Atmospheric environment volume 40 issue 23 2006 doi 10 1016%2fj atmosenv 2006 03 044 masahide aikawa; takatoshi hiraki; jiro eiho vertical atmospheric structure estimated by heat island intensity and tempor

Atmospheric Environment 40 (2006) 4308–4315
Vertical atmospheric structure estimated by heat island intensity
and temporal variations of methane concentrations in ambient
air in an urban area in Japan
Masahide Aikawa
Ã
, Takatoshi Hiraki, Jiro Eiho
Hyogo Prefectural Institute of Public Health and Environmental Sciences, 3-1-27 Yukihira-cho, Suma-ku, Kobe, Hyogo 654-0037, Japan
Received 18 January 2006; received in revised form 28 March 2006; accepted 31 March 2006
Abstract
The vertical atmospheric structure was studied and evaluated based on the distribution and variation of the air
temperature in an urban area in Japan. A difference was observed in the annual mean diurnal variation of the air
temperature between the urban site and a suburban site. The maximum and minimum temperatures were 1:64

C at 1:00
and 1:17

C at 15:00, respectively, resulting in an estimated intrinsic heat island intensity of 0.47 ð¼ 1:6421:17Þ

C. The
height of the temperature inversion layer was approximately 90 m above the ground, based on the intrinsic heat island
intensity in an area where no vertical air temperature was available. The temporal variations of the methane concentrations
in ambient air and the contribution of automobile emissions were estimated and well accounted for by the postulated
temperature inversion layer.
r 2006 Elsevier Ltd. All rights reserved.
Keywords: Urban heat island; Air temperature; Air pollution; Methane; Urban area; Japan
1. Introduction
The urban heat island phenomenon has been
studied all over the world with the objective of
limiting increases in air temperature (e.g., Oke,
1973; Oke and Maxwell, 1975; Gotoh, 1993; Saitoh

et al., 1996; Yamashita, 1996; Oke et al., 1999).
Some studies have demonstrated that urban air
temperatures increase more on their own than
they do as a result of climate change and that the
rapid development of urban areas influences the
magnitude and patterns of heat islands (Hinkel
et al., 2003; Zhou et al., 2004; Weng and Yang,
2004; Fujibe, 2004). On the other hand, the urban
heat island phenomenon has been studied in terms
of vertical atmospheric structure (e.g., Bornstein,
1968; Bornstein and Azie, 1981; Draxler, 1986; King
and Russell, 1988; Saitoh et al., 1996; Shahgedano-
va et al., 1997). The vertical atmospheric structure is
closely related to air pollution (Aikawa et al., 1996;
Sahashi et al., 1996). In the present study, data sets
including air tempe rature and concentrations of air
pollutants such as methane were analyzed to
investigate the relationship of the air temperature
with concentrations of air pollutants and to identify
any factors which control temporal variations of air
pollutant concentrations in the atmosphere in the
ARTICLE IN PRESS
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doi:10.1016/j.atmosenv.2006.03.044
Ã
Corresponding author. Tel.: +81 78 735 6930;
fax: +81 78 735 7817.
E-mail address:
(M. Aikawa).

Hanshin area, which is a 10 Â 10 km area between
the cities of Osaka and Kobe, two of the largest
cities in Japan. The findings are reported below.
2. Experimental
2.1. Survey sites
Air temperature and methane concentrations were
measured at two environmental monitoring stations in
twocitiesintheHanshinarea:Amagasaki(StationA:
135

24
0
58
00
E; 34

43
0
19
00
N) and Nishinomiya (Station
N: 135

21
0
18
00
E; 34

45

0
54
00
N). The two stations are in
a10Â 10 km area. The locations o f the stations are
shown in Fig. 1. The Hanshin area is between Osaka
City (population 2; 634; 000=222 km
2
) and Kobe City
(population 1; 520; 000=551 km
2
). Th e H an shin area is
characterized by intensive industrial development and
dense populations. Mt. Rokko (altitude 931 m), which
runs east and west, is located in Kobe City. Station N
is at the east end of the mountain range. Station A is
in an urban area, whereas Station N is at a suburban
ARTICLE IN P RESS
Fig. 1. Location of environmental monitoring stations. Station A and Station N are located in Amagasaki City and Nishinomiya City,
respectively.
M. Aikawa et al. / Atmospheric Environment 40 (2006) 4308–4315 4309
site. The two stations are located less than 50 m above
sea level (a.s.l.).
2.2. Air temperature and air pollutant concentrations
The conditions for the measurement of air
temperature and methane concentrations are sum-
marized as follows:
Station A. The air temperature was measured on the
grass-covered roof of a five-story building (about 19 m
above the ground), where a thermometer shelter was

installed. The methane concentrations were measured
in the same building by using a non-methane hydro-
carbon monitor (HCM-4A, Shimadzu C orp., Kyoto,
Japan). The air inlet was about 15 m above the ground.
Station N. T he air temperature was measured on
the concrete roof of a two-story building (about 8 m
above the grou nd) by using a forcibly aspirated
shelter. The methane concentrations were measured in
the same b uilding by using a non -methane hydro-
carbon monitor (HCM-4A, Shimadzu Corp., Kyo to,
Japan). The air inlet was about 8 m above the ground.
2.3. Survey period and data acquisition
The data measured in 2004 were used for analyses.
All of the parameters were measured hourly.
3. Results and discussion
3.1. Temporal variation of methane concentrations in
ambient air
Methane in ambient air is one of the main gases
related to climate change. The lifetime of methane
in ambient air is approximately 10 years (IPCC,
2001), which is longer than those of other air
pollutants such as NO þ NO
2
ðNO
x
Þ (1 day) and
CO (65 days) (Seinfeld, 1986). NO
x
and CO are
among the most important air pollutants in urban

areas because they are emitted by automobiles.
Sahashi et al. (1996) demonstrated the nitrogen-
oxide layer over a heat island. However, when
considering kinetic behaviors of air pollutants in
ambient air, air pollutants with longer lifetimes are
advantageous because complicated atmospheric
chemical reactions can be avo ided, suggesting that
methane is more favorable for study purposes than
NO
x
and CO. In addition, natural sources ac-
counted for approximately 40% of total methane
sources (IPCC, 2001), and trans portation contrib-
uted 1.1% of methane emissions in Japan (CGER/
NIES, 2004), indicating that there is a smaller
influence of methane emissions from mobile sources
on methane concentrations in ambient air compared
with other air pollutants such as NO
x
and CO.
Fig. 2 sh ows the annual average t emporal varia-
tions of methane concentrations in ambient air at
Station A and Station N. In general, the methane
concentrations in ambient air were low during the
daytime and high a t night, and the lowest and the
highest concentrations appeared at 16:00 and
7:00–8:00, respectively. Fig. 3(a)and(b)showthe
seasonally average t emporal variation s o f m ethane
concentrations in ambient a ir at St ation A and Station
N, respectively. The methane concentration in ambi-

ent air in winter (December–February) showed two
maximum peaks in the temporal variations at mid-
night (23:00–1:00) a nd in the morning (7:00–9:00),
that in summer (June–August) showed one maximum
ARTICLE IN PRESS
174
176
178
180
182
184
186
1:00
2:00
3:00
4:00
5:00
6:00
7:00
8:00
9:00
10:00
11:00
12:00
13:00
14:00
15:00
16:00
17:00
18:00

19:00
20:00
21:00
22:00
23:00
Time
CH
4
concentration /ppm
StationA StationN
Fig. 2. Annual average temporal variations of methane concentrations in ambient air at Station A and Station N.
M. Aikawa et al. / Atmospheric Environment 40 (2006) 4308–43154310
peak in the morning (7:00–9:00), and t hose in spring
(March–May) and in a utumn (September–October)
showed transitional temporal variations between
winter and summer. Aikawa et al. (1996) reported
similar annual and monthly/seasonal temporal varia-
tions of th e methane co ncentration s in Nago ya City
(population 2; 202; 000=326 km
2
), Japan, and dis-
cussed the temporal variations in relation to the
stability of the atmosphere, suggesting that atmo-
spheric stability i s r elated to the temporal variatio ns o f
the methane concentrations in ambient air in the
current study area.
ARTICLE IN P RESS
Spring
170
175

180
185
190
1 3 5 7 9 11131517192123
Time
CH
4

concentratin /ppm
Summer
170
175
180
185
190
1 3 5 7 9 11131517192123
Time
CH
4

concentratin /ppm
Autumn
170
175
180
185
190
1 3 5 7 9 11131517192123
Time
CH

4

concentratin /ppm
Winter
170
175
180
185
190
1 3 5 7 9 11131517192123
Time
CH
4

concentratin /ppm
(a)
Spring
170
175
180
185
190
1 3 5 7 9 11131517192123
Time
CH
4

concentratin /ppm
Summer
170

175
180
185
190
1 3 5 7 9 11131517192123
Time
CH
4

concentratin /ppm
Autumn
170
175
180
185
190
1
3
5 7 9 11131517192123
Time
CH
4

concentratin /ppm
Winter
170
175
180
185
190

1 3 5 7 9 11131517192123
Time
CH
4

concentratin /ppm
(b)
Fig. 3. Seasonally average temporal variations of methane concentrations in ambient air at Station A (a) and Station N (b).
M. Aikawa et al. / Atmospheric Environment 40 (2006) 4308–4315 4311
3.2. Mean air temperature and diurnal variation
The annual mean values of the daily mean air
temperatures at Station A and Station N in 2004 are
summarized in Table 1. The annual mean air
temperature at Station A ð17:6

CÞ was higher than
that at Station N ð15:8

CÞ. One reason is the
difference in elevation. Therefore, the annual mean
air temperature had to be corrected to consider the
effect of the elevation of the location. A moist-
adiabatic lapse rate of 0:6

C=100 m was considered.
The result of the correction is shown in Table 1. The
difference after the correction was 1:5

C. Aikawa
et al. (2006) demonstrated that the difference in

mean air temperatures at Station A and Station N
from 1990 to 2003 was approximately 0:4

C,
smaller than that in 2004. Japan Meteorological
Agency measured the air temperatures in Osaka:
representative urban site ð135

31:1
0
E; 34

40:9
0
NÞ,
approximately 9 km east–southeast from Station A
and in Sanda: suburban site ð135

12:7
0
E; 34

53:6
0
NÞ,
approximately 16 km northwest from Station N.
The difference of the annual mean air temperature
between Osaka and Sanda was 3:04

C: mean and

3:10

C: median for the duration of 1990–2003. On
the other hand, the difference of the annual mean
air temperature between Osaka and Sanda in 2004
was 3:40

C. The difference in 2004 was also larger
than that in the duration of 1990–2003 in the survey
by Japan Meteorological Agency, similar to the
current resul ts.
Fig. 4 shows the diurnal variations in the air
temperature as illustrated by the corrected hourly
air temperature at each station. The differences in
the diurnal variations between Station A and
Station N were maximum and minimum at 1:00
ð1:64

CÞ and 15:00 ð1:17

CÞ, respectively, with a
mean difference of 1:47

C. Assuming that urban
heat island intensity is defined as the difference in
the air temperatures at Station A and Station N, the
urban heat island intensity was strongest at 1:00 and
weakest at 15:00.
3.3. Height of postulated temperature inversion layer
Bornstein (1968) demonstrated that a tempera-

ture inversion layer covered New York City at
approximately 310 m above the ground. Aikawa et
al. (1996) reported that a temperature inversion
layer was formed at approximately 60 m above the
ground in Nagoya City, Japan. Bornstein (1968)
measured the vertical air temperature profile by
helicopter, while Aikawa et al. (1996) showed the
vertical air temperature profile based on measure-
ments taken at the Nagoya TV Tower, 180 m above
the ground. In the current study, no air temperature
data were availab le to evaluate a vertical air
temperature profile. Therefore, the height of a
postulated temperature inversion layer was calcu-
lated by using the distribution of the air temperature
ARTICLE IN PRESS
Table 1
Measured and corrected annual mean values of daily mean air
temperatures in 2004
Station A Station N
Measured ð

CÞ 17.6 15.8
Corrected ð

CÞ 17.6 16.1
12
13
14
15
16

17
18
19
20
21
1:00
2:00
3:00
4:00
5:00
6:00
7:00
8:00
9:00
10:00
11:00
12:00
13:00
14:00
15:00
16:00
17:00
18:00
19:00
20:00
21:00
22:00
23:00
0:00
Time

Air temperature /°C
Station A StationN
Fig. 4. Diurnal variations of air temperature corrected by elevation at Station A and Station N.
M. Aikawa et al. / Atmospheric Environment 40 (2006) 4308–43154312
shown in Section 3.2. The following assumptions
were made:
Assumption (i). A temperature inversion layer
would exist in the current study area.
Assumption (ii). A temperature inversion layer
with a maximum height would be formed in the air
at the site of Station A.
Assumption (iii). A temperature inversion layer
would be formed on the ground condition at the site
of Station N.
In general, solar radiation is strong in the
daytime, leading to an active vertical mixing of the
air by convection. In contrast, there is no solar
radiation at night, resulting in the formation of a
temperature inversion layer by radiation cooling.
Therefore, it is appropriate to discuss the height of
the temperature inversion layer based on the
distribution of the nighttime air temperature. In
the current study area, even in the daytime, when
there should have been active vertical mixing, a
difference in the air temperature ð1:17

CÞ was
observed between Station A (urban area) and
Station N (suburban area) as shown in Section
3.2. The 1:17


C difference in the air temperature
was presumably due to an intrinsic difference
caused by the characteristics of the sites. Therefore,
when the height of the postulated temperature
inversion layer in nighttime is discussed, the
1:17

C difference in the air temperature in the
daytime should be subtracted from the difference in
the nighttime. Bornstein (1968) clarified that the
height of the base of the crossover layer was 310 m
and the average intensity of the urban heat island ,
as measured by the magnitude of the temperature
difference between urban and rural sites, was 1:6

C.
Saitoh et al. (1996) observed and simulated the heat
island intensity ð5

CÞ and the height of the cross-
over phenomenon (1000 m) in metropolitan Tokyo.
Shahgedanova et al. (1997) showed the heat island
intensity ð123

CÞ and the height of the urban
boundary layer (85–128 m) in Moscow. The statis-
tics shown by Bornstein (1968) were used for the
following calculation since the studies of Bornstein
and Saitoh et al. (1996) yielded similar calculation

results. Taking into account the statistics shown by
Bornstein and the above-mentioned essential differ-
ence in the corrected hourly air temperatures at
Station A and Station N ð1:6421:17 ¼ 0:47

CÞ, the
height of the postulated temperature inversion layer
in the current study area can be calculated as
follows: height of postulated tempe rature inversion
layer ¼ 0:47=ð 1 :6 = 310Þ¼91 m.
3.4. Relationship of temporal varia tions of methane
concentrations with postulated temperature inversion
layer
Aikawa et al. (1996) reported the relationship
between the temporal variations of methane con-
centrations and the lifted temperature inversion
layer, and they accounted for the temporal varia-
tions of methane concentrations by the formation
and disappearance of the lifted temperature inver-
sion layer. The seasonally average temporal varia-
tions of the methane concentrations in the current
study were so similar to those found in the study by
Aikawa et al. (1996) that the temporal variations of
methane concentrations could be also accounted for
by the lifted postulated temperature inversion layer
introduced in Section 3.3.
On the other hand, in the seasonally average
temporal variations of the methane concentrations
in ambient air during the winter season, as shown in
Fig. 3(a) and (b), a small shoulder was observed in

the morning (6:00–10:00). The contribution of
methane from automobiles to the total emission of
methane was generally not large (IPCC, 2001).
However, considering a relatively small and urba-
nized area such as that in the current study, the
contribution of automobile emissions would not be
negligible. The small shoulder would appear in the
morning as a result of a combination of automobile
emissions and the formati on of the lifted postulated
temperature inversion layer found in the current
study area. Fig. 5 shows the average temporal
variations of methane concentrations on weekdays
and weekends. The shoulders on weekdays were
larger than those on weekends at both sites, strongly
suggesting that the shoulder results from the
contribution of automobile emissions.
Seasonal differences in the air temperature at
midnight (23:00–1:00) and in the early morning
(7:00–9:00) between Station A and Station N are
summarized in Table 2. The corrected seasonal
differences in the air temperature at midnight and in
the early morning in winter (0.60 and 0:22

C,
respectively) were larger than those (À0:04 and
0:05

C) in summer, suggesting that the heat island
phenomenon was observed in winter both at mid-
night and in the early morning in the current study

area, while the heat island intensity was small or
nonexistent in summer both at midnight and in the
early morning. Aikawa et al. (1996) also reported
the temperature inversion layer almost never
formed in Nagoya City during the relevant time in
ARTICLE IN P RESS
M. Aikawa et al. / Atmospheric Environment 40 (2006) 4308–4315 4313
summer. The seasonal variation of the heat island
phenomenon and the formation of the postulated
temperature inversion layer would result in the
small shoulder in the morning in winter.
3.5. Estimation of contribution of automobile
emissions
The contribution of automobile emissions to the
temporal variations in methane concentrations was
estimated based on the road traffic census data
taken in 1997. Fig. 6 shows an outline of the traffic
volume on the main roads on weekdays
(7:00–19:00). To estimate the contribution in the
morning, one-third of the traffic volume shown in
Fig. 6 was distributed as the morning traffic volume.
The traffic volume shown in Fig. 6 included all types
of vehicles, including gasoline- and diesel-powered
passenger cars as well as small and large diesel-
powered freight vehicles. The methane emission
factor was calculated by taking into account the
types of vehicles and constituent ratio of vehicles. It
was assumed that the travel distance of vehicles in
the current study area was 10 km. It was also
assumed that the diffusion volume was one-fourth

of the volume of a circular cone with a 10 km radius
and 90 m height. Under these assumptions, the
estimated concentration of methane for the
shoulder peak in the morning was 2.6 ppm. In
contrast, the observed shoulder methane concentra-
tion on weekdays was approximately 3 ppm, which
shows that the shoulder methane concentration was
estimated fairly well.
ARTICLE IN PRESS
178
180
182
184
186
188
190
1 7 9 1011121314151617181920212223
Time
CH
4
concentration /ppm
Weekend inStation A
Weekdayin Station A
Weekend inStation N Weekdayin Station N
654328
Fig. 5. Average temporal variations of methane concentrations on weekdays and weekends at Station A and Station N.
Table 2
Seasonal difference of air temperature between Station A and
Station N in midnight and early morning
a

Difference of air temperature
Summer Winter
23.00–1.00 7.00–9.00 23.00–1.00 7.00–9.00
Measured ð

CÞ 1.13 1.22 1.77 1.39
Corrected
b
ð

CÞÀ0.04 0.05 0.60 0.22
a
Midnight and early morning means 23.00–1.00 and 7.00–9.00,
respectively.
b
Measured values are corrected by subtracting intrinsic
difference ð1:17

CÞ.
10 km
10 km
R171
R2
R43
H.H.
M.H.
25,000
26,000
45,000
52,000

33,000
42,000
36,000
63,000
Fig. 6. Outline of traffic volume on main roads in the daytime
(7:00–19:00) on weekdays in the current study area. R2, R43,
R171, M.H., and H.H. show National Routes No. 2, No. 43, No.
171, Meishin Expressway, and Hanshin Expressway, respectively.
The numerals show the traffic volume of vehicles (vehicles/
daytime (12 h)).
M. Aikawa et al. / Atmospheric Environment 40 (2006) 4308–43154314
4. Conclusions
The temporal variations of methane concentrations
in ambient air in an urban area were studied in
relation to air temperature distribution and the
estimated p ostulated vertical a tmospheric structure.
The average temp oral variations in methane concen-
trations showed seasonal c haracteristics. In w inter, th e
observed sho ulder peak was due to the contribution of
automobile emissions. T he shoulder peak concen tra-
tion could be estimated fairly well by considering the
contribution of automobile emissions and the postu-
lated vertical a tmospheric structure estimated b y the
measured air temperature distribution.
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