The Hidden Diurnal Cycles of Atmospheric Methane at the Top of Mt. Fuji
Chang-Feng Ou-Yang1，Shungo Kato2，Kojiro Shimada3，Hiroshi Okochi3，Ting-Hung Yu4，Neng-Huei Lin1
1. National Central University, Taiwan， 2. Tokyo Metropolitan University, Japan,
3. Waseda University, Japan, 4. Academia Sinica, Taiwan
１．Introduction
Methane (CH4) is the second most abundant anthropogenic
greenhouse gas next to CO2, accounting for approximately 16.5%of the global radiative forcing as reported by NOAA/GMD in2018. According to the AR5 published by IPCC, a CH4 has a 28times greater global warming potential than a CO2 over a 100-year
period (GWP100)1). This study presents the temporal variation of
atmospheric CH4 measured at the Mt. Fuji Research Station (FRS,35.37°N, 138.73°E, 3776 m a.m.s.l.) in the summer of 2019. The
observation period of the campaign was July 12 to August 20,
2019.
2．Methods
Ambient CH4 was measured continuously using a cavity ringdown spectrometer (G1301, Picarro, USA). The instantaneous
data points were taken at a frequency of approximately 2 s andfurther calculated into hourly averages. A series of NOAA/GMD
tertiary standards ranging from 1599 to 2024 ppb using theNOAA04 scale was served for the calibration of CH4 prior to thecampaign. Meteorological data such as wind speed (WS) areprovided by the FRS.
Three-day (72 h) backward trajectories at a frequency of 1
synoptic time per day (15:00 UTC) are computed using theNOAA/ARL HYSPLIT4 model. Four groups are identified
accordingly as China, Japan, Pacific, and Korea (sort inchronological order).

Table 1 Statics of CH4 and WS for individual trajectorygroupsNumber of DaysCH4 (ppb)

WS (m/s)

China

3

1850.2±46.0

4.77±2.35

Japan

16

1811.4±30.3

4.03±4.11

Pacific

181766.4±21.7

1.55±0.21

Korea

3

1812.6±3.0

3.35±3.70

All

39

1791.5±36.0

3.34±3.21

3．Results and Discussion
The observational results are illustrated in Fig. 1. Mean
concentration of CH4 during the campaign is 1791.5±36.0 ppb,
with a maximum and a minimum value of 1903.3 ppb and 1733.5

ppb, respectively. The mean values of CH4 and WS for eachtrajectory group are calculated as shown in Table 1. Group China
has the highest level of CH4 (1850.2±46.0 ppb) and WS(4.77±2.35 m/s) on average, whereas group Pacific has the lowest
level of CH4 (1766.4±21.7 ppb) and WS (1.55±0.21 m/s).
As suggested in the literature2), mountain-valley breezes aresuppressed at Mt. Fuji due to the minimal radiation from the small
surface area of the isolated mountaintop. However, a veiled diurnal
cycle of CH4 was found during a clam period (July 29 – August 8)of the campaign with lower wind speed. Reduced CH4 inassociation with elevated H2O was observed around noontime,
which was likely driven by uplifting winds. The mean diurnal
variation of CH4 during the clam period is thus treated as a model
for calculating the similarity for each day except the first (July 12)
and the last (August 20) one. The similarity is evaluated based on
Minkowski distance metric (𝐷𝑀𝑖𝑛𝑘 ) as given as follows3):
1𝑟𝑟𝑝𝑞
𝐷𝑀𝑖𝑛𝑘 = (∑|𝑓𝑡 − 𝑓𝑡 | )𝑡=1𝑝𝑞
where 𝑓𝑡 and 𝑓𝑡 are the two individual time series at time 𝑡𝑁

and 𝑁 is the number of samples in the time series. A Chebyshev
distance is defined (𝑟 = ∞), which is more sensitive to outlier
values due to the non-linear character of the dataset. Regardless ofthe discrepancies between different originations of air masses, thesimilarity increases against daily mean wind speed with aregression coefficient (R2) of 0.5 as illustrated in Fig. 2, meaningthat the diurnal cycle of CH4 could be more distinct when the windspeed is low.
4．Conclusion
The results of continuous measurement of atmospheric CH4
conducted at the FRS during July 12 – August 20, 2019, waspresented. A minor diurnal feature of CH4 hidden in the time series
was observed during the calm period of low wind speed, which
has been analyzed by the similarity test in this study.
References
1) IPCC (2013). Climate Change 2013: The Physical Science
Basis, Cambridge University Press, Cambridge and NewYork.
2) Igarashi, Y., Sawa, Y., Yoshioka, K., Takahashi, H., Matsueda,＿＿＿＿＿＿＿＿＿＿＿＿＿＿＿＿＿＿＿＿＿
連絡先：Chang-Feng OU-YANG cfouyang@cc.ncu.edu.tw

H., Dokiya, Y. (2006). Seasonal variations in SO2 plumetransport over Japan: Observations at the summit of Mt. Fuji
from winter to summer. Atmos. Environ., 40, 7018-7033.

a)3)

Jain, A.K., Murty, M.N., Flynn, P.J. (1999). Data clustering:
A review. ACM Comput. Surv., 31, 264-323.

b)Fig. 1 a) Results of the continuous measurements during the 2019 campaign. b) Mean diurnal cycles of
CH4 and H2O during the calm period July 29 – August 8.

Fig. 2 Relationship between natural logarithms of similarity (1/𝐷𝑀𝑖𝑛𝑘 ) and mean WS (m/s) for each day
during the 2019 campaign