Atmospheric Environment 40 2006 4308–4315Vertical atmospheric structure estimated by heat island intensity and temporal variations of methane concentrations in ambient air in an urban ar
Trang 1Atmospheric 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 , 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 1:64C at 1:00 and 1:17C at 15:00, respectively, resulting in an estimated intrinsic heat island intensity of 0.47 ð¼ 1:6421:17ÞC 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
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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 temperature 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
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doi: 10.1016/j.atmosenv.2006.03.044
Corresponding author Tel.: +81 78 735 6930;
fax: +81 78 735 7817.
E-mail address: Masahide_Aikawa@pref.hyogo.jp
(M Aikawa).
Trang 2Hanshin area, which is a 10 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
two cities in the Hanshin area: Amagasaki (Station A:
1352405800E; 344301900N) and Nishinomiya (Station N: 1352101800E; 344505400N) The two stations are in
a 10 10 km area The locations of the stations are shown inFig 1 The Hanshin area is between Osaka City (population 2; 634; 000=222 km2) and Kobe City (population 1; 520; 000=551 km2) The Hanshin 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
Fig 1 Location of environmental monitoring stations Station A and Station N are located in Amagasaki City and Nishinomiya City, respectively.
Trang 3site 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 Corp., Kyoto,
Japan) The air inlet was about 15 m above the ground
Station N The air temperature was measured on
the concrete roof of a two-story building (about 8 m
above the ground) by using a forcibly aspirated
shelter The methane concentrations were measured in
the same building by using a non-methane
hydro-carbon monitor (HCM-4A, Shimadzu Corp., Kyoto,
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 NO2 ðNOxÞ (1 day) and
CO (65 days) (Seinfeld, 1986) NOx 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 avoided, suggesting that methane is more favorable for study purposes than
NOx and CO In addition, natural sources ac-counted for approximately 40% of total methane sources (IPCC, 2001), and transportation 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 NOxand CO
Fig 2 shows the annual average temporal varia-tions of methane concentravaria-tions in ambient air at Station A and Station N In general, the methane concentrations in ambient air were low during the daytime and high at night, and the lowest and the highest concentrations appeared at 16:00 and 7:00–8:00, respectively Fig 3(a) and (b) show the seasonally average temporal variations of methane concentrations in ambient air at Station 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) and in the morning (7:00–9:00), that in summer (June–August) showed one maximum
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
Fig 2 Annual average temporal variations of methane concentrations in ambient air at Station A and Station N.
Trang 4peak in the morning (7:00–9:00), and those in spring
(March–May) and in autumn (September–October)
showed transitional temporal variations between
winter and summer Aikawa et al (1996) reported
similar annual and monthly/seasonal temporal
varia-tions of the methane concentravaria-tions in Nagoya City
(population 2; 202; 000=326 km2), Japan, and dis-cussed the temporal variations in relation to the stability of the atmosphere, suggesting that atmo-spheric stability is related to the temporal variations of the methane concentrations in ambient air in the current study area
Spring
170
175
180
185
190
Time
Summer
170 175 180 185 190
Time
Autumn
170
175
180
185
190
Time
Winter
170 175 180 185 190
Time
(a)
Spring
170
175
180
185
190
Time
Summer
170 175 180 185 190
Time
Autumn
170
175
180
185
190
Time
Winter
170 175 180 185 190
Time
(b)
Fig 3 Seasonally average temporal variations of methane concentrations in ambient air at Station A (a) and Station N (b).
Trang 53.2 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 ð17:6CÞ was higher than
that at Station N ð15:8CÞ 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 0:6C=100 m was considered
The result of the correction is shown inTable 1 The
difference after the correction was 1:5C Aikawa
et al (2006) demonstrated that the difference in
mean air temperatures at Station A and Station N
from 1990 to 2003 was approximately 0:4C,
smaller than that in 2004 Japan Meteorological
Agency measured the air temperatures in Osaka:
representative urban site ð13531:10E; 3440:90NÞ,
approximately 9 km east–southeast from Station A
and in Sanda: suburban site ð13512:70E; 3453:60NÞ,
approximately 16 km northwest from Station N
The difference of the annual mean air temperature
between Osaka and Sanda was 3:04C: mean and
3:10C: 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 3:40C 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 results
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 ð1:64CÞ and 15:00 ð1:17CÞ, respectively, with a mean difference of 1:47C 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 available 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
Table 1
Measured and corrected annual mean values of daily mean air
temperatures in 2004
Station A Station N
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
Fig 4 Diurnal variations of air temperature corrected by elevation at Station A and Station N.
Trang 6shown 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 ð1:17CÞ was
observed between Station A (urban area) and
Station N (suburban area) as shown in Section
3.2 The 1:17C 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
1:17C 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 1:6C
Saitoh et al (1996)observed and simulated the heat
island intensity ð5CÞ and the height of the
cross-over phenomenon (1000 m) in metropolitan Tokyo
Shahgedanova et al (1997) showed the heat island
intensity ð123CÞ 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 0:47CÞ, the
height of the postulated temperature inversion layer
in the current study area can be calculated as
follows: height of postulated temperature inversion
layer ¼ 0:47=ð1:6=310Þ ¼ 91 m
3.4 Relationship of temporal variations 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 concentravaria-tions by the formation and disappearance of the lifted temperature inver-sion layer The seasonally average temporal varia-tions of the methane concentravaria-tions 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 formation 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 0:22C, respectively) were larger than those (0:04 and 0:05C) 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
Trang 7summer 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 6shows 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 6was distributed as the morning traffic volume
The traffic volume shown inFig 6included 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
178 180 182 184 186 188 190
Time
6 5 4 3
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 morninga
Difference of air temperature
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
ðCÞ 0.04 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
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)).
Trang 84 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 postulated vertical atmospheric structure
The average temporal variations in methane
concen-trations showed seasonal characteristics In winter, the
observed shoulder peak was due to the contribution of
automobile emissions The shoulder peak
concentra-tion could be estimated fairly well by considering the
contribution of automobile emissions and the
postu-lated vertical atmospheric structure estimated by the
measured air temperature distribution
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