The World Meteorological Organisation (WMO) established the Global Atmosphere Watch (GAW) Programme in 1989. The scientific goals of GAW relate to investigating the role of atmospheric chemistry in global climate change, and include: understanding the complex mechanisms with respect to natural and anthropogenic atmospheric change; and improving the understanding of interactions between the atmosphere, ocean, and biosphere.American Meteorological Society; Stockholm Universit

Chambers, SD

Crawford, J

Kang, CH

Werczynski, S

Williams, AG

Zahorowski, W

English

ANSTO Publications Online

P2.5          INFLUENCE OF TURBULENT MIXING AND AIR CIRCULATION IN THE LOWER 
ATMOSPHERE ON FETCH AREAS OF SELECTED WMO ATMOSPHERE 
WATCH BASELINE AIR POLLUTION STATIONS 
 
 
W. Zahorowski1, S. Chambers1, C-H. Kang2, J. Crawford1, S. Werczynski1 and A.G. Williams1
1 Australian Nuclear Science and Technology Organisation, PMB 1, Menai, NSW 2234, Australia 
2 Department of Chemistry, Cheju National University, Ara-Dong 1, Jeju 690-756, Republic of Korea 
 
 
1. INTRODUCTION 
 
The World Meteorological Organisation 
(WMO) established the Global Atmosphere 
Watch (GAW) Programme in 1989. The 
scientific goals of GAW relate to investigating 
the role of atmospheric chemistry in global 
climate change, and include: understanding the 
complex mechanisms with respect to natural 
and anthropogenic atmospheric change; and 
improving the understanding of interactions 
between the atmosphere, ocean, and 
biosphere. 
 
At present there are 24 “global” stations in 
the GAW network, tasked with the mission to 
make reliable, comprehensive observations of 
the chemical composition and selected physical 
characteristics of the atmosphere on global and 
regional scales. With a view to characterising 
long term (decadal +) trends in the baseline 
composition of the global atmosphere, global 
GAW stations are situated in remote locations 
and considered representative of large 
geographic fetch areas. The network of global 
stations is supplemented by numerous 
“regional” and “contributing” stations worldwide.   
 
When interpreting results from these 
stations it is important first to consider how 
seasonally changing fetch regions and local 
mixing affect long-term observations at each 
site, and how comparable observations are 
between stations in the network in a global 
context. 
 
The combined influence of terrestrial 
source distribution, species’ entrainment into 
air parcels analysed at the stations, and the 
parcels’ transport in the lower atmosphere, 
result in station-specific geographic areas 
(fetch regions) which are over- or under-
represented in station records. Hence, 
interpreting species’ variability requires, at 
least, that extremes of fetch areas be 
independently delineated and their temporal 
variability understood and documented. To 
these ends, the aim of this study is to compare 
and contrast seasonal changes in local mixing 
and regional-scale air mass fetch between 
three GAW stations (two global and one 
regional) using hourly radon concentration data 
collected between Jan-2002 and Dec-2006. 
 
Mauna Loa Observatory (MLO; Hawaii), 
and Cape Grim Baseline Air Pollution Station, 
(CGBAPS; Tasmania, Australia), are well 
established global GAW stations, and Gosan 
Station (Jeju Island, South Korea) is a regional 
GAW station that is playing an increasingly 
important role in the investigation of natural and 
anthropogenic pollution emissions from East 
Asia. 
 
2. METHODS 
 
Irrespective of whether it is natural or 
anthropogenic, most atmospheric pollution has 
terrestrial sources. Due to the transient nature 
and often complex source-sink mechanisms of 
many aerosols and other atmospheric 
constituents, we chose to use the passive 
tracer radon-222 – an unambiguous indicator of 
terrestrial influence on an air mass – to 
investigate fetch regions and local mixing at 
each site. 
 
Radon is a naturally occurring, 
radioactive, noble gas, with a low solubility in 
water. It is emitted from terrestrial surfaces at 
an approximately constant rate on diurnal 
timescales and the emissions are assumed to 
be uniform on local to regional scales. 
Terrestrial emissions of radon are at least two 
orders of magnitude higher than oceanic 
emissions, enabling independent differentiation 
between air parcels predominantly influenced 
by land or water. The half-life of radon (3.8-
days) is comparable to the lifetimes of many 
short-lived atmospheric species and is optimum 
for local mixing studies, since it is long 
compared with typical turbulent timescales (≤1-
hour), but short enough to constrain the 
concentration of radon in the free troposphere 
to be 1-3 orders of magnitude lower than near 
surface values. The combination of these 
properties makes radon an excellent tracer for 
atmospheric transport and mixing studies. 
 
Atmospheric radon concentration is 
measured at each site using 1500 L dual flow 
loop, two filter detectors (Zahorowski et al., 
2005); although at CGBAPS a 5000 L detector 
is also in operation. These kinds of detectors 
are designed for multi-year deployment and 
provide continuous hourly radon concentration 
data. They have a 45 minute response time 
with a lower limit of detection of 10 and 40 
mBq m3 for the 5000 L and 1500 L models, 
respectively. 
 
We used seasonal plots of diurnal 
composite radon concentration at each site to 
investigate the influence of local mixing and 
topography on routine measurements. In 
addition we used radon concentrations in 
conjunction with back-trajectory analyses to 
characterise seasonal changes in fetch region 
for each site instead of wind sector analysis, 
since the direction of an air mass’ final 
approach to a site was often not representative 
of its long-term fetch. 
 
10-day back trajectories were generated 
hourly for each site using the PC version of 
NOAA Air Resources Laboratory’s (ARL) 
HYbrid Single-Particle Lagrangian Integrated 
Trajectory model HYSPLIT [version 4.6; 
Draxler and Hess, 1998] and a Visual Basic 
batch processing tool. The resulting collection 
of trajectories was incorporated into a database 
along with the calibrated hourly radon 
concentrations. Groups of trajectories 
delineating fetch regions corresponding to 
extremes of terrestrial influence were then 
selected by season based on corresponding 
radon concentrations (≥90th percentile: high 
events; ≤10th percentile: low events).  
 
3. RESULTS AND DISCUSSION 
 
3.1 Cape Grim Baseline Air Pollution Station 
 
Located at 90m above sea level (asl) on 
an exposed bluff on the northwest coast of 
Tasmania, and more than 30 km from the 
nearest significant township, Cape Grim is an 
ideal site for observing long term changes in 
the baseline global atmosphere. South of 
Tasmania the Southern Ocean offers 
uninterrupted oceanic fetch for air mass transit 
times in excess of two weeks. The site has 
three main fetch regions, which are broadly 
categorised into the following wind direction 
sectors: oceanic, 190 ≤ θ < 280; mainland 
Australia, 280 ≤ θ < 90; Tasmania, 90 ≤ θ < 
190). Since changes in air mass origin between 
these fetch regions are driven synoptically 
(timescale 1-2 weeks), as opposed to changes 
in regional or seasonal circulation patterns, the 
oceanic fetch region is well represented in 
measurements throughout all months of the 
year. 
On an annual average basis almost half of 
all air masses arriving at Cape Grim have 
experienced long-term oceanic fetch and would 
thus be suitable for characterising baseline 
atmospheric conditions for the Southern 
Hemisphere. 
 
The frequent occurrence of oceanic fetch 
conditions at Cape Grim has a strongly 
stabilising effect on the depth of the 
atmospheric boundary layer. Consequently, 
only a weak concentration / dilution signal for 
atmospheric constituents was found as a result 
in diurnal changes in mixing depth. A 
composite plot based on all hourly observations 
indicated a weak diurnal signal (amplitude 0.06 
Bq m-3) in the CGBAPS radon record. This 
signal was characterised by a broad morning 
maximum and an afternoon minimum. 
Repeating the analysis using observations 
made only during times of oceanic fetch, when 
baseline monitoring occurs, the amplitude of 
the diurnal cycle became negligible (0.01 Bq 
m-3). Therefore baseline sampling at CGBAPS 
during oceanic fetch conditions can be 
performed independently of the time of day. 
 
Due to the nature of synoptic systems that 
pass Cape Grim, after a wind direction change 
to the oceanic sector radon concentrations 
typically decrease for the first 12-24 hours to a 
relatively stable value representative of the 
baseline atmosphere. Usually, after 4-5 days in 
the oceanic sector, radon concentrations then 
begin to rise once more preceding a change in 
fetch sector. Consequently, air masses which 
persist in the oceanic sector for 2-4 days 
(inclusive) are referred to as “deep” oceanic 
events and are most suited to baseline 
sampling. 
 
The 10-day fetch region for deep oceanic 
events covers a large portion of the southern 
Indian Ocean, extending 8000-10,000 km 
WSW of Cape Grim (Figure 1). Very little 
seasonal variability was observed in the shape 
or extent of this oceanic footprint for deep 
oceanic events. 
 
Long (oE)
La
t(
o S
)
40 60 80 100 120 140 160
0
20
40
60
0 24 48 96 192 384
 
Figure 1: 1°x1° 
resolution 
back 
trajectory 
density 
function for 
CGBAPS deep 
oceanic 
events. 
 Long (o)
La
t(
o )
120 130 140 150 160
-50
-40
-30
-20
n
1089
990
891
792
693
594
495
396
297
198
99
30
0
 
Figure 2: 1°x1° resolution back trajectory 
density function of CGBAPS Mainland 
sector observations. 
 
Non-oceanic events, mainland Australian 
events in particular, can be used to 
characterise anthropogenic emissions and to 
estimate emissions inventories of climatically 
active gases from southern Australia (Figure 2). 
Consequently the size and seasonal evolution 
of the CGBAPS terrestrial footprint is of 
particular interest. The portion of mainland 
Australia represented by mainland events is 
limited primarily to the Victorian region (Figure 
2). A strong seasonal variability in the CGBAPS 
terrestrial footprint was observed (Figure 3), 
such that only from May through August of 
each year do mainland events spend sufficient 
time over land to be considered representative 
of regional terrestrial emissions. 
 
0
10
20
30
40
1 2 3 4 5 6 7 8 9 10 11 12
Month of composite year
Ti
m
e 
ov
er
 la
nd
 (%
)
   Tasmania
   Mainland
 
Figure 3: Monthly mean fraction of 10-day 
trajectories corresponding to Mainland 
sector events that were in contact  
with land. 
 
3.2 Mauna Loa Observatory 
 
Mauna Loa Observatory is situated on the 
northern slope of the Mauna Loa volcano, 
Hawaii, in the central eastern Pacific Basin. At 
an elevation of 3,394 m asl it is not subjected to 
the effects of changing atmospheric mixing 
depths in the same sense as most low lying 
stations, and provides an opportunity for direct 
sampling of air from the northern hemisphere’s 
lower troposphere. On the windward side of 
Hawaii the undisturbed oceanic fetch extends 
6000-8000 km. At face value the remote 
location of this site, as well as minimal 
influence of vegetation and human activity, 
provide an ideal setting for monitoring 
constituents of the global atmosphere that are 
implicated in climate change. However, despite 
the setting, the suitability of MLO as a platform 
to observe long-term changes in the baseline 
global atmosphere is strongly time dependent. 
 
The composite diurnal MLO radon 
concentration exhibits a pronounced diurnal 
signal characterised by a broad early morning 
minimum and sharp mid-afternoon maximum 
(Figure 4). While the amplitude of this signal is 
small (0.08 Bq m-3) it is almost 180º out of 
phase with diurnal signals of atmospheric 
constituents at lower elevation sites. The 
nature of this signal is attributable to local 
topographic effects, specifically, the anabatic 
and katabatic winds on the slopes of the 
Mauna Loa volcano. During the day, anabatic 
winds (flowing up the face of the volcano) 
contaminate MLO observations with radon and 
other trace species emitted locally from the 
island. At night katabatic flow dominates. Cool 
air flows down the face of the volcano, drawing 
tropospheric air to the site. Only under these 
conditions (ie. during the nocturnal sampling 
window shown in Figure 4), are observations 
representative of tropospheric conditions. 
100
125
150
175
200
Hour of day
R
ad
on
 (m
B
q 
m
-3
)
6 1812
 
Figure 4: Diurnal composite radon 
concentration at MLO. Hashed bar indicates 
nocturnal tropospheric sampling window. 
 
In addition to the diurnal restrictions on 
tropospheric sampling at MLO, air mass fetch 
analysis indicated large seasonal changes in 
the footprint of sampled tropospheric air (Figure 
5 a,b). In the northern hemisphere summer 
fetch regions were constrained almost entirely 
within the Pacific Basin. Under these 
conditions, and within the nocturnal sampling 
window, observations would be representative 
of northern hemisphere baseline 
concentrations. However, at other times of the 
year – particularly during the winter/spring 
transition and mid autumn when the 
tropospheric jet stream is positioned near 30ºN 
– fetch regions for tropospheric air can reach 
deep into continental Asia. When assisted by 
the jet stream, air with a strong terrestrial 
signature (that has been lifted by cold fronts or 
deep convective processes over Asia), can 
reach MLO in less than a week. Under these 
conditions, even during the suggested 
nocturnal sampling window, measurements 
may not be representative of global baseline 
values. An inspection of the corresponding 
radon concentrations would be required to 
determine the degree of terrestrial influence. 
Furthermore, depending on the position of the 
jet stream, fetch regions within continental Asia 
can shift from the south (dominated by 
anthropogenic pollution sources) to central Asia 
(dominated by natural sources of aerosols). 
 
 
0
20
40
60
La
tit
ud
e 
(0 N
)
 
0
20
40
60
Longitude (0)
La
tit
ud
e 
(0 N
)
120E 150E 180 150W
 
Figure 5: Example of 
typical Asian 
continental and 
Pacific Basin fetch 
regions for MLO 
tropospheric 
samples. 
 
3.3 Gosan Station 
 
The Gosan Station radon detector is 
situated on a bluff at 50 m asl on the west 
coast of Jeju Island, South Korea. The Chinese 
mainland lies approximately 500 km to the west 
across the Yellow Sea, the Korean Peninsula 
approximately 100 km to the north, and Japan 
approximately 300 km to the east. Beyond Jeju 
Island the only open oceanic fetch to this site is 
from the south to southeast across the East 
China Sea into the Pacific Ocean.  The site lies 
near the latitudinal centre of observed winter-
monsoon pollution outflow events from 
continental Asia and was a focal point for a 
large proportion of the international research 
effort during the first Intensive Operation Period 
of the Asian Aerosol Characterisation 
Experiment (ACE-Asia; Huebert et al., 2003). 
 
A marked diurnal signal (amplitude 0.85 
Bq m-3) is evident in the composite annual 
radon record for this site. This signal is 
characterised by a brief morning maximum and 
a broad afternoon minimum, consistent with a 
significant terrestrial influence on the local 
atmospheric boundary layer. When considered 
by season (Figure 6) the range in amplitude of 
the diurnal signal is from 0.44 Bq m-3 in winter 
(when the most recent fetch is over the Yellow 
Sea) to 1.39 Bq m-3 in summer (when the most 
recent fetch of Pacific Ocean air masses is 
terrestrial, 20 to 60 km across Jeju Island). 
 
Gosan Station is a regional (not global), 
GAW network site. Based on the magnitude 
and nature of the diurnal cycle at Gosan, 
measurements in the early to mid-afternoon 
(when atmospheric boundary layer depths are 
maximised) would be most representative of 
regional atmospheric conditions of the 
prevailing fetch region. 
 
0
1000
2000
3000
4000
5000
0 4 8 12 16 20
Hour of composite day
R
ad
on
 (m
B
q 
m
-3
)
Winter Spring
Summer Autumn
 
Figure 6: Diurnal composite plots by season 
at Gosan Station. 
 
From mid-August, through winter, to mid-
May, air mass fetch to Gosan Station is 
primarily off the Asian continent. For the 
remainder of the year fetch is primarily oceanic 
(Figure 7). Only for a short time (4-6 weeks of 
each year in summer) is the station fetch 
almost exclusively from the Pacific Ocean via 
the East China Sea. This is the sole window of 
opportunity for measurements at Gosan Station 
to be representative of near-baseline conditions 
for the northern hemisphere. For the months 
when fetch is primarily from the Asian continent 
the regional signal observed at Gosan Station 
is of mixed origins, representing the Korean 
Peninsular, northeast China, Mongolia and 
Siberia. Briefly (for 2-3 weeks in spring) the 
site’s fetch region shifts to the west (eastern 
and central China). 
 
100 120 140 160
10
20
30
40
50
60
La
tit
ud
e 
(o
N
)
Longitude (oE)
 Winter
 Summer
 
Figure 7: Examples of Gosan Station fetch 
regions for winter terrestrial fetch (blue)  
and summer oceanic fetch (red). 
 
4. CONCLUSIONS 
 
The Cape Grim Baseline Air Pollution 
Station is an ideal global GAW station. There 
are minimal anthropogenic sources of pollution 
nearby and almost half of the hourly 
observations each year represent conditions 
over an extensive unperturbed fetch of the 
Southern Ocean that shows little seasonal 
variability in extent or location. Events 
occurring within deep oceanic conditions can 
be sampled independently of time of day, and 
deep oceanic conditions are statistically well 
represented throughout all months of the year. 
 
The Mauna Loa Observatory is ideally 
situated to sample directly from the lower 
troposphere of the northern hemisphere. 
However, anabatic winds associated with the 
local steep topography limit sampling of 
unperturbed tropospheric air to an 8-hour 
temporal window of each day. Furthermore, for 
more than half of each year the fetch of 
tropospheric air masses reaches well into 
continental Asian. This is of particular concern 
to baseline sampling efforts during the 
winter/spring transition, and in mid-autumn, 
when the combination of frontal uplift / deep 
convection over Asia and the tropospheric jet 
stream, transport air with a strong terrestrial 
signature to MLO in less than a week. 
Observations of northern hemisphere baseline 
conditions are best made at MLO during the 
northern hemisphere summer within the 
nocturnal sampling window. Or other times of 
the year when radon observations indicate 
minimal terrestrial influence. 
 
The Gosan Station exhibits the greatest 
terrestrial influence of the three sites in this 
study. Consequently, observations at this site 
would be most representative of the regional 
atmosphere when mixing depth is maximised 
(early to mid afternoon). There is a narrow 
window of opportunity each summer for quasi-
baseline sampling at this site. For the 
remainder of the year fetch regions for this site 
are in continental Asia, primarily northeast 
China, and around the Mongolia-Siberian 
border. Unfortunately, terrestrial fetch regions 
for this site lie close to, or beyond, the upper 
latitude threshold of regions thought to 
contribute significantly to Asian pollution 
outflow events across the Pacific Basin. 
 
5. REFERENCES 
 
Draxler, R. R. and Hess, G. D. 1998. An 
overview of the HYSPLIT 4 modelling system 
for trajectories, dispersion and deposition. 
Australian Meteorological Magazine, 47, 295-
308.  
 
Huebert, B. J., Bates, T. Russell, P. B., 
Shi, G., Kim, Y. J., Kawamura, K., et al. 2003. 
An overview of ACE-Asia: strategies for 
quantifying the relationships between Asian 
aerosols and their climatic impacts. J. 
Geophys. Res., 108 (D23), 8633-8633. 
 
Zahorowski, W., S. Chambers, T. Wang, 
C.-H. Kang, I. Uno, S. Poon, S.-N. Oh, S. 
Werczynski, J. Kim and A. Henderson-Sellers. 
Radon-222 in boundary layer and free 
tropospheric continental outflow events at three 
ACE-Asia sites. Tellus, 57B, 124-140, 2005. 
 


Influence of turbulent mixing and air circulation in the lower atmosphere on fetch areas of selected WMO Global Atmosphere Watch baseline air pollution stations.

http://apo.ansto.gov.au/dspace/bitstream/10238/2852/2/139979.pdf

Influence of turbulent mixing and air circulation in the lower atmosphere on fetch areas of selected WMO Global Atmosphere Watch baseline air pollution stations.

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