ARTICLE IN PRESS

1352-2310/$ - se

doi:10.1016/j.at

$Special Issue

Japan, 11–13 M�Correspond

Tsukuba, Ibara

E-mail addr

Atmospheric Environment 38 (2004) 7041–7054

www.elsevier.com/locate/atmosenv

Ten years of atmospheric methane observations at a highelevation site in Western China$

L.X. Zhoua,b,�, D.E.J. Worthyc, P.M. Langd, M.K. Ernstc, X.C. Zhange,f,Y.P. Wena, J.L. Lig

aChinese Academy of Meteorological Sciences (CAMS), 46 Zhong-guan-cun South Street, Beijing 100081, ChinabNational Institute for Environmental Studies, Tsukuba, Ibaraki 305-8506, Japan

cMeteorological Service of Canada, 4905 Dufferin Street, Toronto, Ont., Canada M3H 5T4dNOAA Climate Monitoring and Diagnostics Laboratory, Boulder,CO, USA

eSchool of Physics, Peking University, Beijing 100871, ChinafQinghai Meteorological Bureau, Xining 810001, China

gSchool for Environmental Sciences, Peking University, Beijing 100871, China

Received 20 September 2003; received in revised form 16 January 2004; accepted 4 February 2004

Abstract

In this paper, the continuous (1994–2001) and discrete air sample (1991–2001) measurements of atmospheric CH4

from the Waliguan Baseline Observatory located in western China (361170N, 1001540E, 3816 m asl) are presented and

characterized. The CH4 time series show large episodic events on the order of 100 ppb throughout the year. During

spring, a diurnal cycle with average amplitude of 7 ppb and a morning maximum and late afternoon minimum is

observed. In winter, a diurnal cycle with average amplitude of 14 ppb is observed with an afternoon maximum and

morning minimum. Unlike most terrestrial observational sites, no obvious diurnal patterns are present during the

summer or autumn. A background data selection procedure was developed based on local horizontal and vertical

winds. A selected hourly data set representative of ‘‘baseline’’ conditions was derived with approximately 50% of the

valid hourly data. The range of CH4 mixing ratios, annual means, annual increases and mean annual cycle at Waliguan

during the 1992–2001 were derived from discrete and continuous data representative of ‘‘baseline’’ conditions and

compared to air samples collected at other Northern Hemisphere sites. The range of CH4 monthly means of

1746–1822 ppb, average annual means of 1786.7710.8 ppb and mean annual increase of 4.574.2 ppb yr�1 at Waliguan

were inline with measurements from sites located between 301 and 601N. There were variations observed in the CH4

annual increase patterns at Waliguan that were slightly different from the global pattern. The mean CH4 annual cycle at

Waliguan shows an unusual pattern of two gentle peaks in summer and February along with two small valleys in early

winter and spring and a mean peak-to-peak amplitude of �11 ppb, much smaller than amplitudes observed at most

e front matter r 2004 Elsevier Ltd. All rights reserved.

mosenv.2004.02.072

of the Eighth International Conference of Atmospheric Sciences and Applications to Air Quality (ASAAQ), Tsukuba,

arch 2003.

ing author. Center for Global Environmental Research, National Institute for Environmental Studies, 16-2 Onogawa,

ki 305-8506, Japan. Tel.: +81-298-50-2769; fax: +81-298-51-4732.

ess: zhou.lingxi@nies.go.jp (L.X. Zhou).

ARTICLE IN PRESSL.X. Zhou et al. / Atmospheric Environment 38 (2004) 7041–70547042

other mid- and high-northern latitude sites. The Waliguan CH4 data are strongly influenced by continental Asian CH4

emissions and provide key information for global atmospheric CH4 models.

r 2004 Elsevier Ltd. All rights reserved.

Keywords: Atmospheric CH4; Local winds impact; Background data selection; CH4 variation characteristics; Source and sink influence

1. Introduction

The increase in the atmospheric burden of CH4 and its

impact on the Earth’s radiative balance are well

documented (Crutzen, 1995; Etherridge et al., 1998;

Lelieveld et al., 1998; Rasmussen and Khalil, 1984). CH4

is an important greenhouse gas and a key molecule

influencing atmospheric chemistry. Consequently, there

is considerable interest in the atmospheric CH4 budget

and how it has been changing with time. Global

atmospheric CH4 observations have also been combined

with atmospheric transport and chemistry models to

predict the spatial and temporal distribution of methane

in the atmosphere (CMDL, 2002; Levin et al., 1999;

Dlugokencky et al., 1993, 1995; Fung et al., 1991).

Systematic worldwide measurements of tropospheric

CH4 began in 1978 (Blake et al., 1982; Rasmussen and

Khalil, 1981). Over time, many other global continuous

and discrete air sampling measurement programs

were initiated, including the National Oceanic and

Atmospheric Administration, Climate Monitoring and

Diagnostics Laboratory (NOAA CMDL) trace gas

measurement programs (Dlugokencky et al., 1994a;

Steele et al., 1987; WMO, 1997, 2003a, b). The existing

air sampling networks do not provide sufficient spatial

coverage to constrain emission rates from most CH4

sources. To derive individual source apportionments

and to better address the role of the terrestrial biosphere

in the global atmospheric cycle of methane, it will be

necessary to increase the number of measurement sites in

mid-continental regions (CMDL, 2002; Globalview-

CH4, 2001; WMO, 2001).

In 1991, NOAA CMDL began a weekly air-sampling

program at Waliguan, located in the Tibetan Plateau of

Western China. In 1993, a new baseline observatory was

established at Waliguan. In 1994, the observatory was

officially opened by the Chinese Meteorological Admin-

istration (CMA) as China’s first long-term research

station for the continuous monitoring of greenhouse

gases, ozone, aerosols and meteorology. The opening of

the Waliguan Observatory marked the end of several

years of planning and coordination, and the beginning

of a new phase of continuous atmospheric chemistry

monitoring in China’s interior. Waliguan is an official

World Meteorological Organization (WMO) Global

Atmospheric Watch (GAW) station—one of 22 baseline

observatories around the world, which includes Alert,

Canada; Mauna Loa, Hawaii; South Pole; and Cape

Grim, Australia (WMO 2001, 2003a; Zhou et al., 1998,

2003). The Waliguan Observatory is situated in an

important geographical region and the measurements

from Waliguan provide essential information on sources

and sinks from within the Eurasia continent.

In this paper, the continuous (1994–2001) and discrete

air sample (1991–2001) measurements of atmospheric

CH4 from Waliguan are presented and characterized.

The measurements show CH4 to be highly variable. The

features of the data are discussed and a methodology for

data selection is developed. From the selected data

record, daily, monthly and annual averages are derived.

These values are compared with CH4 data results

recorded at other global observational sites. The CH4

results from Waliguan show unique features that will

provide additional constraints for atmospheric models

to help improve the understanding of CH4, particularly

over the Asian continent.

2. Experiment

2.1. Sampling site

The Waliguan Baseline Observatory is located within

a remote region of western China at 361170N, 1001540E

and 3816 m asl. The United Nations and the Chinese

Government provided funding for the station. Fig. 1

shows the topographical distribution for 100 km around

Waliguan. Located at the edge of northeastern part of

the Tibetan Plateau, the area surrounding the station

has been untouched, maintaining its natural environ-

ment of sparse vegetation along with arid and semi-arid

grassland and some desert regions. Yak and sheep

grazing is the main activity during summer with small

agricultural regions located in the lower valley area. The

population density is less than 6 people km�2 and the

station is relatively isolated from industrial and popu-

lated centers.

2.2. Methane measurement system

Atmospheric CH4 is measured quasi-continuously

using a Hewlett Packard (HP) 5890 Series II gas

chromatograph (GC) equipped with a flame ionization

detector (FID). An HP 3396 Series II integrator is used

to acquire and process the FID signal and in conjunc-

tion with an HP 19405A Sampler/Event Control

ARTICLE IN PRESSL.X. Zhou et al. / Atmospheric Environment 38 (2004) 7041–7054 7043

Module (S/ECM), is used to control the automated

operation of the analysis. The processed and raw data

are stored on an external HP 9122C disk drive. The data

Fig. 2. Configuration of the Wa

Fig. 1. Topographical map of the surrounding area (100 km)

around Mount Waliguan. The triangular markings represent

mountain peaks (meters asl). The scattered dotted area

indicates desert regions.

are regularly transferred to a host computer for

further processing and archiving. A schematic illustra-

tion of the configuration of the GC at Waliguan is

shown in Fig. 2.

The automated sampling module is designed to

sample from separate gas streams (standard tanks and

ambient air) supplied to the instrument. Ambient air is

delivered to the GC at approximately 5 L min�1 by a

KNF Neuberger N2202 vacuum pump via a dedicated

0.95 cm o.d. sample line from an 80 m intake line

attached to an 89 m steel triangular tower located

approximately 15 m from the main observatory. The

residence time of the ambient air from the top of the

tower to the instrument is �30 s. The ambient air is first

passed through a 7 mm stainless steel membrane filter

located upstream of the pump and then (after the pump)

passed through a pressure relief valve set at �1 atm to

release excess air and pressure. The ambient air is then

dried to a dew point of approximately �60 1C by passing

it through a glass trap submerged in a �701C methanol

bath. All standard gases are supplied to the GC from

pressurized 37.5 L treated aluminum alloy cylinders

fitted with high-purity, two-stage gas regulators. Stain-

less steel tubing (0.32 cm o.d.� 0.22 cm i.d.) is used for

the standard gas sample line and the ambient sample line

after the cold trap.

Selection of ambient sample or standard gas is

performed by electrical actuation of a 1/1600 4-port

stream selection valve equipped with a corresponding 8

liguan gas chromatograph.

ARTICLE IN PRESSL.X. Zhou et al. / Atmospheric Environment 38 (2004) 7041–70547044

position electrical actuator (for intermediate off posi-

tions). The sample and standard gas flow rates are

controlled at �250 mL min�1 using individual needle

valves. Sample gas is passed through the Valco

Instruments Co., Inc. 6-port sampling valve, fitted with

a 3 mL sample loop, and vented to the room via a 5 m 1/

1600 o.d. stainless steel isolation coil. After a 30 s purging

flow, the selection valve is rotated to an intermediate

off position to allow the gas in the sample loop to

equilibrate with atmospheric pressure and the valve oven

temperature. Following an equilibration time of 30 s, the

sampling valve is switched to ‘‘inject’’ position allowing

the column carrier gas (499.999% ultra high purity

Nitrogen) to transfer the contents of the sample loop to

the stainless steel analytical column (3/1600 o.d.� 100)

packed with Porapak QS (100–120 mesh) maintained at

401C. Upon leaving the column, the N2 carrier gas is

mixed with H2 (50 mL min�1) and is passed to the inlet

of FID where it is combusted with 625 mL min�1 of

clean air provided from a clean air generator. A

50 mL min�1 carrier flow provides a retention time

of �2.5 min for CH4.

A four-injection sequence of a low standard, ambient

air, high standard, and ambient air injection is

programmed into the integrator and S/ECM (64

ambient air measurements per day). The injection

sequence is repeated every 45 min. After each 45 min

sequence, the HP 3396 integrator generates a report

consisting of retention times, peak heights, peak widths

and peak areas for each individual injection using a

customized integration procedure programmed into the

integrator. The information is then stored on an external

disk drive (HP 9122C). The ambient CH4 mixing ratio is

derived (using a separate processing routine) by multi-

plying its measured peak height by the average mixing

ratio response factor (mixing ratio/height count) deter-

mined from the assigned CH4 mixing ratios and

measured peak height counts of the two bracketing

standards. The validity of this method has been

demonstrated by others (Dlugokencky et al., 1995;

Steele et al., 1987).

All CH4 measurements are reported in nmol mol�1

(abbreviated ppb, or parts per billion, mole fraction),

dry air. Ambient CH4 measurements are only included

in the final data set if the re-evaluation of the second

standard gas relative to the first standard is within

10 ppb of its assigned value and if the peak width for all

injections is less than 0.015 min. Approximately 90% of

the data were accepted using the criterion. The data are

also manually inspected and examined using quality

control routines before being accepted as valid measure-

ments. Subsequent data averaging intervals (hourly,

daily, etc.) are calculated using valid data only. The

analytical precision of the measurement system is

approximately 0.2% based on replicate injections of

standard gas (Zhou et al., 1998). Calibrations of the

working standards are done using station primary

standards every 6 months. Working standard gases have

also shown to be stable with an absolute deviation of less

than 2 ppb based on calibrations using external calibra-

tion gas tanks (Zhou, 2001).

The standard scale at Waliguan for the CH4

measurement system is relative to a CH4 measurement

scale maintained by the Meteorological Service of

Canada (MSC) in Toronto (Worthy et al., 1998). MSC

calibrated and provided the initial station primary

standard tanks as well as in-house secondary standard

tanks. The MSC CH4 scale has been compared with the

NOAA CMDL CH4 scale through several inter-com-

parison experiments. Methane mixing ratios determined

by the MSC scale is a factor of 1.0151 greater

(approximately 25 ppb at ambient level) than those

determined by the NOAA CMDL CH4 scale (Global-

view-CH4, 2001; Worthy et al., 1998). An inter-

comparison experiment in 2001 organized by the

WMO GAW World Calibration Centre (WCC) for

Methane in Asia and the South-West Pacific (http://

gaw.kishou.go.jp/wcc/ch4) reported shifts of 0.53%

(�1790 ppb transfer standards) and 0.79% (�1940 ppb

transfer standards) respectively, of the Waliguan CH4

standards refer to its assigned values. The continuous

CH4 measurements used in this paper have been

adjusted and are reported on the NOAA CMDL

scale and data are available at URL http://www.cams.

cma.gov.cn/camscgi/ch4.

The discrete air samples collected at Waliguan are

measured for CH4 (and other trace gas species) by

NOAA CMDL Carbon Cycle Greenhouse Gases group

(CCGG) in Boulder, CO, USA Two samples are

collected in series using glass containers and a portable,

battery powered sampling apparatus (Dlugokencky et

al., 1994a). The samples are measured by an HP6890 GC

with flame ionization detection. Methane measurement

repeatability is approximately 1 ppb. All CH4 measure-

ments are referenced to the NOAA CMDL CH4

reference scale and data are available at URL ftp://

ftp.cmdl.noaa.gov/ccg/ch4/flask.

In addition to weekly air samples, NOAA CMDL

CCGG operates GC systems for measuring atmospheric

CH4 at Mauna Loa, Hawaii and Barrow, Alaska. The

average difference (71s) between the discrete and

semi-continuous hourly averaged measurements is

(0.079.8) ppb at Barrow and (�0.775.0) ppb at Mauna

Loa (Dlugokencky et al., 1995).

2.3. Wind speed and direction

The 89 m tall triangular steel tower is instrumented for

horizontal wind direction and speed measurements at

10, 20, 40 and 80 m using R.M. Young, model RMY-

05103 anemometers. An R.M. Young model RMY-

27106T anemometer located at the 80 m level records the

ARTICLE IN PRESSL.X. Zhou et al. / Atmospheric Environment 38 (2004) 7041–7054 7045

vertical wind speed. Surface wind observations began in

July 1994 with a sampling frequency of 2 Hz. The

meteorological measurements are recorded on a Camp-

bell Scientific model CR-21X datalogger. The datalogger

reduces the high-frequency meteorological readings to

5 min average intervals. The data are transferred every

hour to the local host computer for storage and further

processing.

0

5

10

15

20

25

30

-20 -15 -10 -5 0 5 10 15 20 25

CH4 difference (ppb, flask minus continuous)

Freq

uenc

y (%

)

Fig. 4. A comparison of the discrete and continuous measure-

ment records at Waliguan over the period of 1994–2001. The

distribution represents the difference between the NOAA

CMDL flask pair averages and the corresponding continuous

hourly averaged values (110 counts matched).

3. Results and discussion

3.1. Methane data

All valid hourly averaged atmospheric CH4 mixing

ratios from Waliguan for the period of 8 August 1994–5

September 2001 are shown in Fig. 3. Also shown are the

valid CH4 discrete sample pair averages from the NOAA

CMDL air-sampling program at Waliguan for the

period of 7 May 1991–21 December, 2001. Large gaps

in the continuous record are due to system malfunctions

and two periods of main building reconstruction (5

June–7 November 1997 and 2 February 1998–29 May

2000). A discussion of sampling site characteristics, data

selection, and the CH4 diurnal cycle, seasonal cycle,

trend and inter annual variability observed at Waliguan

is given below.

3.2. Comparison of CMDL CH4 measurements to

continuous measurements

A comparison of the discrete air samples and

continuous measurements at Waliguan is shown in

Fig. 4. The in situ data were smoothed using a 3-h

running mean to minimize instrumental noise. The

distribution represents the difference between the sample

pair average and the corresponding continuous

1700

1725

1750

1775

1800

1825

1850

1875

1900

1991 1992 1993 1994 1995 1996

Y

CH

4 (p

pb)

Fig. 3. Hourly averaged atmospheric CH4 mixing ratios from Walig

shown are the discrete CH4 flask pair averages from the NOAA CMD

December, 2001. The dots present hourly data (19840 h in total) and

smoothed hourly averaged value. There were 110

periods in which a paired air sample value matched an

in situ hourly record (1994–2001). The comparison of

the discrete air sample and continuous data resulted in a

mean difference (discrete minus continuous, �2s) of

(�0.6671.54) ppb. For the most part, the distribution is

relatively equally distributed around zero and thus

certifies the relative quality of both data sets.

3.3. Short-term variability in the continuous CH4 record

Variability in CH4 is commonly observed at northern

hemisphere (NH) observational sites. The variability

occurs over a multitude of time scales ranging from

synoptic (2–5 days) to seasonal and annual. The

variation can be generally attributed to one or more

factors including changes in nearby or far away

emissions and sinks of CH4 and variations in atmo-

spheric transport (Dlugokencky et al., 1993, 1995;

Shipham et al., 1998; Tohjima et al., 2002; Worthy

et al., 1998).

1997 1998 1999 2000 2001 2002

ear

uan for the period of 8 August 1994–5 September 2001. Also

L flask air-sampling program for the period of 7 May 1991–21

open circles denote discrete data (556 counts in total).

ARTICLE IN PRESSL.X. Zhou et al. / Atmospheric Environment 38 (2004) 7041–70547046

Hourly and monthly average CH4 mixing ratios and

standard deviations were derived using the individual

valid ambient CH4 measurements at Waliguan. Episodic

events in CH4 of �100 ppb and lasting from 1 to 20 days

were commonly observed (Zhou, 2001). These episodes

are also consistently observed throughout the year,

unlike that at other NH sites such as at the high Arctic

site in Alert, Canada (Worthy et al., 1994). To illustrate

the observed short-term variability, a frequency dis-

tribution of standard deviations of the daily means

derived using the selected hourly data values

(1994–2001) is shown in Fig. 5. The most common

standard deviation for daily means occurs at 6 ppb.

However, the distribution has a long tail with daily

standard deviations greater than 10 ppb occurring more

than 30% of the time, illustrating significant natural

daily variation.

As observed at other mountainous sites, Waliguan

experiences both upslope and downslope wind flow.

Shortly after sunrise, the ground surfaces warm resulting

in air being drawn up from lower levels. At night the

ground surfaces become cooler and air is drawn down-

wards. In principle, the downward airflow is more

representative of large scale well-mixed free tropospheric

air. However, the effects of upslope and downslope wind

flow at Waliguan result in a typical pattern of elevated

CH4 being observed at night, with elevations in CH4 up

to 10 ppb higher than during the day. When the

nighttime data in CH4 is excluded, the observed

standard deviation in the daily means is reduced.

Waliguan is a high plateau site away from anthropo-

genic sources with many high mountains in between. As

we will examine it later in this paper, horizontal winds

from east of Waliguan (Xining city, etc.) typically bring

a down slope airflow (night time, elevated CH4);

horizontal winds from west of Waliguan (desert clean

area) however bring an upslope airflow (Zhou et al.,

2003). The nighttime elevated CH4 at Waliguan might

mainly attribute to influence of local terrain and

transport.

During winter at Waliguan, the variability in CH4,

CO2, CO and black carbon (BC) often show similar

0

2

4

6

8

10

12

14

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20CH4 standard diviation of the daily means (ppb)

Freq

uenc

y (%

)

Fig. 5. Occurrence of standard deviations in the daily means

derived using the selected hourly data values over the period of

1994–2001 (1065 days in total).

patterns (even on time scales of less than a day) (Zhou,

2001; Zhou et al., 2003). Using the simple argument that

BC and CO are primarily produced by combustion of

fossil fuels and biomass burning and with the site

located at a drought temperate climatic zone surrounded

primarily with low grasslands and mountainous regions

(and thus minimal contributions from natural combus-

tion sources), it is reasonable to further conclude that

the origin of the episodic increases in CH4 at Waliguan

in winter are a result of transport. Similar observations

and arguments have been made in previous studies

(Conway et al., 1993; Hansen et al., 1989; Harris et al.,

2000; Hopper et al., 1994; Worthy et al., 1994).

3.4. Average diurnal variation and impact of local surface

winds on CH4

For 1994–1996, an hourly CH4 residual data set

(sometimes referred to as detrended and deseasoned

data) was constructed by subtracting the monthly means

from each hourly averaged value (within that month) for

the purpose of evaluating average diurnal variability and

impact of local surface wind on CH4. For convenience

of plotting, a CH4 value of 1810 ppb was added to each

hourly mean residual. Hourly averaged wind direction

and speed values were derived from 5 min averages. The

hourly adjusted residual CH4 and surface wind values

were then partitioned according to season with the

months of March, April and May as spring, June, July

and August as summer, September, October and

November as autumn and December, January and

February as winter. The average diurnal variation and

impact of wind on the continuous CH4 observations was

investigated using these partitioned data sets.

3.4.1. Average diurnal variation

Fig. 6 shows the average diurnal pattern by season

(1994–1996) for CH4 at Waliguan. In spring, a diurnal

amplitude of 7 ppb is observed with the maximum

occurring in the morning and minimum during the late

afternoon. In winter, a diurnal range of 14 ppb is

observed with the maximum occurring in the afternoon

and a minimum during the early morning, opposite to

that observed in spring. No significant diurnal patterns

are seen for the summer and autumn periods. The small

diurnal patterns observed at Waliguan indicate weak

local CH4 sources. This is unlike diurnal patterns

observed at some other NH continental sites (Dlugo-

kencky et al., 1993, 1995; Shipham et al., 1998; Tohjima

et al., 2002; Worthy et al., 1998).

3.4.2. Impact of local surface winds on CH4

The combination of continuous CH4 measurements

with meteorological data can improve the relationship

between CH4 emissions and transport (Dlugokencky

et al., 1993, 1995). The aim of the Waliguan CH4

ARTICLE IN PRESS

0 2 4 6 8 10 12 14 16 18 20 22 24

Time (Autumn, BJT)

0 2 4 6 8 10 12 14 16 18 20 22 24

Time (Autumn, BJT)

0 2 4 6 8 10 12 14 16 18 20 22 24

Time (Summer, BJT)

1800

1804

1808

1812

1816

1820

0 2 4 6 8 10 12 14 16 18 20 22 24

Time (Winter, BJT)

CH

4 (p

pb)

1800

1804

1808

1812

1816

1820

CH

4 (p

pb)

1800

1804

1808

1812

1816

1820

CH

4 (p

pb)

1800

1804

1808

1812

1816

1820C

H4

(ppb

)

Fig. 6. Seasonally averaged diurnal CH4 cycles at Waliguan (in Beijing time, abbreviated as BJT) for the period of 1994–1996.

BJT=UTC+8 h=Waliguan local time+1 h.

L.X. Zhou et al. / Atmospheric Environment 38 (2004) 7041–7054 7047

program is to obtain information representative of large

well-mixed volumes of the atmosphere. Therefore, a

focus is made towards obtaining CH4 measurements

representative of background conditions and removing

data affected by nearby CH4 sources. Wind patterns are

used to categorize these two situations. The seasonal

wind directions observed at Waliguan are shown as wind

rose plots in Fig. 7. In winter the prevailing wind

directions at Waliguan are NW–W and WSW–SW

(55%) and in summer prevailing winds are from the

NE–ENE–E–ESE (40%). The prevailing wind directions

in spring and autumn are typically SW–WSW–W and

ENE–E–ESE, respectively. Overall, calm conditions

have an occurrence of less than 2%.

The seasonal CH4 hourly mixing ratios segregated by

horizontal wind direction is shown in Fig. 8. Fig. 9

shows similar results but weighted by frequency of wind

occurrence. This method of weighting shows the relative

importance of wind directions to the annual atmospheric

trace species loading at Waliguan (Zhou et al., 2003).

The results in Figs. 8 and 9 show that during the spring,

autumn and winter, CH4 is increased by 10–20 ppb when

winds originate from the NE–ENE–E. The results also

show an additional 10 ppb elevation from southern wind

flows in winter. A more complicated situation occurs in

summer with greater CH4 mixing ratios (approximately

15 ppb) occurring when winds are from the ENE–E–ES-

E–SE, a region with more plantations and higher

population density, and smaller elevated CH4 (5 ppb)

occurring from winds in WNW–NW direction.

Previous studies at Waliguan (Tang et al., 1999; Zhou

et al., 2001, 2003) showed the relationship of CO2, CO

and BC with surface winds and long-rang transport.

These results indicated that the highest BC and CO

concentrations (slightly different seasonally) occur with

air masses originating from the NNE and NE. These

higher values were attributed to emissions more than

500 km away from the Yellow River Canyon industrial

area northeast of Waliguan. Similar patterns are

observed in the CH4 data record. Thus, in winter we

categorize ‘‘clean’’ representative CH4 levels when air

originates from the SW–WSW–W–WNW (55% occur-

rence). Higher contaminated CH4 levels measured in

summer are associated with winds from the NE–E-

NE–E–ESE (40%).

Five categories were formed for horizontal wind speed

measurements: p0.5 m s�1 (40.5 to p3) m s�1, (43 to

p6) m s�1, (46 to p10) m s�1 and readings 410 m s�1.

Wind speeds p0.5 m s�1, or 410 m s�1 occur less than

10% of the time (Zhou et al., 2003). Depending on the

season, CH4 mixing ratios can be strongly influenced by

horizontal wind speeds. Generally, lower CH4 levels are

accompanied with greater wind speeds in winter, spring

and autumn. For example, CH4 observations associated

with wind speeds from the (43 to p6) m s�1 category

are 12 ppb lower in winter and spring and 7 ppb lower in

autumn compared to CH4 observations associated with

calm conditions. Similarly, CH4 mixing ratios associated

with wind speeds 410 m s�1 have values 9 ppb lower in

winter and 5 ppb lower in spring and autumn, when

compared to the (43 to p6) m s�1 wind speed category.

The results suggest that for these three seasons, higher

wind speeds (and possible longer transport) result in a

more diluted (or better mixed) CH4 signal. The story is

ARTICLE IN PRESS

Spring (Calm 1.08%)

0

5

10

15

N

NNENE

ENE

E

ESE

SE

SSE

S

SSWSW

WSW

W

WNW

NW

NNW

Summer (Calm 1.61%)

0

5

10

15

20N

NNE

NE

ENE

E

ESE

SE

SSE

S

SS W

SW

WSW

W

WNW

NW

NNW

Autumn (Calm 2.34%)

0

5

10

15N

NNE

NE

ENE

E

ESE

SE

SSE

S

SSW

SW

WSW

W

WNW

NW

NNW

Winter (Calm 1.67%)

05

10152025

N

NNENE

ENE

E

ESE

SE

SSE

S

SSWSW

WSW

W

WNW

NW

NNW

Fig. 7. Seasonal wind-rose distribution patterns at Mt. Waliguan for the period of 1994–1996.

L.X. Zhou et al. / Atmospheric Environment 38 (2004) 7041–70547048

different in summer with calm conditions resulting in a

CH4 decrease of 2 ppb and a CH4 increase of 2 ppb with

wind speeds 410 m s�1, compared to CH4 results

associated with winds in the (43 to p6) m s�1 category.

There were no measurable differences observed between

either of the p0.5 m s�1 (40.5 to p3) m s�1, or (46 to

p10) m s�1 with the (43 to p6) m s�1. The results for

summer suggest that the elevated CH4 is likely due to

local transport from sources within the surrounding

region. It is also reasonable to conclude that, in summer,

data associated with wind speeds greater than 10 m s�1

or during calm conditions are not representative of free

tropospheric background conditions.

Five groups were also formed for vertical wind speed

measurements: p–1 m s�1 (4–1 to p–0.3) m s�1,

(4�0.3 to p+0.3) m s�1 (4+0.3 to p+1) m s�1 and

4+1 m s�1 (negative numbers indicate downslope air-

flow). Vertical wind speeds greater than 71 m s�1

occurred less than 10% of the time and showed no

obvious seasonal dependence. Due to the influence of

local terrain around Waliguan, horizontal winds origi-

nating from the east result in downslope airflow

whereas, horizontal winds originating from the west

tend to bring upslope air (Zhou, 2001; Zhou et al.,

2003). Comparing springtime CH4 mixing ratios asso-

ciated with vertical speeds in the (4–0.3 to

p+0.3) m s�1 range and upslope air flow (4+0.3 to

p+1) m s�1 and 4+1 m s�1 results in a 5 and 15 ppb

decrease in CH4, respectively, while downslope air flow

p–1 m s�1 (4–1 to p–0.3) m s�1 results in a 2 and 5 ppb

increase in CH4. In autumn, upslope airflow results in a

3 ppb decrease while downslope airflow results in a 5 ppb

increase. In winter, similar results are observed with

upslope airflow resulting in a 4–5 ppb decrease and

downslope airflow resulting in a 3–7 ppb decrease. In

summer, upslope airflow results in a 2 ppb decrease

while downslope airflow (p–1 m s�1), results in a 2 ppb

increase. The results of both the vertical and the

horizontal wind analysis shows that elevated CH4 levels

are primarily attributable to the eastern regions and

lower CH4 levels are associated with transport and well-

mixed air from the western clean air sectors.

ARTICLE IN PRESS

Spring (CH4, ppb)

17 80

18 00

18 20

1840N

NNE

NE

ENE

E

ESE

SESSE

S

SSW

SW

WSW

W

WNW

NW

NNW

Summer (CH4, ppb)

17 80

18 00

1820

1840N

NNE

NE

ENE

E

ESE

SE

SSE

S

SSWSW

WSW

W

WNW

NW

NNW

Autumn (CH4, ppb)

17 80

1800

1820

1840N

NNENE

ENE

E

ESE

SESSE

S

SSW

SW

WSW

W

WNW

NWNNW

Winter (CH4, ppb)

1780

1800

18 20

1840N

NNE

NE

ENE

E

ESE

SE

SSE

S

SSW

SW

WSW

W

WNW

NW

NNW

Fig. 8. Seasonal CH4 hourly mixing ratios segregated by horizontal wind direction for the period of 1994–1996.

L.X. Zhou et al. / Atmospheric Environment 38 (2004) 7041–7054 7049

3.5. Data selection

To evaluate the trend and annual cycles of CH4

observed at Waliguan, it is important that CH4 mixing

ratios representative of large, well-mixed volumes of the

atmosphere are extracted from the data record. The

determination of ‘‘baseline’’ conditions is also necessary

to minimize the effect of local sources and sinks on the

long-term variations in the data (Dlugokencky et al.,

1995; WMO, 2001). To identify hourly CH4 data

representative of ‘‘baseline’’ conditions at Waliguan,

the following rules were applied:

(1)

All hourly CH4 values associated with wind origi-

nating from the ENE–E (�10–20%) were excluded.

(2)

During winter, all hourly CH4 values associated with

wind originating from the S (less than 3%) were

excluded.

(3)

All hourly records during calm wind conditions and

for summer wind speeds greater than 10 m s�1 (less

than 10%) were excluded.

(4)

All hourly records with vertical wind speed greater

than 71 m s�1 (�5%) were excluded.

(5)

All hourly records that differed by more than 3sfrom a curve fitted to all valid hourly averaged data

(8 August 1994–5 September 2001) were excluded

(�10–15%) to eliminate instrument noise.

Approximately 50% of the data were removed

after these five data criteria were applied. The remaining

50% of data were categorized as selected ‘‘baseline’’

or ‘‘background’’ CH4 values. Daily, monthly and

annual means were derived using this selected data

set.

3.6. Background characteristics

3.6.1. Range of CH4 mixing ratios, annual means and

annual increases

Approximately 75% of the global source of atmo-

spheric CH4 is in the NH (Dlugokencky et al., 1994a;

Steele et al., 1987; WMO, 2003b) and CH4 mixing ratios

observed in the NH can be as large as 150 ppb greater

than CH4 mixing ratios observed in the southern

hemisphere (SH) (Dlugokencky et al., 1994a; Steele

et al., 1987; WMO, 2003b). The ranges of monthly CH4

ARTICLE IN PRESS

Table 1

Estimated range of atmospheric CH4 monthly means and average (7 2s) annual means at Waliguan and selected NOAA CMDL NH

sites, 1992–2001

Selected NH sites Latitude Longitude Altitude (m asl) Range of CH4

monthly mean

values (ppb)

10-year average

(7 2s) annual

means (ppb)

ALT 82N 62W 210 1760�1860 1816.878.2

BRW 71N 156W 11 1776�1869 1826.678.4

MHD 53N 10W 25 1762�1848 1804.1710.4

UUM 44N 111E 914 1776�1858 1812.8710.0

NWR 40N 105W 3475 1731�1816 1777.679.5

WLG 36N 101E 3810 1746�1822 1786.7710.8

TAP 36N 126E 20 1757�1929 1843.579.6

IZO 28N 16W 2360 1718�1807 1765.779.2

MLO 19N 155W 3397 1700�1781 1746.8710.7

Spring (CH4 loading)

-6000

-3000

0

3000

6000N

NNE

NE

ENE

E

ESE

SE

SSES

SSW

SW

WSW

W

WNW

NWNNW

Summer (CH4 loading)

-6000

-3000

0

3000

6000N

NNENE

ENE

E

ESE

SE

SSES

SSW

SW

WSW

W

WNW

NW

NNW

Autumn (CH4 loading)

-6000

-3000

0

3000

6000N

NNE

NE

ENE

E

ESE

SE

SSES

SSW

SW

WSW

W

WNW

NWNNW

Winter (CH4 loading)

-6000

-3000

0

30 00

6000N

NNENE

ENE

E

ESE

SE

SSE

S

SSW

SW

WSW

W

WNW

NWNNW

Fig. 9. Same as Fig. 8 but weighted by frequency of wind occurrence.

L.X. Zhou et al. / Atmospheric Environment 38 (2004) 7041–70547050

values and 10-year average (�2s) annual means (using

NOAA CMDL weekly CH4 data) from selected NH

sites (Waliguan included) from 1992–2001 are listed in

Table 1. At Waliguan, the range of CH4 monthly means

of 1746–1822 ppb and a 10-year average annual mean

value of 1786.7710.8 ppb was derived over the period of

January 1992–December 2001, the same 10-year period

as derived at other selected NH sites. By comparison

with ranges of CH4 monthly means at Alert (ALT),

Barrow (BRW), Mace Head (MHD), Ulaan Uul

(UUM), Niwot Ridge (NWR), Tae-ahn Peninsula

(TAP), Izana (IZO) and Mauna Loa (MLO), the

ARTICLE IN PRESSL.X. Zhou et al. / Atmospheric Environment 38 (2004) 7041–7054 7051

observed result at Waliguan was more inline with CH4

observations observed between 301 and 60 1N.

Table 2 lists annual means and annual increases of

atmospheric CH4 determined from NOAA CMDL air

sample measurements at selected NH sites (Waliguan

included) during 1992–2001. The annual means were

derived using data from within each calendar year

(January–December) using the NOAA CMDL weekly

CH4 air sample data. NOAA CMDL uses curve fits to

the weekly air sample data to calculate monthly and

annual means (Dlugokencky et al., 1994a). Methane at

Waliguan has increased on average (�2s) by

4.574.2 ppb yr�1 over the 10-year time period

(1992–2001) and is comparable to the range of CH4

annual increases observed at other sites located at

301–60 1N during the same time period. The annual

increases at Waliguan also show significant inter annual

variability. For example, a decrease of �1.9 ppb was

observed in 2000 and decreases of �1.1 ppb were

observed in 1992 and 1996. A sharp increase of

13.4 ppb was observed in 1993 and an increase of

12.2 ppb was observed in 1998. These large CH4

variations at Waliguan were also similar to variations

observed at Alert, Barrow, Niwot Ridge and Mauna

Loa.

Relevant studies (Dlugokencky et al., 1994a, b, 1998,

2001; Etherridge et al., 1998; Khalil and Rasmussen,

1993; WMO, 2003b) addressed that the increase in the

atmospheric burden of CH4 is due to human activities.

Large variations in the annual increases of CH4 have

been observed in recent decades. Mean annual increases

Table 2

Estimated atmospheric CH4 annual means and annual growths at W

Year CH4 (ppb) ALT BRW

1992 Annual mean 1802.8 1807.8

Growth �3.6 0

1993 Annual mean 1799.2 1807.8

Growth 10.9 14.7

1994 Annual mean 1810.1 1822.5

Growth 1.3 2.5

1995 Annual mean 1811.4 1825.0

Growth 2.5 �1.0

1996 Annual mean 1813.9 1824.0

Growth 0.3 1.6

1997 Annual mean 1814.2 1825.6

Growth 11.3 13.0

1998 Annual mean 1825.5 1838.6

Growth 6.1 0.8

1999 Annual mean 1831.6 1839.4

Growth �2.6 �2.3

2000 Annual mean 1829.0 1837.1

Growth 1.1 1.1

2001 Annual mean 1830.1 1838.2

1992–2001 (7 2s) Mean annual growths (ppb yr�1) 3.074.0 3.474.6

are lower in the 1990s compared to the 1980s. According

to literatures (Dlugokencky et al., 1994b, 2001; Ether-

idge et al., 1998; WMO, 2003b), the annual average

global increase was 11 ppb yr�1 for 1984–1990 and

5 ppb yr�1 from 1991 to 2001. For the period of

1991–2001, the maximum global annual increases were

observed in 1991 with a 13 ppb increase and in 1998 with

a 12 ppb increase. The largest decrease was observed

from 1991 to 1992 with an 11.5 ppb yr�1 decrease. These

large changes were attributed to the impact of the

eruption of Mt. Pinatubo in 1991 and an associated

change in global mean temperatures.

Large-scale variations in atmospheric CH4 are pri-

marily due to changes in the balance between sources

and sinks of CH4. It has been postulated that the overall

decrease in the CH4 growth value observed during the

1990s was due to global CH4 sources approaching a

steady-state relationship with the OH removal process

(Dlugokencky et al., 1998; Etherridge et al., 1998).

Overall, for 1992–2001, the CH4 growth value at

Waliguan is similar to that observed globally, but a

strong influence from continental Asian sources is also

seen. This further supports the importance and unique-

ness of the Waliguan CH4 data record in improving our

understanding of the distribution and magnitude of CH4

sources and sinks in Asia.

3.6.2. Monthly mean time series and mean annual cycle

A time series of monthly mean CH4 mixing ratios at

Waliguan from 1991 to 2001 is shown in Fig. 10. The

monthly means were derived using selected continuous

aliguan and selected NOAA CMDL NH sites, 1992–2001

MHD UUM NWR WLG TAP IZO MLO

1782.6 1792.9 1756.2 1765.4 1836.2 1757.5 1724.1

4.1 4.3 4.1 �1.1 16.5 3.4 3.0

1786.7 1797.2 1760.3 1764.3 1852.7 1760.9 1727.1

6.4 4.9 4.6 13.4 �15.5 �15 9.7

1793.1 1802.1 1764.9 1777.7 1837.2 1745.9 1736.8

�0.7 1.4 11.8 4.6 �15.5 9.0 3.6

1792.4 1803.5 1776.7 1782.3 1821.7 1754.9 1740.4

15.0 9.0 �0.9 3.8 8.3 7.7 0.7

1807.4 1812.5 1775.8 1786.1 1830.0 1762.6 1741.1

�3.8 �4.1 4.2 �1.1 1.3 �5.8 9.6

1803.6 1808.4 1780.0 1785.0 1831.3 1756.8 1750.7

8.3 14.1 6.9 5.2 22.0 17.9 5.0

1811.9 1822.5 1786.9 1790.2 1853.3 1774.7 1755.7

8.5 3.3 6.1 12.2 6.4 4.9 7.7

1820.4 1825.8 1793.0 1802.4 1859.7 1779.6 1763.4

�0.3 4.9 �3.0 5.2 �3.5 0.4 0.7

1820.1 1830.7 1790.0 1807.6 1856.2 1780.0 1764.1

2.2 1.4 2.6 �1.9 0 4.5 0.5

1822.3 1832.1 1792.6 1805.7 1856.2 1784.5 1764.6

4.474.4 4.473.8 4.073.3 4.574.2 2.279.7 3.077.0 4.572.8

ARTICLE IN PRESS

Jan Feb Mar Apr May June July Aug Sept Oct Nov Dec

Month

-40

-30

-20

-10

0

10

20

CH

4 (p

pb)

Fig. 11. Mean annual CH4 cycle at Waliguan from 1991 to

2001. The dots and curve fitting are derived by CH4 data for

each month in 1991–2001, with standard deviation as error bar.

The dashed line shows the MBL reference mean annual cycle

for the same period.

y = 4.8639x - 7926.4

R2 = 0.7864

1740

1750

1760

1770

1780

1790

1800

1810

1820

1830

1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002

Year

CH

4 (

pp

b)

Fig. 10. Monthly mean CH4 mixing ratios at Waliguan from 1991 to 2001. A 3-month running mean and a liner regression

(y=4.8639x�7926.4, R2=0.7864) was passed through all the monthly data values in order to illustrate the seasonal variation and long-

term trend more clearly.

L.X. Zhou et al. / Atmospheric Environment 38 (2004) 7041–70547052

and discrete air sample data. A total of 89 monthly mean

CH4 values (May 1991–December 2001) were derived

from the discrete samples and 52 monthly mean CH4

values were derived using continuous data. A compar-

ison of the overlapping discrete and continuous monthly

data records resulted in a mean difference (discrete

minus continuous, �2s) of (1.9372.57) ppb.

At NH sites, the phase and amplitude of the CH4

annual cycle exhibits interannual variability. The phase

and amplitude of the CH4 annual cycle also varies with

latitude (Dlugokencky et al., 1994a; Globalview-CH4,

2001). The mean annual CH4 cycle observed at

Waliguan is shown in Fig. 11, also shown is the

1992–2001 Marine Boundary Lever (MBL) reference

mean annual cycle (Globalview-CH4, 2001) for the

similar NH latitudinal band. The mean annual CH4

cycle at Waliguan was derived using the detrended

monthly mean values by subtracting a 12-month

running mean from each data value and then averaging

all the January values (independent of year), then all the

February values and so on. Average peak-to-peak

amplitudes of CH4 annual cycles observed at Waliguan

as well as other NH sites during the same 10-year time

period are listed in Table 3.

The CH4 annual seasonal cycle observed at all the

sites (except Waliguan) reach a minimum in summer and

a maximum in late fall to mid-winter. At Waliguan,

however, there is little seasonal variation. The mean

annual cycle is similar in both the summer and winter

periods, with two gentle peaks in summer and in

February along with two small valleys in early winter

and spring are observed at Waliguan. The seasonal cycle

at Waliguan shows interannual variation but for the

most part higher CH4 levels are observed in summer.

The Waliguan average CH4 peak-to-peak amplitude is

approximately 11 ppb, much smaller than the other NH

sites. Relative to the 1992–2001 MBL mean annual

cycle, the mean annual CH4 cycle at Waliguan in

summer was �30 ppb higher and �20 ppb lower in

winter. These differences are large and lend further

support to the uniqueness of this data set.

Mean annual cycles of atmospheric CH4 and inter-

annual variation in this cycle are a result of changes in

sources and sinks of atmospheric CH4 as well as

variations in transport (Dlugokencky et al., 1993, 1997;

Khalil, 1983). There are both large natural (i.e. wetland)

and anthropogenic (i.e. fossil fuels) sources of CH4. The

main sink of CH4 is the chemical destruction with the

hydroxyl (OH) radical in the troposphere, strongest

during the summer months. A previous study (Ma et al.,

2002) estimated Waliguan chemical budget of ozone in

January and July 1996 by using a box model constrained

by trace gases mixing ratios observed at the site. The

study however, suggested weak summer photochemical

capacity in both free troposphere and boundary layer air

ARTICLE IN PRESS

Table 3

Estimated atmospheric CH4 seasonal cycle amplitudes at Waliguan and selected NOAA CMDL NH sites, 1991–2001

Selected sites ALT BRW MHD UUM NWR WLG TAP IZO MLO

Latitude 82N 71N 53N 44N 40N 36N 36N 28N 19N

Longitude 62W 156W 10W 111E 105W 101E 126E 16W 155W

Altitude (m asl) 210 11 25 914 3475 3810 20 2360 3397

CH4 Amplitude (ppb) 53 43 33 28 30 11 25 31 29

L.X. Zhou et al. / Atmospheric Environment 38 (2004) 7041–7054 7053

masses conditions at Waliguan. In addition, Waliguan is

a remote high altitude site, ruminants’ emissions are a

short period summer activities at surroundings, and no

rice agriculture in arid and semi-arid western China. The

fact that CH4 seasonal variation of a summer minimum

and winter maximum were not seen at Waliguan suggests

that the area is subject to a CH4 source in summer, likely

attributed to ruminants at the high plateau meadow in

summer as well as subsequent air mass transport from

surrounding regions especially prevailing eastern winds

in summer.

4. Conclusions

The CMA continuous and NOAA CMDL discrete air

sample measurements of atmospheric CH4 at the Wali-

guan global baseline observatory are in good agreement.

The CH4 time series shows large episodic events lasting

many days during winter and summer. The events are due

to atmospheric transport from anthropogenic source

regions. The seasonal variation in horizontal and vertical

winds had a distinct impact on the observed CH4. High

horizontal wind speeds (greater than 10 m s�1) or calm

condition as well as large vertical wind speeds (greater

than 71m s�1) had the largest impact on the CH4.

Background (air originating from wind sectors

SW–WSW–W–WNW) and contaminated (winds from

sectors NE–ENE–E–ESE) conditions were categorized. A

background data selection procedure was developed

based on local surface winds. A selected hourly data set

representative of ‘‘baseline’’ conditions was derived with

approximately 50% of the valid hourly data. The range of

CH4 monthly mean distributions, annual means and

mean increase per year at Waliguan during the period of

1992–2001 were consistent with measurements from other

sites between 301 and 601N. The diurnal, seasonal and

year-to-year growth value characteristics of CH4 observed

at Waliguan, however, often showed unique patterns that

could be attributed to the combined influence of multiple

nearby CH4 sources, as well as air mass transport from

the surrounding regions. The Waliguan CH4 data likely

contain a strong Asian influence and may help refine our

knowledge of CH4 sources especially from within the

inland plateau of China.

Acknowledgments

We thank operators at the Waliguan Observatory for

their diligent efforts in operating discrete air sampling,

in situ CH4 analytical and meteorological equipments at

the remote high plateau. We also thank the WMO

Environment Division for coordinating the GAW

programme at Waliguan. An anonymous reviewer’s

valuable comments and suggestions are also appre-

ciated. This work was supported in part by the Japanese

Society for Promotion of Science (JSPS) post-doctorial

fellowship program (PB01736), the United Nation’s

GEF fund (GLO/91/G32), the CMA’s financial routine,

and the China Ministry of Science and Technology’s

research project (G99-A-07).

References

Blake, D.R., Mayer, E.W., Tyler, S.C., Makide, Y., Montague,

D.C., Rowland, F.S., 1982. Global increase in atmospheric

methane concentrations between 1978 and 1982. Geophy-

sical Research Letters 9, 477–480.

CMDL, 2002. Climate monitoring and diagnostic laboratory.

2000–2001 Summary Report, Boulder, CO, USA, NOAA/

CMDL No.26, pp. 28–50.

Conway, T.J., Steele, L.P., Novelli, P.C., 1993. Corre-

lations among atmospheric CO2, CH4 and CO in the

Arctic, March 1989. Atmospheric Environment 27A,

2881–2894.

Crutzen, P.J., 1995. On the role of CH4 in atmospheric

chemistry: sources, sinks and possible reductions in anthro-

pogenic sources. Ambio 24 (1), 52–55.

Dlugokencky, E.J., Harris, J.M., Chung, Y.S., Tans, P.P.,

Fung, I., 1993. The relationship between the methane

seasonal cycle and regional sources and sinks at Tae-ahn

Peninsula, Korea. Atmospheric Environment 27A (14),

2115–2120.

Dlugokencky, E.J., Steele, L.P., Lang, P.M., Masarie, K.A.,

1994a. The growth rate and distribution of atmospheric

methane. Journal of Geophysical Research 99 (D8),

17021–17043.

Dlugokencky, E.J., Masarie, K.A., Lang, P.M., Tans, P.P.,

Steele, L.P., Nisbet, E.G., 1994b. A dramatic decrease in the

growth rate of atmospheric methane in the Northern

Hemisphere during 1992. Geophysical Research Letters 21

(1), 45–48.

ARTICLE IN PRESSL.X. Zhou et al. / Atmospheric Environment 38 (2004) 7041–70547054

Dlugokencky, E.J., Steele, L.P., Lang, P.M., Masarie, K.A., 1995.

Atmospheric CH4 at Mauna Loa and Barrow Observatories:

presentation and analysis of in situ measurements. Journal of

Geophysical Research 100 (D11), 23103–23113.

Dlugokencky, E.J., Masarie, K.A., Tans, P.P., Conway, T.J.,

Xiong, X., 1997. Is the amplitude of the methane seasonal

cycle changing? Atmospheric Environment 31 (1), 21–26.

Dlugokencky, E.J., Masarie, K.A., Lang, P.M., Tans, P.P.,

1998. Continuing decline in the growth rate of the atmo-

spheric methane burden. Nature 393, 447–450.

Dlugokencky, E.J., Walter, B.P., Masarie, K.A., Lang, P.M.,

Kasischke, E.S., 2001. Measurements of an anomalous

global methane increase during 1998. Geophysical Research

Letters 28 (3), 499–502.

Etherridge, D.M., Steele, L.P., Francey, R.J., Langenfelds, L.,

1998. Atmospheric methane between 1000 A.D. and

present: evidence of anthropogenic emissions and climatic

variability. Journal of Geophysical Research 103 (D13),

15979–15993.

Fung, I., John, J., Lermer, J., Matthews, E., Prather, M., Steele,

L.P., Fraser, P.J., 1991. Three-dimentional model synthesis

of the global methane cycle. Journal of Geophysical

Research 96, 13033–13065.

Globalview-CH4, 2001. Cooperative Atmospheric Data Integra-

tion Project-Methane. NOAA/CMDL, Boulder, CO, USA.

Hansen, A.D.A., Conway, T.J., Steele, L.P., Bodhaine, B.A.,

Thoning, K.W., Tans, P.P., Novakov, T., 1989. Correlation

among combustion effluent species at Barrow, Alaska:

aerosol black carbon, carbon dioxide, and methane. Journal

of Atmospheric Chemistry 9, 283–299.

Harris, J.M., Dlugokencky, E.J., Oltmans, S.J., Tans, P.P.,

Conway, T.J., Novelli, P.C., Thoning, K.W., Kahl, J.D.W.,

2000. An interpretation of trace gas correlations during

Barrow, Alaska, winter dark periods, 1986–1997. Journal of

Geophysical Research 105 (D13), 17267–17278.

Hopper, J.F., Worthy, D.E.J., Barrie, L.A., Trivett, N.B.A.,

1994. Atmospheric observations of aerosol black carbon,

carbon dioxide, and methane in the high arctic. Atmo-

spheric Environment 28, 3047–3054.

Khalil, M.A.K., 1983. Sources, sinks and seasonal cycles of

atmospheric methane. Journal of Geophysical Research

(C9) 88, 5131–5144.

Khalil, M.A.K., Rasmussen, R.A., 1993. Decreasing trend of

methane: unpredictability of future concentration. Chemo-

sphere 26 (1–4), 803–814.

Lelieveld, J., Crutzen, P.J., Dentener, F.J., 1998. Changing

concentration, lifetime and climate forcing of atmospheric

methane. Tellus 50B (2), 128–150.

Levin, I., Glatzel, M.H., Marik, T., Cuntz, M., Schmidt, M.,

Worthy, D.E., 1999. Verification of German methane inven-

tories and their recent changes based on atmospheric observa-

tions. Journal of Geophysical Research 104, 3447–3456.

Ma, J.Z., Zhou, X.J., Tang, J., Zhang, X.S., 2002. Estimates of

the ozone budget for ozone at Waliguan Observatory.

Journal of Atmospheric Chemistry 41, 21–48.

Rasmussen, R.A., Khalil, M.A.K., 1981. Atmospheric

methane: trends and seasonal cycles. Journal of Geophysical

Research 86, 9826–9832.

Rasmussen, R.A., Khalil, M.A.K., 1984. Atmospheric methane

in the recent and ancient atmospheres: concentrations,

trends, and interhemispheric gradient. Journal of Geophy-

sical Research 89, 11599–11605.

Shipham, M.C., Bartlett, K.B., Crill, P.M., Harriss, R.C.,

Blaha, D., 1998. Atmospheric CH4 measurements in central

New England: an analysis of the long-term trend, seasonal

and diurnal cycles. Journal of Geophysical Research 103

(D9), 10621–10630.

Steele, L.P., Fraser, P.J., Rasmussen, R.A., Khalil, M.A.K.,

Conway, T.J., Crawford, A.J., Gammon, R.H., Masarie,

K.A., Thoning, K.W., 1987. The global distribution of

methane in the troposphere. Journal of Atmospheric

Chemistry 5 (2), 125–171.

Tang, J., Wen, Y.P., Zhou, L.X., 1999. Study of black carbon

aerosol in western China. Chinese Quarterly Journal of

Applied Meteorology 10 (2), 160–170.

Tohjima, Y., Machida, T., Utiyama, M., Katsumoto, M.,

Fujinuma, Y., 2002. Analysis and presentation of in situ

atmospheric methane measurements from Cape Ochi-ishi

and Hateruma Island. Journal of Geophysical Research

107, ACH X-1–10.

WMO, 1997. Report of the 9th WMO meeting of experts on

CO2 concentration and related tracer measurement techni-

ques. No.132, Victoria, Australia.

WMO, 2001. Strategy for the implementation of the Global

Atmosphere Watch Programme (2001–2007), a contribution

to the implementation of the WMO long-term plans.

No.142, Geneva, Switzerland.

WMO, 2003a. Report of the 11th WMO/IAEA meeting of

experts on CO2 concentration and related tracer measure-

ment techniques. No. 148, Tokyo, Japan.

WMO, 2003b. World Data Center for Greenhouse Gases

(WDCGG) data summary. WDCGG No. 27, Tokyo,

Japan.

Worthy, D.E.J., Trivett, N.B.A., Hopper, J.F., Bottenheim,

J.W., Levin, I., 1994. Analysis of long range transport

events at Alert, N.W.T., during the Polar Sunrise Experi-

ment. Journal of Geophysical Research 99 (D12),

25329–25344.

Worthy, D.E.J., Levin, I., Kuhlmann, A.J., Hopper, J.F.,

Ernst, M.K., 1998. Seven years of continuous methane

observations at a remote boreal site in Ontario, Canada.

Journal of Geophysical Research 103 (D13), 5995–6007.

Zhou, L.X., 2001. Study on the background characteristics of

major greenhouse gases over continental China. Ph.D.

Thesis, Peking University, China.

Zhou, L.X., Tang, J., Zhang, X.C., Worthy, D.E.J., Emst, M.,

Trivett, N.B.A., 1998. In-situ gas chromatographic mea-

surement of atmospheric methane and carbon dioxide. Acta

Scientiae Circumstantiae 18, 356–361.

Zhou, L.X., Tang, J., Ernst, M., Worthy, D.E.J., 2001.

Continuous measurement of baseline atmospheric CO in

western China. Environmental Science 22, 1–5.

Zhou, L.X., Tang, J., Wen, Y.P., Li, J.L., Yan, P., Zhang, X.C.,

2003. The impact of local winds and long-range transport

on the continuous carbon dioxide record at Mount

Waliguan, China. Tellus 55B, 145–158.