Introduction
In order to quantitatively understand the global carbon cycle, long-term
observations of atmospheric CO2 concentrations have been performed at
“background sites” where the observed air is less affected by local sinks
and sources, such as Mauna Loa in Hawaii and Antarctica (Keeling et al.,
1989, 2001; Conway et al., 1994). On the other hand, the
importance of observations at “regional sites” has also been recognized
when analyzing regional sinks and sources using an inverse model with data
from various ground observation sites, such as Siberia and China (Maksyutov
et al., 2003; Saeki et al., 2013; Zhang et al., 2014).
Globally, atmospheric CO2 concentrations are currently observed at
about 180 sites, including both background and regional sites, although
their distribution is not uniform (WMO, 2015). For example, there are
insufficient sites in Southeast and East Asia, regions in which economies
have grown rapidly in this decade. In particular, China is now known as the
country with the greatest CO2 emissions in the world (CDIAC
00001_V2016; Boden et al., 2016), so more observation sites
are needed. Representative sites for the long-term observation of
atmospheric CO2 concentrations in Japan are located in the higher- and
lower-latitude coastal areas of Ochi-ishi, Minamitori Island, Hateruma
Island, and Yonaguni Island (Fig. 1; Watanabe et al., 2000; Mukai et al.,
2001).
Location of Mt. Fuji (35.21∘ N, 138.43∘ E) and of other sites
representative for the long-term observation of atmospheric CO2 concentration
in Japan (small black dots), and the five areas used for back-trajectory
analysis of air mass origins (Siberia, China, Southeast Asia, around Japan, and Pacific Ocean).
Midlatitude regions of Japan are mostly affected by regional air mass
transport via the prevailing westerlies from the Asian continent. However,
suitable CO2 observation sites are lacking because of the distribution
of industrial and populated areas. Obtaining CO2 observations in the
midlatitude Asian region would be advantageous for monitoring changes in
the carbon cycle, because many countries in this region are growing
economically with associated increases in their CO2 emissions.
Mt. Fuji, the highest mountain in Japan (3776 m a.s.l.), is a well-suited
site for monitoring CO2 concentrations. Its summit is located in the
free troposphere, which means it is not affected directly by air at ground
level for most of the year (Tsutsumi et al., 1994). Mt. Fuji is positioned
in the center of Japan (Fig. 1), and the air masses that pass over it are
mainly affected by the regional air characteristics of the Asian continent
(Igarashi et al., 2004). Nakazawa et al. (1984) performed observations of
CO2 concentration at the summit in October 1980 and July–October 1981,
with the following findings: (i) the CO2 concentration was not
influenced by wind direction; (ii) diurnal variation of CO2
concentration was not observed; (iii) CO2 concentrations were in close
agreement with vertical profiles derived from aircraft measurements near
Sendai, Japan; and (iv) CO2 variations were caused by air masses
transported from different regions such as the Pacific Ocean and the Asian
continent; (v) The observed annual rate of increase was comparable with the
rate derived from aircraft measurements over Japan at an equivalent
altitude to the summit. Therefore, they suggested that observations obtained
at the summit of Mt. Fuji could be considered representative of the
free-tropospheric CO2 concentration in the midlatitude Asian region. Sawa
et al. (2005) performed continuous observations of CO2 and CO at the
summit from September 2002 to February 2003 and from May 2003 to May 2004,
with the following findings: (i) many episodic events showed large
enhancements of CO2, and (ii) episodic enhancements clearly associated with
increased CO peaks were observed at the same time.
After 2004, CO2 observations at the summit of Mt. Fuji were interrupted
because of the shutdown of manual operations at the Mt. Fuji weather station
by the Japan Meteorological Agency (JMA). Although the JMA continued
automatic meteorological observations, commercial power has only been
available during 2 months in the summer since 2004. Therefore, it became
difficult to maintain whole-year observations at the station without a power
supply and a heating and air-conditioning system.
In this study, we developed a CO2 measurement system that can be
operated without gridded electricity, even under the harsh conditions found
at the summit of Mt. Fuji, where the weather can be extremely cold in winter
and other severe weather conditions such as high wind velocity and frequent
lightning are encountered. It is impossible for an ordinary person to go
there from September until July. Low atmospheric pressure (about 650 hPa)
makes manual maintenance work difficult and also causes reduced sensitivity
in infrared (IR) absorption detection in the system. Our system was insulated to
protect it against the cold temperatures and designed to use battery power
for over 10 months of each year without manual maintenance. To minimize
power consumption, our system was configured to measure CO2
concentrations for only 1.5 h day-1. To maintain long-term observations,
the operational system included an auto-charging system for 100 batteries
and a satellite communication system. Since 2009, we have successfully
obtained CO2 data using this system on Mt. Fuji.
In this paper, we evaluate the system performance (stability and
sustainability in operation) in addition to the analytical viewpoint of
repeatability and accuracy of the measurements. We present our data for
CO2 concentration at the summit of Mt. Fuji for 6 years and
characterize the concentration variation in relation to regional air masses,
comparing our data with those from the Mauna Loa Observatory (MLO), Hawaii,
and other datasets.
Methods
Location
Mt. Fuji is the highest mountain in Japan, located in the middle of mainland
Japan (35.21∘ N, 138.43∘ E, 3776 m a.s.l.) on the
Pacific side and isolated from other mountain ranges (Fig. 1). It has not
erupted since 1707, and no gas emission from the crater has been observed at
the mountain summit. There is a small outer rim (200 m height, 800 m
diameter) surrounding the crater at the top, with no vegetation above 2500 m. At the summit,
the coldest daily temperature observed was -35 ∘C in February 1981. The maximum daily temperature is about 17 ∘C
in August. There is generally a strong wind, which averages about 12 m s-1; during the passage of a typhoon, wind speed can peak at
> 70 m s-1.
The CO2 measurement system: (a) picture of the system installed
in the third building at the Mt. Fuji station, (b) schematic of gas and electricity
flows. The thin lines and arrows show the direction of electric current in the
summer mode, and the thick lines and arrows show it in the winter mode.
(c) Insulation method used for the main measurement component, and (d) measurement sequence.
The first weather station on Mt. Fuji was constructed on the edge of the
highest outer rim at Kengamine in 1936. In 1970, the old building was
replaced by the present buildings. Until October 2004, four officers of the
JMA stayed for 3 weeks at a time, collectively
working throughout the year to make weather observations. Compared to other
high-altitude stations such as the MLO in Hawaii and Jungfraujoch in
Switzerland, the station at Mt. Fuji is more difficult to access because
vehicular transportation is not available. In 2004, manual observations
stopped because of the termination of radar observations in 1999 and the
change in the role of weather observations at Mt. Fuji due to new technology,
including automated weather observations and satellites. After that, a
non-profit organization (NPO) started to rent the Mt. Fuji weather station
and has managed it for scientific research since 2007.
Currently the Mt. Fuji station is open only in July and August for several
proposed research activities (http://npo.fuji3776.net/) and for the
maintenance of weather instruments; the NPO provides some staff at the
station. Commercial electricity is provided only during this period for
safety reasons, because there are no workers on site during the rest of the
year. Our measurement system was installed in the third building of the
station on 20 July in 2009. Because it includes several instruments,
insulation boxes, and batteries (about 3 t total), we asked to use a
specialized bulldozer for transportation. We can only access the station
during summer for the installation and maintenance of the system and to
exchange our standard gas cylinders and other items.
Measurement system
Figure 2 shows the CO2 measurement system, a schematic of the gas and the
electricity flows, and the insulation method for the main measurement
component. The system consists of the main measurement component, power
control devices for battery charging and power switching, and 100 shielded
lead acid batteries (12 V; G42EP: Enersys Co. Ltd.; service temperature
ranges from -40 to 80 ∘C). The main measurement
component (CO2-MTF1, Kimoto Electric Co. Ltd.) includes a
non-dispersive infrared sensor (NDIR; Li-840, LI-COR Co. Ltd.), air pumps
(CM-15-12: E. M. P-Japan Ltd.; 1.2W), a drying system using a membrane
(Flemion: SWG A01-18: AGC Engineering Co. Ltd.), and a desiccant (Silica
gel; 2000 mL). Each device in the measurement component was selected for
minimal electricity consumption.
The main measurement component (30 × 30 × 30 cm) was
covered with foam insulation (Phenovaboard: Sekisui Chemical Co. Ltd.) and
placed in a 100 L plastic insulated box (RPS-100NF: REMACOM Co. Ltd.),
which was additionally covered with 15 cm thick Phenovaboard. The
temperature at the summit of Mt. Fuji drops below -20 ∘C in
winter. Although the control circuit boards had been tested under such low
temperatures in the laboratory, the Li-840 sensor requires a certain
temperature (50 ∘C) in the Li-840's cell when starting the
measurements. Furthermore, the diaphragm rubber (chloroprene) of the air
pumps could be damaged at such low working temperatures. Therefore, the main
measurement component was specially insulated to maintain a suitable working
temperature and to save energy for starting the Li-840. In addition, when
the temperature of the measurement component falls below 0 ∘C, a
small internal heater maintains the temperature above 0 ∘C when
the pumps start. Each battery was wrapped in plastic film and placed in a
corrugated cardboard box, which itself was wrapped in plastic film in case
of liquid leakage and for heat insulation.
An air inlet was mounted on a water tank (about 3 × 4 × 3 m), which was located just next to the third building of the station. The
water tank was partly covered by wooden boards for protection from snow but
with sufficient space for air to penetrate between the wooden cover and the
tank. If the air inlet had been placed completely outside, it would have
been difficult to take in air during the winter because snow would cover the
whole inlet. Air was drawn from the air inlet through about 20 m of
1/4 in. polytetrafluoroethylene (PTFE) tube with a 7-micron filter (Swagelok
SS-4F-7) to the station by an air pump with a flow rate of about 1.5 L min-1. In general, metal tubing such as stainless steel or aluminum is
better than PTFE for measuring CO2. However, because the tube was
installed outside (between the water tank and the third building), it has
to be inconspicuous or transparent because Mt. Fuji is a national park and
installation of artificial things is strictly limited by law. Therefore, we
selected a transparent PTFE tube but used a slightly higher flow rate for
pumping air to minimize contamination from the material. We conducted a leak
check every summer and replaced the inlet filter. We also placed a data
communication antenna (AT1621-142W-THCF- 000-00-00-NM: AeroAntenna
Technology Inc., Chatsworth, CA, USA) for the Iridium satellite (Iridium
9601: KDDI Co. Ltd.; 7.5 W max) in the small space within the wooden cover
of the water tank. We constructed two sets of the main measurement
component, which were switched each summer after 1 year of operation; the
non-operational pumps and other equipment were then readied for their next
deployment the following year.
Electrical power system
The electrical power line to the station, which runs from the foot of Mt.
Fuji, is fairly old as it was mainly installed more than 40 years ago.
Furthermore, since the power cables are buried in volcanic rocks which can
easily collapse during avalanches, the cables are likely to be damaged.
Therefore, at the beginning of every summer, the power line has to be
checked and repaired prior to use.
It is too difficult to construct wind turbines or solar panels outside the
station under the legal limitations of the national park and the harsh
natural conditions at the summit. At present, only a few small solar panels
are installed in a limited space for weather observation. Therefore, we
needed another electrical source, like batteries, which could operate the
measurement system for the remaining 10 months. We investigated two types of
battery, lithium and lead acid; the latter was more reliable for autonomous
operation. According to the electrical power needed for the system, we
decided to use 100 batteries (12 V/G42EP) specially suited for the cold
environmental conditions.
These batteries were connected in parallel, and the voltage was automatically
checked when the measurement system started. When the voltage was below 10.5 V,
the system halted operation to protect the batteries. The electrical
power system has a device to switch from “winter mode” to “summer mode”.
In winter, we used the 100 batteries for powering the system, whereas in
summer we used another battery connected to a battery charger powered via
the commercial supply (see Fig. 2b).
In summer (July–August), the 100 batteries (two series of 50 batteries)
were charged by two specially designed charging controllers. After the
battery connection is changed from “power mode” to “charge mode”, each
controller starts charging a pair of batteries. Normally, each controller
can charge 50 batteries, one pair after another, within a 3-week period. At
the end of August, the power system was switched to winter mode, and the
system operated using the 100 batteries until the following July.
Measurement sequence
Because of the limited power supply during winter, the measurement system
was configured to operate for only about 3.5 h day-1 (in addition,
measurements are only taken during 1.5 h day-1). We initially selected the
time period 14:00–17:28 Japan Standard Time (JST) from 20 July 2009 to
19 July 2010. However, we subsequently changed the operational time to
21:00–00:28 JST, to avoid local daytime influences from transportation of
the air mass around Mt. Fuji that might affect the CO2 concentration
over the summit; this is similar to how baseline condition is obtained at
MLO.
The measurement sequence is summarized in Fig. 2d. The sequence is
controlled by the control board (MC-mini, Kimoto Electric Co. Ltd.).
Briefly, at 21:00 JST, the temperature of the measurement system is checked
and the heater operates for an hour if the temperature is less than
0 ∘C. The Li-840 starts at 21:30 JST and the cell temperature
increases to about 50 ∘C within 30 min (14 W max). Then, starting
at 22:00 JST, room air is introduced into the Li-840 (3.6 W) for 2 min using a
small pump with a flow rate of 50 mL min-1. This process is intended to
purge the inside measurement line with new room air and stabilize the
conditions (e.g., the temperature and drying system) before outside air is
introduced. During this time outside air is drawn into a manifold by a
medium-volume pump at a flow rate of 1.5 L min-1. Then, an aliquot of
just 50 mL min-1 is introduced into the drying and measurement system
regulated by a mass flow controller for 8 min. Next, three working standard
gases (about 360, 390, and 420 ppm in a 10 L aluminum cylinder), prepared by
the Japan Fine Products Co. and calibrated against the NIES09 CO2 scale
(Machida et al., 2009), are measured for 4 min each to calibrate the system.
Generally, this sequence is repeated four times. According to the results of
the sixth World Meteorological Organization (WMO)/International Atomic Energy Agency (IAEA)
Round Robin intercomparison (NOAA/ESRL, 2016), we
know the scale difference between NIES09 and NOAA. Although the NIES09 scale
was lower than the NOAA scale by 0.04–0.09 in a range of 376–404 ppm, both
scales are fairly close to each other. We used special regulators
(TORR-1300: Nissan Tanaka Co.) which are able to work stably within 15 %
deviation at the 0.1 MPa level even in a large temperature range from -25 to
50 ∘C.
At 23:28 JST, the Iridium satellite data communication is activated for 1 h
until the data (192 bytes) are sent successfully. Communications become
difficult under bad weather conditions, such as cloud cover at the summit,
so the 1 h period allows for reliable data communication. If data
cannot be sent at that time, the device tries again the next day. Actually,
in 2009–2010 we used ORBCOMM satellite communication, but because of poor
transmissions we changed to Iridium.
In addition, data were stored as 10 s averages to a CompactFlash (CF) card in the
system. The derived concentration was based on the average of the data from
the second, third, and fourth cycles; in other words, the daily average is
based on 3 × 6 min (because we discarded the first 2 min data for 8 min data) = 18 min of observations.
Continuous measurement in summer and flask sampling
Because this system monitors CO2 mole fraction only for a short period
(22:00–23:28) of the day, we needed to evaluate the daily variation of
CO2 concentration to clarify how this measurement represents the
CO2 concentration over Mt. Fuji. In the summers of 2012 and 2013, we
set up a simple continuous CO2 measurement system, connected to the
inlet line of the measurement system. It comprises a second Li-840,
flow-adjusting parts, a gas selector, and four standard gases (NIES09
CO2 scale). Although this system has no drying component, the dry air
base CO2 mole fraction is calculated from the H2O concentration
measured by the Li-840 itself with a good repeatability (below 1 %); the
repeatability of the CO2 measurement was found to be better than 0.3 ppm
(signal-to-noise ratio was below 0.1 ppm), which is sufficient to evaluate the
CO2 daily variation.
Furthermore, to evaluate the accuracy of the measurement system, we compared
the CO2 mole fraction between the battery-powered measurement system
and flask samples analyzed later in the laboratory. Air was collected by
1.5 L glass flask four times per year (12 times in total) during July and August
2009–2011 near the inlet of the battery-powered measurement system when the
system measured CO2 mole fraction. This flask sampling was done by
pumping through the glass flask with a small air pump for about 10 min with
a flow rate of about 2 L min-1 and shutting both inlet and outlet valve after
stopping the pump. So, the pressure in the flask was about 650 hPa. By the
end of the summer season, we brought the flask to the laboratory in NIES and
analyzed the CO2 mole fraction by NDIR (Li-7000, LI-COR Co. Ltd.) with
a metal bellows pumping system (MB-21, Senior Aerospace Metal Bellows) to
send air in the flask to the NDIR.
Other datasets of CO2 data
For comparison with the Mt. Fuji CO2 data, we obtained daily data from
the MLO, which is located at a similar altitude (19.54∘ N,
155.58∘ W, 3397 m a.s.l.; Tans and Keeling, 2016;
www.esrl.noaa.gov/gmd/ccgg/trends/). We also used data from aircraft
measurements obtained via the Comprehensive Observation Network for Trace
Gases by Airliner (CONTRAIL) project (Machida et al., 2008). We selected
CONTRAIL data over Japan in the region 34–36∘ N,
136–141∘ E at an altitude of 3600–3900 m during 2009–2013. The
CO2 concentration trend was calculated according to the method of
Thoning et al. (1989) with a cut-off frequency of 667 days (0.5472 cycles yr-1) for a fast Fourier transform filter.
Weather data
The temperatures in the room and inside the measurement system were
monitored by the system itself, while the system measured CO2
concentration from the outside air. The external temperature at the summit
of Mt. Fuji was measured by the JMA, and the data were taken from the JMA
website (http://www.data.jma.go.jp/obd/stats/etrn/index.php).
Location of Mt. Fuji (35.21∘ N, 138.43∘ E) and of
other sites representative for the long-term observation of atmospheric
CO2 concentration in Japan (small black dots), and the five areas used
for back-trajectory analysis of air mass origins (Siberia, China, Southeast
Asia, around Japan, and Pacific Ocean).
Before the cylinder installation
After the cylinder replacement
Change of concentration
Cylinder no.
Date of
Calibrated
Pressure of
Date of
Date of
Date of
Re-calibrated
Pressure of
Change
Change rate
calibration
value
cylinder
installation
replacement
re-calibration
value
cylinder
amount of the
(ppm yr-1)
(ppm)
(MPa)
(ppm)
(MPa)
concentration
(ppm)
CPC-00449
17-Jun-2009
368.86
10.8
16-Jul-2009
24-Jul-2011
19-Aug-2011
368.82
3.4
-0.03
-0.02
CPC-00447
17-Jun-2009
383.10
11.0
16-Jul-2009
24-Jul-2011
19-Aug-2011
383.24
4.0
0.14
0.06
CPC-00448
17-Jun-2009
403.45
10.8
16-Jul-2009
24-Jul-2011
19-Aug-2011
403.54
3.2
0.09
0.04
CPC-00445
10-Jun-2011
367.94
11.4
25-Jul-2011
25-Jul-2013
6-Aug-2013
368.02
0.8
0.09
0.04
CPC-00450
10-Jun-2011
383.46
11.5
25-Jul-2011
25-Jul-2013
6-Aug-2013
383.42
3.6
-0.04
-0.02
CPC-00451
10-Jun-2011
402.29
11.5
25-Jul-2011
25-Jul-2013
6-Aug-2013
402.37
3.4
0.08
0.04
CPC-00043
23-Jun-2013
367.10
12.8
26-Jul-2013
1-Jul-2016
10-Jul-2016
367.12
2.5
0.02
0.01
CPC-00448
23-Jun-2013
393.17
12.6
26-Jul-2013
1-Jul-2016
10-Jul-2016
393.12
2.5
-0.05
-0.02
CPC-00449
23-Jun-2013
418.59
12.8
26-Jul-2013
1-Jul-2016
10-Jul-2016
418.44
2.5
-0.15
-0.05
CPC-00445
15-Jun-2016
389.18
13.2
2-Jul-2016
CPC-00450
15-Jun-2016
409.15
13.2
2-Jul-2016
CPC-00451
15-Jun-2016
429.16
13.2
2-Jul-2016
(a) The difference in CO2 concentration for (1) outside air
and (2) standard gas, between the individual measurement values (averaged
for 10 s) and over the entire measurement periods, and (b) standard
deviation of standard gas measurement values during 2–4 min.
Backward-air-trajectory analysis
In order to assess the sources for regional air masses affecting the
station, we calculated backward air trajectories using the METEX system
(Zeng and Fujinuma, 2004) available via the website of the Center for Global
Environmental Research, National Institute for Environmental Studies
(http://db.cger.nies.go.jp/metex/trajectory.jp.html). METEX uses
horizontal and vertical wind speed from European Centre for Medium Range
Weather Forecast (ECMWF) analyses on a 0.5∘ × 0.5∘ mesh to calculated 72 h trajectories.
Results and discussion
Analytical performance of the system at the station
CO2 data from the Li-840 were recorded every 1 s and averaged over
10 s. Signals from the Li-840 for the outside air and the working
standard gases are shown in Fig. 3a. The data of the first 2 min had
high variation because of the purging and exchanging of air during this
period, so we discarded the data of the first 2 min and averaged the
rest. The standard deviation of measurement values for the working standard
gas is shown in Fig. 3b. The noise level for 2 min averages of this
system was usually lower than 0.1 ppm.
(a) Linearity tests between the apparent measured values of the
standard gas and the reference standard values (assigned values), and (b) CO2 residuals.
The calibration using three working standard gases was done repeatedly every
day. These working gases were calibrated by our NIES calibration system in
the laboratory with a high accuracy below 0.1 ppm, which was confirmed by a
WMO/IAEA Round Robin inter-comparison. Certified values for the standard
gases prior to use and after use (i.e., 2–3 years later) are shown in Table 1. Because the standard gas was pressurized in relatively small cylinders
(i.e., 10 L aluminum cylinder), the drifts in CO2 concentration over 2–3 years were relatively large. However, their values still showed sufficient
concentration stability, mainly better than 0.1 ppm.
Calibration curves using these standards, as measured by the system, are
shown in Fig. 4a. Because the main measurement unit was exchanged every
year and standard gas series were changed every 2 or 3 years, apparent
output values measured and corresponding calibration curves were different
in each year. Since the span of the Li-840 was rather stable, calibration
curves were also stable during a year. The calibration curve was made by
linear regression for simplicity. Figure 4b shows the difference between the
assigned values of standards and calibrated values for each calibration
curve made by the system. Calibration was done four times a day, and the
differences in each calibration curve were averaged over 1 year; the
standard deviations were also calculated and plotted. Deviations from the
calibration curve were fairly small (lower than 0.05 ppm), and their
variations were also smaller than 0.05 ppm, suggesting that measurements
were sufficiently precise in accordance with WMO recommendations for this period.
Every summer we replaced the inlet filter and drying cartridge of silica gel
in addition to exchanging the main measurement unit. There was no problem
with filtering and drying the air during any 1 year of operation. Over
90 % of the silica gel was still blue after 1 year, indicating it had
not been exhausted. As mentioned in the experimental section, pumps
installed in the system were replaced by new ones annually and also worked
properly.
Daily summary data transmission by Iridium was also done automatically. We
found that bad weather conditions (clouds and rain) sometimes influenced the
success ratio of satellite transmission. During the rainy season from June
to July, the success ratio was 0.4–0.5. During poor weather, the system
attempted communication with the satellite at least twice a day on average
to send daily data. For other months the success ratio was as high as
0.6–0.7. When using the ORBCOMM satellite, the ratio was lower than 0.2; it
was harder to send data and caused extra power consumption, although all the
data could be stored on a CF card. Therefore, the Iridium communication
system we subsequently adopted seemed better for sending our data.
Daily averages of ambient temperatures outside the building,
within the observation room, and inside the insulation box, along with daily
voltage readings for the batteries. Temperatures inside the insulation box
and observation room were measured by the CO2 measurement system at the
same time that atmospheric CO2 concentrations were measured. The
temperature outside the building was measured by JMA; data were taken from
the JMA website (http://www.data.jma.go.jp/obd/stats/etrn/index.php). One hundred batteries
(12 V) were connected in parallel, and the voltage was checked when the
measurement started.
Operation with 100 batteries over 6 years
The total power consumption of the CO2 measurement system was estimated
to be about 3 A for 3 h day-1. However, if a single battery can
last 42 ampere hours (Ah), then 100 batteries could operate for 467 days
under ideal conditions. As mentioned earlier, however, the ambient
temperature on Mt. Fuji is low in winter and so reduces battery capacity. In
addition, the operation of the small heater in the system shortens the
duration of operation further.
Figure 5 shows daily data for the outside temperature on Mt. Fuji, within the
observation room, and within the measurement system protected by the
insulation box. The outside temperature was very low until April, sometimes
below -20 ∘C. The average temperature in January was -19.8 ± 4.4 ∘C. The room temperature
(-15.4 ± 1.9 ∘C) was about 5 ∘C higher than the outside
temperature.
The temperatures of the batteries were similar to the ambient temperature,
which have affected their capacities. On the other hand, the temperature
inside the insulation box was 15 ∘C higher than the room
temperature. Even in winter, the average temperature of the system remained
just above 0 ∘C because small amounts of heat were produced by
each device (e.g., the pump, circuit board, and Li-840 sensor) during the
operational time, which was retained within the insulated box. Consequently,
the system's small heater rarely operated in winter. The CO2
measurement system was able to measure CO2 concentrations stably in
spite of the low-temperature conditions.
Figure 5 also shows the voltage of the batteries during the operation. The
voltage just after charging in summer was 13.4–13.6 V but decreased
gradually to 12.0–12.2 V by July of the following year. After March, the
voltage seemed to recover as the temperature increased toward summer. Over
the 6-year period, voltages less than 10.5 V were not observed. Therefore,
measurements of CO2 concentration were not interrupted by power
shortages. In terms of battery condition, the G42EPs were very stable even
though they were used for over 6 years, although the lowest voltage
decreased from 12.3 to 12 V. The automatic charging system was very
effective with respect to simplifying maintenance, as it could charge all
batteries within 3 weeks. Thus, we concluded that our CO2 measurement
system, developed for the specific conditions of Mt. Fuji with its harsh
weather and limited power supply, operated successfully for 6 years with
no interruptions except for short periods of maintenance during summer. In
total, we recorded 2219 days of data from July 2009 to December 2015 (a
total of 2354 days), which corresponds to 94 % of the period.
At the station, we often observed lightning in the summer. To prevent severe
damage of the instruments at the station by voltage spikes conducted through
the power cable, the Mt. Fuji weather station sometimes (once or twice per
summer season) stopped grid electricity and changed the source to a small
generator at the station. At such times, our charging system stopped and
waited for the recovery of the grid electricity before restarting
automatically. On two occasions, the operation of the system was interrupted
by damage to a power board in the main control unit because of lightning
(2 April–23 July 2012 and 1–18 August 2014). Particularly in the summer
season, there are often thunderstorms observed at the summit because the
station is in the cloud layer. Recently, we connected a new ground line
between the observation building and the water tank facility to reduce the
risk of lightning passing through the Iridium antenna cable. However, we are
still unsure whether this will be sufficient to mitigate the risk of a
lightning strike affecting the measurement system.
Evaluation of the sampling time in terms of the daily variation of
CO2 concentration at the summit of Mt. Fuji
The CO2 concentrations at the summit of Mt. Fuji were observed
continuously in the summers (July–August) of 2012 and 2013. Figure 6 shows
the difference between the hourly and daily averages of CO2
concentration during the observed periods. The hourly averages of the
difference showed random variations within a range of about 0.5 ppm with no
clear diurnal variation. In the 2012 data, concentration tended to decrease
slightly (less than 0.4 ppm) before noon, although this tendency was not
seen in 2013. At the MLO, afternoon data are usually excluded because they
include vegetation effects. However, since no clear diurnal cycle could be
observed, we conclude that the timing of the air sampling at Mt. Fuji did not
affect the monthly and yearly averages. Although the sampling time was
changed from daytime to nighttime in July 2010, we believe that this change
did not affect the results of the trend analysis. Nakazawa et al. (1984) and
Sawa et al. (2005) also reported that daily variation of CO2
concentration at the summit of Mt. Fuji was very small. Tsutsumi et al. (1994) observed O3 concentration at the summit of Mt. Fuji and reported
that the small daily variation observed was influenced by a mountain–valley
wind system in summer; however, the variation was much smaller than at MLO,
Niwot Ridge in the USA (40.05∘ N, 105.59∘ W, 3528 m a.s.l.), and Hakkoda
(40.41∘ N, 140.51∘ E, 1324 m a.s.l.) in Japan. Igarashi et al. (2004) observed SO2 variation at the
summit of Mt. Fuji, and although they did not find any daily variation, they
did reveal episodic large-distance transport from the Asian continent.
According to observations at the summit of Mt. Fuji by Sawa (2005), the
short-term cycle of CO concentration variation was related to neither wind
direction nor wind velocity, though its variation was considered to be
affected by regional-scale pollution over Asia. Thus, the elevation of an
observing station must be sufficiently high to minimize surface influences
from local pollution and vegetation.
Difference between hourly average and daily average CO2
concentration during the observed periods: (a) 2012 and (b) 2013. Median value (the line in the box), inner 50th percentile of the value
(box), and inner 95th percentile of the value from the daily average for
each hour of the day for the CO2 mixing ratio at Mt. Fuji.
Comparison between CO2 concentrations measured by the
battery-operated CO2 measurement system and in the flask-sampling
experiment, and the difference in CO2 concentration between both
results.
Comparison with the flask sampling data and aircraft measurements over
Japan
In general, even if measurements are thoroughly calibrated, biases cannot
always be excluded, for example due to leaks in the sampling line or other
measurement issues. To assess the accuracy of our measured CO2 values,
we therefore conducted comparison experiments using flask sampling and
analysis. The air was collected into a glass flask at the same time as the
measurement system worked at the summit, usually during daytime in the
summers of 2009–2011. For this comparison, we ran the measurement system
four times a day, including periodical timing.
CO2 values from the system were very similar to those collected by the
flask sampling experiment (Fig. 7). The mean differences were almost zero
(0.04 ppm) with a standard deviation of 0.15 ppm. The variations were partly
caused by our sampling method for glass flasks, which only provided low
atmospheric pressure of the air in the flask, making the analytical
precision worse sometimes. Thus, we conclude that our measurement system
could measure CO2 mole fraction fairly accurately.
To study how well our CO2 data represented regional characteristics
around Japan not only in summer but over the whole year, we also compared
them to data observed by the CONTRAIL project. We selected CONTRAIL data,
taken on the same day as our measurements, within the region
34–36∘ N, 136–141∘ E, at altitudes of 3600–3900 m
during 2009–2013. Multiple daily points were averaged as daily data.
Figure 8 shows a scatterplot between both datasets. The data scatter around
the 1:1 line, and the average difference was only 0.05 ± 2.13 ppm. On
average, the datasets thus show good agreement, but the standard deviation
is relatively large. This is likely influenced by the difference in the
measuring time, as our measurements were taken at night and CONTRAIL's were
taken throughout the day, and by the variable distance from Mt. Fuji. It is not
straightforward to compare these data directly given such differences in
spatial and temporal sampling. However, Fig. 8 suggests that the
concentrations in both datasets are highly consistent on average, implying
that our dataset was showing regionally representative CO2
concentration. Therefore, we conclude that the summit of Mt. Fuji can be
considered a suitable location for sampling free-tropospheric air over the
Eastern Asian region throughout the year, unaffected by local wind blowing
from the valley around the Mt. Fuji weather station.
Scatterplots of CO2 concentration at Mt. Fuji and obtained
by CONTRAIL.
Effects of air mass origin on seasonal variation in CO2
Observations of CO2 mole fraction measured at the summit of Mt. Fuji
(2009–2015) are plotted together with daily data from CONTRAIL and MLO in
Fig. 9 (monthly average data are shown in Supplement Table S1; daily data will be
shown on a website:
http://db.cger.nies.go.jp/portal/overviews/index?lang=eng). A seasonal
minimum clearly occurs in September and a maximum in April or May, though
local minimum concentrations (episodic low concentration) were often seen in
July. The seasonal amplitude of variation at Mt. Fuji was the same as the
amplitude in the CONTRAIL data and about 18 ppm higher than the amplitude at
MLO (8 ppm). In general, the seasonal amplitude of CO2 variation is
greater over land areas than oceanic areas because of the influence of
photosynthesis and respiration of local and/or regional vegetation, and
anthropogenic CO2 emissions.
Japan has a seasonal wind pattern (i.e., the monsoon), and the CO2
concentration must be affected by the wind direction and the origin of the
air mass. To clarify the characteristics of the seasonal variation of
CO2 concentration at Mt. Fuji, we plotted the difference between the
daily concentration at Mt. Fuji and MLO (ΔCO2) in Fig. 10. The
CO2 concentration levels at both stations were similar from July to
September, when oceanic air prevails over Japan, although occasional lower
concentrations occurred in July at Mt. Fuji. Conversely, the CO2
concentration from December to March at Mt. Fuji was generally higher than
at MLO.
(a) Daily average of CO2 concentration and the trend of
CO2 concentration (solid line), and (b) growth rates for data from Mt.
Fuji, CONTRAIL, and Mauna Loa Observatory (MLO) 2009–2015. Atmospheric
CO2 concentration at Mt. Fuji was measured at 22:00–23:28 JST,
compared with the daily average of atmospheric CO2 concentration for
CONTRAIL and MLO data. CONTRAIL data were selected over Japan in the region
34–36∘ N, 136–141∘ E at an altitude of 3600–3900 m
during 2009–2013, and Mauna Loa Observatory is located at a similar altitude
(19.54∘ N, 155.58∘ W, 3397 m a.s.l.).
To characterize the difference between the two sites, we performed a
trajectory analysis for the daily data obtained at Mt. Fuji. We divided the
surrounding region into five areas (Siberia, China, Southeast Asia, Pacific
Ocean, and around Japan) of air mass origins according to a 72 h trajectory
analysis, as shown in Fig. 1. The distribution of the air mass origins
showed clear differences in each season. In the summer air masses came from
various areas, but in winter air masses mainly originated from China and in
some cases from Siberia and Southeast Asia. The ΔCO2
concentrations for June–August and January–March are shown in Fig. 11 for
the five areas of origin. Siberian air masses always carry lower
concentrations than the background air arriving at MLO. In addition, Chinese
air sometimes (three or four times per season) had much lower concentrations
than Siberian air (i.e., up to 10 ppm less), but it also sometimes (once per
season) had concentrations up to 7 ppm higher than at MLO in summer. Such
characteristics might be explained by CO2 uptake by the vegetation of
Siberia and China and by CO2 addition from anthropogenic emissions over
the Asian continent. Conversely, air originating from the Pacific Ocean and
the areas near Japan and Southeast Asia showed similar concentrations to
MLO. Therefore, we concluded that lower summertime CO2 concentrations
(compared to MLO) were related to the origin of the air mass, especially
those from Siberia and China, which are influenced by terrestrial ecosystems.
In winter, air from all areas except the Pacific Ocean showed higher
concentrations than at MLO, indicating some influence from the continent,
or even Southeast Asia. In particular, air originating from China sometimes
(three or four times per season) showed much higher concentrations than air
from other areas, suggesting that anthropogenic CO2 was added to the
air over China in winter. In addition, this season corresponded to the
season of biomass burning on the Indochinese Peninsula (Sawa, 2005), which
may also influence this higher concentration for the Southeast Asian sector.
Sawa et al. (2005) reported that CO2 concentrations at Mt. Fuji were
2 ppm lower than the level at Minamitori Island in summer but 3 ppm
higher in winter; this island is located 2000 km southeast of Mt. Fuji and
5000 km west of MLO (Watanabe et al., 2000). They stated that the air mass
over Mt. Fuji must be influenced by continental air. Similarly, at Hateruma
Island, which is much closer to China, the influence from the continent was
clearly observed, especially in winter (e.g., Tohjima et al., 2010).
Difference between daily CO2 concentrations at Mt. Fuji and
Mauna Loa Observatory, 2009–2014.
Overall, the seasonal variation of CO2 concentration at Mt. Fuji was
characterized by values lower than MLO in summer, associated with air
carried from Siberia and Asia in summer, and values higher than MLO in winter,
associated with air from China which is influenced by emissions over the
Asian continent.
The yearly variation in seasonal characteristics of CO2 concentration
shown in Fig. 10 also revealed some interesting tendencies. For example, the
negative values of ΔCO2 concentration in summer (June–August)
became especially large in 2013 and 2014. Looking at the air masses shown in
Fig. 11b, the frequency of air masses originating from China increased in
these 2 years, while the Pacific Ocean decreased its ratio. Accordingly,
ΔCO2 became relative low in 2013 and 2014 because air masses
originating from Siberia and China carried relatively low CO2
concentrations.
(a) Differences between daily CO2 concentration in summer
(June–August) and winter (January–March) at Mt. Fuji and Mauna Loa
Observatory for five areas of air mass origin (Fig. 1), and (b) percentage
of air mass origin in summer (June–August) and winter (January–March) at
Mt. Fuji from 2010 to 2014.
Trend analysis
Nakazawa et al. (1984) and Sawa et al. (2005) reported that CO2 mole
fraction on average was 337.8 and 338.8 ppm in October 1980 and 1981,
respectively, and 374.8 ppm in October 2003. Therefore, the CO2
concentration at the summit of Mt. Fuji increased by as much as 62 ppm over
the 35 years from 1980 to 2015. We calculated the annual rate of increase
since 2009 as 1.5–2.7 ppm yr-1 (Fig. 9). This was reasonably matched
with the growth rate at MLO (Tans and Keeling, 2016). These rates were
slightly higher than those in the 1980s (1.0–2.3 ppm yr-1), because
anthropogenic CO2 emissions have increased to > 10 Gt-C yr-1 in recent years, while they were only as high as 5 Gt-C yr-1
in the 1980s (Le Quéré et al., 2015). In particular, higher growth
rates were observed in 2010, 2012, and 2015. In general, El Niño years
lead to a higher global surface temperature, which is connected to an
increased rate of growth of CO2 because of accelerated plant
respiration over land, weakened photosynthesis activity, and occurrence of
biomass burning (van der Werf et al., 2008). However, 2012 was exceptional
as it was not an El Niño year. As for 2015, a clear El Niño occurred
and frequent biomass burning events were observed especially in Southeast
Asia, so the increase growth rate toward 2015 was reasonable.
These recent growth rate variations were almost the same as at MLO,
suggesting that the long-term stability for our CO2 observation system
is reliable. However, the trend line itself (average concentration) was
higher at Mt. Fuji than that of MLO. As mentioned in the last section,
because of influences by the wind patterns and air mass origins, the annual
average concentration at Mt. Fuji was about 1 ppm higher than MLO.
Therefore, the trend line of Mt. Fuji exceeded 400 ppm around October 2014,
while MLO exceeded this mark in March 2015, as shown in Fig. 9.
Conclusions
We developed a battery-powered CO2 measurement system and installed it
in 2009 at the weather station on the summit of Mt. Fuji, where there is no
grid-based electricity from September to June. This system has reliably
observed year-round atmospheric CO2 mole fractions from 2009 to 2015,
except for brief interruptions due to lightning damage; CO2 data were
obtained for 94 % of all days of the observation period. We improved the
lightning countermeasures by grounding between the buildings, but safer
technology may be needed to prevent lightning incidents from disrupting
future longer monitoring periods.
For sustainable and easy operation, we also developed an automatic
battery-charging system, which could charge all 100 batteries automatically within
3 weeks. This was essential for continuous observations at Mt. Fuji, because
researchers cannot stay at the station during the 3-week charging period.
The performance of these batteries was still good after 6 years. In the near
future, however, we plan to replace the batteries with new ones.
Even though the room temperature declined to -20 ∘C in winter,
the system worked properly. The insulation box kept the main unit warm
enough and the pumps worked safely for 1 year at a time; the small heater
for increasing temperature rarely needed to operate, helping reduce power
needs. Replacing the whole measurement unit and pumps every summer and the
maintenance of used pumps were very important for reliable operation during
the 10-month winter period when access is not possible. In addition, having
an extra unit at hand meant that we could quickly respond to sudden
incidents (usually by lightning) and could exchange the system very quickly
if these occurred in the summer, like in 2015. The daily satellite data
transmission was also successful after changing the system from ORBCOMM to
Iridium.
We conclude that our system could measure CO2 concentration accurately
and stably using a calibration strategy with daily calibrations with three
working standard gases, which were fairly stable in their concentration for
over 2 years. The high accuracy was confirmed by a comparison with
independent flask samples collected at the station.
We also compared our measured values of CO2 at Mt. Fuji with
CONTRAIL-derived data and found that both datasets were in good agreement
in the area relatively close to Mt. Fuji, despite the inclusion of data
obtained around the Tokyo area. This suggested that data obtained at Mt.
Fuji were comparable with other background data and that these data could be
highly representative. Because of power limitations, the system obtained
data only during a limited time period from 22:00 to 23:28. Even so, because
daily variation in CO2 concentration was found to be very small
(< 0.5 ppm), our data can be used as a representative daily dataset
in this region, as confirmed by the good agreement with the CONTRAIL data.
Historically, CO2 concentrations at Mt. Fuji have increased by about
62 ppm from 1980 to 2015, similar to MLO. The recent rate of increase in
CO2 concentration was found to be 1.5–2.7 ppm yr-1, also
consistent with the rate at MLO. However, there were seasonal differences
between the two sites. In summer, the CO2 concentration at Mt. Fuji is
about 2–10 ppm lower than at MLO, whereas in winter the CO2
concentration at Mt. Fuji is about 2–12 ppm higher. Such differences were
related to the large variation in seasonal origins of air masses over Japan
as determined by the regional wind patterns.
Even in the same season, CO2 characteristics depended on air mass
origin. For example, air masses originating from Siberia and China in summer
showed lower CO2 concentrations than other origins, while air masses
from China and Southeast Asia in winter had relatively high values. Such
regional source and sink characteristics could be clearly monitored in the
CO2 concentration at Mt. Fuji, suggesting that the developed system can
be applied for long-term regional environmental monitoring.