Asian dust and pollution transport—A comprehensive observation in the downwind Taiwan in 2006

Atmospheric Research 95 (2010) 19–31

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Atmospheric Research j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / a t m o s

Asian dust and pollution transport—A comprehensive observation in the downwind Taiwan in 2006 Shuenn-Chin Chang a,b, Charles C.-K. Chou c, Wei-Nai Chen c, Chung-Te Lee d,⁎ a b c d

Environmental Protection Administration, Taiwan, ROC National Defense Medical Center, School of Public Health, Taiwan, ROC Research Center for Environmental Changes, Academia Sinica, Taiwan, ROC Graduate Institute of Environmental Engineering, National Central University, Taiwan, ROC

a r t i c l e

i n f o

Article history: Received 26 October 2008 Received in revised form 26 May 2009 Accepted 31 July 2009 Keywords: Asian dust Asian continental outflow Long-range transport Aerosol property Aerosol supersite

a b s t r a c t This study analyzed the air pollution episode which affected Taiwan on March 19 and 20, 2006, an event confirmed to have been caused by Asian dust (AD). During this AD event, the maximum hourly values of PM2.5 and PM2.5–10 particles at 136 and 218 µg m− 3, respectively, were recorded at the Taipei aerosol supersite (TAS) located in northern Taiwan. Moreover, of Taiwan's 74 air quality monitoring stations, 65 (on March 19) and 29 stations (March 20) recorded daily average concentrations of PM10 which exceeded Taiwan's air quality standard of 125 µg m− 3. This AD event not only greatly increased the PM2.5 and PM2.5–10 concentrations, but the gaseous pollutants transported by the northeast monsoon were predicted to adversely affect Taiwan as well. In addition to ground measurements, an aerosol layer was detected in Taipei, the thickness of which was verified to be about 800 m through the light detection and ranging (LIDAR) method. During the AD-affected period (defined as the situation where the concentration of PM2.5–10 maintains a high value), the black carbon (BC) fraction of PM2.5 was reduced from 15.7 to 5.6%; sulfate fraction of PM2.5 increased from 7.1 to 19.8%; and the nitrate fraction of PM2.5 was reduced slightly from 10.3 to 8.1% at the TAS site. Moreover, the particles' number size spectra showed a decrease in the amount of particles with a diameter of less than 200 nm, indicating that the rise of the concentration of PM2.5 increased along with the increase of PM2.5 particles with diameters larger than 200 nm. This study demonstrates that anthropogenic pollutants from the Asian mainland including nitrates, sulfates, BC, gaseous pollutants (CO, SO2, and O3), and other fractions of fine particles, would influence the downwind regions as the AD is transported. © 2009 Elsevier B.V. All rights reserved.

1. Introduction The loess areas in northwest China are believed to be important sources of Asian dust (AD) (Huebert et al., 2003). The dust storms triggered by cold air masses passing over northern China and Mongolia were well recognized to significantly enhance PM10 concentrations over Korea, Japan, and Taiwan during the annual winter and spring seasons (Lee et al., 2004; Lin, 2001; Xiao et al., 1997; Lin et al., 2004; Lin et al., 2005; Liu et al., 2006; Lee et al., 2006). The AD ⁎ Corresponding author. E-mail address: [email protected] (C.-T. Lee). 0169-8095/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.atmosres.2009.07.012

is frequently transported in a southward direction by a strong and cold high-pressure system affecting the air quality in Taiwan (Lee et al., 2006). The impact of PM10 (particulate matter with an aerodynamic diameter smaller than or equal to 10 µm) from the AD on northern Taiwan is about 15% on a yearly basis (Liu and Shiu, 2001). On the average, there are four to five dust events and 6.1 dust days in a year in Taiwan (Liu et al., 2006). Chang and Lee (2007a) estimated that 7210 t of PM10 were transported by the AD to the Taipei Metropolitan Area in 2001. Yang (2002) stated that the highest monthly mean concentration of PM10 at 60 to 70 µg m− 3 in northern Taiwan from March through May 1994–1999 was due to AD. As

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expected, the major components of the enhanced PM10 were from the mineral dust present during the event. However, one-quarter of the PM10 concentration was estimated to be made of nitrate and sulfate (Liu and Shiu, 2001). Lee et al. (2006) showed that sulfate was the major constituent of PM2.5 in the transported dust in Taiwan during an AD event in 2002. Meanwhile, concerns over the effect of AD events on public health are increasing. Chen et al. (2004) found that the increase of respiratory deaths among Taipei residents was associated with dust brought in during the AD periods which occurred from 1995 to 2000. Moreover, Lei et al. (2004) showed that an exposure to particulate matter during a dust event could increase lung inflammation and injury in pulmonary hypertensive rats. For these reasons, it is important to reveal the pollutant constituents during AD events as basis for further investigation on their effects on human health. Although several studies have already observed PM2.5 transported from AD in the past, most of these only reported on the daily integrated aerosol samples (Tsai and Chen, 2006). To our knowledge, no study on high time-resolution of aerosol and gaseous pollutants has been conducted in detail in relation to an AD event in the downwind Taiwan. This lack of detailed description on pollutant progression during dust transport is tantamount to the inability to comprehensively understand the dust effect. For instance, Asian dust events were reported to increase cardiopulmonary emergency visits

during the dust-affecting periods in Taipei when ambient PM10 concentrations were above 90 µg m− 3 (Chan et al., 2008). The coupling of gaseous pollutants and aerosol components would greatly help in resolving the dust health effect. Consequently, the aim of this study is to describe the dynamics of how an AD event evolves as the air mass from China moves over Taiwan. In addition to PM10 hourly mass data, we used PM2.5 hourly mass data; hourly data on the PM2.5 content of nitrate, sulfate, black carbon (BC); hourly gaseous pollutants such as carbon monoxide (CO), sulfur dioxide (SO2), ozone (O3), and even shorter aerosol size distributions; and optical properties to infer the characteristics and evolution of the AD event. 2. Sources of monitoring data and instruments The Taiwan Air Quality Monitoring Network (TAQMN) was established by TEPA in September 1993. It includes 71 monitoring stations in Taiwan with three satellite stations located in distant islands, as shown in Fig. 1. Sixty-six automated ambient air quality monitoring stations used in this study were set up about 3 to 15 m above the ground. We utilized the hourly data collected by the TEPA air quality monitoring vans including data on CO, nitrogen oxide (NOX = NO + NO2), SO2, O3, and particulate matters (PM10 and PM2.5). All gaseous pollutant monitors designated by the US EPA as equivalent or reference methods were

Fig. 1. Locations of the monitoring stations adopted in this study; from Taiwan Air Quality Monitoring Network.

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manufactured by Ecotech Pty. Ltd. (Blackburn, Victoria, Australia), except for the non-dispersive infrared method for CO analyzers, which was manufactured by Horiba Instruments Incorporated (Irvine, CA, USA). The instruments used were as follows: an APMA-360CE gas filter correlation ambient CO analyzer (RFCA-0895-160) for CO, an Ecotech 9841B chemiluminescence NO–NO2–NOX Analyzer (RFNA-1292-090) for NOX, an Ecotech 9850B UV fluorescence SO2 Analyzer (EQSA-0193092) for SO2, an Ecotech 9810B UV absorption O3 Analyzer (EQOA-0193-091) for O3, Tapered Element Oscillating Microbalance (TEOM) 1400a Monitors (EQPM-1090-079) manufactured by Rupprecht and Patashnick Company Incorporated (Albany, NY, USA), a Metone BAM 1020 PM10 Beta gauge automated particle monitor (EQPM-0798-122), and a Verewa PM10 Beta Attenuation Monitor (EQPM-0990-076) for both PM10 and PM2.5. Monitors for both PM10 and PM2.5 came from the same manufacturer and were installed in a monitoring station. The instruments were well operated and maintained to ensure data quality. Scheduled quality control procedures included daily zero and span checks, biweekly precision checks, quarterly multiple-point calibrations, and data validations. In addition, an independent quality assurance program and a performance audit were also regularly undertaken. The tasks of carrying out the quality assurance program and instrument maintenance of the monitoring stations were outsourced to a qualified independent laboratory. Each monitoring instrument was set to conduct automatic daily zero and span checks. Personal maintenance or adjustment was performed for each instrument when more than 2% of zero or 10% of the span drift occurred. The calibration gases for CO, SO2, and NO are traceable to the US NIST standards within ±2%. In addition to the automatic daily zero and span checks, the ozone monitor was calibrated by a transfer standard (calibrator), which was carried to the site each season. The transfer standard was maintained by the first class Standard Reference Photometer # 30, traceable to US NIST. Moreover, the monitoring data were validated by an automatic computer system controlled by experienced personnel. The aerosol composition data were retrieved from the TEPA's TAS site (the Sin-Jhuang (SJ) air quality monitoring station located 200 m from the TAS site shown in Fig. 1). Data from eight air quality monitoring stations plus the TAS site (shown as star signs in Fig. 1) were adopted in this study. The TEOM (Tapered Element Oscillating Microbalance) monitor is a US EPA designated PM10 equivalent method (EQPM-1090079) with valid measurements from above 5 µg m− 3 to several g m− 3 (Rupprecht and Patashnick Co., Inc, 2002; Jaques et al., 2004). All the instruments for aerosol speciation were equipped with a PM10 inlet, followed by a sharp cut cyclone to collect ambient PM2.5. Both aerosol nitrate and sulfate concentrations from R&P 8400 N Ambient Particulate Nitrate (NO− 3 ) Monitor and 8400S Ambient Particulate Sulfate (SO42−) Monitor have a base line stability of 0.4 µg m− 3 and a measurement resolution of 0.2 µg m− 3 (Rupprecht and Patashnick Co. Inc, 2001a, b). Meanwhile, the BC concentration in aerosols was measured using an Aethalometer™ (Model AE-31, Magee Scientific Co., CA, USA). The instrument measured light absorption due to aerosols at seven wavelengths. However, only the “standard” outputs of BC concentration derived from the 880 nm measurements were presented in this paper; these measurements were retrieved with a temporal resolution of 1 h. Aerosol

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volume size distribution was calculated by assuming sphericity in particle shape from the particle number measured by the PMS PCASP-X Aerosol Spectrometer (Particle Measuring Systems Co, Inc., 2001). This spectrometer measured particles over a size range from 0.1 to 10.0 µm in 31 size channels. A He– Ne (632.8 nm) laser was installed into the instrument for detecting particles with a minimum detectable size of 0.1 µm. For aerosol number size distribution, a TSI 3936 L22 SMPS system (TSI Incorporated, St. Paul, MN, USA) provided scanned aerosol size distribution covering 0.012 to 0.55 µm in 106 size channels. The SMPS system uses a bipolar charger in the Electrostatic Classifier to charge the particles to a known charge distribution; the charged particles are then classified according to their electrical mobility to traverse an electrical field, and counted by a condensation particle counter. The 10-minute time interval was retrieved for the data monitored by aerosol size distributions. In addition, the vertical profiles of the aerosols' backscattering coefficient and depolarization ratio were measured every 15 min using LIDAR installed at the National Taiwan University campus in Taipei. The warrant for the application of LIDAR data to explain the air quality variations over the Taipei Basin has been previously presented (Chou et al., 2007). 3. Unusual PM10 and PM2.5 episodes over Taiwan on March 19 and 20, 2006 An unusual episode of PM10 and PM2.5 was observed in Taiwan on March 19 and 20, 2006. On March 19, high PM10 and PM2.5 concentrations were detected in the northern region. On March 20, southern Taiwan was also covered with high aerosol concentrations, which affected the entire country. On March 19, 65 stations discovered daily average concentrations of PM10 exceeding 125 µg m− 3, the air quality standard for particulate matter in Taiwan. These 65 stations cover a large part of the country, as shown in Fig. 2(a), accounting for 85.5% of the total existing stations. On March 20, 29 stations revealed similar conditions. These stations account for 38.2% of the total existing stations and are mainly located in southern Taiwan (Fig. 2(b)). Given that no biomass burning in the rice paddy fields or significant industrial accidents during that period were reported, we could consider the episode to be a long-range transport event. Comparing the time periods when such high PM2.5 and PM2.5–10 concentrations (calculated by subtracting PM2.5 from PM10) occurred in different areas in Taiwan, the transport phenomena of PM2.5 and PM2.5–10 could then be shown. Fig. 3(a), (b) shows the spatial distributions for the daily average concentrations of PM2.5 on March 19 and 20, while the related daily average concentrations of PM2.5–10 are shown in Fig. 3(c), (d), respectively. The maximum concentrations of the southern PM2.5 and PM2.5–10 occurred about 12 h later than those of the northern region, whose average moving speed was 3.8 m s− 1 corresponding to the average wind speed of 4.0 m s− 1 of the three stations located in the west coast: the Wanli (WL) station in the north, the Siansi (SS) station in the west, and the Linyuan (LY) station in the south. Hence, the pollutants have been transported from the north to the south along the movement of air masses. In this pollution episode, PM2.5 concentration was distributed in a more uniform manner than that of PM2.5–10, although it was

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Fig. 2. Contour plot of PM10 over Taiwan during the 2006 AD event, on (a) March 19, 2006 and (b) March 20, 2006.

the concentration of the latter which increased the most. For the two stations in the east, the Hualien (HL) and Taitung (TT) stations, and the Hengchun (HC) station in the southernmost, their PM2.5 and PM2.5–10 variations were similar to those

observed in the west, wherein the PM2.5 concentration rose earlier than that of PM2.5–10. On the other hand, the maximum concentration of the southern PM2.5 occurred later than that of the northern PM2.5–10.

Fig. 3. Contour plot of the particulate matter over Taiwan during the 2006 AD event, (a) fine particle (PM2.5) dated 2006/03/19; (b) fine particle (PM2.5) dated 2006/03/20; (c) coarse particle (PM2.5–10) dated 2006/03/19; and (d) coarse particle (PM2.5–10) dated 2006/03/20.

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4. Identification of the AD event on March 19, 2006 To obtain a better understanding of how the March 19 and 20 episode took place, we analyzed the concentrations of air pollutants measured at the SJ station beside the TAS site. Fig. 4(a) compares the variations in hourly concentrations of PM10 measured at the Yangming (YM, located in Yangmingshan National Park, 850 m above sea level, see Fig. 1), SJ, and WL stations. Data recorded from these three stations prove that the phenomenon of increased PM10 concentration occurred at the same moment (March 19). Subsequent discussions revealed that the enhancement of PM10 at these three stations was distantly transported from the same source on March 19. According to the characteristics of past ADs that have affected Taiwan's air quality, the WL and YM stations could take the lead in monitoring the long-range transport of AD and peak PM10 concentrations (Liu et al., 2006). The YM station has been commonly used to determine the air quality influenced by air pollution of long-range transport because it has not yet been influenced by local pollution; in addition, the long-term average concentration of PM10 discovered here comprise less than 20 µg m− 3 (Chang and Lee, 2007a; Lee

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et al., 2006; Liu et al., 2006). Fig. 4(a) also shows that the high PM10 concentrations recorded at the SJ station on March 17 and 18 are due to the accumulated local pollution. This is because the WL and YM stations, located in the background area, were not affected by the increased concentration of pollutants from the metropolitan area. Further comparisons of the variations of gaseous pollutant concentrations found at the SJ, WL, and YM stations showed that on March 17 and 18, or March 20 and 21, only the SJ station located in the metropolitan area showed an increase in the primary pollutant concentrations of CO, SO2, and NOX; meanwhile, the two background stations, the WL and YM stations, had no apparent concentration phenomena accumulation. These can be seen in Fig. 4(b)–(d). Furthermore, during the local pollution accumulation period, the O3 concentration observed at the SJ station showed a typical day and night concentration variations that are significantly different from those obtained from the background stations (Fig. 4(e)). At 09:00 on March 19, when CO and SO2 at the SJ station peaked at 1.2 ppm and 21 ppb, respectively, the background WL and YM stations also showed similar values of 0.93 ppm and 20 ppb as shown in Fig. 4(b), (c). This shows that the air quality in Taipei during this period was controlled by the

Fig. 4. Comparisons of air pollutant concentrations among those recorded at Sinjhuang (SJ) station and at the Wanli and Yangming (WL and YM) stations, (a) PM10; (b) CO; (c) SO2; (d) NOX; and (e) O3.

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Fig. 5. Time series of wind speed and wind direction recorded from the Sinjhuang (SJ) station from March 17 to 21, 2006.

external factors from the long-range transport, causing the metropolitan and background stations' pollutant concentrations to record almost the same levels. According to the surface weather maps, a cold front passed over Taiwan on March 18; on this day, the wind speed strengthened at 19:00, with a change in wind direction that

turned a southwesterly wind to a northeasterly one. On March 19, a full-day wind speed higher than 3 m s− 1 was recorded; this persisted until 06:00 on March 20 when the wind direction remained northeasterly (Fig. 5). These indicate that on March 19, the Taipei area was mainly influenced by the impact of the cold north air. Fig. 6 shows the five-day backward trajectories (Draxler and Rolph, 2003; based on NOAA http://www.arl.noaa.gov) at the TAS site from the time when the maximum concentration of PM10 took place (15:00 on March 19). This indicates that the air masses impacting Taipei on March 19 had passed on March 17–18, initially through the desert and loess area in China. Moreover, using the NASA Ozone Monitoring Instrument (OMI) aerosol index (http://toms.gsfc.nasa.gov/), northern China was shown to have had a large-scale dust storm during the March 17–18 period (shown in Fig. 7). The data observed by NASA AeroNet also showed the phenomenon of substantial increase in northern China's aerosol optical thickness (Aerosol Optical Thickness, AOT) (http://aeronet.gsfc.nasa.gov/). The above analyses imply that on March 19, the high

Fig. 6. Five-day backward trajectories (the end heights of the back trajectories were 300, 1000, and 1500 m, respectively) from the NOAA HYSPLIT model (Draxler and Rolph, 2003) recorded at the Taipei aerosol supersite (TAS site) at 15:00 (local time) on March, 19, 2006.

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confined within the mixed layer, suggesting that the transport of the polluted air parcel was dominated by advection, and that convection was depressed in the subsidence zone of the high-pressure system. This observation is consistent with the observation at the YM station and the calculated trajectories shown in Fig. 6. Moreover, a dense 800-meter aerosol layer will certainly result in a high AOT. The emergence of the aerosol layer indicates the arrival of the high AOT air masses which were spread over northern China on March 18 (Fig. 7). Fig. 8(b) shows that on March 19, there occurred a significant increase in the depolarization ratio of the suspended particles (about 20%), which is in line with the optical properties of dust particles (Iwasaka et al., 2003). However, the related high value of the depolarization ratio occurred mainly from 12:00 to 17:00, which was nonidentical to the increase of backscattering. It can then be speculated that on March 19, the incident of pollution impacting Taipei's air quality could be influenced by two distinct air masses. Before 12:00, air masses were mainly composed of fine particles, resulting in backscattering brought about by such fine particles which have a high efficiency of visible light scattering. After 12:00, it was mainly the dust which caused the increase of the depolarization ratio. Additionally, data from the LIDAR observation showed that the internal and external parts of the atmosphere's mixed layers exhibited different types of sand and dust transports, and that the significant increase of dust concentration in the mixed layer only began from the afternoon of March 19. However, starting from the early hours of that particular morning, a high concentration of dust particles above the mixed layer was observed (between 1 and 2 km), as the dust particles floated in the air moving towards south. Afterwards, as it was affected by the rainfall, the LIDAR observation of this pollution incident could only be conducted until the evening of March 19. Nevertheless, the LIDAR measurements have provided the vertical profile and the critical evidence for the contribution of AD on the high PM10 event. These measurements are therefore in agreement with the results of the trajectory analysis and the surface weather observations. 5. Observations on the air pollutants during the AD period at the Taipei aerosol supersite 5.1. Mass concentration and size spectra of PM Fig. 7. NASA Ozone Monitoring Instrument (OMI) aerosol index (http://toms. gsfc.nasa.gov/) on March 17 and 18, 2006.

concentration of particles recorded in Taiwan was mainly affected by the dust which came from northern China. Although there is an annual average of four to five dust events and 6.1 dust days in Taiwan (Liu et al., 2006), we only observed four days during which recorded values exceeded Taiwan's air quality standard of PM10 at 125 µg m− 3 due to AD events from 1994 to 2006 as shown in Table 1. This demonstrates that the transport of AD on March 19, 2006 was an unusual event. In Fig. 8(a)–(b), one can see from the data observed by the Taipei LIDAR station during this same period that from 06:00 to 12:00 on March 19, a sudden increase of aerosol backscattering in the mixed layer became visible, an aerosol layer thickness of about 800 m. Apparently, the aerosols were

According to the hourly PM2.5 and PM2.5–10 concentrations measured at the TAS site from March 17 to March 21, 2006, the PM2.5 concentration was higher than that of PM2.5–10 such that the ratios of PM2.5/PM10 were over 0.5 on March 17 and March 18 as shown in Fig. 9(a). However, on March 19, changes completely different from those of the two previous days took place. Starting from 20:00 on March 18, brought about by the northeast monsoon effect, there was a rapid increase of both PM2.5 and PM2.5–10 concentrations, with PM2.5 even reaching a maximum concentration of 136 µg m− 3 at 09:00 on March 19. Starting from 10:00 on March 19, PM2.5 concentration declined, while that of PM2.5–10 continued to rise, resulting in the higher concentration of the latter compared to the former. This resulted to the ratio of PM2.5/PM10 being less than 0.5. The PM2.5–10 concentration which began to increase at 12:00, peaked at 16:00 on March 19 to a value of 218 µg m− 3,

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Table 1 Days in which air quality exceeded Taiwan's air quality limit of PM10 (125 µg m− 3) due to the AD events from 1994 to 2006; the data were recorded at the Yangming (YM) station in northern Taiwan. Jan 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 Total

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

1⁎

1⁎ 1⁎

Nov

Dec

Total

1

1⁎

1

1⁎

1

2⁎

1⁎

1 1

2 5

The asterisks are the recorded values exceeded Taiwan's air quality standard of PM10 at 125 μg m– 3 due to AD events.

and the ratio of PM2.5/PM10 dropped to about 0.2. This indicates that the impact of AD results in a substantial increase of PM2.5–10 concentration (Lee et al., 2006). On March 17 and 18, fine particles with number size spectra of less than 200 nm presented significantly high concentrations as shown in Fig. 9(b). It is common knowledge that most of

Fig. 8. Time series of Taipei aerosol vertical profiles from March 17 to 20, 2006 (a) backscattering coefficient and (b) depolarization ratio.

Fig. 9. Aerosol characteristics measured at the Taipei aerosol supersite (TAS site), (a) variations of PM2.5, PM2.5–10, and PM2.5/PM2.5–10; (b) number size spectra; and (c) volume size spectra from March 17 to March 21, 2006.

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Taipei's local air pollution consists of emissions from motor vehicles (Chang and Lee, 2006) and that vehicle emissions of particulate pollutants are mainly small-sized (Gillies et al., 2001; Lough et al., 2005; Kristensson et al., 2004; Mamani-Paco and Helble, 2007; Chang et al., 2009). In this light, this period's particles could be caused by the local vehicle exhausts or the impact of photochemical reactions. From 20:00 on March 18 to 06:00 on March 20, due to the effect of the northeast monsoon, the number concentration of particles less than 200 nm was much lower than those recorded for March 17 and March 18. This indicated that although the concentration of PM2.5 continued to rise, the sources of pollution changed from local emissions to a long-range transport when affected by the northeast monsoon. In addition, because of the transport of dust, the gaseous pollutants would likely manifest heterogeneous reactions on existing dust surface. Thus, nucleation would not easily occur during the transport. Apart from the substantially reduced number of particles (less than 200 nm) during the impact of the AD period, the peak size of the corresponding particulate volume rose from less than 1 µm during the local pollution period to 2–3 µm during the AD period (Fig. 9(c)). This change presents a similar time variation of the mass concentration related to this study (Fig. 9(a)) and is consistent with Chun et al.'s (2001) study on the AD transfer to South Korea, which stated that AD duration decreased the number of small-sized particles. It is worth mentioning that the PM2.5 peak influencing at 09:00 on March 19 occurred earlier than the PM2.5–10 peak influencing at 16:00. Hence, the leading PM2.5 prior to PM2.5–10 of the AS must be due to the anthropogenic industrial or urban vehicle emissions of particulate pollution transported from the Asian continent. 5.2. Variations of the gaseous pollutant concentrations The Taipei metropolitan area's CO is comprised mainly by the emissions of motor vehicles, which can be used as the tracer for the local primary emissions (Chang and Lee, 2007b; Baumgardner et al., 2004). Taking the station SJ located at the metropolitan area in Taipei as an example, one can observe the changes in the concentration of gaseous pollutants as shown in Fig. 10(a). March 17–18 and March 20–21, mark the duration of local pollution accumulation in this area. The CO concentration accumulated by as much as 3.27 ppm, which is much higher than that during the impact period of AD while the SO2 concentration accumulated by as much as 67 ppb. However, during the impact period of AD at 09:00 on March 19, PM2.5 reached a maximum value, and CO and SO2 also showed high values of 1.2 ppm and 21 ppb, respectively. March 19 was a Sunday, hence, local pollution emissions were lower. Moreover, high morning wind had a speed of over 4 m s− 1 indicating that the high concentrations of CO and SO2 were from China's longrange transport. The O3 concentration demonstrated typical day and night concentration variations during the local pollution accumulation period: on the night of March 17, due to the NO titration effect, O3 concentration decreased to 2 ppb, while it increased to 82 ppb at noon of March 18. These are shown in Fig. 10(a). It should be noted that the O3 concentration began to rise at the onset of the impact of AD at around 20:00 on March 18. This reached a maximum concentration of 77 ppb on the early morning of March 19. As no photochemical

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Fig. 10. Comparisons of air pollutant concentrations recorded at the Sinjhuang (SJ) station, (a) time series plots of O3, SO2, and CO and (b) time series plots of NO, NO2, and NO2/NOX from March 17 to March 21, 2006.

reaction occurs at night, the high concentration of O3 occurring in both the evening and early morning also implies that this could be caused by the long-range transport. The level of NOX detected to be present over Taipei's metropolitan area is mainly caused by motor vehicles (Chang and Lee, 2006). During the local pollution accumulation period, NO and NO2 concentrations increased by as much as 111 ppb and 127 ppb, respectively, much higher than the impact of AD as shown in Fig. 10(b). The fresh emissions from local sources would cause the lower ratio of NO2/NOX. When the local emissions accumulated high concentrations of NO, the NO2/NOX ratio became even lower than 0.4 (such as that detected at 24:00 on March 17). However, on March 18, the NO2/NOX ratio was greater than 0.8, mainly due to the impact of the northeast monsoon. At 09:00 on March 19, when the maximum value of PM2.5 was reached and CO and SO2 also reached their high values of 1.2 ppm and 21 ppb, respectively, the NO and NO2 concentrations were only 1 ppb and 18 ppb, respectively. This is different from the characteristics of the local pollution accumulation period in which CO and NOX concentrations simultaneously increased (Chang and Lee, 2007c). Given that NOX in the air will react to form nitrates, its reaction speed is greater compared to when SO2 will react to form sulfates and transform CO to CO2. Comparing the high CO and SO2 with low NOX values, it clearly shows that NOX in the dust air mass would be transferred in the transport process and that the strong air current would dilute the local concentration of NOX emissions. 5.3. Variations of aerosol speciation concentrations Related past analyses of particulate components present during the impact of AD mostly utilized filter-based samples

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which could only be used for the time-average analysis of aerosol compositions and not as a measure for understanding the timing changes of each component. The hourly concentrations of sulfate, nitrate, and BC of PM2.5 measured at the TAS site are shown in Fig. 11. During the local pollution accumulation period, the maximum sulfate concentration was only 11.4 µg m− 3, but reached as high as 27.1 µg m− 3 during the AD period. Meanwhile, the sulfate concentration, identical to that of PM2.5, began to rise from 20:00 on March 18, and the maximum concentration was consistent with that of PM2.5, indicating that sulfate was one of the major components during the long-range transport. On the other hand, during the local pollution period, maximum nitrate concentration was at 17.0 µg m− 3, but reached as high as 15.6 µg m− 3 during the AD period. Nitrate concentration began to rise from 20:00 on March 18 as well; its concentration was similar and consistent with that of PM2.5. That NOX concentration was low during the impact of AD, while that of nitrate was consistent with the variation of other aerosol species reveal that nitrate was transformed from NOX during transport. From March 17–18 or March 20–21, when the local accumulation pollution took place, the BC peak concentration was as high as 17.1 µg m− 3 or at least over 10 µg m− 3. As BC was caused by the incomplete combustion of organic matters, the high-concentration BC was similar to the gaseous pollutants mentioned above, thereby representing the local pollution emissions. On March 19, during the AD impact period, the maximum concentration of PM2.5 at 136 µg m− 3 was higher than 112 µg m− 3 during the local pollution accumulation period on March 17, while BC was only 7.9 µg m− 3, a figure much lower than that recorded from the local pollution accumulation period (17.1 µg m− 3). This shows that although the long-range transport AD would have also brought BC, its related proportion of PM2.5 is clearly lower than the other components, such as sulfate and nitrate. To understand the enhancement of aerosol composition before and after the arrival of AD in Taiwan, this study hereby defines 00:00 on March 17 to 19:00 on March 18 as the “before dust” impact period of AD. Furthermore, the period from 20:00 on March 18 until 16:00 on March 19 (when the concentration of PM2.5–10 maintained a high value) is to be called the “during dust” period. Finally, the period between 17:00 on March 19 and 24:00 on March 21 will be called the “after dust” period, pertaining to that after the impact of AD. The scattering plots of PM2.5 versus sulfate, nitrate, as well as BC for the three periods are shown in Fig. 12(a)–(c),

Fig. 11. Variations of PM2.5 sulfate, nitrate, and BC concentrations at the Taipei aerosol supersite (TAS site) from March 17 to March 21, 2006.

Fig. 12. Correlations of the selected PM2.5 species with PM2.5 recorded at the Taipei aerosol supersite (TAS site) before, during, and after the AD event, (a) PM2.5 sulfate; (b) PM2.5 nitrate; and (c) PM2.5 BC.

respectively. Fig. 12 demonstrates that sulfate, nitrate, and BC are well correlated with PM2.5 at the “before dust,” “during dust,” and “after dust” periods. This indicates that all these three components affect the variations of aerosol concentration. Fig. 12(a) shows the relationship between PM2.5 and sulfate before the impact of AD, wherein the sulfate concentration was approximately 7.1% of PM2.5; during the impact of AD, where the sulfate of PM2.5 increased to 19.8%; and after the impact of AD, where the sulfate ratio of PM2.5 dropped to 14.1%. During the impact of AD, the SO2 value of 21 ppb was lower than the local pollution accumulation of 67 ppb, but the sulfate concentration of 27.1 µg m− 3 was higher than the local accumulated pollution, which was 11.4 µg m− 3. This indicates that, apart from the local sulfate, SO2 conversion during AD long-range transport results in a significant increase in the sulfate to PM2.5 ratio. Therefore, the PM2.5 control should consider the impact of the regional pollution transport. Fig. 12(b) shows the relationship between PM2.5 and nitrate. Prior to the impact of AD, the nitrate was approximately 10.3% of PM2.5; during the impact of AD, the nitrate to PM2.5 ratio slightly decreased to 8.1%; and after the impact of

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AD, the nitrate to PM2.5 ratio increased to 10.8%. The concentration of the nitrate ratio of PM2.5 during the local pollution period was higher than that during the impact of AD. During this latter period, the contribution of nitrate to PM2.5 was lower than that of sulfate, which might have been influenced by the transfer process in which the air mass coming from high latitude to low latitude would encounter the increase of temperature, thereby resulting in nitrate evaporation. However, when it moved close to Taiwan, volatile nitric gas would react with the increase of ammonia to form nitrate, leading to a slight increase in aerosol concentration (Chuang et al., 2008). Fig. 12(c) shows the relationship between PM2.5 and BC. Before the impact of AD, BC was approximately 15.7% of PM2.5; during the impact of AD, BC was approximately only 5.6% of PM2.5; and finally, after the impact of AD, the BC ratio of PM2.5 returned to 18.4%. This shows that the BC to PM2.5 ratio is about 16% to 18% during the local pollution period. This is because the primary pollutants caused by local emission sources were higher. During the impact of AD, the concentration of the primary pollutants decreased significantly, demonstrating that less BC came from the long-range transport.

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6. Discussion Since 98% of CO is emitted from motor vehicles in the Taipei metropolitan area (Chang and Lee, 2007b) CO can be assumed to be a non-reactive tracer for mobile sources by neglecting its photochemical formation during air mass transport (Grosjean, 1989). In addition, CO may last for a relatively longer period (Holloway et al., 2000). Therefore, the CO concentration may also reflect the dispersion condition of the atmospheric environment. An analysis of the ratios of pollutants to CO may help evaluate primary combustion emissions (Maria et al., 2003) as well as evaluate the dilution effect via the dispersion process. The increase of the ratios of pollutants to CO represents the increase in pollutant concentration due to chemical reaction occurring during the transport process. From Fig. 10(a), it can be seen that during the dust period, both CO and SO2 have a peak concentration, implying that the long-range transport pollutants included CO. Thus, the increased ratios of pollutants to CO during the dust period indicate not only primary emissions but also the inclusion of relatively high extraneous pollutants in the AD transport. The ratios of various pollutants to CO in the Taipei metropolitan area are shown in Fig. 13. Before and after the

Fig. 13. Ratios of pollutants to CO before, during, and after the AD event in the Taipei metropolitan area, (a) NOX/CO; (b) PM2.5/CO; (c) BC/CO; (d) sulfate/CO; and (e) nitrate/CO.

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impact of AD, NOX/CO was 0.074 ± 0.025 and 0.091 ± 0.026, respectively. However, during the impact, it was only 0.034 ± 0.009, as shown in Fig. 13(a). As the majority of CO and NOX existing in Taipei's metropolitan areas are caused by the pollution from motor vehicles, both are considered to be highly correlated (Chang and Lee, 2007c); however, during the impact of AD, the ratio of NOX/CO decreased significantly, which may confirm that NOX would be lost in the transport process due to secondary reactions. During the impact of AD, PM2.5/CO was 0.091 ± 0.019, which was significantly higher than 0.044 ± 0.018 (before) and 0.025 ± 0.014 (after) (Fig. 13(b)). This implies that during the impact of AD, PM2.5 was not only emitted from the Taipei metropolitan area's sources but was also brought about by the continental primary sources and secondary reactions which were transported along with AD (Chuang et al., 2008). In the “before dust,” “during dust,” and “after dust” periods, BC/CO was 0.006 ± 0.002, 0.005 ± 0.001 and 0.004 ± 0.001, respectively. This indicates no significant difference, as shown in Fig. 13(c). The ratio of the two pollutants did not change due to the similar dispersion and diluted effects in the transport process. As Lim and Turpin (2002) observed, BC and CO in the metropolitan area are both caused by vehicle emissions. In their study on the city of Atlanta, they found that elemental carbon and CO varied consistently. Similarly, this phenomenon was also found in the transport of AD in this study. During the impact of AD, sulfate/CO was 0.018 ± 0.004, which was significantly higher than the 0.007 ± 0.004 value before the impact of AD or the value of 0.003 ± 0.002 after the impact of AD. This implies that, due to the secondary reactions in the long-range transport, significant sulfate concentration could be found during the impact of AD, as shown as Fig. 13(d). During the impact of AD, nitrate/CO was 0.006 ± 0.003, a value which was relatively high compared to 0.004 ± 0.003 before the impact of AD or 0.002 ± 0.002 after the impact of AD. Park et al. (2005) studied seasonal and short-term variations in particulate nitrate in Baltimore and pointed out that the high contribution of local nitrate formed at rush hours in the morning or at night when a high concentration of NOX was emitted and at the time when a high concentration of ozone was formed. During the AD-affected period under low NOX concentration, a higher concentration of nitrate reveals that it could be transported to the downwind areas along with AD, as shown in Fig. 13(e). Comparing Fig. 13(e) and Fig. 12(b), one can find that during the impact of AD, the ratio of nitrate/CO increased by 50% more than the ratios before and after the impact of AD, while the nitrate fraction in PM2.5 exhibited no significant change. This implies that compared with CO, the nitrate concentration in the atmosphere during long-range transport had increased relatively. However, the increase of nitrate was not as much as compared with PM2.5. Nitrate in the atmosphere mainly come from the oxidation of NOX, particularly from HNO3. Related studies have shown that dust particles react with the acidic gases under a high-humidity environment during transport, such that the partition of the gas-solid phase nitrate could be changed, leading to an increase of aerosol nitrate (e.g., Sullivan et al., 2007). Since the lifetime of gaseous HNO3 may be shorter than that of PM2.5, gaseous HNO3 reacting with dust particles to form aerosol may be

transported to remote areas. Given that this study focuses on the monitoring of PM2.5, the nitrate aerosol formed via the aforementioned reaction caused by HNO3 and dust can therefore be observed in small quantities, while more nitrates should be presented in the coarse particles (Kim and Park, 2001). This result, however, also provided an observational evidence for the heterogeneous reaction hypothesis. 7. Conclusions This study, applying a combination of ground-based observations of particulate pollutants and gaseous pollutant concentrations and five-day backward trajectories, shows the impact of AD on Taiwan. Additionally, based on the hourly aerosol components observed in Taipei and the observations of vertical profiles via LIDAR, the variations of aerosol compositions transported and the related time series changes are described. As a result, among Taiwan's 74 air quality monitoring stations, 65 stations on March 19 and 29 stations on March 20 respectively recorded their average concentrations of PM10 during the impact of the AD; data show that the values exceeded Taiwan's air quality standard. Influenced by the impact of such an AD event, not only were the high concentrations of PM2.5 and PM2.5–10 transported from north to south, but the gaseous pollutants, including SO2, CO, and O3, were transported by the northeast monsoon as well, eventually affecting the entire Taiwanese region. While a maximum PM2.5 concentration was reached, the relatively high concentration levels of SO2, CO, and O3 demonstrated that, in addition to fine particles transported along with AD, the above-mentioned gaseous pollutants were also presented. However, the NOX concentration did not increase significantly because it might have been transformed into nitrate during transport. In addition, the hourly observations of the aerosol components and size distribution at the TAS site revealed that in the first half of the impact of AD, PM2.5 concentration increased, including those of PM2.5 BC, PM2.5 sulfate, and PM2.5 nitrate. At the same time, the aerosol volume peaked with a diameter of 2–3 µm, and significant enhancements in sulfate and nitrate levels were also observed. After PM2.5 reached its peak concentration, the PM2.5–10 peak concentration appeared, indicating that it was the leading edge of the dust airflow that drove man-made pollution particles to the downwind areas. Comparing the entire impact duration of AD and the period before the arrival of AD, PM2.5 sulfate concentration increased, while that of BC decreased. The ratios of the three components of PM2.5 to CO showed that long-range transport sulfate and nitrate were significantly increased during the impact of AD, while no apparent change in the BC content was observed. During the impact of AD, the number concentration of particles with diameters smaller than 200 nm decreased significantly, showing that the increase in the concentration of PM2.5 was caused by the larger size of particles in PM2.5. In sum, at the start of autumn until spring of the following year, AD and man-made pollutants undergo long-distance transportation, thereby causing significant impact on the downwind areas. The strategies for protecting the air quality of the downwind areas should therefore consider both the influence of dust and air pollutants.

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