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Measurement and Analysis of a Multiday Photochemical Smog Episode in the
Pearl River Delta of China
TAO WANG
AND JOEY
Y. H. KWOK
Regional Air Monitoring and Research Group, Department of Civil and Structural Engineering, The Hong Kong Polytechnic University,
Kowloon, Hong Kong, China
(Manuscript received 12 February 2002, in final form 16 September 2002)
ABSTRACT
Recent measurements of a photochemical episode in September of 2001 in the Pearl River delta (PRD) were
analyzed to gain insight into the meteorological and chemical processes affecting ozone (O 3 ) concentrations in
the subtropical southern China coast. High concentrations (.120 ppbv) of O 3 were observed at a rural coastal
site in western Hong Kong for six consecutive days, with maximum 1-h O 3 concentration reaching 191 ppbv
and visibility decreasing to 1.8 km. Comparison with O 3 data obtained from six other sites in the region indicated
the regional nature of the O 3 pollution. Examination of synoptic charts showed that this unusually severe and
prolonged pollution episode was induced by a quasi-stationary tropical cyclone in the East China Sea that caused
air subsidence and stagnation over the PRD. Weak northerly winds were observed from radiosonde and at a
mountaintop site, but surface winds showed a complex pattern owing to land–sea breezes and the topography
effects. The measurements of O 3 , carbon monoxide (CO), sulfur dioxide (SO 2 ), nitric oxide (NO), and total
reactive nitrogen (NO y ) at the western Hong Kong site were analyzed to show the possible sources and emission
characteristics of O 3-laden plumes. The daytime high concentrations of O 3 and other pollutants were caused by
the diffusion/advection of urban plumes under light north-northeast winds; and their reduced concentrations in
the late afternoon were due to the stronger sea breezes. The large values of CO/NO y and SO 2 /NO y on some
days implied the contribution of regional emissions to the high O 3 in western Hong Kong. The data from the
western site were compared with those from an eastern site to illustrate the spatial variability of air pollutants
in the coastal environment of the study region.
1. Introduction
Photochemical smog is characterized by the formation of high concentrations of oxidants and aerosols in
the atmosphere by chemical reactions involving oxides
of nitrogen (NO x ), carbon monoxides (CO), volatile organic compounds (VOCs), and sunlight. Ozone (O 3 ) is
the most important photochemical oxidant and, at high
concentrations, has adverse effects on human health,
agricultural crops, and forests (NRC 1991). Gaseous
nitrogen dioxide and aerosols formed by organics can
cause significant reduction in visibility during a severe
smog episode. Studies have shown that meteorological
factors, such as sunlight, vertical mixing, temperature,
and wind can strongly influence the chemical formation
of O 3 , transport of O 3 and the precursor pollutants, and
evaporative emissions of VOCs (NRC 1991). Studies
have also indicated that O 3 chemistry and the effects of
meteorological conditions can differ from one place to
another, depending on the characteristics of emission
Corresponding author address: Dr. T. Wang, Dept. of Civil and
Structural Engineering, The Hong Kong Polytechnic University,
Hung Hom, Kowloon, Hong Kong, China.
E-mail: [email protected]
q 2003 American Meteorological Society
and climate in a geographical region of interest. Most
of the previous large-scale studies of O 3 were conducted
in North America and Europe, and there have been far
fewer such investigations on the Chinese subcontinent.
The Pearl River delta (PRD) is situated on the southern China coast and is a home to several large Chinese
cities, including Hong Kong. This region has experienced astonishing economic and industrial developments in the past two decades. The large emission of
NO x and VOCs, coupled with the subtropical climate,
has resulted in high concentrations of ground-level O 3 .
Several studies of O 3 pollution in Hong Kong have been
carried out previously. Wang et al. (1998) reported the
observations of O 3 episodes in 1994 made at an eastern
coastal site and found that the highly variable ozone
concentrations were correlated with the reversals of
wind directions. Kok et al. (1997) presented the measurement results obtained in the autumn of 1994 aboard
an aircraft and showed increased ozone concentrations
in the western and northwestern sectors and a ‘‘convergence’’ pattern of wind flow around Hong Kong.
Wang et al. (2001) analyzed surface O 3 data collected
in 1996 from five sites and showed again higher ozone
concentrations in the western part of Hong Kong. That
study also indicated an apparent association of higher
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405
FIG. 1. Map of the Pearl River delta showing Tai O and other air-quality and wind-monitoring
stations whose data were used in the study.
ozone with changes of surface wind direction in western
Hong Kong. These previous investigations also showed
some evidence suggesting transport of regional pollution.
To understand better the causes of ozone pollution
previously found in western Hong Kong, an enhanced
chemical measurement site was established in the summer of 2001 in a rural/coastal area on Lantau Island in
Hong Kong. In the beginning of the field study, a photochemical episode was observed. This episode was different from the previously reported cases in that the
recent episode showed a higher hourly O 3 (191 ppbv)
and lasted for six consecutive days with hourly O 3 exceeding the U.S. National Ambient Air Quality Standard
(NAAQS) of 120 ppbv. In addition, very poor visibility
(visual range ,2.0 km) was recorded in Hong Kong
during the episode. In this study, we analyze O 3 and
other chemical data from the above site, as well as from
other locations in the region, to understand better 1) the
regional characteristics of O 3 pollution in the southern
China coastal region, 2) the relation of O 3 pollution to
complex coastal wind flow, and 3) the sources and emission characteristics of O 3-laden plumes. Another objective of this study is to gather an integrated dataset and
interpretations against which detailed modeling studies
can be performed and compared. We will show O 3 data
collected from seven sites in the southern part of the
PRD. We will present the measurements of O 3 , CO,
nitric oxide (NO), total reactive nitrogen (NO y ), and
sulfur dioxide (SO 2 ) at Tai O in western Hong Kong
and compare them with the results obtained at an eastern
coastal site.
2. Experiment
a. The Tai O study site
Tai O is a sparsely populated coastal area on Lantau
Island, situated roughly in the north–south centerline of
the Pearl River estuary with Hong Kong’s urban center
32 km to the east and Macau/Zhuhai, China, to the west
at about the same distance (Fig. 1). The study site is
located on a hill 80 m above sea level, overlooking the
Pearl River estuary to the west and north and the South
China Sea to the south. Local emissions from Tai O are
small because of the sparse population and the light
traffic to the area. Major sources of emissions from traffic and power plants in the region are located in the
east, north, and southwest directions.
The three largest population centers in the PRD are
Hong Kong (population: 6.7 million) and Shenzhen
(population: 4 million) in the southern part of the Pearl
River delta and Guangzhou (population: 10 million) in
the north. There are a number of midsized cities in the
region with populations ranging from 280 000 to
430 000. The region’s coal-fired power plants, airports,
and seaports are mainly located along the two sides of
the Pearl River.
The PRD region has a complex terrain. The Nan Ling
range separates the PRD in the north from the rest of
China, and the areas that surround the delta on the western and eastern sides are also mountainous. The complex
topography is particularly evident in Hong Kong, whose
land area is covered with 70% mountains, with the highest peak [Tai Mo Shan (TMS)] at 957 m above the sea
level. Victoria Harbor separates Hong Kong Island and
the Kowloon Peninsula. There are a number of outlying
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islands in Hong Kong, with Lantau being the largest.
Under the conditions of light regional winds, sea–land
breezes are frequently observed around Hong Kong
(Yeung et al. 1991; Zhang and Zhang 1997; Liu and
Chan 2001).
b. Measurement techniques
Measurement instruments were housed in a laboratory situated on a cliff. Ambient air samples were drawn
through a 10-m-long perfluoroalkoxy (PFA) Teflon tube
(outside diameter: 12.7 mm; inside diameter: 9.6 mm).
The inlet of the sampling tube was located 3 m above
the rooftop of the laboratory. The other end of the sample tube was connected to a PFA-made manifold with
a bypass pump drawing air at a rate of 15 L min 21 . The
intake of the analyzers for O 3 , CO, SO 2 , and NO was
connected to the manifold; the NO y channel used a separate Teflon line (outside diameter: 6.4 mm), which was
connected to an enclosure placed outside at about 1.5
m above the rooftop.
The analyzers for measuring O 3 , CO, and SO 2 had
been previously used in field studies at a rural site in
eastern China (Wang et al. 2002). A brief description
is given below.
Ozone was measured using a commercial UV photometric instrument [Thermo Environmental Instruments, Inc. (TEI), model 49] that had a detection limit
of 2 ppbv and a 2-sigma (2 s) precision of 2 ppbv for
a 2-min average. Sulfur dioxide was measured by pulsed
UV fluorescence (TEI model 43S), with a detection limit
of 0.06 ppbv and 2-s precision of 3% for ambient levels
of 10 ppbv (2-min average). The uncertainty was estimated to be about 9%. Carbon monoxide was measured
with a gas filter correlation, nondispersive infrared analyzer (Advanced Pollution Instrumentation, Inc., model
300) with a heated catalytic scrubber (as purchased) to
convert CO to carbon dioxide for baseline determination. Tests showed that nearly 100% of the water vapor
was able to pass through the converter, although it could
take a few minutes for the signal to reach an equilibrium.
In our study, zeroing was conducted every 2 h, each
lasting 12 min. The 2-min data at the end of each zeroing
were taken as the baseline. The detection limit was 30
ppbv for a 2-min average. The 2-s precision was about
1% for a level of 500 ppbv (2-min average), and the
overall uncertainty was estimated to be 10%. Both NO
and NO y were detected with a modified commercial molybdenum oxide (MoO)/chemiluminescence analyzer
(TEI, model 42S). The modification was made to relocate the internal catalytic converter to a separate enclosure that was placed near the sample inlet of NO y
(located at about 1.5 m above the rooftop) to reduce the
loss of NO y in the sample line prior to the catalytic
converter. The enclosure also housed a temperature controller (for the MoO catalyst) and solenoid valves for
zero and span tests; NO y was converted to NO on the
surface of MoO maintained at 3508C, which was sub-
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sequently measured by the chemiluminescent detector.
The instrument automatically switched among zero, NO,
and NO y mode. A bypass pump was installed in the NO y
line to maintain a flow of 1.5 L min 21 through the converter when the instrument was in the NO mode. The
analyzer had a detection limit of 0.05 ppbv. The 2-s
precision of this instrument was 4% (for NO 5 10
ppbv), and the uncertainty was about 10%.
The analyzers were calibrated by injecting scrubbed
ambient air (TEI, model 111) and a span gas mixture.
A National Institute of Standards and Technology
(NIST) traceable standard (Scott-Marrin, Inc.) containing 156.5 ppmv CO (62%), 15.64 ppmv SO 2 (62%),
and 15.55 ppmv NO (62%) was diluted using a dynamic
calibrator (Environics, Inc., model 6100). The NO y conversion efficiency on MoO was checked using a 5.34ppm N–propylnitrate standard (Scott-Marrin, Inc.). A
datalogger (Environmental Systems Corporation, model
8800) was used to control the zero/span calibration and
to collect 1-s data, which were averaged to 1-min values.
c. Other data sources
The Hong Kong Observatory (HKO) operates a network of ground-based weather stations over the Hong
Kong territory and an upper-air station at King’s Park
in Kowloon. In this study, we obtained and used radiosonde and surface wind data from the selected sites in
the above network. We also used visibility data recorded
by trained HKO observers at the Hong Kong International Airport, Lantau, Hong Kong, which is located 6
km northeast of the Tai O site. In addition, O 3 data
collected at several stations of the Hong Kong Environmental Protection Department (HKEPD) Air Quality Monitoring Network were utilized, together with O 3
and surface wind data from Taipa Granda, Macau, to
show a larger spatial picture on ozone and wind flow.
We also made use of chemical measurements at our
Cape D’Aguilar (Hok Tsui) research station, which is
located on the southeast coast of Hong Kong Island (Fig.
1). The reader is referred to Wang et al. (1998) for a
detailed description of the site. The measurement techniques used at this station were identical or similar to
those employed at Tai O.
3. Results and discussion
a. Observations of smog episode
The pollution event was observed during 14–19 September 2001. Figure 2 shows the hourly O 3 at Tai O
and the inverse of visual range (km 21 ) obtained at the
airport. The O 3 concentrations exceeded the NAAQS of
120 ppbv or the Hong Kong Air Quality Objective of
122 ppbv on each day and reached 180 and 191 ppbv
on 15 and 19 September, respectively. Both are the highest values that have been reported so far in Hong Kong.
As shown in the section below, O 3 concentrations at Tai
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FIG. 2. Hourly O 3 concentrations at Tai O and the inverse of visual range observed at the
Hong Kong International Airport between 14 and 19 Sep 2001.
O were the highest in comparison with those recorded
during the same period at other locations in Hong Kong
and Macau. In addition to the high O 3 concentrations,
very poor visibility was observed. The hourly visual
range dropped to below 8 km for each day and to as
low as 1.8 and 2.5 km on 15 and 19 September, respectively.
Examination of previously obtained data suggests that
mid-September is the beginning of the high-ozone season in the subtropical PRD region. Figure 3 shows the
time series of daily maximum 1-h O 3 measured in 1996
at a coastal site (Sha Lo Wan), which is 6 km northeast
of the Tai O site. It is relevant to note that there have
been significant changes in land use in northern Lantau
since 1996, including the commission of the new international airport in 1998 and the establishment of a
new town, Tung Chung.
Figure 3 shows a clear seasonal pattern of the mean
O 3 concentrations in the study region. For most of the
time, O 3 concentrations were the lowest in the summer
months (June, July, and August) although occasionally
very high O 3 concentrations could occur in the same
season. The generally low O 3 concentration in summer
is known to be the result of the Asian monsoon, which
brings in clean oceanic air from the Tropics and unstable
rainy weather. The frequent occurrence of high O 3 concentrations in autumn (September, October, and November) has been attributed to the subsidence of air mass
associated with an approaching tropical cyclone (low
pressure system) in the western Pacific and/or a high
pressure system over the continent (e.g., Wang et al.
1998, 2001). Such synoptic conditions have been found
to be conducive to chemical production and accumulation of O 3 .
It can be seen from Fig. 3 that, when compared with
the 1996 data, the 2001 episode showed higher O 3 concentrations and persisted for a much longer period of
time. An O 3 episode observed in Hong Kong normally
lasts for 1–2 days.
b. Comparison with O 3 data from Macau and other
sites in Hong Kong
To obtain a regional perspective of this episode, we
compared O 3 data from Tai O with those collected from
other locations in Hong Kong and Macau. In general,
O 3 concentrations in urban areas were found to be lower
than at the perimeters and in rural areas, because of
titration of O 3 by fresh emissions in urban centers. Figure 4 shows hourly O 3 results from six sites in Hong
FIG. 3. Time series of daily maximum 1-h O 3 concentration in 1996 at a coastal site
(Sha Lo Wan).
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FIG. 4. Ozone concentrations measured at Tai O and six other locations in Hong Kong and Macau.
Kong and one site in Macau (see Fig. 1). Among these
sites, Tap Mun (TM) and Hok Tsui (HT) are normally
upwind of the urban areas in the region under the prevailing east-northeast flow conditions; the central/western (CW) and Yuen Long (YL) sites are located within
or adjacent to populated urban districts. Tai O and Tung
Chung are on the much less populated Lantau Island,
but the latter is located in a new suburban town and
near the airport. The site in Macau (Taipa Grande) is
located in a suburban coastal area overlooking the Pearl
River estuary to the east.
Several interesting features are indicated from Fig. 4.
First, on most of the episode days, O 3 concentrations at
different locations showed a similar upward and downward trend and there was no obvious time lag in the
occurrence of daytime O 3 peaks at the different sites.
Such a feature suggests that the increased O 3 was a result
of in situ or in transit photochemistry induced by the
same general meteorological conditions, as opposed to
the transport of ozone produced elsewhere. Second, the
figure shows that the highest O 3 was recorded in Tai O
for every day during the episode, suggesting that ozone
pollution is most severe in the western rural part of Hong
Kong. Third, despite a similar general trend of O 3 at
the different sites, there were obvious site-to-site variations across the network. For example, the two sites
in western Hong Kong (Tai O and Tung Chung) showed
closer resemblance (both showing a narrower O 3 peak),
whereas the two eastern sites (Tap Mun and Hok Tsui)
has broader maxima. At the Macau site on the western
side of the Pearl River estuary, O 3 variation was gen-
erally similar to that for the two western Hong Kong
sites. However, on a smaller timescale, O 3 concentrations at one site can be very different from those at
other sites, which is attributed to the difference in smaller-scale circulation and/or local emission characteristics
at the sites. This topic is examined in greater detail in
a later section.
c. Synoptic weather conditions
A large body of research has shown the important
roles of meteorological conditions in the formation of
smog. These conditions include a well-defined boundary
layer, subsidence inversion, light winds, high temperatures, and intense solar radiation. In North America
and Europe, high-O 3 conditions are often associated
with slow-moving anticylones (e.g., NRC 1991). In the
subtropical part of eastern Asia, different synoptic situations often apply, as mentioned previously. Figure 5
shows the synoptic charts for the episode days of this
study, indicating the influence of a tropical storm/typhoon, Nari, which originated and meandered over the
East China Sea. According to the Hong Kong Observatory, Nari was one of the most unusual tropical cyclones to affect Hong Kong in recent years (HKO 2001).
It intensified and weakened on four occasions and
showed sharp returns for four times near the island of
Okinawa. The lifetime of Nari was almost 15 days,
which was the third longest of all tropical cyclones to
affect Hong Kong (HKO 2001). The prolonged lifetime
of this low pressure system in the East China Sea was
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FIG. 5. Synoptic weather charts for 13–20 Sep 2001.
thought to be the synoptic cause of the observed 6-day
pollution episode. The pollution was terminated on 20
September by the strong winds and rainy weather as
Nari approached the PRD region.
Table 1 shows some meteorological parameters re-
corded by the Hong Kong Observatory for the episode
days in comparison with the normal values for September. It can be seen that, on the episode days, temperature
was generally higher and the relative humidity was
mostly lower when compared with the monthly normal.
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TABLE 1. Meteorological parameters during episode and normal days (source: Hong Kong Observatory).
14 Sep
15 Sep
16 Sep
17 Sep
18 Sep
19 Sep
Monthly normal
Max
temperature* (8C)
Mean relative
humidity* (%)
Amount of clouds*
(%)
Prevailing wind
direction** (8)
Prevailing wind speed**
(m s21 )
29.8
32.5
32.4
32.6
32.9
33.8
30.3
82
72
59
68
75
68
78
61
47
6
9
31
65
63
190
290
010
230
260
280
090
0.36
0.80
0.54
0.99
1.21
0.98
1.48
* HKO Headquarters.
** Waglan Island.
Also, fewer clouds were observed during the episode.
These conditions are thought to be the result of heating
in the large-scale descending air mass occurring at the
outskirts of the low pressure system. In addition, under
the influence of the counterclockwise flow around the
low pressure system, wind changed from prevailing east
to north and northwest during the episode, with reduced
speeds. These conditions (high temperature, clear sky,
and weak winds) are normally considered to favor O 3
photochemical formation and buildup.
d. Atmospheric vertical structures
To learn about the vertical wind profile and atmospheric thermal stability, we examined radiosonde data
obtained in Kowloon at 0800 LST. Figure 6 shows temperature, relative humidity, and wind as a function of
altitude between 0 and 3 km. The wind profiles for 1400
LST were also examined but are not shown because
they gave results similar to the morning profiles. (The
afternoon sounding did not measure the temperature
FIG. 6. Temperature, relative humidity, wind direction, and wind speed as a function of altitudes at 0800 LST in Hong Kong between 13
and 20 Sep 2001. Temperature profiles in the evening (2000 LST) are also shown.
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FIG. 7. Time series of vector winds at a mountaintop station (Tai Mo Shan) and four surface sites: Tai O, Lau Fau Shan (northwest Hong
Kong), Waglan Island (southeast Hong Kong), and Taipa Grande, Macau.
profile.) It can be seen from Fig. 6 that, during the
episode, winds in the lowest few hundred meters were
generally from west to north as compared with northeast
before the episode (13 September). The vertical profiles
indicate different directions at higher altitudes (e.g., 17–
19 September). The general northerly winds in the lower
atmosphere as shown in radiosonde were consistent with
the winds recorded at TMS (957 m above sea level).
The wind at this mountaintop station (see Fig. 7) indicated northwest–north–northeast flow throughout the
episode.
The temperature profile in Fig. 6 reveals the presence
of a temperature inversion at 0800 LST below 500 m
between 16 and 19 September. For comparison, the
evening (2000 LST) profile was also included. It can
be seen that the inversion layer moved to a higher altitude at a later time of the day on some days (e.g., 18
September) but disappeared on 19 September. This feature may be attributed to the daytime heating of the
surface that would increase the depth of the mixing
layer. It is interesting to see that on 14 September the
inversion was indicated in the evening profile but not
in the morning one. In a previous study, Wang et al.
(2001) examined the mixing heights derived from the
radiosonde data by the HKO and found no significant
difference in them for high- and low-O 3 days during
September–November 1996. The exact reason for this
result was unclear. It is possible that the occurrence of
high O 3 concentrations requires that several meteorological conditions are met. Wang et al. (2001) showed
that the high-O 3 days tended to have stronger solar radiation and weaker wind. Figure 6 also shows an abrupt
change in relative humidity across the temperature inversion.
e. Complex pattern of surface wind
Although the radiosonde and mountaintop station
suggested a large-scale wind from west–north during
the episode, surface stations revealed a complex lowlevel wind pattern. Figure 7 shows vector wind measured at Tai O, Lau Fau Shan (LFS in the northwest),
and Waglan Island (WGL in the southeast), Hong Kong,
and Taipa Grande, Macau. Diurnal variations of winds
were clearly indicated at all the surface sites. The two
sites in the Pearl River estuary (Tai O and Taipa Granda)
showed a similar diurnal pattern. In general winds were
east–south (sea breeze) from the afternoon to early
morning of the next day and then switched to north–
west (land breeze) with reduced speeds. The timing for
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the wind changes slightly differed, and wind speeds on
the Hong Kong side tended to be smaller than in Macau.
Within the Hong Kong territory, winds at the eastern
Waglan Island site showed a diurnal pattern similar to
the western sites on some days (e.g., 16 September) but
different on other days (e.g., 19 September). The complex surface winds are believed to be a combined result
of synoptic wind, thermally driven sea–land breezes,
and local terrain effects. The sea–land breeze circulation
has been found to play an important role in the dispersion and transport of air pollutants in coastal areas, such
as Los Angeles, California (e.g., Blumenthal et al. 1978;
Harley et al. 1993), and Athens, Greece (e.g., Lalas et
al. 1987; Moussiopoulos et al. 1995). In Hong Kong,
Wang et al. (2001) found that, during September–November 1996, 44% of the days showed wind reversals
in western Hong Kong, and much higher 1-h concentrations (mean value: 92 ppbv) were found on these days
than on days with a uniform wind flow (mean value:
39 ppbv). In the following section, we closely examine
the chemical and wind measurements from Tai O to
show the relation of air pollutants to wind flow and to
shed some light on the emission characteristics of the
O 3-laden air masses at the site.
f. Temporal variations of O 3 and other pollutants in
relation to surface winds
The temporal profiles of O 3 and related pollutants can
provide valuable information about the chemical and
dynamic processes that have affected O 3 variation at a
site. Studies have shown that high O 3 concentration at
a nonurban site can be a result of several processes.
These include downward mixing of O 3-rich air aloft in
the morning hours, daytime chemical production by precursors from rural and dispersed urban emissions, and
advection of photochemically aged urban plumes (e.g.,
Kleinman et al. 1994; Frost et al. 1998; Cheung and
Wang 2001). Figure 8 shows the time series of 1-min
measurements of O 3 , CO, NO/NO y , NO y , and SO 2 , as
well as the hourly vector wind for the Tai O site.
It can be seen that O 3 reached a daily peak in the
afternoon period (1300–1500 LST). The rate of O 3 increase in the morning at this coastal site is smaller than
the O 3 enhancement normally seen at inland continental
sites (e.g., Parrish et al. 1993; Cheung and Wang 2001).
Other gases also showed higher concentrations during
daytime. The temporal variations of air pollutants correlated closely with wind changes over a diurnal period.
Increased amounts of air pollutants were observed with
light north-northeast winds, and the lower pollutant
amounts were associated with south-southwest winds
of larger speeds. These results suggest that the increased
amounts of O 3 and other pollutants resulted from the
diffusion/advection of urban plumes under weak northnortheast winds and that their reduced concentrations
in the later afternoon and at night were due to stronger
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sea breezes, which brought less polluted air from the
ocean. The NO/NO y ratio can serve as an indicator for
the degree of photochemical processing of an air mass
since its recent injection of freshly emitted NO. Figure
8 shows that NO/NO y peaked in the morning with increasing NO y , suggesting fresh urban emissions being
transported to the study site. The ratio dropped to near
zero in a well-processed air mass in the afternoon.
It is interesting to notice that there was apparently a
small buildup of nighttime CO over the 6-day period,
whereas no such a trend was indicated for O 3 and other
primary pollutants. This observation may be attributed
to the chemical and/or depositional loss of O 3 , SO 2 , and
NO y in nighttime air masses.
A closer examination of Fig. 8 indicates a rapidly
changing concentration of O 3 during some periods. For
example, on 14 September, the O 3 mixing ratio increased
from 60 to 113 ppbv between 1300 and 1400 LST and
dropped sharply (with other pollutants) at about 1600
LST. A similar case was seen on 16 September when
O 3 increased from 61 to 124 ppbv between 1200 and
1300 LST. Probably the most interesting case is for 19
September. The O 3 mixing ratio showed a step-by-step
increase between 1030 and 1330 LST. During this period, O 3 amounts jumped by 35–60 ppbv in ;30 min
and held constant for 60 min in between. The sharp
increases in O 3 concentrations are more likely due to
drastic changes of air masses as opposed to photochemical formation of O 3 within the same air mass. The
indications of airmass changes were more clearly seen
in the primary pollutant data. On 14 September, for
example, CO, NO y , and SO 2 amounts increased with
that of O 3 , whereas they dropped sharply with increasing O 3 on 16 and 19 September.
Carbon monoxide has been found to be a good tracer
of urban plumes because of its relatively long lifetime
(;1 month in summer) and the fact that it is not as
easily removed by wet and dry processes as are SO 2
and NO y . On 14 and 18 September, O 3 reached a maximum in the afternoon with a concurrent CO peak,
whereas on 15–17 September, O 3 approached a daily
peak with a sharp decrease in the amounts of CO and
other primary pollutants. The latter observation appears
to suggest ozone production reached a maximum in diluted urban plumes. It is interesting to see that on 19
September both O 3 and CO reached very high concentrations in the afternoon but the NO y concentration was
much lower than that in the morning. This pattern may
be due to the removal of NO y in the ‘‘aged’’ afternoon
air mass and/or the different emission characteristics in
the morning and afternoon air masses.
To have additional insights into the variation of air
pollutants in other coastal settings in the region, we
examined the concurrent measurement of O 3 , CO, NO,
NO y , and SO 2 obtained at the Hok Tsui research station,
which is approximately 40 km east of Tai O. Figure 9
shows 1-min measurement results on these species and
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FIG. 8. Time series of 1-min measurement of O 3 , CO, NO/NO y , NO y , SO 2 , and hourly wind at Tai O between 14
and 19 Sep 2001.
the hourly wind recorded at Waglan Island, about 5 km
southeast of the Hok Tsui site.
The fast-changing concentrations of air pollutants are
more obvious at this site than at the western Tai O. It
is very interesting to see several narrow O 3 peaks on
15, 17, and 19 September, each lasting for about 1 h.
Some of these peaks occurred at a very late time of the
day (;1900 LST), indicating the transport of aged air
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FIG. 9. Time series of 1-min measurement of O 3 , CO, NO/NO y , NO y , SO 2 , and hourly wind at Hok Tsui between
14 and 19 Sep 2001.
parcels to the site. On 17–19 September, a large peak
of NO y and SO 2 were observed in the morning with
high NO/NO y ratios indicating the arrivals of fresh urban
plumes, whereas their amounts decreased sharply in the
afternoon when the wind shifted to southwesterly and
O 3 reached maxima. Of interest is that CO amounts did
not indicate a drastic decrease on 17 and 18 September.
The sharp drops in the NO y and SO 2 amounts in the
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WANG AND KWOK
afternoon may provide additional evidence of their removal in afternoon air masses. At this site, many NO y
and SO 2 spikes were observed in the afternoon and are
believed to be due to ship emissions. The temporal variations of air pollutants were very different from the
profiles at western Tai O. The observations from these
two sites provide a useful database for comparison with
future modeling studies in this complex coastal area.
g. Chemical species ratios
The concentration ratios of certain chemical species
can provide useful insights into the origin(s) of emission
sources that contribute to O 3 pollution. Examination of
CO/NOy and SO 2 /NO y may be particularly helpful in
distinguishing air originating in Hong Kong versus that
from other cities in the PRD because of the different
emission characteristics. It has long been known that the
SO 2 /NO x emission ratio is characteristically high on the
China mainland because of the burning of coal containing
a high sulfur content. Air-quality monitoring data obtained by the HKEPD indicated typical SO 2 /NOx ratios
of 0.1 (ppbv/ppbv) in the urban atmosphere of Hong
Kong (information at the time of writing available online
at http://www.epd-asg.gov.hk/Aqr00/Aqr00e.pdf), and a
much higher ratio of 0.4 (ppbv/ppbv) was observed in
urban Guangzhou (Zhang et al. 1998). The CO/NOy ratios
have also been found to be very different. Aircraft measurements made around Hong Kong in 1994 showed a
CO/NOy ratio of 3.3 (ppbv/ppbv) in plumes generated from
Hong Kong’s urban area and a much larger value of 16
(ppbv/ppbv) in air masses from Shenzhen (Lind and Kok
1999). The very low CO/NOy in Hong Kong has been
attributed to the large fleet of diesel vehicles. The higher
CO/NOx and SO 2/NOx ratios in the PRD are also indicated
in the emission inventories. The inventories for 2000 (at
the time of writing available online at http://www.cgrer.
uiowa.edu/EMISSIONpDATA/indexp16.html) suggest a
SO 2 /NO x ratio of 0.79 (ppbv/ppbv) for Guangdong province and 0.31 (ppbv/ppbv) for Hong Kong, and a CO/
NOx ratio of 11 (ppbv/ppbv) as compared with 1.2 (ppbv/
ppbv) for Hong Kong. These values were derived from
combined point and mobile sources.
In this study, we computed the enhancement of the
concentration ratios of SO 2 and CO to NO y , D[SO 2 ]/
D[NO y ] and D[CO]/D[NO y ], by subtracting nighttime
background levels from those during their morning
peaks (0600–0900 LST) and afternoon O 3 maxima. The
results are shown in Table 2. The morning ratios are
better indicators of the original emission ratios because
the measured NO y had not been subject to significant
removal between sampling and injection of fresh emissions. Table 2 shows that, in general, D[CO]/D[NO y ]
and D[SO 2 ]/D[NO y ] in the afternoon are higher than in
the morning. This difference could be explained by the
depletion of NO y in an aged air mass, but we could not
rule out the possibility of sampling PRD air masses that
would contain abundant CO and SO 2 relative to NO y .
TABLE 2. Morning and afternoon chemical species ratios
at Tai O.
D[SO 2 ]/D[NOy ]
(ppbv/ppbv)
14
15
16
17
18
19
Sep
Sep
Sep
Sep
Sep
Sep
0.23
0.37
0.07
0.74
0.42
0.15
(0.50)*
(0.33)
(0.32)
(1.2)
(0.72)
(0.35)
D[CO]/D[NOy ]
(ppbv/ppbv)
9
.21
7
.15
16
7
(15)
(21)
(22)
(24)
(18)
(30)
* Afternoon values in parentheses.
On three days (15, 17, and 18 September), large values
of D[SO 2 ]/D[NO y ] (0.37–0.74) and D[CO]/D[NO y ] (15–
21) were observed in the morning, which implies that
the PRD emissions may have contributed significantly
to the high O 3 observed on these days. On two other
days (16 and 19 September), however, much lower ratios
were found in the morning, with D[SO 2 ]/D[NO y ] of
0.07–0.15 and D[CO]/D[NO y ] of about 7, suggesting a
strong contribution of local Hong Kong emissions. The
above results imply that the high ozone observed at Tai
O was produced in air masses that had been significantly
influenced by PRD emissions or had been injected with
both Hong Kong and regional emissions.
4. Summary and conclusions
In this study, we have presented recent measurement
of O 3 and related air pollutants during a persistent photochemical smog observed in the southern part of the
Pearl River delta. Ozone concentrations exceeded the
NAAQS of 120 ppbv for 6 consecutive days at a coastal
site, Tai O, in western Hong Kong, with very poor visibility also recorded during the episode. Examination of
O 3 data from six other stations indicated the regional
nature of O 3 formation. A slow-moving tropical cyclone
over the East China Sea was shown to have caused largescale air stagnation over Hong Kong for a prolonged
period, bringing in conditions (e.g., higher temperature
and solar radiation because of fewer clouds) that were
conducive to the O 3 formation and buildup. Although
the large-scale winds were from the north-northwest,
surface winds were light and variable, with a strong
influence of sea–land breezes. The high concentrations
of O 3 and other pollutants (CO, SO 2 , and NO y ) found
in the morning and early afternoon were due to diffusion/advection of urban plumes in weak north-northeast
land breezes. Their reduced amounts in late afternoon
were attributed to the stronger south-southwest sea
breezes that brought in less polluted air. The high values
of SO 2 /NO y and CO/NO y on some days suggested the
contribution of PRD regional emissions to the high O 3
found in western Hong Kong. The chemical measurements at this site and another eastern coastal site revealed rapidly changing chemical compositions and
large spatial variability in the concentrations of air pol-
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lutants. Indications of NO y removal were observed during the periods of sea breezes.
Like many of the previous studies conducted in coastal regions, this work has illustrated the complex nature
of wind flow and air pollution transport in a southern
China coastal area. The data that were available to this
study (i.e., mainly along the coast) are somewhat limited
when compared with some of the large-scale investigations carried out in North America and Europe. More
spatial high-resolution monitoring and modeling are
needed, particularly for the inland areas of the Pearl
River delta. Such information would give a better picture on the three-dimensional distribution of wind flow
and the transport of photochemical pollutants. For instance, it is conceivable that the O 3-laden air observed
in western Hong Kong could be transported to inland
locations by the strengthening sea breezes in late afternoon; therefore, data from the inland areas would reveal
the extent of sea-breeze penetration and O 3 pollution
transport. The observations presented in this study form
a valuable database for comparison with the results from
future modeling investigations for the study region.
Acknowledgments. We thank Steven Poon and Vincent Cheung for their invaluable contributions in setting
up and operating the field measurements. We are grateful
to Professor Y. S. Li for his support of the Tai O field
study. We thank A. J. Ding for helpful discussions. We
thank the Hong Kong Observatory for providing meteorological data and the Hong Kong Environmental
Protection Department for providing O 3 data in Hong
Kong, and we thank the Macau Geophysical and Meteorological Bureau for providing O 3 and wind data for
Macau. This research was funded by the Research
Grants Council of the Hong Kong Special Administrative Region (Project PolyU5062/99E and PolyU5059/
00E). Additional financial support for the field study
was provided from the Hong Kong Polytechnic University.
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