subsequently, there was an abrupt de-
crease of the inside scattering coefficient. This definitely indicates that
the RTP was not gradually engulfed by air with a smaller aerosol number
density. Rather, there was a relatively abrupt decrease of the aerosol
number density which occurred during the aerosol burst. It is also import-
ant to note in Figure 5 that the inside relative humidity was about 45
percent at the beginning and the end of the observational period. Conse-
quently, it is not likely that the decrease of the scattering coefficient
can be attributed to a shrinking of the aerosols.
From sundown to sunup, there was an increasing trend of the outside
scattering coefficient. Since the inside scattering coefficient was roughly
constant during this period, the increasing trend of the outside scattering
coefficient was most likely predominately due to a growth of the aerosols
resulting from the increasing relative humidity. In contrast to the previ-
ous two examples, the evidence for an increase of the aerosol number
density between sundown and sunup is debatable.
From sunup until the maximum of the outside scattering coefficient at
about 0800 EST, the outside relative humidity increased from about 92 to 95
percent. From 0800 until 0900 EST, the outside relative humidity decreased
from 95 to 94 percent during the first 15 minutes, remained relatively
constant for the next 30 minutes, and subsequently increased to a maximum of
about 96 percent by 0900 EST. From 0900 EST until the end of the period at
1400 EST, there was a decline of the relative humidity to about 48 percent.
With this information on the relative humidity, a better understanding of
the aerosol burst is possible.
From sunup until the maximum of the aerosol burst, there was a rela-
tively small increase of the inside scattering coefficient. Consequently,
the increase of the outside scattering coefficient is primarily due to the
growth of the aerosols which results from the 3 percent increase of the
19
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outside relative humidity.
From 0800 until 0815 EST, there was a relatively rapid decrease of the
outside scattering coefficient. Evidently, this decrease was in response to
the 1 percent decrease of the relative humidity. During this time there is
no evidence of a decrease of the aerosol number density since the inside
scattering coefficient was relatively constant even though the inside
relative humidity was constant.
From about 0845 to 0900 EST, the outside relative humidity increased
from 94 to 96 percent. There is no expected increase of the outside
scattering coefficient. Rather, there is a continued decrease. To explain
this anomaly, the only plausible explanation appears to be that the aerosol
number density was decreasing. The inside scattering coefficient supports
this conjecture since it decreases rapidly during this period.
As a matter of interest, a portion of the direct solar radiation
profile is shown in Figure 5. Dips in the profile indicates that there were
a few clouds. The weather station at RDU reported scattered clouds at 8 km.
For this case, it is not apparent that the scattered clouds had any influ-
ence on the relative humidity. However, at times, the influence of clouds
on the relative humidity can be easily noted since the clouds retard the
evaporation of moisture. Consequently, it is possible for clouds to influ-
ence the aerosol burst.
According to the above analysis, "clean" air began arriving at the RTP
at about 0845 EST. At 0840 EST, the records indicate that there was a
transition from calm winds (<2 ms~l) to the customary daytime fluctuating
winds. At that time, air above the nocturnal inversion can be mixed down-
ward. If this air has a relatively small aerosol number denisty, the air
near the surface would be diluted by mixing and the decrease of the aerosol
number density could be understood.
After 1400 EST until sunset on November 23, 1980, the inside and
outside scattering coefficients were essentially the same and about 0.03
20
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km~l. Thus, the RTP was bathed with relatively "clean" air for at least 7
hours. At times after a thunderstorm, the scattering coefficient will be as
small as 0.03 knrl. However, these periods seldom last over an hour.
Consequently, it would appear that there must be a large source region for
the "clean" air. Information concerning a possible source region was
obtained by consulting the weather maps.
On November 23, 1979 at 0700 EST, there was a low pressure system
centered over Lake Superior. A relatively slow moving cold front, which was
roughly oriented in a north-south direction from Lake Superior to Western
Louisiana, was associated with the low pressure system. Precipitation,
associated with the front, was quite extensive and back of the front. On
November 23, 1979, the front was about 400 km to the west of the RTP. Maps
of the upper winds indicated that the winds had been from the west and
southwest for at least 3 days. Thus, the weather maps indicated that a
possible source region of "clean" air did exist since rainfall would be
expected to purge the atmosphere of aerosols.
Presumably, the "clean" air was transported to the RTP above the
nocturnal temperature inversion and brought to the surface during the
aerosol burst by turbulent mixing processes. In any case, the "clean" air
could not be associated with the passage of a front or a change of the wind
direction. This example most likely represents a case of which there has
been a long distance, transport (^500 km) of "clean" air to the RTP.
3.2 AEROSOL BURST, FUMIGATION, AND VISIBILITY DETERIORATION
For each of the examples previously presented, a relatively small
protuberance was observed on the profile of the outside scattering coeffi-
cient. It was located after the maximum of the profile which resulted from
the growth and shrinking of the aerosols due to the influence of the rela-
tive humidity. The protuberance was observed to be due to an increase and
subsequent decrease of the inside scattering coefficient which resulted from
an increase and subsequent decrease of the aerosol number density. Pro-
cesses associated with solar heating are the most plausible explanation for
21
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the variations of the aerosol number density which will now be discussed.
When solar heating commences after sunup, atmospheric mixing begins and
will increase as solar heating increases with time. As a result of mixing,
aerosols aloft will be diffused and transported toward the ground. To
account for an increase of the inside scattering coefficint after sunup, the
aerosols aloft must have a larger aerosol number density, at least initial-
ly, than aerosols closer to the ground. As the mixing layer increases in
depth, the aerosol number density must eventually decrease by dilution.
It is of interest to note that the transition from relatively calm
winds (<2 ms~l) to the usual daytime fluctuating winds occurs roughly at
the time of the maximum of the inside scattering coefficient. In addition,
by acoustic soundings, it has been observed that the time of the maximum of
the inside scattering coefficient is roughly the time that thermal plumes
begin penetrating the base of the nocturnal temperature inversion. At that
time, aerosols are more easily diffused upward with a corresponding decrease
of the aerosol number density. After the nocturnal temperature inversion
dissipates, it would be anticipated that aerosols are distributed nearly
uniform in the vertical from the ground to the top of the planetary boundary
1ayer.
The processes discussed in the literature to explain the fumigation of
smoke after sunuplS are the same as the processes which were presented as
a explanation of the increase and subsequent decrease of the inside scatter-
ing coefficient after sunup. During fumigation, there is an initial in-
crease and subsequent decrease of the aerosol number density which also is
responsible for the variations of the inside scattering coefficient. Thus,
the protuberance, which was observed on the outside scattering coefficient
profiles of the examples presented, appears to be due to the phenomenon of
fumigation. In part, the prominence of the protuberance will depend on the
magnitude and vertical distribution of the aerosol number density. An
example of a more prominent protuberance is shown in Figure 6.
22
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b, km'
1
P
en
P
•-J
P
oa
s
s
m NI
m o
o
m o
O N>
IS>
'* %
I
'^-.^
\ a y-
_ m /
Z r- I
*
\.
\
l
I -
c
'-I
CD
— I O
rr o
CD CD
03 I?
05 CD g
- cn -r
Q. ?
._. O
n Q3 C
S = i
rr CL
CO <
r O h--
- C O
CD
H- O
- = cn
25 " K
-< i- Q.
5. Q. ™
CD
cn 93
S 5.
w
< Q.
CD CD
RELATIVE HUMIDITY, percent
23
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In Figure 6, there is an increasing trend of the inside and outside
scattering coefficients from sunset to sunup. For the inside scattering
coefficient, the increasing trend must be attributed to an increasing trend
of the aerosol number density. For the outside scattering coefficient, the
increasing trend must be attributed to the growth of the aerosols resulting
from the increasing trend of the relative humidity and to the increasing
trend of the aerosol number density. At sunup, the contributions due to an
increase of the aerosol number density and the growth of aerosols to the
outside scattering coefficient were approximately 0.12 and 0.14 km~l,
respectively.
The first prominent peak on the outside scattering coefficient is due
to the growth and shrinking of the aerosols under the influence of the
relative humidity. The second prominent peak corresponds to the protuber-
ance on the outside scattering coefficient profiles discussed earlier. Its
prominence in Figure 6 is apparently due to the existence of a relatively
large aerosol number density aloft. The transition from relatively calm
winds (<2 ms~l) to the usual daytime fluctuating winds occurred at about
0930 EST. The expected decrease of the scattering coefficient was also
observed to start occurring for reasons previously discussed. The source of
the aerosols could not be determined. It should be noted that the scatter-
ing coefficient at 1400 on December 11, 1980 was approximately equal to the
scattering coefficient 24 hours later which indicates that there was not net
change of the aerosol number density.
It has been shown that there is a linear relation between the visibil-
ity observations at RDU and the inverse of the scattering coefficient
observation at RTP.16 There was also an excellent correlation between
these parameters. Thus, the visibility observed at RDU is usually repre-
sentative of the visibility at the RTP. Consequently, it is of interest to
examine the visibility observed at RDU during an aerosol burst. Table 1
presents the hourly visibility observations during the aerosol burst for the
four examples which have been presented.
24
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TABLE 1. HOURLY VISIBILITY DURING AN AEROSOL BURST
DATE
October 16, 1979
February 11, 1980
November 23, 1979
December 12, 1980
TIME
(EST)
0453
0551
0653
0754
0853
0953
0751
0850
0951
1051
0554
0656
0755
0852
0950
0651
0748
0853
1003
1050
VISIBILITY
(miles)
10
7
7
7
8
10
12
6
6
10
15
7
10
10
20
12
5
7
7
10
In Table 1, it will be noted that, during each aerosol burst, the
visibility was smaller than it was before and after the aerosol burst.
Thus, as would be expected, the visibility decreases during an aerosol
burst. This decrease of the visibility is due to an increase of the scat-
tering coefficient during an aerosol burst. In part, the meteorological
conditions for an aerosol burst to occur after sunup is that the preceeding
night should be calm (<2 ms~l) and the sky should be essentially free of
clouds. It is of interest to note that these meteorological conditions have
been enunciated as being necessary for the phenomenon of visibility deteri-
oration during a winter morning.17 There can be little doubt that an
aerosol burst is responsible for the phenomenon of visibility deterioration
during the morning.
25
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SECTION 4
SUMMARY OF CONCLUSIONS
It will be instructive to summarize the significant conclusions con-
cerning the growth and shrinking of aerosols and the variations of the
aerosol number density deduced from nephelometer observations at the RTP.
The conclusions are intended to be only appropriate for anticyclonic pres-
sure systems. Four examples of the observations have been presented. In
particular, the conclusions pertain to meteorological conditions such that
the atmosphere is stable during the night. In addition, the sky has few
clouds and the wind is relatively calm.
The conclusions are:
o The outside scattering coefficient increases nearly monotinically
from sundown until sunup. In general, aerosol growth and an in-
crease of the aerosol number density are responsible for the in-
creasing trend of the outside scattering coefficient.
Occasionally, an increasing trend of the aerosol number density is
the only process responsible for the increasing trend of the outside
scattering coefficient during the night. While aerosol growth is
most likely due to an increasing trend of the relative humidity, the
processes responsible for an increasing trend of the aerosol number
density are vague.
o For an aerosol burst, which occurs within a 2 to 3 hour period
after sunup, the outside scattering coefficient is a composite of
contributions by the growth and shrinking of aerosols and the
increase and decrease of the aerosol number density.
26
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Contributions to the outside scattering coefficient by the growth
and shrinking of the aerosols occur due to the increase and decrease
of the relative humidities. Variations of the relative humidity are
primarily due to the evaporation of moisture by solar radiation
heating. The moisture was deposited at ground level during the
night.
The processes responsible for the increase and decrease of the
aerosol number density are the same as the processes responsible for
the phenomenon of fumigation. These processes result from turbulent
mixing of the atmosphere due to solar radiation heating.
The maximum contribution to the outside scattering coefficient, by
the growth and shrinking of aerosols, occurs before the maximum
contribution by the increase and decrease of the aerosol number
density.
o The phenomenon of visibility deterioration during the morning is due
to the phenomenon of an aerosol burst.
27
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REFERENCES
1. Orr, C., F. K. Hurd. and W. J. Corbett, 1958: Aerosol size and rela-
tive humidity. J. Colloid Science, 13. pp. 472-482.
2. Covert, D. A., R. J. Charlson, and N. C. Ahlquiest, 1972: A study of
the relationship of chemical composition and humidity to light scat-
tering by aerosols. J. Appl. Meteor., 11, pp. 968-976.
3. Winkler, P. and C. Junge, 1972: The growth of atmospheric aerosol
particles as a function of the relative humidity - I. Method and
measurements at different locations, _J._ Rech. Atm. (memorial Henri
Dessens), pp. 617-638.
4. Garland, J. A., J. R. Branson, and L. C. Cox. 1973: A study of the
contribution of pollution to visibililty in a radiation fog. Atmos.
Environ., 7, pp. 1079-1092.
5. Winkler, P., 1973: The growth of atmospheric aerosol particles as a
function of the relative humidity - II. An improved concept of mixed
nuclei. Aerosol Sci.. 4, pp. 373-387.
6. Sinclair, D., R. J. Countess, and G. S. Hoopes. 1974: Effect of
relative humidity on the size of atmospheric aerosol particles. Atmos.
Environ., _8, pp. 1111-1117.
7. Fitzgerald, J. W., 1975: Approximation formulas for the equilibrium
size of an aerosol particle as a function of its dry size and composi-
tion and the ambient relative humidity. J. Appl. Meteor., 14, pp.
1044-1049.
8. Tang. I. N., 1976: Phase transformation and growth of aerosol parti-
cles composed of mixed salts. J. Aerosol Sci., 7_, pp. 361-371.
9. Hanel, G. and B. Zankel. 1979: Aerosol size and relative humidity:
Water uptake by mixtures of salts. Tell us, 31, pp. 478-486.
10. Charlson, R. J., A. H. Vanderpol. D. S. Covert. A. P. Waggoner, and N.
C. Ahlquist, 1974: H2S04/(NH4)2S04 background aerosol optical
detection in St. Louis region. Atmos. Environ., 8:, pp. 1257-1267.
11. Fox, D. L., M. R. Kuhlman, and P. C. Reist, 1976: Sulfate aerosol
formation under conditions of variable light intensity, Co 11id and
Interface Science. Vol. II- Aerosols, Emulsions and Surfactants,
Academic Press, New York, pp. 185-196.
28
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12. Miller, D. F., W. E. Schwartz, J. L. Gemma, and A. Levy 1975: Haze
formation: Its nature and origin. Final Report EPA 650/3-65-010
NERC, Research Triangle Park, North Carolina 27711. 100 pp.
13. Saunders, P. M., 1964: Sea smoke and steam fog. Quart. J. R. Met.
Soc., JK), pp. 156-165.
14. Wessels, H. R. A., 1979: Growth and disappearance of evaporation
fog during the transformation of a cold air mass. Quart. _J. j}. Met.
Soc., 105, pp. 963-997.
15. Hewson, E. W., 1945: The meteorological control of atmospheric pollu-
tion by heavy industry. Quart. jj. R:. Met. Soc., _71, pp. 266-282.
16. Griff ing, G. W., 1980: Relation between the prevailing visibility
nephelometer scattering coefficient and sunphotometer turbidity
coefficient. Atmos. Environ., 14, pp. 577-584.
17. Saunders, W. E., 1971: Visibility deteriorations during winter morn-
ings. Met. Mag., 100, pp. 149-155.
29
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
2.
3. RECIPIENT'S ACCESSION-NO.
4. TITLE AND SUBTITLE
DEPENDENCE OF NEPHELOMETER SCATTERING COEFFICIENTS ON
RELATIVE HUMIDITY
Evolution of Aerosol Bursts
5. REPORT DATE
6. PERFORMING ORGANIZATION CODE
7. AUTMOR(S)
8. PERFORMING ORGANIZATION REPORT NO.
George W. Griffing
9. PERFORMING ORGANIZATION NAME AND ADDRESS
10. PROGRAM ELEMENT NO.
CDTA1D/03-1327 (FY-81)
(same as block 12)
11. CONTRACT/GRANT NO.
12. SPONSORING AGENCY NAME AND ADDRESS
Environmental Sciences Research Laboratory - RTP, NC
Office of Research and Development
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
13. TYPE OF REPORT AND PERIOD COVERED
In-house
14. SPONSORING AGENCY CODE
EPA/600/09
15. SUPPLEMENTARY NOTES
16. ABSTRACT
Observations on the temporal dependence of the nephelometer scattering
coefficient on relative humidity are presented and discussed for four different
cases. For each case, the weather at the Research Triangle Park, North Carolina was
dominated by an anticyclonic weather system. By taking simultaneous nephelometer
scattering coefficient observation at two different relative humidities, it was
possible to conclude that with nocturnal stable atmospheric conditions:
o In general, the scattering coefficient increases from sundown to sunup due to
aerosol growth and an increasing trend of the aerosol number dens.ity:
o In general, the relatively rapid increase and subsequent decrease of the
scattering coefficient during a 2 to 3 hour period after sunup is due to a
relatively rapid aerosol growth and shrinkage, and a relatively rapid in-
crease and decrease of the aerosol number density.
The latter behavior of the scattering coefficient was called an aerosol burst.
The relationship between an aerosol burst, fumigation, and early morning visibility
deterioration is also discussed.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lOENTlFIERS/OPEN ENDED TERMS C. COSATI Field/Group
18. DISTRIBUTION STATEMENT
RELEASE TO PUBLIC
19. SECURITY CLASS (ThisReport)
UNCLASSIFIED
21. NO. OF PAGES
38
20. SECURITY CLASS (Thispage)
UNCLASSIFIED
22. PRICE
EPA Form 2220-1 (9-73)
30
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