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Atmospheric Environment 39 (2005) 4779–4791 www.elsevier.com/locate/atmosenv

Observational studyof ozone and carbon monoxide at the summit of (1534 m a.s.l.) in central-eastern China Jian Gaoa,b, Tao Wanga,Ã, Aijun Dinga, Chunbo Liub

aDepartment of Civil and Structural Engineering, The Hong Kong Polytechnic University, Hong Kong, China bEnvironment Research Institute, University, , Shandong 250100, China

Received 2 February2005; accepted 27 April 2005

Abstract

We report measurement results of ozone (O3) and carbon monoxide (CO) obtained from 5 Julyto 23 November 2003 at the summit of Mount Tai (1534 m a.s.l, 36.251N, 117.101E) in the Shandong Peninsula of China. The studywas carried out to gain insights into regional O3 pollution and air-mass transport in the highlypopulated North China (Huabei) Plains. The average mixing ratio was 58 (716) ppbv for O3 and 393 (7223) ppbv for CO during the studyperiod. The monthlyvariations in O 3 and CO exhibited a similar pattern, i.e. high in summer and low in autumn. While such a seasonal pattern for O3 is a common phenomenon in manyrural areas in the Northern Hemisphere and is mainlydue to seasonal changes in solar radiation and temperature, the higher CO levels in summer were attributed to the difference in dynamic transport and the evolution of PBL heights in summer and autumn over this region. An examination of diurnal variations in O3 indicated evident daytime photochemical production, especially in summer. The evolution of PBL heights and mountain-valleybreezes also had a large impact on the diurnal patterns of O 3 and CO. O3 and CO showed a moderatelygood positive correlation ( r ¼ 0:60) in Julywith a slope of 0.08 ppbv/ppbv, which is much lower than the slope (0.3–0.4 ppbv/ppbv) observed in North America. A back trajectoryanalysis showed that air masses mainlyoriginated from the North China Plains or were re-circulating over the Shandong Peninsula, collectivelyaccounting for 76% of the air masses sampled. Summertime air traveling in the lower troposphere over northern China had the highest concentrations of O3 and CO, revealing that the lower troposphere in northern China was significantlypolluted in summer due to strong convection transporting PBL pollution to the free troposphere. The trajectories also showed notable contributions from eastern China, central China, and countries in northeast such as and Japan. r 2005 Published byElsevier Ltd.

Keywords: Tropospheric O3; Northern China; Photochemical production; Long-range transport; Back trajectoryanalysis

1. Introduction O3 at ground levels have adverse effects on human health and vegetation, such as crops and forests (NRC, Tropospheric ozone (O3) is a trace gas that has 1991). O3 plays a key role in determining the oxidizing important implications for air quality, tropospheric capacityof the atmosphere and is also a greenhouse gas chemistry, and climate change. High concentrations of contributing to global warming (Bojkov, 1988). In the lower troposphere over industrialized regions, O3 is ÃCorresponding author. Tel.: +852 2766 6059; mainlyproduced from photochemical reactions invol- fax: +852 2334 6389. ving oxides of nitrogen (NOx), carbon monoxide (CO), E-mail address: [email protected] (T. Wang). and volatile organic compounds (VOCs) in the presence

1352-2310/$ - see front matter r 2005 Published byElsevier Ltd. doi:10.1016/j.atmosenv.2005.04.030 ARTICLE IN PRESS

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of sunlight. Apart from being a precursor to O3,CO inventories (Wang et al., 2002, 2004a) and a comparison helps regulate the oxidizing capacityof the atmosphere of the chemical transport model results with the ground- byreacting with OH radicals ( Novelli et al., 1994, 1998). based and/or TRACE-P aircraft measurements (Carmi- Due to its relativelylong atmospheric lifetime of 1–2 chael et al., 2003b; Palmer et al., 2003; Tan et al., 2004; months, CO is also an ideal tracer for anthropogenic Y.X. Wang et al., 2004) have indicated that emissions of pollution and has been used to aid in interpretations of CO have been substantially( 450%) underestimated in chemical measurements (Jaffe et al., 1997; Parrish et al., the bottom-up inventories of Streets et al. (2003). It has 1991, 1998; Wang et al., 1996, 1997) and in validating been suggested that the burning of biomass/biofuel and/ chemical transport models (Carmichael et al, 2003a; Liu or the combustion of coal from domestic and industrial et al., 2003; Tan et al., 2004; Y.X. Wang et al., 2004). sectors could be the missing source(s) of CO (Carmi- Rapid urbanization and industrial developments in chael et al., 2003a; Tan et al., 2004; Wang et al., 2002, China in the past two decades are thought to have led to 2004a; Y.X. Wang et al., 2004). These analyses made use a significant increase in the emissions of O3 precursors of surface measurements at two surface stations where (Akimoto and Narita, 1994; Streets and Waldhoff, long-term data were available within the Chinese main- 2000). This will likelyfoster severe photochemical O 3 land. It is apparent that data on CO in other parts of pollution in major urban centers and the surrounding China will be needed in order to obtain a more robust regions, which will have adverse effects on human health conclusion on the source sectors and the geographical and agricultural crops and forests. It is thus imperative extent of the missing source(s). In particular, it would be that systematic research be carried out to investigate highlydesirable to obtain measurements made in the current and future trends in O3 pollution. North China (Huabei) Plains, where the emissions of Some measurement studies on surface O3 in China anthropogenic trace gases and aerosols are expected to have been reported, most of which were based on data be the greatest in the country. Data on O3 pollution on a collected in a rural area (Lin’an) in eastern-central regional scale are also needed to evaluate the impact of China and in a southern coastal area (Hong Kong). Luo O3 on human health and agriculture in this vast and et al. (2000) analyzed 1-year data collected during highlypopulated region. 1994–1995 at four rural sites in eastern China and During July–November 2003, we conducted measure- showed frequent occurrence of high-O3 events during ments of O3 and CO for 5 months (and of additional the autumn season. A recent studycarried out in trace gases and fine aerosols during an intensive 4-week 1999–2001 in the Yangtze River delta region indicated period toward the end of the study) at the summit of that average levels of O3 peaked in earlysummer (May) Mount Tai, the highest mountain in the North China (Cheung and Wang, 2001; Wang et al., 2001a). The O3 Plains (see Fig. 1). This mountain-top location was concentrations observed in these rural areas have selected for our studybecause the high elevation would suggested that the O3 concentrations were high enough allow us to sample air masses more representative of to cause damage to Chinese corps (Chameides et al., regional sources of pollution, examine transport from 1999; Cheung and Wang, 2001). For the southern other regions in Asia, and studyexchanges between the coastal area, long-term (412 month) measurements of planetaryboundarylayer(PBL) and the free troposphere. surface O3 and CO have been reported in several studies Another purpose of the studywas to collect data in (Lam et al., 2001; Wang et al., 2001c; Wang et al., eastern-central China for comparison with measurements 2004b). These studies confirmed that a seasonal average concurrentlymade at a high-altitude site in western China peak of O3 in southern China occurs in the autumn (Mount Wailiguan, Lat: 36.281N Lon: 100.901E, 3816 m season. This was attributed to meteorological conditions above sea level (a.s.l.)). In this paper, we present the favoring the transport and photochemical formation of hourlymeasurements of O 3 and CO from the entire study O3 in autumn. Intensive studies have also been carried of nearly5 months at Mount Tai. We will compare the out to elucidate chemical and transport processes that data from Mount Tai with those obtained from other influence O3 concentrations and pollution episodes in high-elevation sites and from aircraft studies in eastern subtropical coastal regions (Wang et al., 1997, 2001b, Asia, and examine variations in diurnal and monthly 2003a, b; Wang and Kwok, 2003). timescales and the relationship between levels of trace An interesting finding in the previous studies is levels gases and air-mass transport patterns. of CO in eastern China of approximatelytwo to three times greater than those measured in rural areas of North America and Europe. An analysis of concurrently 2. Experiments and methodologies measured pollution tracers such as methyl chloride and soluble fine potassium indicated strong emissions of CO 2.1. Measurement site from the burning of biomass/biofuel in rural Lin’an in eastern-central China (Wang et al., 2002, 2004a). A The measurement site was in a meteorological comparison of these measurements with emission observation station located at the summit of Mount ARTICLE IN PRESS

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Fig. 1. Location of the observation site at the summit of Mount Tai in central-eastern China and the inventoryof CO emissions (Streets et al., 2003).

Tai with an elevation of 1534 m a.s.l. (in Fig. 1). The 3 days during the last month of the measurement summit overlooks the cityof Tai’an (population: through manual injections of a span gas produced by 500,000) 15 km to the south. The cityof Ji’nan (capital diluting a NIST traceable standard (from Scott-Marrin of Shandong province, population: 2.1 million) is Inc., California) with ultra pure air in a cylinder (Scott- situated 60 km to the north. A few villages also can be Marrin Inc., California). The sensitivityof both instru- seen from the north. As a famous tourism spot, Mount ments did not change in a significant wayover the 5- Tai receives a large number of visitors in the summer month period (O3 o1%; CO o4%). The concentrations months (June–September), and there are some local of CO were also determined from analyses of 18 whole emissions from small restaurants and temples. However, air samples collected in stainless steel canisters at the site our measurement site is situated in the less frequently (4 taken in Julyand 14 in November), and were analyzed visited eastern part of the summit. The measurement byDr. Donald Blake at the UniversityCalifornia-Irvine instruments were housed in a room on the top floor of using gas chromatographic methods. The two data sets the Meteorological Station. The intakes of the O3 and agreed verywell in the observed range of 250–1200 ppbv CO analyzers were located 2.3 m above the rooftop ([CO]API ¼ 1.01[CO]canister+38, r ¼ 0:97). (approximately15 m above the ground). The two individual Teflon lines (length: 5.6 m; 1/400 outside diameter) were used as sample lines. Each line had an 3. Results and discussion in-line particulate filter to prevent particles from entering into the instruments. 3.1. Monthly and diurnal variations and the relation between O3 and CO 2.2. O3 and CO instruments 3.1.1. Overall pattern The O3 and CO instruments have been used in a The time series of the 8-h moving average of the O3 number of previous studies (see Wang et al., 2001a, b; and CO mixing ratio are presented in Fig. 2 together Wang and Kwok, 2003 for more detailed descriptions). with the air temperature and relative humidityfor the Briefly, O3 was measured with a commercial UV period from 5 Julyto 23 November 2003. Here, the 8-h photometric analyzer (Thermo Environment Instru- moving averages were used in order to focus on the ments Inc., Model 49) that had a detection limit of variations in the trace gas levels caused bychanges in 2 ppbv and a 2-sigma (2-s) precision of 2 ppbv for a 2- large-scale air masses. The data capture rate for O3 and min average. CO was measured with a gas filter CO was 94% and 75%, respectively. No data on CO correlation, a non-dispersive infrared analyzer (Ad- were available between 25 August and 25 September due vanced Pollution Instrumentation Inc., Model 300) with to the malfunction of the CO analyzer. Fig. 2 shows that a heated catalytic scrubber for baseline determination, Mount Tai experienced frequent rainyand humid which was conducted every2 h. The detection limit was weather with relativelyhigh temperatures in summer 30 ppbv for a 2-min average, with a 2-s precision of and decreasing temperatures and drier weather towards about 1% for a level of 500 ppbv (2-min average). The the end of the study. Cold fronts were observed during overall uncertaintywas estimated to be 10%. the later period of the studysuch as on 11–14 October, The two analyzers were calibrated in our home lab in 21–23 October, 2–4 November, and 7–9 November. Hong Kong before and after the field experiment. In Both O3 and CO exhibited large day-to-day variations. addition, the CO instrument was calibrated on site every A number of high O3 days (with an 8-h mixing ratio ARTICLE IN PRESS

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exceeding US NAAQS 8-h 80 ppbv for O3) occurred genic sources in the North China Plains and mayalso during the first 3 months, while O3 levels decreased in indicate much larger emissions of CO in China than in the cold season. For CO, some spikes of over 800 ppbv developed countries (Wang et al., 2001a). (e.g., on 31 October, 15 and 20 November) were The pattern of summer high and autumn low for O3 observed, indicating of the impact of a large point has been observed in manymid-latitude locations. This source or fresh plumes of pollution transported to pattern is mainlyattributed to the more intense solar Mount Tai. radiation and higher temperatures often associated with the presence of weak, slow-moving, and persistent high-

3.1.2. O3 and CO characteristics in the summer and pressure systems, which favor the photochemical pro- autumn seasons duction of O3 (Logan, 1989). In summer, the northern Table 1 gives the statistics for O3 and CO for each and eastern parts of China are often dominated bya month from Julyto November 2003. O 3 showed a well- subtropical high, situated over the western Pacific, defined monthlypattern with apparent high values in leading to weak winds and hot weather. The elevated summer (60–65 ppbv) and reduced levels in autumn O3 levels observed in the summer at Mount Tai can thus (48–50 ppbv). These values are generallyhigher, espe- be explained bythe transport of photochemically ciallyfor CO, than those previouslyobtained at other processed boundarylayerair masses to the mountain rural mountain-top sites in mid-latitude locations of the peak due to mountain-valleybreezes and/or the growth Northern Hemisphere. For example, Kajii et al. (1998) of convective PBL. In autumn, with decreases in solar reported summer mean O3 and CO mixing ratios of radiation and temperature, and reductions in photo- 45–50 and 150–200 ppbv, respectively, at Happo (1840 m chemical production and PBL heights, the peak of the a.s.l.) in Japan during 1994–1996. At Mount Cimone mountain received less impact from PBL pollution, (2160 m a.s.l.) in Italy, mean O3 and CO mixing ratios leading to lower concentrations of O3 at Mount Tai were in the range of 50–68 and 100–140 ppbv, respec- compared to the summer. Such an O3 pattern of summer tively( Fischer et al., 2003). The much higher levels of high and autumn low has also been observed at other CO at Mount Tai are consistent with fact that the mountain-top sites, such as Hohenpeissenberg, Ger- mountain is situated in closer proximityto anthropo- many(998 m a.s.l.) ( Low et al., 1990) and Arosa, Temperature ( 30 100 80 20 60 10 40 RH

RH (%) 0 20 ° Temperature C) 0 -10 120 O3 1400 100 CO

1200 CO (ppbv) 80 1000 60 800 (ppbv)

3 600

O 40 400 20 200 0 0 5 Jul 15 Jul 25 Jul 4 Aug 14 Aug 24 Aug 3 Sep 13 Sep 23 Sep 3 Oct 13 Oct 23 Oct 2 Nov 12 Nov 22 Nov

Fig. 2. Eight-hour moving average of the O3 and CO mixing ratios at the summit of Mount Tai, together with the temperature and relative humidityfrom 5 Julyto 23 November 2003.

Table 1

Monthlystatistics of O 3 and CO at Mount Tai for the period from Julyto November 2003

JulyAugust September October November

O3 CO O3 CO O3 CO O3 CO O3 CO

Mean 65 439 60 395 62 — 50 357 48 371 Median 63 405 60 364 61 — 48 302 47 286 Max 120 1175 103 950 104 — 93 1196 86 1518 Min 36 85 19 51 17 — 23 16 17 26 S.D. 17 206 13 198 15 — 12 212 11 248 ARTICLE IN PRESS

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Switzerland (1860 m a.s.l.) (Staehelin et al., 1994). It trace gases) has also been observed at Niwot Ridge, should be noted that to obtain a full picture on the Colorado (Oltmans and Levy, 1994). In contrast, the seasonal behavior of O3 (and CO) at Mount Tai, data of reduced daytime O3 concentrations, due to deposition a longer period (e.g., X1 year) will be needed. and chemical loss of O3, were observed for mountain- With regard to CO, its trend in summer and autumn is top sites in more remote areas, such as Mauna Loa different from that observed at flatland sites (Wang et (Oltmans and Komhyr, 1986). al., 2001a) which often show the lowest concentrations occurring in summer and the highest in winter. 3.1.4. The relationship between O3 and CO in different These winter maxima can be attributed to weaker months vertical mixing, slower chemical destruction rates, and The relationship between O3 and CO can be used to a higher demand for energyfor heating during the cold evaluate the amount of O3 produced due to photo- season, as compared to summer. The pattern of summer chemical processes in the sampled air masses (Parrish et high and autumn low for CO at Mount Tai can be al., 1991; Wang et al., 2001a). Figs. 4a–d show the attributed to a different degree of impact bythe scattering plots of O3 and CO for different months. boundarylayerair in the two seasons, similar to the Similar to Wang et al. (2004a), here we applied the case for O3. That is, in the cold season, the daytime PBL reduced major axis (RMA) method to calculate regres- heights over the North China Plains were compressed sion statistics (slope and intercept) rather than using and were often below the summit of Mount Tai standard linear regression, because both O3 and CO (1534 m), and mountain-valleybreezes could not fully were subject to errors of measurement. It can be found develop due to weak solar radiation and strong regional that the CO and O3 at Mount Tai generallyhad a background winds. Hence, the transport of PBL air positive correlation during the studyperiod, consistent pollutants to this site was less frequent in autumn than with other studies carried out in rural sites (Fischer et in summer, resulting in reduced concentrations of CO in al., 2003; Poulida et al., 1991; Wang et al., 2001a). The the autumn season. Visual observations indeed con- slope (i.e. DO3/DCO) decreased from July(0.084 ppbv/ firmed that the studysite was often above the top of ppbv) to November (0.045 ppbv/ppbv). Parrish et al. pollution haze of the PBL in November whereas the (1991) suggested that the slope between O3 and CO polluted PBL air masses (often containing high moist- provides information on O3 production per molecule of ure) frequentlyimpacted the mountain-top during CO emitted. Therefore, the decrease in the slope reflects daytime in July and August. a reduction in the photochemical production efficiency of O3. The summertime slope of 0.08 ppbv/ppbv is in 3.1.3. Diurnal variations in O3 and CO good agreement with that observed at Lin’an in Diurnal variation of O3 and CO can yield useful eastern China, but is much smaller than those insights into factors/processes relating to the emission, (0.3–0.4 ppbv/ppbv) observed in North America (Parrish dilution/transport, chemical transformation, and de- et al., 1993), supporting the contention that high O3/CO position of these gases. Fig. 3a and b shows the average ratios are a unique characteristic of photochemically diurnal cycles in the warm season (July, August, and processed air in China (Wang et al., 2001a). The smaller September) and the cold season (October and Novem- O3–CO slopes in China are due to larger CO to NOx ber) in 2003. In summer, O3 showed relativelystable and slopes of 40–50 ppbv/ppbv (Wang et al., 2002) and the high values (63 ppbv) from midnight to the early similar O3–NOx slopes of 8–13 ppbv/ppbv (Wang et al., morning hours and then a decrease at sunrise. After 2001a). (For consistency, these slopes have been reaching a minimum (58 ppbv) at 0900 (local time LT), converted to the values based on the reduced major it showed a daytime buildup reaching a maximum of axis method.) about 67 ppbv at 1600 LT. In autumn, O3 also Fig. 4 indicates that the correlation coefficient showed higher levels during daytime, but the morning between O3 and CO decreased graduallyfrom Julyto trough disappeared with a smaller diurnal difference November, and was mainlycaused bythe occasional compared to summer. For CO, the main feature transport of fresh plumes to the site in autumn, was a double hump structure, with the first peak corresponding to the CO spikes shown in Fig. 2. appearing between 0100 and 0300 LT and second (major) one in the afternoon. The elevated afternoon 3.2. Influence of long-range atmospheric transport on the levels of O3 and CO implythe transport of boundary- abundance of O3 and CO layer pollution to the summit of the mountain due to a daytime upwind slope and the growth of convective Apart from the mountain-valleybreezes, long-range PBL. The cause for the nighttime peak is not entirely transport can also stronglyinfluence variations in the clear, but perhaps is related to changes in small to concentrations of trace gases bybringing in pollutants mesoscale scale dynamical transport. The feature of emitted from distant regions. To investigate enhanced afternoon concentrations of O3 (and other such transport characteristics at Mount Tai and its ARTICLE IN PRESS

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70

65

60

55 (ppbv) 3 O 50

45

40 0 6 12 18 24 (a) Hours (LST)

500

450

400

CO (ppbv) 350

300

250 0 6 12 18 24 (b) Hours (LST)

Fig. 3. Diurnal variations in the average (a) O3, (b) CO mixing ratios (open circle: summer; solid circle: autumn) and one standard error (whiskers).

impact on the measured O3 and CO levels, a back compute trajectories for 72-h durations (i.e., 3 days). trajectoryanalysiswas carried out for the whole Four trajectories were determined each daywith the studyperiod. ending points of the trajectories at the summit of Mount Tai for the whole 5-month period. 3.2.1. Back trajectory calculation and air-mass With about 600 trajectories, we classified the air classification masses reaching Mount Tai into 6 main categories, In this study, we used an online version of the HYbrid byexamining their origins, paths, and the altitudes Single-Particle Lagrangian Integrated Trajectory(HYS- of the trajectories. The typical pathway and the PLIT) model (accessed via NOAA Air Resources percentage of each categoryare shown in Figs. 5a–d. Laboratory’s Real-time Environment Applications and The definitions of these categories are summarized as DisplaySystemWebsite: http://www.arl.noaa.gov/ follows: N and NW—two groups of continental air ready/open/hysplit4.html) to calculate back trajectories. masses from the north and northwest with different The meteorological input for the trajectorymodel was travel distances, CC—air masses passing over central the FNL dataset (reprocessed from NCEP Final China arriving at the site from the south, EA—air Analysis data by ARL) with a 6-h temporal resolution, masses passing over the Korean Peninsula and Japan, about 190 km horizontal resolution, and 13 vertical ME—maritime air masses with relativelyshort journeys layers. Considering a relatively high elevation of the over the eastern coastal areas of China, and LP—air studysite and the large wind speeds, we chose to masses moving slowlyor with a loopytrajectoryover ARTICLE IN PRESS

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140 [O ]= 0.084 [CO] + 28 140 July 3 August [O3] = 0.067 [CO] +33 R = 0.60 120 120 R = 0.56 100 100 80 80

60 O3 (ppbv) 60 O3 (ppbv) 40 40 20 20 0 0 0 400 800 1200 0 400 800 1200 (a) CO (ppbv) (b) CO (ppbv)

140 140 [O ]= 0.045 [CO] + 31 October [O3] = 0.057 [CO] + 30 November 3 R = 0.29 120 R = 0.47 120 100 100 80 80

O3 (ppbv) 60 60 O3 (ppbv) 40 40 20 20 0 0 0 400 800 1200 0 400 800 1200 (c)CO (ppbv) (d) CO (ppbv)

Fig. 4. Scatter plots of O3 vs. CO in (a) July, (b) August, (c) October, and (d) November 2003. The regression statistics are determined using the reduced major axis method.

eastern China. From Figs. 5a–d, it can be seen that the instruction observed at other mountain sites (Bonasoni continental air masses (including N and NW) were the et al., 2000; Stohl et al., 2000), but the CO data suggest a most dominant type of air mass during this period, different origin. Table 2 shows that CategoryN had the accounting for 53% of the total, followed byCategory highest mean CO value (5037230 ppbv) among all the LP (26%), CC (9%), EA (7%), and ME (4%). air-mass groups. In addition, O3 had a moderately strong positive correlation (r ¼ 0:56) with CO in this 3.2.2. Chemical characteristics of different air masses category(not shown), which suggests that the high O 3 in To examine the relationship between the levels of the CategoryN air mass was mainlyderived from trace gases and the origins of air masses at Mount Tai, photochemical production involving anthropogenic pol- we computed the statistics of O3 and CO for all of the lutants, since CO is a good tracer of combustion. above categories of air mass (see Table 2). As the Previous studies have shown that biomass burning in trajectories were calculated every6 h, we averaged the Siberia could give rise to high concentrations of CO and data on trace gas over a 5-h period centered at the O3 in downwind regions of the western Pacific (Kato termination time of each trajectory. The mean O3 mixing et al., 2002; Tanimoto et al., 2000; Pochanart et al., ratio (73717 ppbv) in CategoryN is the highest among 2003). Indeed, an examinations of data on fire counts the six categories. An examination of the cumulative from MODIS (http://daac.gsfc.nasa.gov/data/dataset/ frequencydistribution shows that over 60% of the O 3 MODIS/index.html) and data on CO from MOPPIT data in this group are higher than 65 ppbv. The 3-day (http://eosdatainfo.gsfc.nasa.gov/eosdata/ssinc/mopitt_ back trajectorygiven in Fig. 5a indicates that the air dataprod.shtml) indicated veryactive burning of bio- masses in this group originated in the low or middle mass in northeastern Mongolia and southeastern Siberia troposphere over Mongolia and descended to Mount in July2003 (not shown). However, these burning Tai as theymoved southeastward. This typeof air mass activities did not seem to the main cause for the high looks like previouslyreported cases of stratospheric concentrations of CO and O3 in the CategoryN air-mass ARTICLE IN PRESS

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Fig. 5. Four types of air mass classified by backward trajectories: (a) continental originated: (N) on November 16 at 1400 LT, (NW) on August 12 at 0200 LT; (b) loop trajectories (LP) on October 6 at 2000 LT, marine originated (ME) on September 4 at 1400 LT; (c) south originated passing through central China (CC) on July27 at 2000 LT; (d) eastern Asia originated (EA) on August 17 at 0200 LT. ARTICLE IN PRESS

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Table 2

Statistical data of O3 and CO in each air mass categorycaught at Mount Tai

N(n ¼ 111) NW (n ¼ 169) LP (n ¼ 137) EA (n ¼ 37) CC (n ¼ 49) ME (n ¼ 19)

O3 CO O3 CO O3 CO O3 CO O3 CO O3 CO

Mean 73 503 51 353 57 444 51 324 56 353 41 253 Median 74 502 49 286 56 412 52 260 55 359 40 243 Max 120 1175 93 1518 93 1738 97 1007 87 661 61 421 Min 25 70 17 26 23 87 27 16 36 107 17 107 S.D. 17 230 11 230 12 226 14 205 11 114 10 78

group. This is supported bythe fact that the average CO CO and O3 in CategoryLP can be explained bythe slow mixing ratio in August 2003 in CategoryN (see Fig. 7) movement of air, in which case pollutants emitted from was similar to that in July(528 7213 versus surrounding areas in Shandong and from the highly 5457218 ppbv), despite the fact that there were far industrialized Yangtze River delta region (where the fewer incidences of fire in August (not shown here). largest Chinese city, Shanghai, is located) could be Hence, the elevated levels of CO in this group were transported to the summit. Wang et al. (2001a) reported mostlylikelydue to emissions from northern China, as veryhigh concentrations of CO (577 ppbv) but lower opposed to emissions from the burning of biomass in levels of O3 (34 ppbv) measured at a rural site, Lin’an, in Siberia. that region during the same period in 1999. They Compared with CategoryN, air masses from the suggested that the emissions from this region could northwest contained a lower level of both O3 and CO, cause high levels of O3 in downwind regions after they with a mean value of 51 ppbv (711) and 353 ppbv had undergone sufficient photochemical reactions. (7230), respectively. Fig. 5a indicates that the air CategoryEA, which accounts for 7% of the total masses in this categorymostlycame from the middle trajectories, indicates transport within the PBL from troposphere (at about 600 hPa) over the remote central northeast Asia to northern China. Table 2 shows that Eurasian continent but traveled at a much higher speed, this categorycontained a mean level of 51 714 ppbv and suggesting theyhad a relativelyshort residence time over 3247205 ppbv for O3 and CO, respectively. They are highlypolluted northern China. Nevertheless, both the comparable to those in CategoryNW but much higher O3 and CO levels in this categorywere much higher than than those in CategoryME (O 3:41710 ppbv; CO: those in the air masses of continental origin observed in 253778 ppbv). This result suggests a notable contribu- remote Siberia (Pochanart et al., 2003) and in Oki Island tion of emissions in Korea and Japan, to the air of Japan in the western Pacific (Pochanart et al., 1999), pollution in northern China. suggesting some impacts of emissions in northern China, particularlyfrom urban/industrial areas in the highlands 3.2.3. The monthly behavior of O3 and CO related to long- of Shanxi province and Inner Mongolia. range transport Associated with the Asian summer monsoon, the In the proceeding section, we examined the concen- CategoryCC air mass—which originated in South Asia trations of O3 and CO in different categories of air mass. and passed over Central China—contained moderately However, transport patterns can change significantly high levels of O3 and CO with a mean value of 56 ppbv with seasons with different temperatures, levels of solar (711) and 353 ppbv (7114), respectively. These levels radiation, and rainfall, all of which can also strongly are comparable to those for CategoryNW, suggesting influence emissions, chemical transformations, and the that emissions in Central China had an impact on the removal of trace gases (Lam et al., 2001; Pochanart et abundance of trace gases in this category. al., 1999; Wang et al., 2001a). It is, therefore, of great As for the Categories LP and ME, although both interest to further investigate the monthlyvariations in passed over the eastern coastal regions, the former transport and its impact on the behavior of O3 and CO contained much higher levels of O3 (57712 ppbv) and at Mount Tai. Fig. 6 gives the percentage distributions CO (4447226 ppbv), than the levels of 41710 ppbv for of the six categories of air mass for the 5 months under O3 and 253778 ppbv for CO for CategoryME. The study. It is evident that there were large variations lower levels of O3 and CO in ME can be attributed to within the months in occurrence of each air-mass group. the fact that the air masses of this group travelled faster Consistent with the Asian monsoons, the proportion of and spent less time over coastal areas with strong CategoryNW increased sharplyfrom Julyto November, emissions of pollutants. In contrast, the higher levels of reflecting more frequent transports of air from the ARTICLE IN PRESS

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100 Unclassified 80 ME CC 60 EA 40 LP N 20 NW Percentage of Category (%)

0 July August September October November

Fig. 6. Percentage of occurrence of air-mass categoryin each month.

northwest in the cold season; in contrast, there were was carried out to investigate the characteristics of long- larger proportions of Categories N, ME, and CC in the range transport at Mount Tai and its impact on the summer. There were small month-to-month variations measured levels of trace gases. The major results are in the percentage of CategoryLP, except in August summarized as follows. when a Subtropical High dominated over the region. The mean O3 and CO mixing ratio was 58 and Categories CC and ME were not shown because they 393 ppbv, respectively, which were generally higher than occurred onlyin summer. those observed at other rural mountainous sites in the Figs. 7a and b show the monthlyvariations in the middle latitude of the Northern Hemisphere. The very average O3 and CO mixing ratio in Categories N, NW, high concentrations of CO at this site are consistent with LP, and EA, and a comparison with the corresponding those previouslyobserved at flatland rural sites in China monthlymean values obtained at Lin’an in 1999–2000 (Wang et al., 2001a, 2004b), indicating strong emissions (Wang et al., 2001a, 2002). All categories have similar of CO. The pattern of summer high and autumn low for monthlypatterns for O 3, i.e., high in summer and low in O3 can be attributed to such seasonal changes in autumn. The summer–autumn trend at Mount Tai meteorological conditions as solar radiation and tem- contrasts with that observed at the flatland site of perature. For CO, dynamic transport processes, such as Lin’an, where a higher autumn average was indicated mountain-valleybreezes and long-range transport, and with a yearly maximum in May (Wang et al., 2001a). changes in PBL heights were the main factors affecting For CO, the monthlyvariations are much more the monthlyvariations in CO, which also exhibited a complex. CategoryN showed decreasing concentrations summer high and autumn low pattern. The O3 and CO from July/August to November, suggesting a decrease in concentrations showed an evident daytime maximum, the influence of emissions on the lower troposphere due especiallyin summer, indicating the transport of to weakening convective activities in autumn compared regionallypolluted air masses to the summit during to summer. In comparison, CategoryLP indicated a daytime. O3 and CO showed a moderatelystrong small trend of increase, suggesting more influence from positive correlation (r ¼ 0:60) in Julywith a slope of boundarylayerpollution for this categoryof air mass, 0.08, which is in good agreement with the value observed whose pattern is consistent with that observed at Lin’an at the flatland site Lin’an in the eastern central region of (Wang et al., 2001a). Further studies will be needed to China, but is much smaller compared to those understand the three-dimensional dynamic transport (0.3–0.4 ppbv/ppbv) observed in North America. A back influencing the variations in trace gases measured at trajectoryanalysisshowed that air masses were mainly Mount Tai. originating from northern China (passing over the North China Plains) or were re-circulating over the Shandong Peninsula. Summertime air masses traveling 4. Summary and conclusions in the low troposphere from northern China had the highest concentrations of O3 (73717 ppbv) and CO We presented continuous measurements of O3 and (5037230 ppbv), revealing that the low troposphere in CO obtained from Julyto November in 2003 at Mount North China was significantlypolluted in summer due Tai, the highest Peak in the North China (Huabei) to strong convections transporting PBL pollution to the Plains. The temporal behavior of O3 and CO, including free troposphere. The trajectories also show notable the monthlyand diurnal variations, and the O 3–CO contributions of emissions from Korea and Japan to the correlation were discussed. A back trajectoryanalysis levels of O3 and CO at Mount Tai. ARTICLE IN PRESS

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Fig. 7. Seasonal variations in the mixing ratios and standard deviations (whiskers) of (a) O3 and (b) CO for each air-mass group: N, NW, EA, LP, and Lin’an result (Wang et al., 2001a). ARTICLE IN PRESS

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Acknowledgments Cheung, V.T.F., Wang, T., 2001. Observational studyof ozone pollution at a rural site in the Yangtze Delta of Chian. The authors would like to thank C.N. Poon and H.C. Atmospheric Environment 35, 4947–4958. Cheung for their hard work in setting up the measure- Fischer, H., Kormann, R., Klyupfel, T., Gurk, Ch., Konigstedt, ment site and Y.H. Kwok for his help in processing the R., Parchatka, U., Muhle, J., Rhee, T.S., Brenninkmeijer, data. We are grateful to Professor Wang Wenxing for C.A.M., Bonasoni, P., Stohl, A., 2003. Ozone production and trace gas correlations during the June 2000 MINA- his encouragement and support of this study. We thank TROC intensive measurement campaign at Mount Cimone. Dr. Yin Yongxuan for his help in selecting the site and Atmospheric Chemistryand Physics3, 725–738. deploying the instruments, the Mount Tai meteorologi- Jaffe, D.A., Mahura, A., Kelley, J., Atkins, J., Novelli, P.C., cal station for providing surface meteorological data, Merrill, J., 1997. Impact of Asian emissions on the remote and the NOAA Air Resources Laboratory(ARL) for North Pacific atmosphere: interpretation of CO data from providing the HYSPLIT transport and dispersion Shemya, Guam, Midway, and Mauna Loa. Journal of model. We thank Professor Donald Blake for analyzing Geophysical Research 101, 2037–2048. the whole air samples collected in canisters. This Kajii, Y., Someno, K., Tanimoto, H., Hirokawa, J., Akimoto, research was funded mainlybythe Research Grants H., 1998. 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