Characteristics of trace gaseous pollutants at a regional background station in Northern China

We present measurement results of trace gaseous pollutants obtained at the Shangdianzi (SDZ) Global Atmosphere Watch (GAW) regional station in Northern China, from September 2003 to December 2006. The gases include ozone (O3), nitrogen oxide(s) (NOx=NO+NO2), sulfur dioxide (SO2), and carbon monoxide (CO). During 5 the study period, the mean annual O3 concentrations were 30.1±21.0, 32.8±19.1 and 30.9±19.8 ppbv in 2004, 2005 and 2006. The corresponding NOx values were 14.5±14.0, 11.0±11.3 and 12.7±11.8 ppbv, respectively. The mean annual SO2 concentrations were 5.9±10.0, 6.1±9.9 and 7.6±10.2 ppbv in 2004, 2005 and 2006, while the mean CO levels were 586±415 and 742±558 ppbv in 2005 and 2006. The data 10 obtained at SDZ station are compared with the results measured at other background sites in China as well as abroad. The concentrations of O3, NOx, SO2, and CO at the SDZ background station are found to have clear seasonal and diurnal variations. The impacts of local and remote pollution sources on the regional air quality are assessed using trace gases concentration roses and 3-day back trajectories of air masses arriv15 ing at the SDZ station.


Introduction
Human activities have exerted large impacts on the global environment.Anthropogenic emissions of gaseous and particulate matters have been altering the levels of many atmospheric species, causing air pollution, climate change, and other environmental problems.Air pollution has become one of the important factors affecting public health (Ostro, 2004) and its influences can be of a regional scale or even a global scale (Lelieveld et al., 2002).Changes in the atmospheric compositions have very likely caused the observed global warming (Forster et al., 2007).Large increases in anthropogenic emissions of the photochemical precursors, NO x in particular, have caused substantial increases in the global background O 3 mixing ratios over the past century (Jaffe et al., 2003;Marenco et al., 1994).Acid rain has already been recognized as a regional-scale environment problem in China, and SO 2 is the most important precursor (Wang and Wang, 1995;Wang et al., 2001a;Zhao et al., 1988).CO has an effect on the oxidization of the atmosphere through interaction with hydroxyl radical (OH), which also reacts with methane, halocarbons and tropospheric ozone.
Observation of the changes in background atmospheric composition is an essential way to understand the influence of human activities on the atmospheric environment and global change (Fischer et al., 2003;Jaffe et al., 2003;Jaffe and Ray., 2007;Meng et al., 2007;Tang et al., 2007;Yan et al. 2008).Campaigns have been carried out in the Yangtze Delta and Pearl River Delta, China (Wang et al., 2001a, b;2003a, b;Xu, et al., 2008), showing significant impacts of human activities on the regional air quality.Some studies focused on major gaseous pollutants in Northern China (Ding et al., 2002(Ding et al., , 2008;;Hao et al., 2005;Ma et al., 2004;2006;Meng et al., 2002Meng et al., , 2008;;Wang et al., 2006;Xie et al., 2005;Xu et al., 2005).So far, field measurements of key air pollutants in rural locations in Northern China are comparatively sparse.In this paper, we present measurements of the key reactive gases, i.e., O 3 , NO x , SO 2 , and CO, from a background station in North China, characterize the levels and variations of the gases in the background area of Northern China, discuss the sources and factors affecting the concentrations of these gases.

Measurement site
The Shangdianzi station (SDZ, 40 • 39 N, 117 • 07 E, 293.3 m a.s.l.) is located in the northeast of Beijing, with a distance of about 150 km to the urban area of Beijing.Beijing sits in the northern edge of the North China Plains, with a population of more than 15.8 million in 2006 (Beijing Municipal Bureau of Statistics, http://www.bjstats.gov.cn.About 55 km southwest of SDZ is the nearest township, Miyun town, with a population of about 0.426 million.Within 30 km of the site, there are only small villages in mountainous areas with a sparse population and thus an insignificant anthropogenic emission source.Surrounding the site, fruit trees and corn are grown in the slope fields.
SDZ is one of the WMO/GAW regional background stations in China.Located on the north edge of the North China Plain, the station is setup to capture the atmospheric background data of the North China Plain. Figure 1 shows the location of SDZ and some cities like Beijing, Tianjin, Tangshan, Zhangjiakou, etc. surrounding the SDZ site in Northern China.The northern regions of SDZ are much less inhabited, encompassing of the vast grassland of Inner Mongolia and mountainous rural regions of Hebei province, where the population is relatively sparse and the industrial activities are less prevalent.

Instrumentation
A set of commercial instruments have been used for continuous observation of the reactive gases.O 3 was measured with a UV photometric analyzer (Thermo Environmental Instruments (TEI), Inc., model 49C).NO, NO 2 and NO x were measured with a chemiluminescence analyzer (TEI, model 42CTL), and SO 2 was measured by using a pulsed UV fluorescence analyzer (TEI, model 43CTL).CO was measured with a gas filter correlation analyzer (TEI, model 48C).All instruments were housed in an air-conditioned room.Ambient air is sucked from 1.8 m above the rooftop of the room through a stainless steel manifold using a ventilator and is introduced to the Teflon sample inlets of the instruments.The residence time of air in the manifold is about 20 s.Automatic zero and span checks were done everyday to check for possible analyzer malfunction and calibration drift.More frequent zero checks (every 6 h) for CO analyzer were carried out because of the inherent zero drift of this type of CO analyzer.The multi-point calibrations were performed  at approximately 3-month interval, and before or after any adjustments are taken on the instruments.Reference SO 2 , NO, and CO gas mixtures (National Institute of Metrology, Beijing, China) have been used for span checks and multipoint calibrations of the SO 2 , NO x , and CO analyzers, respectively.The O 3 analyzer has been span-checked using an O 3 source from the dynamic gas calibration system (TEI 146C) and calibrated using O 3 Primary Standard Calibrator (TEI 49CPS), which are traceable to the Standard Reference Photometer (SRP) maintained by WMO World Calibration Centre in Switzerland.Five-minute average data were stored in the data logger and hourly averaged values are presented in this paper.

Observed levels and comparisons with data from other background sites
The statistics of concentrations of trace gases measured at the SDZ site are shown in Levels of pollutants at a regional background station are closely related to the regional population density, industrial activities, efficiency of emission control, etc.To gain a better understanding of air quality of the SDZ area, monthly mean levels of SO 2 , NO 2 , CO, and O 3 at SDZ are compared with those observed at some other regional background stations in China and other countries at similar latitudes (Table 2).In China, there are another two WMO/GAW regional background stations, i.e., Lin'an (LA, 30.18 • N, 119.44 • E, 138.6 m a.s.l.) and Longfengshan (LFS, 44 • 73 N, 127 • 60 E, 310.0 m a.s.l.).The locations of these stations are also marked in Fig. 1.The LA station is situated on the southern edge of the Yangtze Delta region, which is a densely populated and fast developing region.
LFS is a remote site in China's Northeast Plain, a sparsely populated and underdeveloped region.Data in Table 2 indicate that the SDZ site is as polluted as the Lin'an site in terms of the reactive gases and is much more polluted than the LFS site in terms of primary gases.It is not surprising that the background levels of pollutants at SDZ are so high because the North China Plain region, which encompasses the city cluster Beijing-Tianjin-Bohai Bay, is an economically vibrant and densely populated region.
As can be seen in Table 2, the monthly mean NO 2 concentrations at Shangdianzi are higher than those measured at Longfengshan, China, and Pleven, Bulgaria, but much lower than those measured at Burgas, Bulgaria.The monthly mean SO 2 concentrations at Shangdianzi are close to the median level among all stations with SO 2 values in Table 2.The monthly mean CO concentrations at Shangdianzi are close to those measured at Lin'an, China, significantly higher than those measured at Longfengshan, China, and much higher than those measured at Ryori, Janpan.The differences of the monthly mean O 3 concentrations among the stations are not as large as those of primary pollutants concentrations.However, it is noticeable that among all sites listed in Table 2, Shangdianzi shows the largest variability of O 3 .Such large variability is caused by strong influences from emissions in the North China Plain and by the contrast air masses (Lin et al., 2008).

Seasonal variations
The monthly mean concentrations of O 3 , NO x , SO 2 , and CO at SDZ are shown in Fig. 2     September.This year-to-year variation in the seasonal cycle may be due to year-to-year alternation in the meteorological conditions (Wang et al., 2001b).On average, the seasonal variation of O 3 at SDZ shows a double-peak pattern, with the primary and secondary peaks being in June and September, respectively.This seasonal pattern reflects the contribution of photochemical production of O 3 during the period of intensive solar radiation and the influences of Asian monsoon, which brings more clouds and rainfall, and causes lower O 3 in July and August (Wang et al., 2001b;Wang et al., 2008;Xu et al., 2008).
The monthly mean concentration of NO x ranged from 3.5 ppbv in August 2006 to 28.3 ppbv in October 2004 and that of SO 2 ranged from 0.1 ppbv in July 2004 to 15.2 ppbv in February 2006.On the average, NO x and SO 2 showed a pattern of higher concentration in winter and lower concentration in summer.This kind of seasonal cycle is attributable to the seasonal differences in photochemical reduction, removal by rainfall, vertical mixing, etc. Being a station in the middle latitudes in Eastern Asia, SDZ can be characterized by cold/dry winter and hot/humid summer.Precipitation in the areas surrounding SDZ occurs mainly in the summer months.For example, in 2006 about 85% of annual precipitation in the Beijing region occurred in July and August.Therefore, the NO x and SO 2 concentrations at SDZ in summer are lowered by more rapid photochemical reduction, more removal by rainfall, and better vertical mixing.In winter, the concentrations of these gases increase due to smaller removal, strong inversion, and increased emission from heating sources.
The seasonal pattern of CO looks quite different from those of NO x and SO 2 .There are more fluctuations in the seasonal cycle of CO than those of NO x and SO 2 and the seasonal trend of CO is inconsistent with those of NO x and SO 2 .High monthly mean CO concentrations occurred in some warmer months (e.g., June, July, and October, 2006), when NO x and SO 2 were low.
The elevated CO levels at SDZ may be attributed to the transport of regional pollution and also to more intense biomass burning activities in the North China Plain.Very intensive biomass burning occurs during the harvest periods in some major Chinese agricultural regions, such as the North China Plain, leading to higher CO levels.Similar patterns of monthly mean CO and O 3 levels were also observed at the Miyun site (about 50 km from SDZ) in the summer of 2006 (Wang et al., 2008), which are attributed to the influence of the summer monsoonal circulation that develops over the North China Plain in July.When  summer monsoonal circulation develops, more air masses from southwest are transported to SDZ.These air masses containing much higher concentrations of pollutants from Beijing city and other parts of the North China Plain can cause elevated background levels of pollutants at SDZ.The observed data of wind and CO in summer months of 2006 shows that the average CO levels and even the wind speeds from SW-W sector were not much different between June and July, but in July the frequencies of wind from this sector were much more than those in June.Therefore, it is likely that the higher monthly mean of CO in July than in June was related to more frequent transport of CO-rich air from the SSW-W sector.

Diurnal variations
Diurnal variation of atmospheric species gives insight into the interplay of emission and chemical and physical processes operating on a diurnal cycle (Ma et al., 2002b).The average diurnal variations of O 3 , NO x , SO 2 , and CO at SDZ in different seasons are shown in Fig. 3.The diurnal patterns of O 3 for all seasons look similar, with minima and maxima in the early morning and afternoon, respectively.There are two noticeable differences in the diurnal patterns, i.e., the diurnal amplitude and the peak time.The largest diurnal amplitude occurred in summer (38.3 ppbv), followed by fall (29.8ppbv), spring (17.6 ppbv), and winter (9.5 ppbv), respectively.The O 3 concentration peaked in winter, spring/fall, and summer at 15:00, 16:00, and 17:00 Beijing Standard Time, respectively.The late peaking time is believed to be a result of transport of O 3 and precursors from urban area to SDZ.
The diurnal cycles of NO x show valleys around noon and higher values during the nighttime.This phenomenon can be attributed to the day-night differences in the chemical removal of NO x and the height of the mixing layer.The diurnal variations of NO x for winter and fall show little difference between each other, with the maximum of NO x being about two-fold of the minimum, suggesting that the vertical mixing is an important factor for the observed diurnal variation as photochemical removal is much slower in these seasons than in the warmer seasons.The concentrations of NO x in summer and spring were significantly lower than those in winter and fall at corresponding time of day, particularly during the night.The diurnal amplitudes of NO x in summer (4.2 ppbv) and spring (3.9 ppbv) were much smaller than those in winter (9.4 ppbv) and fall (11.3ppbv).
The diurnal variations of SO 2 for all seasons were inconsistent with those of NO x .The diurnal patterns look like those of O 3 , with minimal values in the early morning and maximal values in the afternoon or around noon.An important difference between the diurnal patterns of SO 2 and those of O 3 is that the average peaking time for SO 2 was at the earliest in summer, followed by spring, fall, and winter, respectively, a reversed sequence of the peaking time for O 3 .The diurnal patterns of SO 2 may indicate the existence of a higher SO 2 layer above the nighttime and early morning inversion.The fact that SO 2 peaks earlier in the warmer seasons than the colder seasons support this point because the vertical mixing develops more quickly in the warmer seasons, so that the surface level of SO 2 increases more quickly, leading to earlier peaking of SO 2 in the warmer season than colder seasons.Possible sources of this higher SO 2 layer may be high chimneys in factories and power plants, the major SO 2 emission sources in North China.Additional measurements and modeling studies are necessary for a more robust explanation to the observed diurnal variations of SO 2 .
As can be seen in Fig. 3, the diurnal variations of CO for different seasons look similar to those of NO x , with lower values during the daytime and higher values during the nighttime.The diurnal amplitudes of CO in winter and fall were much larger than those in summer and spring, as seen in the diurnal variations of NO x .Unlike the case of NO x , the average concentrations of CO in summer and spring did not drop to low levels so that there was no large seasonal difference in the average CO concentration.In summer, the average CO concentration changed little with the time of day, showing only a small diurnal variation.As mentioned in the previous section, more intense biomass burning activities and southwesterly winds were responsible for higher CO concentrations.Due to the mountain topography, southwesterly winds dominate during the daytime, favoring the transport of pollutants from the urban area of Beijing to SDZ.Similar results were obtained at a rural site north of Beijing (Wang et al., 2006).

Air mass backward trajectories and corresponding pollutant concentrations
The long-range transport of air pollution has been the topic of scientific research for several decades, and the importance of long-range transport has been increasingly recognized (Cape et al., 2000;Kim et al., 2005;Ma et al., 2002a, b;Meng et al., 2007;Nolle et al., 2002;Pongkiatkul and Kim Oanh, 2007).
To gain an insight into the impact of transport on the reactive gases at SDZ, 72-h air mass backward trajectories were calculated using the HYSPLIT 4 model (HYbrid Single-Particle Lagrangian Integrated Trajectory model version 4.7, http: //www.arl.noaa.gov/ready/hysplit4.html; Draxler and Hess, 1998).The trajectory calculations were done four times each day from 2004 to 2006, with the start times of 00:00, 06:00, 12:00, and 18:00 UTC, respectively.The trajectories were clustered for 2004, 2005, and 2006, respectively, using the cluster analysis function integrated in the HYSPLIT 4 model.Figure 4 shows the results of the cluster analysis, including cluster means and the proportions of different clusters.The trajectories of each year can be grouped into five major clusters.As can be seen in Fig. 4, the cluster means did not vary much from year to year, with cluster 1 indicating air masses from the North China Plain region, cluster 2 from Mongolia or Inner Mongolia passing over Shanxi Province and Hebei Province, clusters 3 and 4 from high altitudes (above 880 m) of Mongolia and Russia passing over Inner Mongolia and Hebei Province, and cluster 5 from Mongolia passing over Inner Mongolia and Hebei Province.
Based on the statistics, in all the years, cluster 1 was the most important for the SDZ site, contributing 37%-48% of air masses.The contributions from clusters 2 to 5 varied largely from year to year.The year-to-year variations in the proportions of different clusters reflect partly the climato-   27.9 11.0 6.3 556 logic differences in the wind systems over the Northeast Asia region, which can influence the transport of pollutants to the SDZ area.To know the seasonal variations in the trajectories, monthly frequency occurrence of each type of air mass arriving at SDZ were calculated and are shown in Table 3.Based on this table, trajectories in cluster 1 can occur in any month but mostly in the summer months, trajectories in clusters 2-4 occur mainly in colder months, and trajectories in cluster 5 seem to be randomly distributed over the year.
Since the emission sources of pollutants are unevenly distributed in the areas surrounding the SDZ site, air masses from different directions contain different levels of pollutants.To characterize the dependences of the pollutants concentrations on air masses, statistics of hourly average concentrations of gaseous pollutants were made for correspond-ing clusters of backward trajectories and are summarized in Table 4. Large differences in the concentrations of primary pollutants exist among the clusters, with cluster 1 corresponding to the highest CO levels and higher NO x and SO 2 levels, cluster 2 corresponding to the highest NO x and SO 2 levels and higher CO levels.
Since cluster 1 represents air masses originating from the North China Plain region, which are quite polluted by industrial, vehicular, and biomass burning emissions, it is not surprising that the highest CO levels and higher NO x and SO 2 were observed in this cluster of air mass.The maximum occurrence of cluster 1 in summer (Table 3), together with the highest CO level, may be responsible for the enhanced CO level in summertime (see Sect. 3.2).Air masses in cluster 2 traveled over China's key coal mining and power www.atmos-chem-phys.net/9/927/2009/Atmos.Chem.Phys., 9, 927-936, 2009 generation regions in Inner Mongolia, Shanxi Province, and Hebei Province (e.g., Datong, Zhangjiakou, etc.).This explains the highest NO x and SO 2 levels corresponding to cluster 2 (Table 4).Although the occurrences of cluster 2 air masses were less than 15% for the whole year, it could be higher than 20% and even 30% in some winter months, contributing to the high concentrations of NO x and SO 2 at SDZ in winter.The data in Table 4 suggest that the NO x , SO 2 , and CO concentrations corresponding to clusters 3 and 4 were low if not the lowest ones.This is attributable to the less polluted air over the northwest sector and its higher traveling heights and velocities.Cluster 5 represents air masses that traveled over a relatively clean sector at lower heights and velocities, corresponding to median levels of NO x and CO and fairly lower levels of SO 2 .
The dependence of the concentration of surface O 3 on air masses was not as strong as those of primary pollutants, as shown in Table 4.However, the O 3 level corresponding to cluster 1 was always highest in all the years, suggesting that pollutants from the North China Plain region contribute most significantly to the O 3 level at SDZ.

Conclusions
In this paper we present measurements of O 3 , NO x , SO 2 , and CO made at the Shangdianzi background station in North China from September 2003 to December 2006.Based on these measurements, the annual mean concentrations were in the range of 30.1-32.8 ppbv for O 3 , 11.0-14.5 ppbv for NO x , 5.9-7.6 ppbv for SO 2 , and 586-742 ppbv for CO.The levels of NO x , SO 2 and CO at SDZ were close or comparable to those observed at some other background sites in polluted regions of the similar latitudes, such as Lin'an, China, Burgas and Pleven, Bulgaria, etc., coinciding with the fact that Shangdianzi is a background site representing North China with large population and consumption of fossil fuels and biofuels.
The concentrations of O 3 , NO x , SO 2 and CO at the SDZ station showed clear seasonal and diurnal variations.The average seasonal variation of O 3 shows primary and secondary peaks in June and September, respectively, reflecting the contribution of photochemical production to O 3 at SDZ during summer and autumn and the influences of Asian monsoon in July and August.However, the year-to-year alternation in the meteorological conditions can cause some year-to-year changes in the seasonal pattern of O 3 .
The seasonal variations of NO x and SO 2 , which show maxima in winter and minima in summer, coincide with the influences of photochemistry and regional climate (cold/dry winter and hot/humid summer) in the areas surrounding SDZ.However, the seasonal pattern of CO does not consist with those of other primary gases (NO x and SO 2 ), with elevated CO levels in some summer months.Intensive biomass burning in the North China Plain region, in combination with the transport of regional pollution by more frequent southwesterly winds, is believed to be the cause for this discrepancy.The diurnal cycles of O 3 in different seasons show peaks between 15:00 and 18:00, a few hours later than those in the urban area of Beijing, suggesting that local production of O 3 is less important than the transport of photochemical aged plume.The diurnal variation of SO 2 shows an unusual pattern, with higher concentrations during the daytime and lower concentrations during the nighttime.This pattern is completely different from those of NO x and CO and does not coincide with the common view.It is hypothesized that there is a SO 2 -rich layer above the nighttime and early morning.This hypothesis remains to be tested in the future.
Backward trajectories of air masses were calculated and analyzed in combination with corresponding pollutant concentrations.Based on the results of this analysis, air masses from the North China Plain region contain the highest concentration of CO and higher concentrations of NO x and SO 2 , and cause the highest O 3 concentration; air masses traveling over the coal mining and power generation regions west of SDZ contain the highest concentrations of NO x and SO 2 and higher concentration of CO.Therefore, transport of air masses from these regions is responsible for the high concentrations of the gaseous pollutants.

Fig. 1 .
Fig.1.Location of the Shangdianzi (SDZ), Lin'an (LA) and Longfenshan (LFS) regional background stations of China, as well as some cities in Northern China.

Fig. 1 .
Fig. 1.Location of the Shangdianzi (SDZ), Lin'an (LA) and Longfenshan (LFS) regional background stations of China, and some cities in Northern China.

Fig. 4 .
Fig. 4. Air mass backward trajectories for 100 m above ground at SDZ in 2004, 2005, and 2006.The proportions of different clusters are shown in the upper-right corner of each figure.

Fig. 4 .
Fig. 4. Air mass backward trajectories for 100 m above ground at SDZ in 2004, 2005, and 2006.The proportions of different clusters are shown in the upper-right corner of each figure.

Table 1 .
Statistics results of the measured trace gaseous pollutants during 2003-2006 at SDZ, China.
changed from 586±415 ppbv in 2005 to 742±558 ppbv in 2006.It can be seen in Table1, NO concentrations are very low at the SDZ station, and NO x exist mainly in the form of NO 2 , representing the character of the cleaner region.

Table 2 .
Comparison with the observations made at other sites (monthly value).

Table 3 .
Monthly frequency occurrence of each type of air mass arriving atSDZ in 2004SDZ in  , 2005SDZ in  , and 2006.   .

Table 4 .
Statistics of average concentrations of gaseous pollutants for each type of air mass arriving at SDZ, China.