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Beijing aerosol: Atmospheric interactions and new
trends
Benjamin Guinot, Hélène Cachier, Jean Sciare, Yu Tong, Wang Xin, Yu
Jianhua
To cite this version:
Benjamin Guinot, Hélène Cachier, Jean Sciare, Yu Tong, Wang Xin, et al.. Beijing aerosol: Atmo-spheric interactions and new trends. Journal of Geophysical Research, American Geophysical Union, 2007, 112 (D14), �10.1029/2006jd008195�. �hal-03190677�
Beijing aerosol: Atmospheric interactions and new trends
Benjamin Guinot,1He´le`ne Cachier,1Jean Sciare,1Yu Tong,2Wang Xin,2and Yu Jianhua2
Received 30 October 2006; revised 15 January 2007; accepted 13 March 2007; published 31 July 2007.
[1] Beijing aerosols are scrutinized as a case study for atmospheric interactions in a complex multisource situation. For the first time, fine (<2 mm) and coarse (>2 mm) aerosols were continuously collected during a time period (20 months) long enough to capture seasonal trends of sources and interactions. Weekly samples were obtained from January 2003 to August 2004 downtown and during 9 months at two periurban sites. Aerosol samples were chemically characterized (black carbon (BC), organic carbon (OC), and major ions) and dust was obtained from mass closure. Concentration data were smoothed and boundary layer height (BLH) corrected in order to better identify sources and processes. All yearlong, the coarse aerosol is dominated by dust (75%) whereas the fine mode is dominated (46%) by carbonaceous particles. Photochemistry is an intense driving force for secondary aerosol formation including secondary organic aerosol (SOA). Dust particles present a reactive surface for secondary aerosol formation from the intense anthropogenic pool of acidic gaseous precursors (SO2, HNO3, and volatile organic compounds (VOCs)). These interactions favor the formation of a very significant coarse fraction for SO4, NO3, and POM, a feature almost never encountered in developed countries. Surprisingly too is the presence of fine NH4NO3in summer. A new result is also that the winter ‘‘heating season’’ appears at present of minor importance with, however, a significant component from domestic heating as traced by BC/OC. In the future, traffic is likely to dominate downtown anthropogenic emissions. Year-to-year variability in meteorological conditions is likely to influence inputs from arid regions and from regional industrial and biomass burning sources.
Citation: Guinot, B., H. Cachier, J. Sciare, Y. Tong, W. Xin, and Y. Jianhua (2007), Beijing aerosol: Atmospheric interactions and new trends, J. Geophys. Res., 112, D14314, doi:10.1029/2006JD008195.
1. Introduction
[2] By 2007, half of the world population will be urban. Urban growth has been experienced in developed countries since the second half of last century, but only for a couple of decades in developing regions. As a consequence, by 2025, urban populations of developing countries are expected to increase four to five times faster than in more developed countries. Rapid urbanization of millions of people will lead to the emergence of 22 megacities (10 millions inhabitants and over) by 2015, including 16 in developing countries [United Nations Population Fund (UNPF ), 2004]. In addi-tion, to health and socioeconomical challenges, such urban-ization implies deep environmental concerns, mainly regarding water and air quality. In countries with rapid industrialization, air pollution is dominated by high con-centrations of atmospheric particles. In 2002, among the 50 largest cities affected by particulate matter (PM), 35 were located in Southeast Asia and most of them (23) in China
[Baldasano et al., 2003], which, on a global scale, figures out the important role of this part of the world.
[3] The study of Beijing atmospheric pollution and tem-poral trends is of particular interest as this megacity has recently undergone deep mutations regarding energy con-sumption. These mutations significantly affect pollutant loadings, atmospheric interactions, and atmospheric chem-ical and radiative balances. Thus the necessary scientific investigations and emission controls which are planned and/or achieved today in the capital city might be considered as case studies and key actions to support the management of other cities.
[4] Source array affecting Beijing atmospheric environ-ment is complex and presents an important temporal varia-bility. Annual PM concentrations are still found at high level (PM10in 2004: 149mg m3) compared with national standards in China (PM10: 100mg m3) and much too high to meet European or US standards (PM10: 40 mg m3 and 20mg m3in 2010). The environmental situation of Beijing has been brought to international attention since the city has been chosen to organize the 2008 Olympic Games. This event certainly accelerates implementation of measures for pollution abatement. However, the municipal government has been tackling air pollution problems since the 1990s and proved in the past how drastic measures can inflect atmo-spheric pollutant concentrations.
1
Laboratoire des Sciences du Climat et de l’Environnement, Gif-sur-Yvette, France.
2
Beijing Municipal Environmental Monitoring Centre, Beijing, China. Copyright 2007 by the American Geophysical Union.
[5] For the last decade, Beijing air particulate pollution has brought a growing volume of scientific publications from the first interests in dust events affecting PM10 in spring [Davis and Jixiang, 2000; Whittaker et al., 2003; Wang et al., 2004] to source apportionment attempts based on PM2.5chemical composition [He et al., 2001; Sun et al., 2004]. But continuous studies covering several months and concerning both the coarse and fine fractions of the aerosol are lacking.
[6] Results presented in this paper originate from a Sino-French scientific collaboration which has been initiated since 2002 between the Municipality of Beijing and the Ile-de-France Region in order to drive an extensive charac-terization of particles and their interactions in the atmo-sphere of the Chinese capital city. Subsequently, the aim has been to identify anthropogenic sources which could be subjected to abatement policy. After a short introduction of issues relative to the economics, energy resource trends, and environment of the capital city, we present chemical measurements performed on weekly aerosol samples col-lected continuously downtown, during the 20-month period from January 2003 to August 2004. The exhaustive chem-ical characterization comprises the dust component, the two carbonaceous fractions (black carbon and organic carbon), and the major ions, and is conducted on two size fractions of the aerosol. It will be shown that this data set provides a relevant tool for identification of the major sources and their characterization and allows to gain original information on atmospheric interactions and processes.
2. Beijing: Economics, Energy Resources, and Environment
2.1. Beijing Urban Area
[7] Beijing (39°480N; 116°280E; 44 m a.s.l.) is located at the northwest border of the Great North China Plain, surrounded by mountains 1500 – 2000 m high in the north and west, about 100 and 50 km far from the urban area, respectively. Agricultural activities girdle the city, especially northward and eastward with wheat, cotton, and corn as the main crops. The industrial area is located southwest and south of Beijing. It includes various activities from tradi-tional industries (metallurgy, machinery, oil and petrochem-ical, power, coal, light, and textile), which should be all moved out of the Fourth Ring Road, or shut down, by 2008. [8] Since the mid-1980s, Beijing has experienced a rapid and high economic growth (7% to 9% of gross domestic product (GDP) per year), which has stimulated and still participates to very significant mutations mainly regarding demography, urbanization, and urban area spreading. Beijing population is approximately 14 million, including an estimated floating population of 3 million people [Leclerc, 2005]. The municipality urban area has expanded up to 30% since the 1980s and is now included within the Fifth Ring Road, which is approximately 40 km in diameter. Over 3600 construction sites are simultaneously underway within this area [National Bureau of Statistics of China, 2004]. At a regional scale, the Hebei Province (70 million inhabitants) girdles two megacities, Beijing and Tianjin (10.3 millions inhabitants), which are located 120 km away from each other. This highly populated region (almost 100 millions inhabitants) spreads over 250,000 km2[Leclerc,
2005] and drives major concerns about regional pollution in general and strong atmospheric pollution by particles espe-cially [He et al., 2002; Aldhous, 2005]. Noteworthy, as anywhere else in East China, the potential for rural depopu-lation in Hebei is particularly high.
2.2. Anthropogenic Sources and Expected Trends [9] Coal is the traditional energy source and is burnt for industrial purposes and home heating or cooking. Its con-sumption is about 28 million tons a year, with an increase during the winter. Coal consumption still represents 65% of Beijing’s energy needs [Energy Information Administration (EIA ), 2003]. Today’s coal quality remains low (with high-sulfur content) and both industrial boilers and domestic settings have poor combustion efficiencies (60% and 20%, respectively). As a result, the use of coal is expected to be responsible for important emissions of sulfur dioxide (SO2) and carbonaceous particles. However, Beijing is phasing out old facilities and shifting toward cleaner coal and industrial settings, along with the use of other energy sources. These measures have visible effects on SO2 concentrations and particle concentrations too [Dan et al., 2004; Wang et al., 2005a; Duan et al., 2006]. In the long term, the situation might improve by diversifying fuel resources such as natural gas and using advanced technologies (gas-based and renewable energy). But in the short term, compared to other alternatives, low-sulfur coal provides the optimum ratio of economical to environmental benefits [Fan and Yu, 2004]. Thus coal is likely to remain the main energy resource in Beijing for the next decades.
[10] Following GDP growth, the vehicle fleet has increased about tenfold since the 1970s, to reach more than two million vehicles in August 2003 [National Bureau of Statistics of China, 2004]. Vehicles severely impact air quality [Hao et al., 2000; Duan et al., 2006] especially at peak hours when congested traffic displays low operating speed with frequent stop-and-go. Projections for the next two decades expect this explosion of car population to continue, with however drastic adapted policies to constrain their emissions. Investments are devoted to promote clean energy-based vehicles, and improvements in public trans-portation are particularly supported. The subway network is also being strongly expanded from two lines in 2003 to expected eight lines in 2008. Since 2002, Beijing has the largest fleet of natural gas buses in the world (200,000 public and private together). Furthermore, 20,000 taxis and 2800 old buses with high particulate emissions have been discarded in 2005. Accompanying this set of policies for cleaner transportation, motorcycles have been forbidden since 1999 within the Fourth Ring Road (but are still very significant outside).
[11] An additional feature of Beijing region is the influ-ence of numerous and aged industrial settings mostly located in the outskirts of the city as most of the industries have been recently banned off Beijing city and are now progressively replaced by new technology industries (http:// www.ebeijing.gov.cn). At last, domestic biomass burning may be too an important contributor to atmospheric pollu-tion, especially in the outskirts in winter.
[12] To anticipate future changes in Beijing atmosphere, it is important to observe the results of previous important policies applied during the two last decades, which have
allowed for decreasing trends in spite of increasing fuel consumption. However, all pollutants do not exhibit same trends. Figure 1 presents the parallel evolutions of selected species: total suspended particles (TSP), SO2, and NOxfrom 1986 to 2004. During this period, population and vehicle fleet have increased. From 1990 to 2003, coal consumption shows a 12% increase too. Conversely, SO2 (and CO) displays an inverse pattern with a sharp decrease since 1998. Aerosol also has been quoted to present a significant decrease trend either for the PM2.5component [Duan et al., 2006] or the carbonaceous OC [Dan et al., 2004] or BC [Wang et al., 2005a] components. Detailed data show that SO2 decrease is particularly significant in winter, which shows the effectiveness of policies regarding coal quality and usage for heating. These results may be explained by the application of a severe abatement policy strategy regarding emissions from coal combustion. Interestingly, this trend implies that coal burnt at present in Beijing region is of better quality, and that industrial settings using coal rely on more modern technologies. Conversely, NOx displays an increasing trend with a possible recent stabilization, where-as the vehicle fleet, which is supposed to be its the most important contributor, has been multiplied by six from 1990 to 2004, with an acceleration of the increase rate since 2001. NOxconcentration illustrates the rising pollution from vehicles. The Law of Air Pollution and Control, which went into effect on September 2001, is, however, expected to induce a decreasing trend during the next decade. At last, particulate concentrations (TSP) remain unchanged despite the numerous construction works in the city, indicating that local dust and anthropogenic emissions might be under control. It may be pointed out that these trends reflect drastic policies applied in Beijing but are not representative of other cities in China.
[13] Finally, measures which may be efficient for anthro-pogenic sources are susceptible to be counteracted by uncontrollable meteorological phenomena. Regional dust affects the Beijing region due to land degradation in Hebei [Tang and Peng, 2002] and long-range transported dust
particles are well known to alter Beijing air quality in spring during dust events. These uncontrolled dust inputs are likely to influence anthropogenic aerosol loads too [Guo et al., 2004; Wang et al., 2005b]. Another striking influence is that of typhoons, which are shown to block the synoptic flow and thus the export of Hebei province pollution as visible on satellite imagery (such as that obtained by the Moderate Resolution Imaging Spectroradiometer (MODIS)) and under such conditions air pollution in Beijing may be very severe. As a consequence of regional climate change, dust inputs and typhoon frequencies might be disturbed with thus a possible impact on Beijing pollution.
3. Experimental
3.1. Sampling Sites and Strategy
[14] Aerosols were continuously studied during 20 months, from 9 January 2003 to 28 August 2004, in Beijing down-town, hereafter termed Beijing Center. The experimental site is located downtown on Chegongzhuang road, 4 km West from Tiananmen Square, between the Second and Third Ring Roads. Samplings were performed on the roof of Beijing Municipal Environmental Monitoring Center (BMEMC), 30 m above the ground. In addition to the BMEMC monitoring network including tapered element oscillating microbalance (TEOM) fitted with a PM10inlet, SO2, NOx, CO analyzers, and meteorological sensors (wind direction, wind speed, temperature, and relative humidity (RH)), two filter sampling lines were added for the analysis of the bulk carbonaceous fraction (black carbon, BC, and organic carbon, OC) and the fine and coarse major ions of the aerosol, respectively. Both sampling lines were operated in parallel for the separation of fine (<2 mm) and coarse (>2mm) particles at controlled flow rate (1 m3h1) in order to obtain the Stack Filter Unit (SFU) prescribed cut-off. Adapted short periods of sampling (4 min h1) were equally distributed through days and nights to obtain a continuous and thus representative pattern, while avoiding filter clog-ging. Samples were integrated over seven days to limit the sample number at a manageable level (n = 84). At last in Figure 1. 1986 – 2004 trends for TSP, SO2, NOx, vehicle fleet, and population in Beijing (data from nine
order to document export of particles, during the January to September 2003 period, two additional sites were similarly investigated at Changping (Ming Tombs) (n = 28) and YuFa (n = 28), about, respectively, 40 km north and south Beijing downtown site (Figure 2). These periurban sites are termed Beijing North and Beijing South in the discussion.
3.2. Chemical Analysis
[15] Aerosols were analyzed for their carbon and ion contents in the two size fractions. The carbonaceous fraction was analyzed on prefired quartz-fiber filters (Whatman QM-A, 47 mm in diameter). Mass weighing and ion extraction were performed using the 47 mm Nuclepore polycarbonate membranes of 0.4 and 8.0 mm porosity. The carbon content was analyzed by coulometry and the BC/OC separation was obtained using a two-step thermal method to determine BC and OC, as described by Cachier et al. [1989]. Prior to analysis, quartz filters were submitted to HCl fumes to remove carbonates, a step which might be crucial for the dust-containing Beijing aerosols. Reproduc-ibility is assessed to be of the order of 5% for OC. All Nuclepore filters were weighed using a Mettler Microbal-ance UMT3 with 1mg sensitivity, after a 24-h equilibration at room temperature and controlled relative humidity below 30% in order to capture the dry aerosol mass. The ion contents were determined by ion chromatography (IC) after a 45 min duration ultrasounding step in ultrapure water to ensure aerosol dissolution [Sciare et al., 2005]. IC analysis provides concentrations for the major soluble inorganic species (Na+, NH4+, K+, Mg2+, Ca2+, Cl, NO3, SO42, and PO43) and some light organic acids including oxalate. No significant on-site contamination has been detected from
blank analyses. The major ion concentrations are given with an average uncertainty of 0.05mg m3.
4. Results and Discussion
4.1. Aerosol Chemical Closure and Mean Composition [16] The chemical mass closure was attempted for each individual sample based on a new simple protocol that has been applied satisfactorily for various urban and semiurban aerosols [Guinot et al., 2006a]. This protocol avoids multi-elemental analysis of mineral dust and generally provides a satisfactory agreement between the gravimetric and the reconstructed masses. For the present work, a correlation has been found with a slope in the range of 0.98 – 1.02, allowing further robust discussion on aerosol sources and processes. The mass closure relies on the separation of the aerosol fine and coarse fractions for which BC, OC, ion contents, and mass are determined. For both fractions, two unknowns have to be solved: the abundance of calcium in dust and the OC/POM conversion factor. As previous works have shown that dust calcium is almost completely solubi-lized during a prolonged ultra-sounding step [Sciare et al., 2005], soluble Ca2+ content is used as a proxy for dust calcium. In Beijing aerosols, Ca2+ is found to be linearly correlated to the missing mass which is the difference between the weighted mass (m) and the concentration mass of analyzed species (BC + POM + ions). The first step applies to the coarse fraction which is dominated by dust. For this fraction, the OC component is minor and its conversion into POM is arbitrarily fixed (1.8). Then a (Ca2+/missing mass) correlation coefficient is obtained (slope in the range 0.05 – 0.025; correlation coefficient Figure 2. Comprehensive map of Beijing Region and the three experimental sites (from Microsoft1
Encarta12006 # 1993 – 2005).
better than 0.80). The second step aims to solve the chemical composition of the fine fraction where POM is the most important component. Fine dust mass is obtained from the fine Ca2+ content and hypothesized to have the same composition as in the coarse fraction which for Beijing aerosols is corroborated by previous works [Sun et al., 2004]. Then the OC/POM conversion factor is obtained by adjustment to the weighed mass. After this treatment of analytical data, the mass closure is obtained for each individual samples with an undetermined component representing less than 5% in mass.
[17] For our interseason experiment, the coarse mass was not available but has been reconstructed from the Ca2+ contents using the conversion factor calculated from the samples obtained during the 1-month intensive winter and summer experiments. The method is sensitive enough to distinguish two types of dust mixtures along the 20-month experiment: one containing 9% in mass of calcium, and a second one characterizing local dust identified to be mostly due to vicinal works (taking place in the June to September 2003 period) with higher calcium contents (13%). Another interesting output of our chemical closure protocol is the OC/POM conversion factor value which here in Beijing was found to be high (1.7) and fairly constant over the whole year. Such a high value reflects functionalized organic aerosols and is quite expected in summer when the secondary formation of organic aerosols takes place significantly. However, for winter months, this value is much more unexpected and could originate from significant emissions produced during coal burning.
[18] Figure 3a presents the mean picture of the aerosol over the 12-month period from August 2003 to August 2004, as its relative chemical mass composition for both fractions. The coarse mode is largely dominated by dust, but anthropogenic aerosols (BC, POM, and major ions), how-ever, account for more than 27% of the average coarse aerosol mass (106.5 mg m3). For the fine mode, POM is the dominant component (38%) and is surprisingly much more important than sulfate (13%). In this fraction, dust still represents a significant component (24%).
[19] Comparison between the Beijing and Paris mean annual aerosol composition, obtained by the same sampling and analytical means, shows similarities in spite of a fourfold difference of aerosol mass (170.1 mg m3 and 42.4mg m3, respectively, for the bulk aerosol, 63.7mg m3 and 15.3mg m3for the fine aerosol). NO3/SO42ratios are, however, lower for Beijing (fine: 0.50; coarse: 0.65) than for Paris aerosol (fine: 0.86; coarse: 1.56) and the majority of European cities (fine: 0.84; coarse: 1.00, from Putaud et al. [2004a]). This result points to the predominant inputs of the traffic source in European cities. Conversely for Beijing, it might indicate major inputs from sulfur-rich sources (possibly coal combustion as the dominant energy supply and industrial emissions) superimposed to traffic emissions, which calls for further sulfur control policies. The slightly higher carbonaceous (BC and OC) aerosol abundance in Beijing fine aerosols and the significant abundance of coarse BC particles may be other indicators of emissions from coal usage.
[20] In order to capture possible changes in the chemical composition of Beijing aerosols, we attempted to compare the results with those from other works. Among the vast
amount of scientific literature on Beijing aerosols, work by He et al. [2001] was found to be suitable for comparison. Indeed, as Beijing undergoes important day-to-day and night-day variability of meteorological conditions, continu-ous sampling over a long period of time is adequate to get a mean picture of the aerosol phase. Both works from He’s group and our work present this type of data for fine particles (PM2.5 and PM2, respectively). The comparison appears particularly relevant as similar continuous sam-plings were performed on a weekly basis during a whole year at the same downtown site (Chegongzhuang road). A careful look at the air mass back-trajectories (http:// Figure 3. (a) Comparison of Beijing and Paris coarse (>2mm) and fine (<2 mm) relative aerosol chemical com-position from a 12-month average. Mean mass concentra-tions are 106.5 and 63.7mg m3for Beijing, and 27.1 and 15.3 mg m3 for Paris. (b) Beijing fine anthropogenic aerosol chemical composition in 1999 – 2000 (adapted from the work by He et al. [2001]) and 2003 – 2004 (this work).
www.arl.noaa.gov/ready/cmet.html) shows that during the two different years of investigation the clean (northern) sector and the polluted (southern) sectors have similar occurrence and thus the site has undergone similar meteo-rological conditions.
[21] To make the comparison between the two fine aerosol data sets as accurate as possible, we first selected in our experiment a 12-month period (August 2003 to July 2004) corresponding to the period chosen for He’s group experiment four years before. Then, the experimental BC/OC split was harmonized following the results obtained from an intercomparison of carbon measurements con-ducted on Beijing aerosol [Guinot et al., 2006a]. It was shown that the two-step method used in this work provides 12% less BC than the thermal optical reflectance (TOR) method used by He et al. [2001] whereas OC is increased by 28%. We next increased the OC-to-POM conversion factor used by these authors from 1.4 to 1.7 following our mass closure results. After these modifications, dust content is evaluated and mass closure can be achieved with a low (4%) undetermined mass. Fine dust is found to be almost the same for the two yearly experiments (12.3mg m3and 15.7mg m3) which confirms our previous assessments on similar meteorological conditions during the two experi-mental time periods. Conversely, the fine aerosol anthropo-genic component appears almost twice lower in 2004 than four years before (50.8 mg m3and 91.5 mg m3, respec-tively). These discrepancies are not found, however, in the aerosol PM10 in which the annual mean concentration displays an 8% decrease only (from 162 mg m3 in 2000 to 149 mg m3 in 2004). Mitigation policies applied in Beijing since 1998 (as replacing coal by natural gas in the downtown areas, see section 2.2) may have reduced signif-icant concentration levels of the fine anthropogenic aerosol. The observed discrepancies are also likely to be due to
different experimental conditions. Different sampling probes (SFU for PM2 in this work and cyclone for PM2.5 in the work by He et al.) with different flow rates were used, and the cut-off efficiency may have been different. But most importantly, samplings were operated at different heights (4 m against 30 m high in the present work) and near ground samples are likely to be more loaded. However, a recent experiment conducted on the Institute of Atmospheric Physics (IAP) Beijing meteorological tower has shown that, in spite of concentrations vertical gradients, aerosols inside the urban canopy display similar chemical composition [Guinot et al., 2006b]. These considerations argue for the relevance for the two data sets of comparison concerning the mean relative chemical composition of the anthropo-genic aerosol. This comparison is shown in Figure 3b, and it may be seen that major species exhibit similar relative abundance with a 3% to 4% difference at most. This result might suggest that the array of dominant anthropogenic sources has not encountered dramatic changes between 2000 and 2004. However, the shift of BC/TC in the fine aerosol from 21% to 28% might point to the increasing influence of traffic emissions and the decreasing influence of domestic use of coal. From the comparison with Paris aerosol shown earlier (Figure 3a), it may be recalled that Beijing fine anthropogenic particles appear chemically close to those found in western countries.
4.2. Beijing Climatology
[22] Beijing climatology is characterized by cyclic and contrasted seasonal conditions (Figure 4 and Table 1). Monthly averages of temperature and relative humidity (RH) are minimum in January (4°C and 10%) and maximum in July and August (30°C and 95%). Although seasonal maximum and minimum appear comparable from 2003 to 2004, temperature weekly variability is significant Figure 4. Records of temperature, relative humidity (RH), precipitation, and boundary layer height
(BLH). Temperature and RH values are normalized. Precipitation unit is millimeter-modified to fit the scale. All values are smoothed.
(20% on average) and RH week-to-week variations often exceed 50%, which calls for careful examination of these two parameters in the interpretation of detailed data. Simi-larly, while the precipitation pattern shows a clear seasonality from dry winter season to rainy summer season, year-to-year variability may be important as, for instance, rainfall was about six times higher from January to March 2003 than during the same period in 2004 (24 mm and 4 mm, respectively). More importantly, to properly picture out seasonal variations of Beijing pollutant concentrations, it appears critical to also consider the influence of the boundary layer height (BLH). Indeed, the BLH displays a significant seasonal variability with weekly average values ranging from 300 m to 910 m (data from Hysplit model http:// www.arl.noaa.gov/ready-bin/mainarc.pl). This variation has a direct influence on atmospheric concentrations, from enhancement in winter (when BLH is minimum) to dilution in summer (when BLH is maximum), and might be con-sidered to account for ventilation effects too. Finally, it may be seen from Figure 4 that BLH variations precede temper-ature changes of a few weeks, suggesting that BLH is primarily triggered by solar radiation.
[23] Wind pattern and air mass trajectories are important agents for atmospheric concentration levels. All the year-long regional winds are driven by a synoptic wind creating a broad Northern flow from WNW to NE. Computation of wind data and back-trajectory statistical analyses from the National Oceanic and Atmospheric Administration (NOAA) Hysplit products [Draxler and Rolph, 2003] suggest that, in the October to April period, wind speeds are often maxi-mum and coupled to the strong Northern flow which therefore drives wind direction at all altitudes. Conversely, during the rest of the year (April to September), average wind speed is 40% slower and the pattern may be different especially during the day when the urban heat island effect is maximum and influences the regional flow. At this period, consequently, the vertical coupling between the different urban boundary layers is poor [Guinot et al., 2006b] and air masses impacting Beijing city predominantly originate from a wide south sector. Atmospheric pollution over Beijing is then expected to be more influenced by the populated and industrialized south and southwest surround-ings as air masses move slowly and at low altitudes before entering the city area.
4.3. Seasonal Variability of Gaseous and Particle Species
[24] In order to facilitate data analysis, weekly concen-tration averages are smoothed and presented in bold curves in the figures whereas actual weekly results are in dot curves. Smoothing consists in a 5-week balance using the following weights: 3 for week W 0, 2 for W ± 1, 1 for W ± 2. Furthermore, in Figures 5 to 10, concentrations are multiplied by the BLH in order to get rid of its influence and are hereafter termed BLH corrected concentrations.
[25] Variations of gaseous and particulate species which are classically monitored were first investigated: NOx, SO2, TEOM PM10, and PM2from filter weighing (Figures 5a, 5b, and 5c). In spite of BLH correction, the SO2pattern remains unchanged with pronounced winter peaks. This is due to both an increased consumption of coal for heating purposes and the lack of oxidation processes at this season. Although (BLH corrected) SO2 (Figure 5c) concentrations in winter appear to increase approximately by a factor of 4, it thus may be underlined that the increase of SO2 emissions is much lower. Conversely, NOx (Figure 5c) and PM10 (Figure 5a) patterns are leveled by BLH correction but display many relative maxima. In comparison with SO2, these species show a much less contrasted pattern with a factor of 2 to 2.5 only between the extreme BLH corrected concentrations. This result indicates that several active sources contribute to Beijing pollution over the whole year. For particles, PM10 and PM2 follow a similar pattern displaying the highest concentrations during the spring dust season with, however, different PM2/PM10 for the two spring seasons of the experiment. High levels of fine particle concentrations in spring confirm that dust inputs influence both coarse and fine aerosols. However, through-out the year, the fine aerosol fraction represents a sustained background of breathable particles in Beijing (36% on average of the TSP and more than 50% in other works as discussed earlier, section 4.1). The PM2/PM10 ratio values recorded for the whole experiment are shown in Figure 5b. It may be seen that this ratio does not display a clear seasonality which reinforces the picture of numerous sources and processes contributing to Beijing aerosols. PM2 presents relative maxima in late spring and fall, possibly driven by biomass burning emissions [Duan et al., 2004; Zheng et al., 2005a, 2005b] and significant concentration levels in summer which are probably due to the photochem-ical formation of secondary particles. Finally, when using the BLH corrected concentrations, the winter season appears surprisingly weak and mostly driven by coarse particles.
[26] Total aerosol optical thickness (AOT) of the atmo-spheric column obtained from the Beijing AERONET website (http://aeronet.gsfc.nasa.gov/) is also reported in Figure 5a. Globally, both AOT and PM display parallel seasonal trends with a general decrease from the spring to the winter season and this interesting covariation points to the prominent role of the highly loaded atmospheric surface layer in the attenuation of solar radiation.
4.4. Deciphering Sources and Processes Through the Study of Individual Species in Beijing
[27] The following discussion is based on chemical data which are presented in Table 2 for annual means of the fine
Table 1. Twelve-Month Mean and Extremum Data From 07 July 2003 to 09 July 2004
Mean Min Max
Temperature (°C) 14.7 3.4 29.5 RH (%) 46.4 9.9 94.5 BLH (m) 586 377 912 Wind Speed (m s1) 7.5 1.5 11.8 Wind Direction (°) 5 – – TSPa 171.7 64.0 466.9 PM10b 131.7 57.9 236.7 PM2a 64.5 22.1 151.2 SO2 57.7 5.2 177.4 NOx 133.6 51.0 298.9 CO (ppm) 1.9 0.8 4.4
Unless otherwise stated, concentrations are inmg m3.
aMasses obtained from filter weighing. b
Figure 5. AOT, NOx, SO2and PM10, and PM2BLH corrected concentrations for the 20-month study. PM10is from TEOM and PM2from filter weighing. (a) Particulate data and AOT, (b) PM2/PM10ratio, (c) gas data.
and coarse fractions. Detailed data are primarily illustrated in Figures 6 – 10.
4.4.1. Fine and Coarse Calcium Variations
[28] Calcium may be used as a reliable tracer for dust inputs [Van der Hoven and Quade, 2002; Putaud et al., 2004b]. Average bulk calcium concentrations are of the order of 8.5 mg m3, which is high compared with other megacities; for example, 2.0mg m3in Paris, 0.25mg m3 in Los Angeles, 0.72 mg m3 in Vienna, 4.1 mg m3 in Lahore (data from Guinot et al. [2006a] and Salam et al. [2003]). Throughout the year, fine calcium concentrations too are also much more important in Beijing (1.2 mg m3) than in Paris for instance (0.2 mg m3). Fine dust mean
concentrations are calculated to be 12.9mg m3(22% of the fine aerosol mass), which therefore points to necessary dust control policies to better meet PM2.5standards. Remarkably, as dust is made of surface-active particles, their abundance in both fractions might favor various interactions with other species, as assessed later in this work.
[29] In Beijing, the dust season is known to be charac-terized by high mean aerosol loadings in spring driven by abrupt dust events. Due to dust enrichment in calcium, these dust events are expected to create high calcium concen-trations. During the 2003 to 2004 period, dust inputs may be assumed to be relatively weaker than average as supported by back-trajectory analysis showing a smaller occurrence of the Northern and Western sectors capable to create desert dust atmospheric transport (49% and 52% for the years 2003 and 2004, compared with 60% for the year 2005). In the calcium record (Figure 6), some dust events are actually visible in spring 2004 only, with high calcium concentra-tions in both fine and coarse fracconcentra-tions (weekly averages may undergo a sixfold increase). This observation is also in agreement with official statistics quoting no dust event during the exceptional spring 2003 season (http://www.cma. gov.cn/). Spring 2003 might be defined as a dust haze season, with no dust event but showing a durable impreg-nation of the atmosphere by suspended dust particles. During a dust haze season, visibility is generally reduced to less than 10 km but remains at acceptable levels compared to with less than 1 km during proper dust storms as in spring 2004. Our 2003 to 2004 record presents also high calcium concentrations from June to September 2003 which are to be related to construction works that occurred close to the sampling site. So it may be assessed that
Table 2. Twenty-Month Mean and Extremum Data From 06 January 2003 to 27 August 2004
Species
Mean Min Max
Coarse Fine Coarse Fine Coarse Fine
Massa 104.96 58.66 33.67 13.64 353.23 148.75 BC 3.05 5.02 0.19 1.16 15.95 16.30 POM 13.61 22.13 2.51 5.06 33.11 50.67 Sulfate 7.57 9.99 0.63 0.33 33.35 32.61 Nitrate 5.26 4.18 0.08 0.28 33.64 14.80 Ammonium 2.22 4.12 0.13 0.08 15.34 19.04 Other ionsb 1.81 1.92 0.10 0.09 8.18 13.17 Calcium 7.27 1.21 1.12 0.02 30.29 5.14 Dust 71.31 12.90 11.20 0.28 302.87 57.12 Fine/Bulkc 0.36 – 0.29 – 0.30 – BC/TC 0.28 0.28 0.11 0.27 0.45 0.35 NO3/SO42 0.72 0.42 0.13 0.08 2.58 1.18 Concentrations are inmg m3. a
Coarse and fine masses are obtained from gravimetric measurements.
b
Other ions comprise K+, Cl, Na+, PO43, and Mg2+. c
From filter chemical analyses: Coarse + Fine aerosol mass fractions.
Figure 6. Coarse and fine calcium BLH corrected concentrations for the 20-month study (note the two different concentration ranges for Y-axis).
calcium data appear as a suitable species to trace dust inputs.
[30] Fine and coarse calcium variations roughly follow the same pattern, but the two spring dust seasons present different characteristics. In 2003, coarse and fine calcium does not exhibit clear covariations whereas in 2004, the dust storms are clearly marked in both calcium fractions. Inter-estingly, the Cafine/Cabulk ratios show some variability during the course of the 2004 dust storm season, which could be related to different dust source regions and different transport pathways. These hypotheses are rein-forced by back-trajectory analysis pointing to different air mass origins, the western air masses exhibiting the highest Cafine/Cabulkratios which may be hypothesized to be due to the major influence of fine loess particles. This could be confirmed by comparison of the metal content (for example, Ca/Al ratio) of the different dust particles as proposed by Sun et al. [2005].
[31] The two different dust seasons might be also char-acterized by a different scavenging efficiency of anthropo-genic species as shown by the NO3/Ca and SO42/Ca mass concentration ratios which are much higher for both fine and coarse modes during the 2003 dust season than during the 2004 season. As an example, NO3/Ca mass ratios are calculated to be 1.7 and 0.4 for the coarse mode and 9.2 and 2.4 for the fine mode showing the same increase tendency in the whole aerosol size range. For a better characterization of dust inputs, however, event-based samplings would be more appropriate than the weekly samplings used in this exper-iment [Sun et al., 2005].
[32] In June 2003, the isolated fine calcium peak might originate from the numerous on-field biomass burning taking place in the southern part of the basin after wheat harvest [Zheng et al., 2005a, 2005b]. The biomass burning emissions are expected to impact Beijing when prevailing winds are in the favorable direction [Duan et al., 2004]. The enhancement of coarse calcium concentrations in the bio-mass burning aerosol is likely to be due to dust resuspension by fires.
[33] For the fine aerosol, biomass burning particulate emissions are known to be enriched in fine calcium [Echalar et al., 1995]. In the Beijing data set, the fine calcium peak is also accompanied by a chloride peak and a significant fine potassium enhancement which suggests the formation of KCl particles as already found in biomass burning aerosols [Echalar et al., 1995; Liu et al., 2000]. The chloride concentration profile is, however, elsewhere very noisy probably due to source region variability and interactions with dust which by its basic nature allows chlorine to remain in the particulate phase (and prevents its escape as HCl). For the following year, biomass burning in May to June appears to poorly affect Beijing aerosol with, however, an influence still visible on both the fine and coarse calcium concen-trations which is not the case for chloride concenconcen-trations. 4.4.2. Carbonaceous Species
[34] As shown from the Beijing mean aerosol chemical composition (Figure 3a), carbonaceous species dominate the anthropogenic species in both size fractions (Table 2). Carbonaceous species in the coarse mode are so abundant that coarse carbon represents more than one third of the total carbon mass. Whereas previous studies have insisted on winter coal burning as the major source of carbonaceous
particles in Beijing [e.g., Dan et al., 2004; Yang et al., 2005], BC and POM (BLH corrected) concentrations mitigate the importance of the heating season. The 20-month data profiles suggest sustained combustion BC and POM inputs all yearlong, pointing to a multiplicity of intense combustion sources and formation processes (Figure 7). Unexpectedly too, BC is not much correlated to POM in both modes. Thus to gain insight about sources, we used the BC to TC ratio which has already been successfully used for this purpose in several works [Cachier, 1998]. The variations of the BC/TC ratio values are shown in the last part of Figure 7. This figure indeed enlightens the different behavior of the two fractions. Despite a similar mean value (0.28) for both the fine and coarse fractions, the two modes appear actually very different. The BC/TC ratios show a marked seasonal-ity, particularly in the coarse fraction, pointing to three periods of interest.
[35] (1) During the heating season, while coal emissions are characterized by high BC/TC values in the coarse fraction (0.40), fine particles have an average BC/TC of 0.25, slightly lower than the annual mean. Different coal burning sources could contribute to both the fine and coarse fractions. The winter season data show that one source is enriched in coarse BC; the other emits more fine POM. Competing inputs from domestic and industrial emissions might explain such a pattern, the domestic usage which is a low temperature combustion, producing always more particles with relatively more organic material [Cooke et al., 1999].
[36] (2) When the dust period starts, BC/TC lowers primarily because POM concentrations enhance significantly. This enhancement might be explained by the scavenging of semivolatile organic gases by dust particles. Interestingly, this phenomenon occurs in both fractions and is more significant in 2003 than in 2004.
[37] (3) In summer in spite of important rainfalls, POM concentrations remain high probably due to intense secondary photochemical formation, and consequently coarse and fine particles display the lowest BC/TC values (respective sum-mer averages: 0.15 and 0.27). In Beijing, the fine aerosol fraction may be sustained by significant nucleation as observed previously [Wehner et al., 2004] and secondary organic aerosol (SOA) formation is one candidate for this process. However, in the coarse mode, SOA formation is likely to be driven by active condensation involving acidic SVOC and preexisting dust. Indeed, photochemistry is expected to form organic acidic precursors from the VOC pool, and interaction with dust would be favored by acid-base reactions [Cachier, 2005].
[38] Oxalate is another organic species of interest. It has both primary and secondary origins [Kawamura and Ikushima, 1993]. Background concentrations are weak but the photochemical season is observed by a sizeable increase of concentrations in both size modes. Although oxalate is a nonvolatile species, its capture by dust particles is highly probable due to its acidic nature which may explain its presence in the coarse mode. Biomass burning too may be a source of primary oxalate aerosol [Baudet et al., 1990; Andreae and Merlet, 2001]. In the present data (Figure 8), oxalate pattern points to the wheat harvest burning season in June which apparently may severely impact Beijing aerosol. Oxalate displays a sharp concentration enhance-ment in June 2003 and this enhanceenhance-ment is accompanied by
Figure 7. BC and POM BLH corrected concentrations for the 20-month study. (a) Coarse fraction, (b) fine fraction, (c) BC/TC ratio for both fractions.
that of chloride and to a lesser extent of potassium, both tracers biomass burning in aerosols (cf. section 4.4.1 and works by Echalar et al. [1995] and Liu et al. [2000]). The 2004 wheat harvest season is also recorded, however, at a lower intensity. Interestingly, oxalate to potassium ratios appear different for two years (2.0 in 2003 and 0.2 in 2004), which may suggest a different meteorological condition or combustion type and/or different dust inputs from resus-pension as dust is also marked by potassium. Specific studies to identify biomass burning would benefit event-basis samplings, as similarly noted for the characterization of dust events (section 4.4.1).
4.4.3. Sulfate and Nitrate
[39] Throughout the study, bulk SO42and NO3 concen-trations display high values (17.6mg m3and 9.6mg m3). Coarse NO3 and SO42 concentrations represent a very significant portion (55% and 43% respectively) of the bulk concentrations, a feature which is never encountered in western cities. Concentrations show parallel variations with no obvious seasonality. The second year of the experiment is marked by a strong decrease of concentrations for both species. For the fine mode for both years, the season of photochemical production (April to September) appears to favor high concentrations (Figure 9). Recalling the volatility of ammonium nitrate, the presence of substantial amounts of fine particulate nitrate in summer is quite surprising and might be explained by either interactions of nitric acid with fine dust or by the presence of a film of water onto particle surface where ammonium nitrate may absorb. Fine NO3 and SO42 do not clearly exhibit parallel trends, and for these species of secondary origin, the ratios SO42bulk/SO2 and NO3bulk/NOxmay be used to provide a proxy of gas-to-particle conversion efficiencies. For both species, higher ratios are found in spring and summer. However, the photochemical season appears more active in 2003 than in 2004 (2003 mean SO42bulk/SO2 and NO3bulk/NOx values: 0.55, 0.12; and 2004: 0.25, 0.09, respectively) which may
partly explain the lower concentrations observed in 2004. RH, which may also influence partition of SO42and NO3 between the gas and the particulate phases, was found to be low and almost similar during the two 2003 and 2004 periods (39% and 34%, respectively).
[40] The importance of NO3and SO42in the aerosol mix (Table 2) reflects primarily the respective influence of mobile and stationary sources. But the overwhelming pre-sence of dust in Beijing is also likely to influence their concentrations in the particulate phase as gaseous precursors (NO2, SO2) and/or oxidized acids (HNO3, H2SO4) may interact with dust particles [Guo et al., 2004; Wang et al., 2005b]. The mean NO3/SO42 ratio values are 0.77 and 0.48 for the coarse and fine fractions, respectively, suggesting that in the coarse mode dust favors the formation of nitrate. The NO3/SO42ratio variations all yearlong might be also a tool to evidence the impact of different sources and the effect of atmospheric processes [Arimoto et al., 1996; Yao et al., 2002]. The observed discrepancy between the two species is now scrutinized in light of the NO3/SO42ratio variations which are presented in Figure 10 as normalized values. The fine mode presents pronounced negative anomalies from April to September. Recalling that nitrate species in this mode is ammonium nitrate, these anomalies may be due to ammonium nitrate volatilization above the observed temperature threshold 15 – 20°C in agreement with recent findings obtained in Europe by Schaap et al. [2004]. However, negative anomalies are more pronounced in 2003. A first explanation may be that more intense photochemis-try in 2003 enhances SO42formation close to the source. A second cause may be due to more pronounced industrial inputs which may increase Beijing downtown concentra-tions, and indeed back-trajectory analyses show that in 2003 spring and summer air masses predominantly passed over the industrial sources located SSW of Beijing. For the fine aerosol, winter and spring positive anomalies may be interpreted as a scavenging of gaseous precursors by the Figure 8. Coarse and fine oxalate concentrations for the 20-month study.
background of fine dust particles, this scavenging being more efficient for nitrate than for sulfate. It may be seen from Figure 10 that the 2003 dust haze season does not show this anomaly, which is in accordance with the absence of significant fine dust at this season. Finally, the important positive anomaly in October could be due to increased formation of ammonium nitrate and reduced formation of sulfate as temperature is getting lower. The coarse mode does not show conclusive trends. In summer, however, the maximum is found in July to August and is concomitant with the negative anomaly in the fine mode. It may be hypothesized that the amount of gaseous HNO3which may not condensate in the fine mode as NH4NO3is neutralized and trapped by the basic coarse dust particles. This
phe-nomenon is more important in 2003 from June to September, as local works occurred close to our sampling site.
4.5. Insights on the Distribution of Pollution in Beijing Region
[41] A comparison between the three sites of the exper-iment was performed for the January to June 2003 period. It must be recalled that this period is off any identified dust event and before the construction works close to the Beijing Center site. Results are presented in Table 3 and show a comparable mean coarse and fine aerosol at the two north and south sites, whereas Beijing Center aerosol presents higher concentrations. A statistical study of back-trajectories shows an important diversity of the regional wind direc-Figure 9. Sulfate and nitrate BLH corrected concentrations for the 20-month study. (a) Coarse fraction,
tions, which supports the homogenization of the pollutants of different origins. The regional background thus originates from either the Beijing source point area (where traffic is important) or from the densely populated and industrialized Hebei region.
[42] The fine mode is chemically similar for the three sites with a 40 – 50% decrease in mass for the two periurban sites. This result confirms the efficient mixing of fine particles in the area. At Beijing South site, however, the higher relative importance of sulfate and nitrate fine aero-sols could be due to the local formation of ammonium nitrate and sulfate from the surrounding industrial emis-sions. Conversely to the fine mode, the coarse aerosol is different and twice less important outside Beijing. From these results, it can be inferred that Beijing city is a source of fine particles spreading over the region and a source of coarse particles which seems to influence primarily the city. The coarse mode in Beijing Center is enriched in secondary inorganic and organic species compared to the two suburban sites. In this mode, the sum of nitrate, sulfate, and POM represents 46% in Beijing Center and 33% outside the city. So it may be further assessed that local dust in Beijing is an efficient scavenger of acidic gaseous precursors and this process is so efficient that sulfate concentrations in the coarse mode are higher than in the fine mode. An important fraction of these coarse heterogeneous particles is likely to rapidly fall out. In the near future, the expected decrease of construction works and, thus, the reduction of local dust might significantly modify the anthropogenic aerosol parti-tion between the coarse and the fine fracparti-tions, and the subsequent gas-to-particle interactions. As a consequence, the export of fine anthropogenic aerosol and/or of its gaseous precursors might be enhanced.
5. Conclusion
[43] To anticipate future changes and thus provide reli-able insights for new abatement policies in Beijing, it is
necessary to picture out the sources and the atmospheric processes which are responsible of the present particulate pollution. For this purpose, aerosols were continuously collected during a long period (from January 2003 to August 2004) for further complete chemical characteriza-tion of two size modes. Mean Beijing bulk (fine and coarse) aerosol mass concentration (163.6 mg.m3) is still found well above the national and international standards and is characterized by a significant fraction of fine particles. Comparison with previous chemical mean concentration data suggest from 2000 to 2004 a significant lowering of coal domestic usage and an increase of traffic emissions.
[44] Concentration data were analyzed in light of prevailing sources and atmospheric processes with focus on seasonality and year-to year variability. As in most megacities, Beijing presents a complex situation where the background of anthropogenic particles is sustained by several combustion Figure 10. Variations of NO3/SO42ratio anomalies. Mean values of NO3/SO42are 0.77 and 0.48 for
the coarse and fine fractions, respectively.
Table 3. Mean Coarse and Fine Concentrations Between January 2003 and August 2003 for Beijing Center, Beijing North, and Beijing South Sites
Species
Center North South
Coarse Fine Coarse Fine Coarse Fine
Massa 101.16 64.30 54.57 31.74 45.74 37.12 BC 2.61 5.83 1.55 4.65 1.12 4.77 POM 14.33 24.22 3.98 19.31 3.84 18.91 Sulfate 12.36 11.97 5.08 6.63 4.29 8.44 Nitrate 9.12 5.00 4.22 4.11 3.46 4.95 Ammonium 3.87 5.31 1.37 3.07 0.76 4.01 Other ionsb 3.80 2.17 1.59 1.52 1.74 2.36 Calcium 6.47 0.87 4.16 0.40 3.41 0.35 Dust 54.64 8.17 35.65 3.38 32.57 3.97 Fine/Bulkc 0.39 – 0.37 – 0.45 – BC/TC 0.24 0.31 0.39 0.31 0.29 0.30 NO3/SO42 0.74 0.42 0.83 0.62 0.81 0.59 Concentrations are inmg m3. a
Coarse and fine masses are obtained from gravimetric measurements.
bOther ions comprise K+, Cl, Na+, PO 4
3, and Mg2+. c
From filter chemical analyses: Coarse + Fine aerosol mass fractions.
sources of comparable intensity. At present, coal combustion remains a significant contributor to Beijing atmospheric pollution all yearlong, with, however, no more marked increase in concentration during the heating season. This new surprising result may enlighten recent abatement policies regarding coal quality and coal usage. Two coal combustion types could be identified (the industrial use and the domestic one) with different characteristics. Photochemistry in Beijing is an intense driving force for secondary aerosol formation including SOA which are the main aerosol anthropogenic component in summer. Although the two years of the experiment were abnormally poor in dust events, dust is found at high concentrations in Beijing aerosol throughout the year in both the coarse and the fine modes (71.3mg m3 and 12.9 mg m3, respectively). As dust is an efficient scavenger of gaseous precursors, the chemical pattern of the anthropogenic aerosol due to condensation processes is partly dependent of its abundance in both modes. Interest-ingly, due to the conjunction of atmospheric impregnation of dust all yearlong and to the importance of the gas pool, the dust-gas interactions sustain a very significant anthropogenic component in the coarse mode (sulfate, nitrate, and POM) probably by condensation of acidic precursor (HNO3, SO2, and volatile organic compounds (VOCs)) onto basic dust particles. In favorable conditions for some species, the anthropogenic coarse component may be more abundant than the fine one. Remarkable too is the presence of fine nitrate aerosol during the summer season where high tem-perature is expected excluding ammonium nitrate formation. This surprising NH4NO3 formation may be due to either dust-HNO3 interactions or to the capture of ammonium nitrate by the water coating of preexisting particles. The year to year and seasonal variability of dust inputs may thus contribute to create a chemical variability pattern of the coarse and fine anthropogenic aerosols.
[45] Furthermore, due to its singular meteorological and topographic situation, Beijing pollution cannot be explained only by its proper sources but is also influenced by emis-sions from the densely populated region around. Imports of industrial pollution frequently occur especially when air masses enter the city from the southeast to southwest sectors. This situation presents a seasonal pattern with a maximum occurrence in July and a year-to-year variability as shown by the two winter-spring chemical data sets of the experiment.
[46] In the future, traffic is likely to dominate downtown anthropogenic emissions and to drive the PM2.5mode and its overwhelming carbonaceous fraction. The sharp decrease of construction works, the translocation of industrial settings outside Beijing, and the extinction of domestic usage of coal are expected to create abrupt changes of the primary and secondary aerosols in the city and to impact pollution export in the Beijing basin. However, if polluting activities remain in the region, dust-gas interactions and intricate exchanges between Beijing city and its outskirts are likely to sustain a significant background of anthropogenic organic and inor-ganic particles inside Beijing at a higher level than in western countries. Modern and efficient industrial settings injecting aerosols and/or gaseous precursors at high altitude might modify this influence and further experimental and modeling studies of pollutant three-dimensional dispersion and interactions are needed. Finally, it must be kept in mind
that regional climate change may affect dust concentrations over Beijing and thus anthropogenic aerosol concentrations too as a significant portion of the anthropogenic component is due to entrainment of gases by dust particles.
[47] Acknowledgments. Authors are indebted to Mathieu Mamers for his contribution to the analytical work and to BMEMC personnel for facilities at the network stations. Thanks to Fre´de´ric Chevallier (LSCE) for providing BLH data, and to Chen Hongbin (IAP-CAS) for the AERONET data. LSCE gratefully acknowledges Ile-de-France Region, CEA, CNRS/ DRI, PRA, and ADEME for their indefectible help.
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