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Journal of Geography, Environment and Earth Science
International
15(2): 1-10, 2018; Article no.JGEESI.41167
ISSN: 2454-7352
Atmospheric Stability across the Lower
Troposphere in Enugu City, Nigeria
D. O. Edokpa
1*
and M. O. Nwagbara
2
1
Department of Geography and Environmental Management, University of Port Harcourt,
Port Harcourt, Rivers State, Nigeria.
2
Department of Soil Science and Meteorology, Michael Okpara University of Agriculture,
Umudike, Abia State, Nigeria.
Authors’ contributions
This work was carried out in collaboration between both authors. Author DOE designed the study and
performed the numerical analysis. Author MON wrote the protocol, wrote the first draft of the
manuscript and reviewed the analyses of the study. Both authors read and approved the final
manuscript.
Article Information
DOI: 10.9734/JGEESI/2018/41167
Editor(s):
(1)
Anthony R. Lupo, Professor, Department of Soil, Environmental, and Atmospheric Science,
University of Missouri, Columbia, USA.
Reviewers:
(1)
Chin-Hsiang Luo, Hungkuang University, Taiwan.
(2)
Eric S. Hall, USA.
Complete Peer review History:
http://www.sciencedomain.org/review-history/24572
Received 10
th
February 2018
Accepted 18
th
April 2018
Published 10
th
May 2018
ABSTRACT
This study surveyed the atmospheric stability pattern in the lower troposphere over Enugu from
2010-2015. The widely and acceptably used Pasquill-Gifford stability scheme was utilized in
evaluating the stability categories. Six-hourly synoptic data parameters for temperature, wind speed
and cloud cover acquired from the Era-Interim platform at 1000 mbar pressure level were used in
the analysis. The data were obtained at 0.125 degree resolution. Results showed that very stable
stability classes D (neutral), E (stable), and F (very stable) conditions occurred during the night and
early hours of dawn. Also, while class D dominated during the wet season, classes E and F
portrayed a reverse trend during the dry season. During the Day, stability classes A (very unstable),
B (moderately unstable) and C (slightly unstable) prevailed, however, (class C) prevailed throughout
the year. While stability class A was dominant from December to January, with its least influence
during the peak of the wet season at noontime, stability classes B and C prevailed during the wet
Original Research Article
Edokpa and Nwagbara; JGEESI, 15(2): 1-10, 2018; Article no.JGEESI.41167
2
season and was lowest at the peak of the dry season. The occurrence of stability class D at 6:00 pm
local time indicates the beginning of transition periods, where increased wind speeds moderates the
effects of heat fluxes from the earth’s surface. The surveyed atmospheric stability conditions in
Enugu city indicates that emissions will be constrained at ground level during the night, where
anthropogenic sources of emissions remain beneath the inversion layer. Nevertheless, where the
sources are beyond the inversion layer, dispersion will take place upward away from ground level.
Therefore, it is compelling that governments, agencies, and industries control emissions from
industries within the city especially at night time to avoid ground level pollutant concentrations that
will affect boundary layer dwellers. Also, potential emitters should be restrained from being sited at
locations where pollutants could be concentrated with sensitive receptors.
Keywords: Atmospheric stability; lower troposphere; Enugu, emission; era-interim; receptors.
1. INTRODUCTION
The tendency of the lower atmosphere to prevent
or enhance altitudinal motion is known as its
stable condition. It is closely linked to how
temperature changes vertically in part of the
troposphere contained in the planetary boundary
layer. Different studies have revealed the level
and degree of stability conditions across
geographical regions [1-5]. Atmospheric stability
has an important effect on the boundary layer by
enhancing local air mass circulation between the
surface and the lower troposphere. The level and
magnitude of stability conditions are by radiation
heating of the ground surface, and the longwave
radiation is emanating from the surface. This
modifies the surface and sensible heat fluxes on
the earth. The recurrent interaction of air masses
within the boundary layer highlights the
atmosphere as a dynamic system. Three
significant stability categories conditions exist;
these are unstable, neutral and stable conditions.
The prevalence of either condition depends on
the energy state of the surface-atmosphere
interface at any point in time. The stability of the
atmosphere is a direct measure of its dispersive
ability. Atmospheric stability plays a vital part in
the turbulence existing in the troposphere and
consequently affects atmospheric diffusion
processes [6]. Atmospheric stability controls the
amount of turbulence within the boundary layer,
which influences the manner air pollutants
dispersed within the lower troposphere [7].
Emission dispersions in the troposphere are
manipulated by altitudinal forcing created by the
existence of stability conditions. Thus, an
assessment of atmospheric stability for any
location is paramount for estimating the times of
high and low ground level emission
concentrations across ground level receptors.
When the lower atmosphere is unstable,
emission dispersion is enhanced and does not
severely impact ground level receptors. When
neutral conditions exist, emissions will continue
unmixed as the atmosphere does not prevent or
improve diffusion. The atmosphere will be likely
to limit emissions mixing during stable conditions
by holding emissions at the ground level due to
temperature inversion. An undulating landscape
like Enugu city, where varied microclimatic
conditions will be prevalent across the spatial
expanse due to pressure differences, will make
the alterations of stability conditions more
frequent diurnally, and increase uneven spatial
precipitation. The undulating terrain over a large
area can significantly impact the weather and
climate pattern of the local environment. This
type of landscape can considerably create
temperature gradients, thereby enhancing the
local atmospheric circulation direction near
earth’s surface, thus affecting the urban wind
environment [8]. This forms the local terrain wind
that could aid mechanical turbulence at the
earth’s surface.
The purpose of this study is to evaluate the
stability conditions of the lower troposphere in
Enugu city using the Pasquill-Gifford stability
technique.
2. MATERIALS AND METHODS
2.1 Description of Study Location
Enugu city is situated within latitudes 6º24
’
–
6º30’N, and longitudes 7º27
’
– 7º32
’
E (Fig. 1) on
an approximate altitude range of 150-250m
above sea level [9,10]. The area is located within
the humid tropical rainforest zone and has two
distinct seasons: wet period (April-October) that
is accompanied by moist warm air from the
Ocean, and; dry period (November-March),
associated with Harmattan north-easterly trade
wind [11]. The month of March is the transition
period to the wet season. The Inter-Tropical
Convergence Zone (ITCZ) influences the distinct
Edokpa and Nwagbara; JGEESI, 15(2): 1-10, 2018; Article no.JGEESI.41167
3
seasons by its south-north oscillation with the
overhead sun. The oscillation of the ITCZ is
accompanied by opposing bands of warm, humid
air masses and the hot and dry continental air
masses. With the area’s location being close to
the dominant of south-west moist air, the area
receives ample average annual precipitation
amounts above 2000 mm [10], with double
maximal rainfall peaks in July and September.
The average precipitation days ranged from 1 in
January to 14, 16, 15 and 18 from June to
September respectively [12]. Average daily
temperature is about 26ºC from July to
September and 29ºC from February to March.
The average monthly hours of available sunshine
is above 200 in November and December, and
below 200 from January to October. The average
relative humidity for the area varies from 47% -
89% throughout the year, with high and low
peaks during the wet and dry seasons
respectively. Average monthly cloud cover in the
area ranged between 4-7 oktas, with higher and
lower values during wet and dry season
respectively. Average daily wind speed ranges
from 0-5m/s, with periods of lower and higher
trends observed during the night and transition
periods respectively. Figs 2-5 show the average
wind direction for the study area from December-
February (DJF) and July-August (JJA) from
midnight to noon local time. The Enugu city,
which lies on the south-eastern foot of Udi hills,
is flanked by hills and valleys in an undulating
manner [13]. This difference in altitude within the
city creates a variation of atmospheric elements
[14].
2.2 Data
The data utilized for this study were acquired
from the universally acceptable reanalysis
Interim data set (Era-Interim) platform.
Temperature, wind speed and cloud cover at
1000 mbar pressure level were retrieved from
2011-2015 at synoptic hours: midnight local time,
6:00am local time, noon local time and 6:00pm
local time at 0.125º grid resolution. The Era-
Interim data are the modern reanalysis data
accessible from 1979 [15]. They are
extremely viable in characterizing the
atmospheric dynamics over West Africa. The
data set is dependable at any point in time, and
also at any spatial level [16]. Mean short wave
radiation for 22 years was acquired for the study
area from NASA’s atmospheric science data
centre.
Fig. 1. Map of study area
Edokpa and Nwagbara; JGEESI, 15(2): 1-10, 2018; Article no.JGEESI.41167
4
Fig. 2. Wind direction pattern for Enugu city at midnight (local time), DJF
Fig. 3. Wind direction pattern for Enugu city at noon (local time), DJF
Fig. 4. Wind direction pattern for Enugu city at midnight (local time), JJA
Edokpa and Nwagbara; JGEESI, 15(2): 1-10, 2018; Article no.JGEESI.41167
5
Fig. 5. Wind direction pattern for Enugu city at noon (local time), JJA
2.3 Method of Data Collection
Pasquill-Gifford (PG) [17] categorized the
atmospheric stability scheme into six classes,
i.e., very unstable (A) to very stable (F). The
category G which indicates extremely stable
conditions was later introduced, and typically
occurs at about 1.4% of the time [18]. Daytime
stability classes were evaluated by relating
surface wind speed at 10 m altitude to solar
radiation, while night-time stability classes were
evaluated by relating surface wind speed to
cloud cover. Daytime solar insolation was
defined as strong, medium, and slight, when
greater than 600 W/m
2
, 300-600W/m
2
, and less
than 300 W/m
2
respectively [19]. The
technique in Tables 1 and 2 designates a method
of determining the stability classes
in the absence of radiosonde data. The
evaluation of atmospheric stability situations
was achieved with the aid of an excel
spreadsheet.
Table 1. Pasquill-Gifford (PG) Day time classification (Classes)
Wind speed
(at 10 m) (m/s)
Day time solar insolation (W/m
2
) Radiation overcast
Strong >600 Moderate 300-600 Slight < 300
<2 A A-B B C
2-3 A-B B C C
3-5 B B-C C C
5-6 C C-D D D
>6 C D D D
Table 2. Pasquill-Gifford (PG) night time classification (Classes)
1Hr before sunset or after sunrise
Cloud cover (Oktas) Night-time
0 -3
4 – 7
8
D F or G F D
D F E D
D E D D
D D D D
D D D D
Source: [1].
Edokpa and Nwagbara; JGEESI, 15(2): 1-10, 2018; Article no.JGEESI.41167
6
3. RESULTS AND DISCUSSION
Average monthly distribution displayed in Table 3
shows that stability class A (very unstable
condition) was noticeable between November
and February, with the highest rate in November,
i.e., 20.8%. The frequency diminished between
March and October, the period of the wet
season, and increased cloud cover in the study
location. Stability classes B and C (moderately
and slightly unstable conditions) were somewhat
uniformly distributed throughout the year. The
high frequency occurrence of stability classes B
and C was 26.7% and 23.4% in September and
August respectively. The lowest rate for both
occurred in November, i.e., 14.7% and 9.2% for
classes B and C respectively. However, the
dominance of class A in November occurred
during the dry season, when more solar
insolation reaches the surface, with less cloud
cover. For the neutral and stable stability
categories (D, E, and F), class D displayed a
stronger pattern from February to November,
while stability classes E and F followed suit
during the same period. Stability class D was
lowest in December, with 9.7% occurrence, and
highest in August, with 36.3% occurrence.
Stability class E was higher in March and lower
in September, i.e., 20.2% and 9.7% respectively,
while that of class F was highest and lowest in
December and March, i.e., 29.7% and 5.1%
respectively (Table 3).
Table 3. Average monthly distribution of stability classes from 2010-2015
Month Frequency of stability class occurrence (%)
A B C D E F G
JAN 12.9 21.1 13.4 12.4 17.1 23.0 0.1
FEB 10.8 15.5 16.0 34.2 14.5 9.0 0.0
MAR 9.3 17.9 16.9 30.6 20.2 5.1 0.0
APR 5.0 23.2 15.3 31.7 17.2 7.6 0.0
MAY 5.2 23.1 15.5 28.0 17.3 10.9 0.0
JUN
1.4
22.8
22.2
26.9
16.4
10.3
0.0
JUL 0.9 23.0 21.1 33.5 13.3 8.2 0.0
AUG 0.4 18.8 23.4 36.3 13.6 7.5 0.0
SEP
1.1
26.7
17.1
34.0
9.7
11.4
0.0
OCT 8.2 26.2 10.8 21.5 14.8 18.5 0.0
NOV 20.8 14.7 9.2 21.4 16.8 17.1 0.0
DEC 11.4 23.0 12.4 9.7 13.6 29.7 0.3
Source: Authors’ Fieldwork, 2016
Fig. 6. Atmospheric stability pattern in Enugu city at midnight (local time)
Edokpa and Nwagbara; JGEESI, 15(2): 1-10, 2018; Article no.JGEESI.41167
7
The Figs 6-9 shows the average stability trend for the
study location at the specified synoptic hours. For the
midnight local time and 6:00 am local time, Stability
class F was dominant from December to January,
and lowest in March and during the wet season
(Fig. 6). Stability class D dominates all through the wet
season, and was lowest during the peak dry season,
i.e., December and January. This shows that stability
class D is closely associated with the period of rainfall
at the study area. The reverse trends observed
during the peak rainy period between stability
classes D and F, compared to the study
result in Port Harcourt [20]. The stability class E
shows a fluctuating pattern between classes D and F,
indicating a switch from neutral to stable atmospheric
conditions.
Atmospheric stability pattern for the area at noon local
time (Fig. 8) shows that stability class C dominates
from February to November. Stability class B
assumes a moderate trend, with lower peaks in
January and December than class A (Fig. 8). Stability
class A was more prevalent in January and
December than classes B and C. This is due to the
increased solar insolation observed during those
months (under low cloud cover). The position of the
ITCZ over the Ocean during December and January
coincides with the position of the sun over the coast of
southern Nigeria, preparing to move the ITCZ towards
the northern region.
At 6:00pm local time, around sunset in the study area,
while stability class B peaks in the month of October
and is lowest in February, class C peaks in March and
is lowest between October – November. Stability
class D portrayed a moderate drift, with low peaks in
January and December (Fig. 9). This trend for class D
is similar to the condition in Port Harcourt [20]. The
occurrence of stability class D indicates the beginning
of transition periods, where increased wind speeds
moderate the effects of the heat fluxes from the
earth’s surface [20].
Fig. 7. Atmospheric stability pattern in Enugu city at 6:00pm (local time)
Fig. 8. Atmospheric stability pattern in Enugu city at noon (local time)
Edokpa and Nwagbara; JGEESI, 15(2): 1-10, 2018; Article no.JGEESI.41167
8
Fig. 9. Atmospheric stability pattern in Enugu city at 6:00pm (local time)
Associating atmospheric stability with pollutant
dispersion, times of stable and very stable
atmospheric conditions (classes E & F), where an
inversion layer exists, will keep impurities and their
associated concentrations continually at ground level.
This occurs when both anthropogenic and natural
sources are located beneath the inversion layers.
Stable atmospheric conditions mean less
atmospheric mixing, and this leads to the build-up of
ground-level emission concentrations at sensitive
receptors. The higher the emissions rate from the
source, the more impactful the effect is on sensitive
ground level receptors. However, the vertical mixing
height of emission concentration levels on the surface
depends on the height of the boundary layer at any
point in time. Boundary layer heights are lower at
night (below 500 m) and higher during the day (above
500 m – 2 km), depending on intensity of the surface
and sensible heat fluxes [21].
Pollutant emissions in Enugu city are from the
combustion of petroleum products and to an
insignificant extent, solid waste disposal by open
burning. As the population increases with the
quest for jobs, ground level pollutant
concentrations will continue to increase. The
degree of air pollution depends on the interaction
between the emitting source, atmospheric
transport, and dispersion, as well as the sensitive
receptor [22]. Atmospheric stability influences the
atmospheric transport component of this
interaction. Due to the high wind speeds in
Enugu between December to March [22],
emission concentrations are rapidly transported
to receptors. With the dominance of stability
class F both at night and the early hours of the
day, ground level emissions will be high at night.
With the dominance of unstable classes A, B and
C during the day, and transition periods under
moderate wind speed, this will ensure the
vigorous mixing of ground level pollutant
concentrations. The slower the wind speed, the
more the emission concentrations will impact
ground level receptors downwind of a source.
The more unstable the atmospheric conditions at
higher wind speeds, the more dispersed the air
pollutants will be on downwind receptors. United
States EPA categorizes lower emission sources
from 0 to10 m and higher emission sources from
10 to above 100 m.
Fig.10. Cumulative stability pattern in Enugu city
Edokpa and Nwagbara; JGEESI, 15(2): 1-10, 2018; Article no.JGEESI.41167
9
The annual cumulative pattern as shown in Fig. 10
shows that the average stability shape for the study
location is placed in the following order:
D>B>C>E>F>A>G.
4. CONCLUSION
Atmospheric stability conditions at any locality
influence what is being transported in the atmosphere
from one point to another. From the evaluated stability
categories of the study area, based on specified
periods, unstable atmospheric conditions are
distributed during the day, while neutral conditions
exist during the night time and early hours of the day.
The effect of surface and sensible heat fluxes, as well
as the undulating environment, influences the stability
pattern of the study location. Findings noted that at
late night and early morning periods, very stable
(class F) conditions dominate between December
and January. While class D dominates during the
rainy season, class F remains low. Stability classes B
and C dominate during the day in the wet season,
while class A remains low. However, during the dry
period, class A is dominant from December to
February. Results suggest that atmospheric stability
patterns in the area, concerning emission dispersion,
indicates that pollutants will be dispersed moderately
during the day, but will be stagnated at night. The
impact of the night time dispersion depends on the
inversion height, as well as the height of the emission
sources.
COMPETING INTERESTS
Authors have declared that no competing
interests exist.
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_________________________________________________________________________________
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Peer-review history:
The peer review history for this paper can be accessed here:
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