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Air pollution episodes associated with East Asian winter monsoons
Science of the Total Environment 409 (2011) 5063–5068
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Air pollution episodes associated with East Asian winter monsoons P.D. Hien a,⁎, P.D. Loc b, N.V. Dao b a b
Vietnam Atomic Energy Agency, 59 Ly Thuong Kiet str. Hanoi, Viet Nam National Hydro-Meteorological Center, 62-A2 Nguyen Chi Thanh str. Hanoi, Viet Nam
a r t i c l e
i n f o
Article history: Received 30 March 2011 Received in revised form 2 August 2011 Accepted 24 August 2011 Available online 16 September 2011 Keywords: PM10 SO2 NO2 CO Continuous monitoring Temperature inversion Nighttime Daytime
a b s t r a c t A dozen multi-day pollution episodes occur from October to February in Hanoi, Vietnam due to prolonged anticyclonic conditions established after the northeast monsoon surges (cold surges). These winter pollution episodes (WPEs) account for most of the 24-h PM10 exceedances and the highest concentrations of gaseous pollutants in Hanoi. In this study, WPEs were investigated using continuous air quality monitoring data and information on upper-air soundings and air mass trajectories. The 24-h pollutant concentrations are lowest during cold surges; concurrently rise thereafter reaching the highest levels toward the middle of a monsoon cycle, then decline ahead of the next cold surge. Each monsoon cycle usually proceeds through a dry phase and a humid phase as Asiatic continental cold air arrives in Hanoi through inland China then via the East China Sea. WPEs are associated with nighttime radiation temperature inversions (NRTIs) in the dry phase and subsidence temperature inversions (STIs) in the humid phase. In NRTI periods, the rush hour pollution peak is more pronounced in the evening than in the morning and the pollution level is about two times higher at night than in daytime. In STI periods, broad morning and evening traffic peaks are observed and pollution is as high at night as in daytime. The close association between pollution and winter monsoon meteorology found in this study for the winter 2003–04 may serve as a basis for advance warning of WPEs and for forecasting the 24-h pollutant concentrations. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Hanoi is among Asian cities experiencing high levels of particulate matter in ambient air (Kim Oanh et al., 2006; Hopke et al., 2008). Concentrations of particulate matter with aerodynamic diameter less than 10 μm (PM10) are about twice the annual mean limit (50 μg m −3), and exceed the 24-h legislation threshold (150 μg m −3) on 45 to 50 days each year (Hien et al., 2002). The particulate pollution level varies substantially over a year. Particulate mass loadings are low in summer (May–September), when the East Asian southeast monsoon driven by tropical maritime air from the Pacific and Indian Oceans brings in hot weather and convective conditions. Conversely, high particulate mass loadings occur in winter (October–March), when Hanoi is impacted by the East Asian northeast monsoon. This monsoon brings continental cold air from the Asiatic high pressure systems to northern Vietnam either through inland China (dry weather) or via the East China Sea (humid weather) (Toan and Dac, 1993). In particular, multi-day winter pollution episodes (WPEs) that account for most of the 24-h PM10 exceedances usually occur in association with nighttime radiation temperature inversion (NRTI) and subsidence temperature inversion
⁎ Corresponding author. Tel.: + 84 913320067, + 84 04 35142215. E-mail address: [email protected] (P.D. Hien). 0048-9697/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.scitotenv.2011.08.049
(STI) in the dry and humid phases of a monsoon cycle, respectively (Hien et al., 2002). Automated air quality monitoring facilities installed in 2002 in Hanoi reveal that these WPEs also involve other pollutants. Data obtained to date show that the 24-h concentrations of particulate (PM10) and gaseous pollutants (SO2, NO, NO2, CO etc.) concurrently rise after the intrusion of cold air (cold surge) and reach maxima at some time in the middle of a monsoon cycle and decline ahead of the next cold surge. All air pollutants exhibit lowest concentrations during cold surges. Song et al. (2006) also found a rather similar trend for PM10 at several monitoring stations in Beijing where the intrusion of cold air from the East Asian anticyclones occurs in average cycles of 4 days. In Hong Kong, particulate pollution episodes also occur in winter when the weather is influenced by the East Asian anticyclones (Air Service Group, 2003; Louie et al., 2005). In Hanoi, recurring northeast monsoon cycles leave a dozen multiday pollution episodes between October and February. Directing emission control measures specifically upon these periods would greatly improve the effectiveness of efforts aimed at combating air pollution in Hanoi and reducing the health risk associated with prolonged and simultaneous exposures to many types of air pollutants. Such a targeted strategy requires reliable advance warnings of WPEs and forecasting of pollutant concentrations. For this purpose, in this study we investigate atmospheric conditions governing the daily and diurnal variations of pollutant concentrations
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during winter in Hanoi. The analysis was done using information on air pollution and meteorology for the winter of October 2003 to February 2004. The study period covering 16 northeast monsoon cycles can be regarded as representing the situation that occurs from year to year in winter in Hanoi. The consistency gained in characterizing winter monsoon conditions associated with WPEs by using combined information on surface and upper-air meteorology and air mass trajectories serves as a basis for predicting oncoming WPEs that can be incorporated into routine weather forecast. Furthermore, the close association between pollution and meteorology found in this study also provides an important justification for developing statistical models capable of forecasting next-day air pollutant concentrations using meteorological parameters as explanatory variables. These models will be presented in a forthcoming paper. 2. Monitoring site and facilities The air quality monitoring station of the National HydroMeteorological Center is located in the Hanoi meteorological garden (21° 03′ N, 105° 48′ E), about 100 m from a road and 6–8 km from industrial suburban areas. Besides surface observations, upper-air radiosonde balloons are released at this garden twice a day at 7:00 am (00:00 GMT) and 7:00 pm (12:00 GMT) providing vertical profiles of atmospheric parameters. PM10 and PM2.5 have been collected at this site since 1998 by using manual air samplers. The data were used for assessing the impact of monsoon regimes on the seasonal variations of particulate pollution (Hien et al., 2002), for comparative studies of air pollution in Asian cities (Hopke et al. 2008), and for source apportionment by receptor modeling (Hien et al., 2004; Bac and Hien, 2009; Cohen et al., 2010). Since 2002 an automatic monitoring station supplied by Fuji Electric was installed at this site for continuous monitoring of both particulate and gaseous pollutants. The methods used for automatic measurements of air pollutants concentrations are based on beta ray attenuation for PM10, UV fluorescence for SO2, chemiluminescence for NO2, non-dispersive infrared absorption for CO, and ultraviolet absorption for O3. The monitoring results on the status and trends of air pollution in Hanoi are reported for the first time in this paper. The hourly data capture for the 2003–2004 monitoring period are 92– 97% for PM10, NO2, SO2, and CO; and less than 70% for O3. Ozone was excluded from this study because the ambient concentration of this secondary pollutant is affected by many other factors, not only by dispersion conditions. The automatic monitoring facility also provides information about surface pressure, daily precipitation, air temperature (T), relative humidity (RH), solar radiation (SR), wind speed (WS) and wind direction. 3. Status of air pollution in Hanoi as derived from continuous air quality monitoring data The annual mean concentrations of PM10, NO2, SO2, and CO are 89, 38, 28, and 1530 μg m −3, respectively. For information, Fig. 1 shows the 2003 mean pollutant concentrations in Hanoi with those in Hong Kong (Air Service Group, 2003), which is about 900 km to the east of Hanoi, and Ho Chi Minh City (HCMC) (Tuan, 2010), about 1200 km south of Hanoi. Under the influence of the same East Asian monsoon regime, pollutant concentrations in the three cities are lowest in July and highest in December (Air Service Group, 2003, Hien et al., 1999). Concerning the northeast monsoon in winter, Hong Kong experiences on average 13 cold surges from November to February (Lam, 1981), while in HCMC cold surges occur less frequently, mainly in December–January. Hanoi has a highest level of PM10 with the annual mean concentration about twice the national standard (50 μg m−3). Besides, from October
Hanoi
HCM City
Hong Kong
100 90
concentration, µg m-3
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80 70 60 50 40 30 20 10 0 PM10
NO2
SO2
CO/100
Fig. 1. Comparison of annual mean concentrations of four criteria pollutants in Hanoi, Ho Chi Minh City and Hong Kong in 2003. Data for Hong Kong and Ho Chi Minh City are averaged over 12 and 4 automatic monitoring stations, respectively. Vertical bars indicate standard errors of the mean concentrations. For Hanoi, data are available only from one station.
2003 to February 2004, the 24-h national standard of 150 μg m −3 was exceeded on 45 days, which were clustered within 11 multiday pollution episodes (Fig. 2). These WPEs involve also SO2, NO2, and CO although at levels within legal limits. The 24-h concentrations of pollutants rise from their lowest levels during the cold surge reaching maxima sometime in the middle of the monsoon cycle and decline thereafter ahead of the next cold surge. Such a behavior reflects the changes of atmospheric conditions in the winter monsoon regime, as shown in the next section.
4. Atmospheric transport and monsoon conditions associated with WPEs 4.1. The East Asian anticyclones and winter monsoons In winter, large areas of East Asia are affected by northerly to northeasterly monsoons driven by continental surface anticyclones, which frequently develop over Lake Baikal in Siberia (Lam, 1976). As Lake Baikal is located at the same longitude as the eastern edge of the Tibetan plateau, strong northerly winds subsiding near Lake Baikal are not blocked by the Himalayas, but drive polar continental cold air southwards across China (Chu, 1978). At any given location a winter monsoon cycle begins with a cold surge, which is usually associated with the passage of a cold front marking the boundary between Asiatic continental polar cold air and Pacific tropical warm air (Lam, 1981). For illustration, Fig. 3 shows the surface isobaric chart with a cold front passing over Hanoi in December. From October 2003 to the end of February 2004, fifteen major cold surges were recorded in Hanoi. The cold surges marked with triangles on the horizontal axes in Fig. 2 were identified using either of the two criteria suggested by Chu (1978) or Lam (1976) for Hong Kong, namely a drop by 2 °C or more in daily mean temperature within 2 days, or a temperature fall of at least 2 °C within an hour. Other atmospheric parameters (pressure, humidity, wind direction, etc.) also change abruptly at a cold surge, but this event can most easily be recognized by high winds (Fig. 2a) that reflect the congestion of the anticyclone's outermost isobars behind the cold front (Fig. 3). Following a cold surge, atmospheric conditions usually proceed through dry and humid phases, according to the position of the anticyclone, and therefore the trajectory of continental cold air arriving in Hanoi. Dry and clear conditions prevail as long as cold air arrives from the anticyclone through inland China. The humid phase with cold air arriving from the sea follows the dry phase as the anticyclone reaches the Pacific coast in its eastward movement over inland China. Finally, the anticyclone dissipates over the sea, marking the northeast monsoon
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Fig. 2. The daily average pollutant concentrations, wind speed (WS), air temperature (T), solar radiation (SR), and relative humidity (RH) in winter. Triangles on the horizontal axis mark the cold surges. NRTIs and STIs are marked in Fig. 2e with long and short vertical bars, respectively. The horizontal line at 150 μg m−3 in Fig. 2e indicates the 24-h national standard for PM10.
recession and the return of maritime tropical air, which may last several days ahead of the new cold surge. 4.2. Air mass back trajectories and related dry and humid phases of a monsoon cycle Air mass back trajectories computed using the HY-SPLIT model (Draxler and Rolph, 2003) help explain the cold air flows and the evolution of atmospheric conditions over dry and humid phases of a monsoon cycle. As an illustration, Fig. 4 depicts three-day air mass back trajectories that are relevant to the changes of atmospheric conditions over monsoon cycles in December 2003. From the 1st to 6th, continental cold air arrived in Hanoi by maritime trajectories. The shift from maritime to inland trajectories caused the cold surge on the 7th, as evidenced by high winds and a large temperature drop in Fig. 2a and b. Cold waves that arrived on
the following days were accompanied by high winds and further temperature drops until the 11th when an additional major cold surge was recorded. The dry phase was established from the 12th to 17th during which inland trajectories brought in dry air and a clear weather (Fig. 2c, d). With maritime trajectories resuming on the 18th, the humid phase replaced the dry phase ahead of the monsoon recession and the next cold surge. The second monsoon cycle in December 2003 started with the cold surge on the 19th followed by the dry phase with inland trajectories from the 20th to 23rd and the humid phase with maritime trajectories from the 24th to 25th. The return of inland trajectories on the 26th started the third monsoon cycle in December 2003, followed by the dry phase on the 27th, and the humid phase from the 28th onward. The anticyclonic dry and humid phases, corresponding to inland and maritime continental cold air trajectories, respectively, also
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Fig. 3. A surface isobaric chart illustrates the passage over Hanoi of a cold front giving rise to a cold surge on 22 December, 2008.
occur in other winter months. However, from October to December, inland trajectories dominate, yielding long dry periods, whereas from January to March cold air arrives in Hanoi mainly from the sea making dry phases shorter and sparser. The frequency of occurrence of winter monsoon surges varies from year to year according to the variability of large scale climatological processes. Chen et al. (2003), for example, observed the influence of ENSO phenomenon on the interannual variation of winter monsoon surge occurrence in Hong Kong, i.e. a higher (lower) occurrence frequency was found during El Niño (La Niña) winters.
4.3. NRTI and STI Upper-air radiosondes released at 7:00 am and 7:00 pm at the monitoring site revealed the association of WPEs with NRTIs in dry phases and with STIs in humid phases (Hien et al., 2002).
In the dry phase, clear sky and light winds caused by anticyclonic sinking air favor NRTI that develops at dusk extending from the ground to about 100–150 m due to rapid surface radiation cooling in the presence of warm air above. Such a surface temperature inversion can actually be observed in the 7:00 pm upper air soundings in the dry phase. As the night progresses, the inversion layer may move up to higher elevations leaving its vestiges at several hundred meters high that can still be observed in the 7:00 am sounding the following day. The inversion layer presumably dissolves as incident solar radiation warms the ground during the day. The mixed layer height obtained from air mass back trajectory calculations (Draxler and Rolph, 2003) varies widely from less than 100 m after sunset to about 600 m at 9:00 am and more than 1200 m in the afternoon. In the humid phase, ground-based NRTIs can no longer occur, but STIs at several hundred meters high are usually found in both the 7:00 am and 7:00 pm soundings. STIs may persist at varying height in both daytime and nighttime. The calculated mixed layer height varies from 200–300 m at night to 500–600 m in daytime. NRTIs and STIs are marked in Fig. 2e with long and short vertical bars, respectively. From October 2003 to February 2004, Hanoi experienced 11 multi-day WPEs, during which NRTI and STI were observed on 21 and 38 days, respectively. Therefore, in terms of air pollution each winter monsoon cycle can usually be divided into three periods, i.e. the low pollution period during the cold surge followed by the two high pollution periods with stagnant atmosphere associated with NRTIs and STIs. Atmospheric conditions are quite different for the three periods, namely high winds during the cold surges; light wind, clear sky, and low humidity during NRTI periods; and light wind, overcast and high humidity during STI periods (Fig. 2a, d). As an illustration, the characteristic atmospheric conditions for the three periods are illustrated in Fig. 5 for the second monsoon cycle in December 2003. Similar patterns of atmospheric conditions are observed for other monsoon cycles in the winter 2003–04. 5. Diurnal behaviors of pollutant concentrations during NRTI and STI periods Pollutant concentrations exhibit different diurnal behaviors during the three periods, as shown by monitoring data for December 2003 in Figs. 6 and 7.
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Fig. 4. Three-day back trajectories of continental cold air arriving in Hanoi in December 2003. The cold surges occurred following the shift from maritime trajectories on the 6th, 18th, and 25th to inland trajectories on the 7th, 19th, and 26th. The return of maritime trajectories on the 18th, 24th, and 28th ended the dry phases and started the humid phases on the 18th, 24th–25th, and from the 28th onward.
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Fig. 5. Evolution of daily average wind speed (WS), air temperature (T), relative humidity (RH) and solar radiation (SR) over a monsoon cycle showing distinct atmospheric conditions for the cold surges, NRTI (full height column) and STI (half height column) periods in December 2003. Triangles on horizontal axis mark the cold surges. Note that on December 27 a cold surge arrived in early morning and a NRTI occurred just in that evening.
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From December 10 to 12, hourly pollutant concentrations were very low due to high winds accompanying cold air arrivals. No near surface temperature inversions were observed on these days. Both the morning and evening traffic pollution peaks were weakly pronounced (Fig. 6). 3 2
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On the 13th, calm conditions suddenly appeared from 5:00 pm and the first NRTI in December was recorded in the 7:00 pm upper-air sounding. The temperature inversion and calm conditions coincided with the evening traffic rush hour and cooking making particulate and gaseous pollutants from traffic and coal burning trapped in a shallow layer just above the ground. Pollutant concentrations drastically soared from 5:00 pm, reached maxima at about 9:00 pm then declined gradually overnight and throughout the daytime reaching the lowest levels at around 4:00 pm ahead of the next day NRTI (Fig. 6). The pollutant loadings occurred predominantly from 6:00 pm to midnight and were much reduced in daytime. The evening peaks occur at about 3.5 h later than the rush hours as a result of pollutant buildup in poor nighttime dispersion conditions (Venkatram and Cimorelli, 2007). In the meantime, the morning pollutant peaks were weakly pronounced suggesting the inversion layer had already dissolved before the morning traffic rush hour. Conversely, in the STI period, from December 28, 2003 to January 3, 2004 (Fig. 7), with the temperature inversion persistent for whole day, the morning traffic peaks were as strong as the evening ones and the pollutant loadings were almost similar in daytime and nighttime. For a better appreciation of the diurnal variations of pollutant concentrations in NRTI and STI periods, Fig. 8 shows the average diurnal trends in 2003. Both the morning and evening traffic peaks clearly show up. The rush hours for motorcycles and gasoline cars, which are the major sources of CO and NO2 in Hanoi, are marked with triangles on the horizontal axis. The rush hours for diesel trucks and buses, which contribute largely to SO2 and PM10 loadings, occur about 1 hour later. According to CENMA (2009), the vehicle fleet of Hanoi is dominated by motorcycles followed by gasoline cars, diesel trucks and buses in the proportions of 1:0.06:0.04:0.008. Based on the above diurnal variations of pollutant concentrations, the average nighttime-to-daytime (N/D) concentration ratios were calculated for NRTI, STI and non-inversion periods (Table 1). The non-inversion period covers the two cold surges on the 7th and 11th and the recession of the precedent monsoon cycle (December 6). The results show the role of nighttime atmospheric dispersion on air pollution in Hanoi. The nighttime contribution to the 24-h pollutant concentration is increasingly significant from non-inversion to STI and NRTI periods. Even in non-inversion and STI periods, pollutant concentrations at night are as high as in daytime despite a considerable reduction of 72% in vehicle emissions at night (CENMA, 2009). The nighttime contribution becomes dominant in NRTI periods, in which pollution levels are about two times higher at night than in daytime. Calm winds and restricted vertical mixing are the two factors responsible for such increased nighttime pollution. Wind speed is much lower during nighttime than in daytime (Table 1). The mixed layer height reduces from more than 1200 m in the afternoon to less
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Fig. 7. Hourly wind speed (m s−1) and pollutant concentrations (μg m−3) in the STI period from 29 Dec. 2003 to 4 Jan. 2004.
Fig. 8. Average diurnal variations of pollutant concentrations in 2003. Triangles on horizontal axis indicate rush hours of commuters using motorcycles and cars according to CENMA (2009). The concentrations of PM10 and CO have been multiplied by 0.5 and 0.02, respectively.
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Table 1 The nighttime-to-daytime ratios (N/D) for pollutant concentrations and wind speed. The non-inversion period (12/6/03–12/12/03) covers the two cold surges on the 7th and 11th.
Non-inversion NRTI STI
Averaging period
SO2
NO2
CO
PM10
WS
12/6/03–12/12/03 12/13/03–12/17/03 12/28/03–01/03/04 2003
0.86 1.6 0.87 0.99
0.94 2.2 1.2 1.27
0.92 2.3 1.1 1.15
0.92 1.9 0.96 1.15
0.94 0.45 0.71 0.77
than 100 m after sunset as a ground-based inversion layer appears. Similar atmospheric dispersion conditions and nighttime pollution episodes were observed by Venkatram and Cimorelli (2007) in the city of Pune, India. This study applied a non-stationary dispersion box model for investigating NRTI pollution episodes of PM10 in that city and found similar diurnal concentration patterns and a large N/D ratio, as observed in this study. 6. Discussion and summary Multi-day pollution episodes involving all types of air pollutants usually occur in Hanoi following the northeast monsoon surges in winter that bring in continental cold air from the Asiatic highpressure anticyclone. This study provides insights into the atmospheric processes underlying the occurrence and evolution of WPEs using continuous monitoring data on air pollutants and surface meteorology as well as information on upper-air soundings and air mass trajectories. Although the analysis was based on monitoring data for the winter of October 2003 to February 2004, atmospheric conditions controlling the daily and diurnal variations in pollutant concentrations during WPEs found in this study occur almost similarly in other winters. The daily concentrations of air pollutants concurrently rise after a monsoon surge reaching their highest levels some time in the middle of a monsoon cycle then declining ahead of the next surge. Each monsoon cycle generally proceeds through a dry phase and a humid phase as continental cold air arrives in Hanoi through inland China then via the East China Sea. WPEs with 24-h PM10 concentrations exceeding 150 g m −3 are associated with NRTI in the dry phase and STI in the humid phase. From October 2003 to February 2004, Hanoi experienced 11 multi-day WPEs, during which NRTI and STI were observed on 21 and 38 days, respectively. Distinctive diurnal patterns of pollutant concentrations were found for NRTI and STI periods as NRTI occurs only at night while STI may also exist in both nighttime and daytime. During each NRTI period, pollutant loadings dramatically rise after sunset reaching maxima sometime before midnight then gradually decline overnight and throughout the daytime before reaching the lowest levels at around 4:00 pm ahead of the next-day NRTI. The evening traffic peak is much more pronounced than the morning one and pollution is about two times higher at night than in daytime despite a considerable reduction of traffic emissions during nighttime. On the contrary, in STI periods, broad morning and evening traffic peaks are observed and pollution is as high at night as in daytime. Among 16 monsoon cycles (cold surges) recorded from October 2003 to February 2004, only 11 multi-day WPEs emerged. Several cold surges were not accompanied by prolonged anticyclonic conditions for NRTI, STI, and multi-day WPE to happen. Especially, from February onward, with the frequent intrusion of maritime air from the East China Sea, the continental anticyclone often quickly weakens after passing northern Vietnam. The analysis in this study suggests
that the daily RH b 60% and WS b 1 mm s −1 are prerequisite for a NRTI to occur after a cold surge. Since the northeast monsoon surges and the daily weather conditions in the dry and humid phases that follow can be reliably forecasted, advance warnings to the public of WPEs can be incorporated in daily weather forecast. Successful prediction of oncoming NRTI episodes would critically support a targeted polution control strategy. In fact, as severe pollution occurs mainly in several hours after sunset, it would be most efficient to concentrate emission reduction measures (such as watering of roadways) in periods just before sunset. Furthermore, because of the close association between pollutant concentrations and meteorological conditions, the forecast of next-day pollutant concentrations can be operationalized using appropriate statistical models, in which meteorological parameters are used as explanatory variables. Acknowledgment The authors gratefully acknowledge the NOAA Air Resources Laboratory (ARL) for the provision of the HYSPLIT transport and dispersion model and/or READY website (http://www.arl.noaa. gov/ready.html) used in this publication. References Air Service Group. A report on the results from the air quality monitoring network (2003). The Government of Hong Kong Special Administrative Region: Environmental Protection Department; 2003. Report Number EPD/TR 02/03. Bac VT, Hien PD. Regional and local emissions in red river delta, Northern Vietnam. Air Qual Atm Health 2009;2:157–67. CENMA. Hanoi Center for Environmental Monitoring and Analysis; 2009. Internal Report (in Vietnamese). Chen TC, Huang WR, Yoon JH. Interannual variation of the East Asian cold surge activity. J Climate 2003;17:401–13. Chu EWK. A method for forecasting the arrival of cold surges in Hong Kong. Royal Observatory Hong Kong; 1978. Technical Note No 43. Cohen DD, Crawford J, Stelcer E, Bac VT. Characterization and source apportionment of fine particulate sources at Hanoi from 2001 to 2008. Atmos Environ 2010;44: 320–8. Draxler RR, Rolph GD. HYSPLIT (Hybrid Single-Particle Lagrangian Integrated Trajectory). Silver Spring, MD: NOAA Air Resources Laboratory; 2003. Model access via NOAA ARL READY Website http://www.arl.noaa.gov/ready/hysplit4.html. Hien PD, Binh NT, Truong Y, Ngo NT. Temporal variations of source impacts at the receptor, as derived from air particulate monitoring data in Ho Chi Minh City, Vietnam. Atmos Environ 1999;33:3133–42. Hien PD, Bac VT, Tham HC, Nhan DD, Vinh LD. Influence of meteorological conditions on PM2.2 and PM2.2–10 concentrations during the monsoon season in Hanoi, Vietnam. Atmos Environ 2002;36:3473–84. Hien PD, Bac VT, Lam DT, Thinh NTHT. PMF receptor modeling of fine and coarse PM10 in air masses governing monsoon conditions in Hanoi, northern Vietnam. Atmos Environ 2004;38:189–201. Hopke PK, Cohen DD, Begum BA, Biswas SK, Ni B, Pandit GG, Santoso M, Chung YS, Davy P, Markwitz A, Waheed S, Siddique N, Santos FL, Pabroa PB, Seneviratne MCS, Wimolwattanapun W, Bunprabob S, Bac VT, Hien PD, Markowicz A. Urban air quality in the Asian region. Sci Total Environ 2008;404:103–12. Kim Oanh NT, Upadhyaya N, Zhuang YH, Hao ZP, Murthy DVS, Lestari P, Villarin JT, Changchua K, Co HX, Dung NT, Lindgren ES. Particulate pollution in six Asian cities: spatial and temporal distributions, and associated sources. Atmos Environ 2006;40:3367–80. Lam CY. 500 mbar troughs passing over Lake Baikal and the arrival of surges at Hong Kong. Hong Kong: Royal Observatory; 1976. Tech. Note No 31. Lam CY. Synoptic patterns associated with the onset of cold surges reaching the South China Sea. Hong Kong: Royal Observatory; 1981. Reprint No 91. Louie PKK, Watson JG, Chow JC, Chen A, Sin DWM, Lau AKH. Seasonal characteristic and regional transport of PM2.5 in Hong Kong. Atmos Environ 2005;39:1695–710. Song Y, Zhang M, Cai X. PM10 modeling of Beijing in the winter. Atmos Environ 2006;40:4126–36. Toan PN, Dac PT. The climate of Vietnam. Hanoi (in Vietnamese): Science and Technology publisher; 1993. Tuan NV. Status and trends of air pollution in Ho Chi Minh City. Project Final Report to Department of Science and Technology; 2010 (in Vietnamese). Venkatram A, Cimorelli A. On the role of nighttime meteorology in modeling dispersion of near surface emissions in urban areas. Atmos Environ 2007;41:692–704.
Contents lists available at SciVerse ScienceDirect
Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv
Air pollution episodes associated with East Asian winter monsoons P.D. Hien a,⁎, P.D. Loc b, N.V. Dao b a b
Vietnam Atomic Energy Agency, 59 Ly Thuong Kiet str. Hanoi, Viet Nam National Hydro-Meteorological Center, 62-A2 Nguyen Chi Thanh str. Hanoi, Viet Nam
a r t i c l e
i n f o
Article history: Received 30 March 2011 Received in revised form 2 August 2011 Accepted 24 August 2011 Available online 16 September 2011 Keywords: PM10 SO2 NO2 CO Continuous monitoring Temperature inversion Nighttime Daytime
a b s t r a c t A dozen multi-day pollution episodes occur from October to February in Hanoi, Vietnam due to prolonged anticyclonic conditions established after the northeast monsoon surges (cold surges). These winter pollution episodes (WPEs) account for most of the 24-h PM10 exceedances and the highest concentrations of gaseous pollutants in Hanoi. In this study, WPEs were investigated using continuous air quality monitoring data and information on upper-air soundings and air mass trajectories. The 24-h pollutant concentrations are lowest during cold surges; concurrently rise thereafter reaching the highest levels toward the middle of a monsoon cycle, then decline ahead of the next cold surge. Each monsoon cycle usually proceeds through a dry phase and a humid phase as Asiatic continental cold air arrives in Hanoi through inland China then via the East China Sea. WPEs are associated with nighttime radiation temperature inversions (NRTIs) in the dry phase and subsidence temperature inversions (STIs) in the humid phase. In NRTI periods, the rush hour pollution peak is more pronounced in the evening than in the morning and the pollution level is about two times higher at night than in daytime. In STI periods, broad morning and evening traffic peaks are observed and pollution is as high at night as in daytime. The close association between pollution and winter monsoon meteorology found in this study for the winter 2003–04 may serve as a basis for advance warning of WPEs and for forecasting the 24-h pollutant concentrations. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Hanoi is among Asian cities experiencing high levels of particulate matter in ambient air (Kim Oanh et al., 2006; Hopke et al., 2008). Concentrations of particulate matter with aerodynamic diameter less than 10 μm (PM10) are about twice the annual mean limit (50 μg m −3), and exceed the 24-h legislation threshold (150 μg m −3) on 45 to 50 days each year (Hien et al., 2002). The particulate pollution level varies substantially over a year. Particulate mass loadings are low in summer (May–September), when the East Asian southeast monsoon driven by tropical maritime air from the Pacific and Indian Oceans brings in hot weather and convective conditions. Conversely, high particulate mass loadings occur in winter (October–March), when Hanoi is impacted by the East Asian northeast monsoon. This monsoon brings continental cold air from the Asiatic high pressure systems to northern Vietnam either through inland China (dry weather) or via the East China Sea (humid weather) (Toan and Dac, 1993). In particular, multi-day winter pollution episodes (WPEs) that account for most of the 24-h PM10 exceedances usually occur in association with nighttime radiation temperature inversion (NRTI) and subsidence temperature inversion
⁎ Corresponding author. Tel.: + 84 913320067, + 84 04 35142215. E-mail address: [email protected] (P.D. Hien). 0048-9697/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.scitotenv.2011.08.049
(STI) in the dry and humid phases of a monsoon cycle, respectively (Hien et al., 2002). Automated air quality monitoring facilities installed in 2002 in Hanoi reveal that these WPEs also involve other pollutants. Data obtained to date show that the 24-h concentrations of particulate (PM10) and gaseous pollutants (SO2, NO, NO2, CO etc.) concurrently rise after the intrusion of cold air (cold surge) and reach maxima at some time in the middle of a monsoon cycle and decline ahead of the next cold surge. All air pollutants exhibit lowest concentrations during cold surges. Song et al. (2006) also found a rather similar trend for PM10 at several monitoring stations in Beijing where the intrusion of cold air from the East Asian anticyclones occurs in average cycles of 4 days. In Hong Kong, particulate pollution episodes also occur in winter when the weather is influenced by the East Asian anticyclones (Air Service Group, 2003; Louie et al., 2005). In Hanoi, recurring northeast monsoon cycles leave a dozen multiday pollution episodes between October and February. Directing emission control measures specifically upon these periods would greatly improve the effectiveness of efforts aimed at combating air pollution in Hanoi and reducing the health risk associated with prolonged and simultaneous exposures to many types of air pollutants. Such a targeted strategy requires reliable advance warnings of WPEs and forecasting of pollutant concentrations. For this purpose, in this study we investigate atmospheric conditions governing the daily and diurnal variations of pollutant concentrations
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during winter in Hanoi. The analysis was done using information on air pollution and meteorology for the winter of October 2003 to February 2004. The study period covering 16 northeast monsoon cycles can be regarded as representing the situation that occurs from year to year in winter in Hanoi. The consistency gained in characterizing winter monsoon conditions associated with WPEs by using combined information on surface and upper-air meteorology and air mass trajectories serves as a basis for predicting oncoming WPEs that can be incorporated into routine weather forecast. Furthermore, the close association between pollution and meteorology found in this study also provides an important justification for developing statistical models capable of forecasting next-day air pollutant concentrations using meteorological parameters as explanatory variables. These models will be presented in a forthcoming paper. 2. Monitoring site and facilities The air quality monitoring station of the National HydroMeteorological Center is located in the Hanoi meteorological garden (21° 03′ N, 105° 48′ E), about 100 m from a road and 6–8 km from industrial suburban areas. Besides surface observations, upper-air radiosonde balloons are released at this garden twice a day at 7:00 am (00:00 GMT) and 7:00 pm (12:00 GMT) providing vertical profiles of atmospheric parameters. PM10 and PM2.5 have been collected at this site since 1998 by using manual air samplers. The data were used for assessing the impact of monsoon regimes on the seasonal variations of particulate pollution (Hien et al., 2002), for comparative studies of air pollution in Asian cities (Hopke et al. 2008), and for source apportionment by receptor modeling (Hien et al., 2004; Bac and Hien, 2009; Cohen et al., 2010). Since 2002 an automatic monitoring station supplied by Fuji Electric was installed at this site for continuous monitoring of both particulate and gaseous pollutants. The methods used for automatic measurements of air pollutants concentrations are based on beta ray attenuation for PM10, UV fluorescence for SO2, chemiluminescence for NO2, non-dispersive infrared absorption for CO, and ultraviolet absorption for O3. The monitoring results on the status and trends of air pollution in Hanoi are reported for the first time in this paper. The hourly data capture for the 2003–2004 monitoring period are 92– 97% for PM10, NO2, SO2, and CO; and less than 70% for O3. Ozone was excluded from this study because the ambient concentration of this secondary pollutant is affected by many other factors, not only by dispersion conditions. The automatic monitoring facility also provides information about surface pressure, daily precipitation, air temperature (T), relative humidity (RH), solar radiation (SR), wind speed (WS) and wind direction. 3. Status of air pollution in Hanoi as derived from continuous air quality monitoring data The annual mean concentrations of PM10, NO2, SO2, and CO are 89, 38, 28, and 1530 μg m −3, respectively. For information, Fig. 1 shows the 2003 mean pollutant concentrations in Hanoi with those in Hong Kong (Air Service Group, 2003), which is about 900 km to the east of Hanoi, and Ho Chi Minh City (HCMC) (Tuan, 2010), about 1200 km south of Hanoi. Under the influence of the same East Asian monsoon regime, pollutant concentrations in the three cities are lowest in July and highest in December (Air Service Group, 2003, Hien et al., 1999). Concerning the northeast monsoon in winter, Hong Kong experiences on average 13 cold surges from November to February (Lam, 1981), while in HCMC cold surges occur less frequently, mainly in December–January. Hanoi has a highest level of PM10 with the annual mean concentration about twice the national standard (50 μg m−3). Besides, from October
Hanoi
HCM City
Hong Kong
100 90
concentration, µg m-3
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80 70 60 50 40 30 20 10 0 PM10
NO2
SO2
CO/100
Fig. 1. Comparison of annual mean concentrations of four criteria pollutants in Hanoi, Ho Chi Minh City and Hong Kong in 2003. Data for Hong Kong and Ho Chi Minh City are averaged over 12 and 4 automatic monitoring stations, respectively. Vertical bars indicate standard errors of the mean concentrations. For Hanoi, data are available only from one station.
2003 to February 2004, the 24-h national standard of 150 μg m −3 was exceeded on 45 days, which were clustered within 11 multiday pollution episodes (Fig. 2). These WPEs involve also SO2, NO2, and CO although at levels within legal limits. The 24-h concentrations of pollutants rise from their lowest levels during the cold surge reaching maxima sometime in the middle of the monsoon cycle and decline thereafter ahead of the next cold surge. Such a behavior reflects the changes of atmospheric conditions in the winter monsoon regime, as shown in the next section.
4. Atmospheric transport and monsoon conditions associated with WPEs 4.1. The East Asian anticyclones and winter monsoons In winter, large areas of East Asia are affected by northerly to northeasterly monsoons driven by continental surface anticyclones, which frequently develop over Lake Baikal in Siberia (Lam, 1976). As Lake Baikal is located at the same longitude as the eastern edge of the Tibetan plateau, strong northerly winds subsiding near Lake Baikal are not blocked by the Himalayas, but drive polar continental cold air southwards across China (Chu, 1978). At any given location a winter monsoon cycle begins with a cold surge, which is usually associated with the passage of a cold front marking the boundary between Asiatic continental polar cold air and Pacific tropical warm air (Lam, 1981). For illustration, Fig. 3 shows the surface isobaric chart with a cold front passing over Hanoi in December. From October 2003 to the end of February 2004, fifteen major cold surges were recorded in Hanoi. The cold surges marked with triangles on the horizontal axes in Fig. 2 were identified using either of the two criteria suggested by Chu (1978) or Lam (1976) for Hong Kong, namely a drop by 2 °C or more in daily mean temperature within 2 days, or a temperature fall of at least 2 °C within an hour. Other atmospheric parameters (pressure, humidity, wind direction, etc.) also change abruptly at a cold surge, but this event can most easily be recognized by high winds (Fig. 2a) that reflect the congestion of the anticyclone's outermost isobars behind the cold front (Fig. 3). Following a cold surge, atmospheric conditions usually proceed through dry and humid phases, according to the position of the anticyclone, and therefore the trajectory of continental cold air arriving in Hanoi. Dry and clear conditions prevail as long as cold air arrives from the anticyclone through inland China. The humid phase with cold air arriving from the sea follows the dry phase as the anticyclone reaches the Pacific coast in its eastward movement over inland China. Finally, the anticyclone dissipates over the sea, marking the northeast monsoon
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Fig. 2. The daily average pollutant concentrations, wind speed (WS), air temperature (T), solar radiation (SR), and relative humidity (RH) in winter. Triangles on the horizontal axis mark the cold surges. NRTIs and STIs are marked in Fig. 2e with long and short vertical bars, respectively. The horizontal line at 150 μg m−3 in Fig. 2e indicates the 24-h national standard for PM10.
recession and the return of maritime tropical air, which may last several days ahead of the new cold surge. 4.2. Air mass back trajectories and related dry and humid phases of a monsoon cycle Air mass back trajectories computed using the HY-SPLIT model (Draxler and Rolph, 2003) help explain the cold air flows and the evolution of atmospheric conditions over dry and humid phases of a monsoon cycle. As an illustration, Fig. 4 depicts three-day air mass back trajectories that are relevant to the changes of atmospheric conditions over monsoon cycles in December 2003. From the 1st to 6th, continental cold air arrived in Hanoi by maritime trajectories. The shift from maritime to inland trajectories caused the cold surge on the 7th, as evidenced by high winds and a large temperature drop in Fig. 2a and b. Cold waves that arrived on
the following days were accompanied by high winds and further temperature drops until the 11th when an additional major cold surge was recorded. The dry phase was established from the 12th to 17th during which inland trajectories brought in dry air and a clear weather (Fig. 2c, d). With maritime trajectories resuming on the 18th, the humid phase replaced the dry phase ahead of the monsoon recession and the next cold surge. The second monsoon cycle in December 2003 started with the cold surge on the 19th followed by the dry phase with inland trajectories from the 20th to 23rd and the humid phase with maritime trajectories from the 24th to 25th. The return of inland trajectories on the 26th started the third monsoon cycle in December 2003, followed by the dry phase on the 27th, and the humid phase from the 28th onward. The anticyclonic dry and humid phases, corresponding to inland and maritime continental cold air trajectories, respectively, also
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Fig. 3. A surface isobaric chart illustrates the passage over Hanoi of a cold front giving rise to a cold surge on 22 December, 2008.
occur in other winter months. However, from October to December, inland trajectories dominate, yielding long dry periods, whereas from January to March cold air arrives in Hanoi mainly from the sea making dry phases shorter and sparser. The frequency of occurrence of winter monsoon surges varies from year to year according to the variability of large scale climatological processes. Chen et al. (2003), for example, observed the influence of ENSO phenomenon on the interannual variation of winter monsoon surge occurrence in Hong Kong, i.e. a higher (lower) occurrence frequency was found during El Niño (La Niña) winters.
4.3. NRTI and STI Upper-air radiosondes released at 7:00 am and 7:00 pm at the monitoring site revealed the association of WPEs with NRTIs in dry phases and with STIs in humid phases (Hien et al., 2002).
In the dry phase, clear sky and light winds caused by anticyclonic sinking air favor NRTI that develops at dusk extending from the ground to about 100–150 m due to rapid surface radiation cooling in the presence of warm air above. Such a surface temperature inversion can actually be observed in the 7:00 pm upper air soundings in the dry phase. As the night progresses, the inversion layer may move up to higher elevations leaving its vestiges at several hundred meters high that can still be observed in the 7:00 am sounding the following day. The inversion layer presumably dissolves as incident solar radiation warms the ground during the day. The mixed layer height obtained from air mass back trajectory calculations (Draxler and Rolph, 2003) varies widely from less than 100 m after sunset to about 600 m at 9:00 am and more than 1200 m in the afternoon. In the humid phase, ground-based NRTIs can no longer occur, but STIs at several hundred meters high are usually found in both the 7:00 am and 7:00 pm soundings. STIs may persist at varying height in both daytime and nighttime. The calculated mixed layer height varies from 200–300 m at night to 500–600 m in daytime. NRTIs and STIs are marked in Fig. 2e with long and short vertical bars, respectively. From October 2003 to February 2004, Hanoi experienced 11 multi-day WPEs, during which NRTI and STI were observed on 21 and 38 days, respectively. Therefore, in terms of air pollution each winter monsoon cycle can usually be divided into three periods, i.e. the low pollution period during the cold surge followed by the two high pollution periods with stagnant atmosphere associated with NRTIs and STIs. Atmospheric conditions are quite different for the three periods, namely high winds during the cold surges; light wind, clear sky, and low humidity during NRTI periods; and light wind, overcast and high humidity during STI periods (Fig. 2a, d). As an illustration, the characteristic atmospheric conditions for the three periods are illustrated in Fig. 5 for the second monsoon cycle in December 2003. Similar patterns of atmospheric conditions are observed for other monsoon cycles in the winter 2003–04. 5. Diurnal behaviors of pollutant concentrations during NRTI and STI periods Pollutant concentrations exhibit different diurnal behaviors during the three periods, as shown by monitoring data for December 2003 in Figs. 6 and 7.
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Fig. 4. Three-day back trajectories of continental cold air arriving in Hanoi in December 2003. The cold surges occurred following the shift from maritime trajectories on the 6th, 18th, and 25th to inland trajectories on the 7th, 19th, and 26th. The return of maritime trajectories on the 18th, 24th, and 28th ended the dry phases and started the humid phases on the 18th, 24th–25th, and from the 28th onward.
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Fig. 5. Evolution of daily average wind speed (WS), air temperature (T), relative humidity (RH) and solar radiation (SR) over a monsoon cycle showing distinct atmospheric conditions for the cold surges, NRTI (full height column) and STI (half height column) periods in December 2003. Triangles on horizontal axis mark the cold surges. Note that on December 27 a cold surge arrived in early morning and a NRTI occurred just in that evening.
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From December 10 to 12, hourly pollutant concentrations were very low due to high winds accompanying cold air arrivals. No near surface temperature inversions were observed on these days. Both the morning and evening traffic pollution peaks were weakly pronounced (Fig. 6). 3 2
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On the 13th, calm conditions suddenly appeared from 5:00 pm and the first NRTI in December was recorded in the 7:00 pm upper-air sounding. The temperature inversion and calm conditions coincided with the evening traffic rush hour and cooking making particulate and gaseous pollutants from traffic and coal burning trapped in a shallow layer just above the ground. Pollutant concentrations drastically soared from 5:00 pm, reached maxima at about 9:00 pm then declined gradually overnight and throughout the daytime reaching the lowest levels at around 4:00 pm ahead of the next day NRTI (Fig. 6). The pollutant loadings occurred predominantly from 6:00 pm to midnight and were much reduced in daytime. The evening peaks occur at about 3.5 h later than the rush hours as a result of pollutant buildup in poor nighttime dispersion conditions (Venkatram and Cimorelli, 2007). In the meantime, the morning pollutant peaks were weakly pronounced suggesting the inversion layer had already dissolved before the morning traffic rush hour. Conversely, in the STI period, from December 28, 2003 to January 3, 2004 (Fig. 7), with the temperature inversion persistent for whole day, the morning traffic peaks were as strong as the evening ones and the pollutant loadings were almost similar in daytime and nighttime. For a better appreciation of the diurnal variations of pollutant concentrations in NRTI and STI periods, Fig. 8 shows the average diurnal trends in 2003. Both the morning and evening traffic peaks clearly show up. The rush hours for motorcycles and gasoline cars, which are the major sources of CO and NO2 in Hanoi, are marked with triangles on the horizontal axis. The rush hours for diesel trucks and buses, which contribute largely to SO2 and PM10 loadings, occur about 1 hour later. According to CENMA (2009), the vehicle fleet of Hanoi is dominated by motorcycles followed by gasoline cars, diesel trucks and buses in the proportions of 1:0.06:0.04:0.008. Based on the above diurnal variations of pollutant concentrations, the average nighttime-to-daytime (N/D) concentration ratios were calculated for NRTI, STI and non-inversion periods (Table 1). The non-inversion period covers the two cold surges on the 7th and 11th and the recession of the precedent monsoon cycle (December 6). The results show the role of nighttime atmospheric dispersion on air pollution in Hanoi. The nighttime contribution to the 24-h pollutant concentration is increasingly significant from non-inversion to STI and NRTI periods. Even in non-inversion and STI periods, pollutant concentrations at night are as high as in daytime despite a considerable reduction of 72% in vehicle emissions at night (CENMA, 2009). The nighttime contribution becomes dominant in NRTI periods, in which pollution levels are about two times higher at night than in daytime. Calm winds and restricted vertical mixing are the two factors responsible for such increased nighttime pollution. Wind speed is much lower during nighttime than in daytime (Table 1). The mixed layer height reduces from more than 1200 m in the afternoon to less
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Fig. 8. Average diurnal variations of pollutant concentrations in 2003. Triangles on horizontal axis indicate rush hours of commuters using motorcycles and cars according to CENMA (2009). The concentrations of PM10 and CO have been multiplied by 0.5 and 0.02, respectively.
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Table 1 The nighttime-to-daytime ratios (N/D) for pollutant concentrations and wind speed. The non-inversion period (12/6/03–12/12/03) covers the two cold surges on the 7th and 11th.
Non-inversion NRTI STI
Averaging period
SO2
NO2
CO
PM10
WS
12/6/03–12/12/03 12/13/03–12/17/03 12/28/03–01/03/04 2003
0.86 1.6 0.87 0.99
0.94 2.2 1.2 1.27
0.92 2.3 1.1 1.15
0.92 1.9 0.96 1.15
0.94 0.45 0.71 0.77
than 100 m after sunset as a ground-based inversion layer appears. Similar atmospheric dispersion conditions and nighttime pollution episodes were observed by Venkatram and Cimorelli (2007) in the city of Pune, India. This study applied a non-stationary dispersion box model for investigating NRTI pollution episodes of PM10 in that city and found similar diurnal concentration patterns and a large N/D ratio, as observed in this study. 6. Discussion and summary Multi-day pollution episodes involving all types of air pollutants usually occur in Hanoi following the northeast monsoon surges in winter that bring in continental cold air from the Asiatic highpressure anticyclone. This study provides insights into the atmospheric processes underlying the occurrence and evolution of WPEs using continuous monitoring data on air pollutants and surface meteorology as well as information on upper-air soundings and air mass trajectories. Although the analysis was based on monitoring data for the winter of October 2003 to February 2004, atmospheric conditions controlling the daily and diurnal variations in pollutant concentrations during WPEs found in this study occur almost similarly in other winters. The daily concentrations of air pollutants concurrently rise after a monsoon surge reaching their highest levels some time in the middle of a monsoon cycle then declining ahead of the next surge. Each monsoon cycle generally proceeds through a dry phase and a humid phase as continental cold air arrives in Hanoi through inland China then via the East China Sea. WPEs with 24-h PM10 concentrations exceeding 150 g m −3 are associated with NRTI in the dry phase and STI in the humid phase. From October 2003 to February 2004, Hanoi experienced 11 multi-day WPEs, during which NRTI and STI were observed on 21 and 38 days, respectively. Distinctive diurnal patterns of pollutant concentrations were found for NRTI and STI periods as NRTI occurs only at night while STI may also exist in both nighttime and daytime. During each NRTI period, pollutant loadings dramatically rise after sunset reaching maxima sometime before midnight then gradually decline overnight and throughout the daytime before reaching the lowest levels at around 4:00 pm ahead of the next-day NRTI. The evening traffic peak is much more pronounced than the morning one and pollution is about two times higher at night than in daytime despite a considerable reduction of traffic emissions during nighttime. On the contrary, in STI periods, broad morning and evening traffic peaks are observed and pollution is as high at night as in daytime. Among 16 monsoon cycles (cold surges) recorded from October 2003 to February 2004, only 11 multi-day WPEs emerged. Several cold surges were not accompanied by prolonged anticyclonic conditions for NRTI, STI, and multi-day WPE to happen. Especially, from February onward, with the frequent intrusion of maritime air from the East China Sea, the continental anticyclone often quickly weakens after passing northern Vietnam. The analysis in this study suggests
that the daily RH b 60% and WS b 1 mm s −1 are prerequisite for a NRTI to occur after a cold surge. Since the northeast monsoon surges and the daily weather conditions in the dry and humid phases that follow can be reliably forecasted, advance warnings to the public of WPEs can be incorporated in daily weather forecast. Successful prediction of oncoming NRTI episodes would critically support a targeted polution control strategy. In fact, as severe pollution occurs mainly in several hours after sunset, it would be most efficient to concentrate emission reduction measures (such as watering of roadways) in periods just before sunset. Furthermore, because of the close association between pollutant concentrations and meteorological conditions, the forecast of next-day pollutant concentrations can be operationalized using appropriate statistical models, in which meteorological parameters are used as explanatory variables. Acknowledgment The authors gratefully acknowledge the NOAA Air Resources Laboratory (ARL) for the provision of the HYSPLIT transport and dispersion model and/or READY website (http://www.arl.noaa. gov/ready.html) used in this publication. References Air Service Group. A report on the results from the air quality monitoring network (2003). The Government of Hong Kong Special Administrative Region: Environmental Protection Department; 2003. Report Number EPD/TR 02/03. Bac VT, Hien PD. Regional and local emissions in red river delta, Northern Vietnam. Air Qual Atm Health 2009;2:157–67. CENMA. Hanoi Center for Environmental Monitoring and Analysis; 2009. Internal Report (in Vietnamese). Chen TC, Huang WR, Yoon JH. Interannual variation of the East Asian cold surge activity. J Climate 2003;17:401–13. Chu EWK. A method for forecasting the arrival of cold surges in Hong Kong. Royal Observatory Hong Kong; 1978. Technical Note No 43. Cohen DD, Crawford J, Stelcer E, Bac VT. Characterization and source apportionment of fine particulate sources at Hanoi from 2001 to 2008. Atmos Environ 2010;44: 320–8. Draxler RR, Rolph GD. HYSPLIT (Hybrid Single-Particle Lagrangian Integrated Trajectory). Silver Spring, MD: NOAA Air Resources Laboratory; 2003. Model access via NOAA ARL READY Website http://www.arl.noaa.gov/ready/hysplit4.html. Hien PD, Binh NT, Truong Y, Ngo NT. Temporal variations of source impacts at the receptor, as derived from air particulate monitoring data in Ho Chi Minh City, Vietnam. Atmos Environ 1999;33:3133–42. Hien PD, Bac VT, Tham HC, Nhan DD, Vinh LD. Influence of meteorological conditions on PM2.2 and PM2.2–10 concentrations during the monsoon season in Hanoi, Vietnam. Atmos Environ 2002;36:3473–84. Hien PD, Bac VT, Lam DT, Thinh NTHT. PMF receptor modeling of fine and coarse PM10 in air masses governing monsoon conditions in Hanoi, northern Vietnam. Atmos Environ 2004;38:189–201. Hopke PK, Cohen DD, Begum BA, Biswas SK, Ni B, Pandit GG, Santoso M, Chung YS, Davy P, Markwitz A, Waheed S, Siddique N, Santos FL, Pabroa PB, Seneviratne MCS, Wimolwattanapun W, Bunprabob S, Bac VT, Hien PD, Markowicz A. Urban air quality in the Asian region. Sci Total Environ 2008;404:103–12. Kim Oanh NT, Upadhyaya N, Zhuang YH, Hao ZP, Murthy DVS, Lestari P, Villarin JT, Changchua K, Co HX, Dung NT, Lindgren ES. Particulate pollution in six Asian cities: spatial and temporal distributions, and associated sources. Atmos Environ 2006;40:3367–80. Lam CY. 500 mbar troughs passing over Lake Baikal and the arrival of surges at Hong Kong. Hong Kong: Royal Observatory; 1976. Tech. Note No 31. Lam CY. Synoptic patterns associated with the onset of cold surges reaching the South China Sea. Hong Kong: Royal Observatory; 1981. Reprint No 91. Louie PKK, Watson JG, Chow JC, Chen A, Sin DWM, Lau AKH. Seasonal characteristic and regional transport of PM2.5 in Hong Kong. Atmos Environ 2005;39:1695–710. Song Y, Zhang M, Cai X. PM10 modeling of Beijing in the winter. Atmos Environ 2006;40:4126–36. Toan PN, Dac PT. The climate of Vietnam. Hanoi (in Vietnamese): Science and Technology publisher; 1993. Tuan NV. Status and trends of air pollution in Ho Chi Minh City. Project Final Report to Department of Science and Technology; 2010 (in Vietnamese). Venkatram A, Cimorelli A. On the role of nighttime meteorology in modeling dispersion of near surface emissions in urban areas. Atmos Environ 2007;41:692–704.