levels of aerosols were due to the mixing with air masses from the Pacific Ocean. Diurnal .... inlet line was heated to 400°C in order to effectively volatilize.
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 116, D15302, doi:10.1029/2010JD015563, 2011
Seasonal and diurnal variations of black carbon and organic carbon aerosols in Bangkok L. K. Sahu,1,2 Y. Kondo,1,3 Y. Miyazaki,4 Prapat Pongkiatkul,5,6 and N. T. Kim Oanh5 Received 28 December 2010; revised 5 April 2011; accepted 9 May 2011; published 4 August 2011.
[1] Measurements of black carbon (BC) and organic carbon (OC) were conducted in Bangkok during 2007–2008. Annual trends of BC and OC show strong seasonality with lower and higher concentrations during wet and dry seasons, respectively. Flow of cleaner air, wet removal, and negligible biomass burning resulted in the lowest concentrations of aerosols in the wet season. In addition to anthropogenic sources, long‐range transport and biomass burning caused higher concentrations in the dry and hot seasons, respectively. Despite extensive biomass burning in the hot season, moderate levels of aerosols were due to the mixing with air masses from the Pacific Ocean. Diurnal distributions exhibit peaks during rush hour marked by minima in the OC/BC ratio and stagnant wind flow. The lowest concentrations in the afternoon hours could be due to deeper planetary boundary layer and reduced traffic. Overall, the concentrations of both BC and OC decrease with the increase in wind speed. The weekend effects, due to reduced emission during weekends, in the concentrations of both BC and OC were significant. Therefore, stricter abatement in vehicular emissions could substantially reduce pollution. A slope of DBC/DCO of 9.8 ngm−3 ppbv−1 for the wet season represents the emission ratio from vehicular sources. The highest of DOC/DBC (3 mg mg−1) in the hot season was due to the predominant influence of biomass burning and significant formation of secondary OC. The levels of BC and OC in Bangkok fall within the ranges of their concentrations measured in the major cities of East Asia. Citation: Sahu, L. K., Y. Kondo, Y. Miyazaki, P. Pongkiatkul, and N. T. Kim Oanh (2011), Seasonal and diurnal variations of black carbon and organic carbon aerosols in Bangkok, J. Geophys. Res., 116, D15302, doi:10.1029/2010JD015563.
1. Introduction [2] Anthropogenic emissions of aerosols and trace gases in Asia are increasing because of rapid economic growth and urban development [Ohara et al., 2007; Zhang et al., 2009]. Black carbon (BC) is a refractory component of carbonaceous particles emitted primarily by incomplete combustion [Antony Chen et al., 2001; Penner et al., 1993; Cooke and Wilson, 1996; Liousse et al., 1996]. BC particles strongly absorb radiation in the visible, near UV, and near IR due to their graphitic structure [Rosen et al., 1978]. Model estimates suggest that the strongly absorbing aerosols have large impacts on regional climate and the hydrological cycle [Menon et al., 1 Research Center for Advanced Science and Technology, University of Tokyo, Tokyo, Japan. 2 Now at Physical Research Laboratory, Ahmedabad, India. 3 Now at Department of Earth and Planetary Science, Graduate School of Science, University of Tokyo, Tokyo, Japan. 4 Institute of Low Temperature Science, Hokkaido University, Sapporo, Japan. 5 Asian Institute of Technology, Pathumthani, Thailand. 6 Now at Department of Environmental Engineering, Faculty of Engineering, King Mongkut’s University of Technology Thonburi, Bangkok, Thailand.
Copyright 2011 by the American Geophysical Union. 0148‐0227/11/2010JD015563
2002; Ramanathan and Carmichael, 2008]. The atmospheric abundance of BC comprises mainly fine particles, including ∼90% fraction of PM2.5 size (particles up to 2.5 mm in aerodynamic diameter), which can be harmful to human health in polluted regions [Lighty et al., 2000]. The major source sectors of carbonaceous aerosols include emissions from transportation, heating, power generation, industrial processes, biofuels, and biomass burning [e.g., Streets et al., 2003; Bond et al., 2004]. In the global budget of BC, the contributions of emissions from fossil fuel, biofuel, and open biomass burning have been estimated to be ∼38%, 20%, and 42%, respectively [Bond et al., 2004]. [3] Organic carbon (OC) which has both primary and secondary sources constitutes a major fraction of the mass of carbonaceous aerosols in the fine mode [e.g., Turpin et al., 2000, and references therein]. OC aerosols can impact climate directly by scattering solar radiation, and they can also act as cloud condensation nuclei (CCN) [Novakov and Penner, 1993; Saxena et al., 1995] and hence influence the climate indirectly by forming cloud droplets. Primary organic carbon (POC) is emitted directly in particulate form by combustion processes, whereas secondary OC is formed via gas‐to‐particle conversion of oxidized products of volatile organic compounds (VOCs) in the atmosphere [Pankow, 1994]. In the global budget of OC, the contributions of fossil fuel, biofuel,
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in Bangkok represent the activities of distinct emission sources than those in typical urban areas of Asia, for example, Beijing and Tokyo in East Asia [Han et al., 2009; Kondo et al., 2006]. The key features of variations observed in the concentrations of BC and OC aerosols are also compared with the measurements reported for other urban areas of Asia.
2. Measurements
Figure 1. Distribution of anthropogenic emissions of BC for the year 2000 at 0.5° × 0.5° resolution [Streets et al., 2003]. The sites of BC measurements discussed in the present study are also shown. and open biomass burning have been estimated to be ∼7%, 19%, and 74%, respectively [Bond et al., 2004]. [4] In the tropical regions of Asia, forest fires and biomass burning are widespread [Christopher et al., 1996; Folkins et al., 1997]. The emissions from anthropogenic, forest fires and open biomass burning sources in Southeast Asia (SEA) contribute substantially to the inventories of carbonaceous aerosols in Asia [Streets et al., 2003; Bond et al., 2004]. According to the estimates presented by Streets et al. [2003] for the year 2000, the total anthropogenic emissions of BC and OC aerosols in SEA were about 0.77 Tg and 3.68 Tg, respectively. In spite of the large emissions of various aerosols and trace gases from SEA, studies of the spatiotemporal variations of these species are very limited compared to other regions of the world where biomass burning is an important source of aerosols and trace gases [Folkins et al., 1997; Christopher et al., 1998]. [5] The growing emissions of various aerosols including carbonaceous species from the megacities of Asia can impact local air quality and climate on regional and global scales. In most of the megacities of the world, the emissions of carbonaceous aerosols are mainly due to the use of fossil fuels in automotive engines and industry. The annual emission map of BC from anthropogenic sources in parts of Asia for the year 2000 is shown in Figure 1. The major sources can be located mostly in the eastern part of China, South Korea and Japan where emissions from urban areas are highest. In the SEA region, the emissions of BC from anthropogenic sources are relatively less; however, the emissions from biomass burning and forest fire sources make major contributions. [6] Thus far, detailed studies characterizing the variations of carbonaceous aerosols near the major source regions in SEA are rare mainly due to the lack of continuous observations [See et al., 2006, and references therein]. The present study is based on continuous observations of PM2.5 carbonaceous aerosols in Bangkok, Thailand during the years 2007–2008. The characteristics of temporal variations of BC and OC aerosols have been discussed in view of the short‐term changes in local meteorology, seasonality of the long‐range transport and strength of local emissions. The observations of aerosols
[7] The mass concentrations of carbonaceous aerosols were measured using a semicontinuous EC‐OC analyzer (RT3006) manufactured by Sunset Laboratory, Inc. (Beaverton, Oregon, USA) [Bae et al., 2004; Kondo et al., 2006; Miyazaki et al., 2006], with 1 h time resolution. Air samples were drawn from an inlet fitted with PM2.5 cyclone (cutoff diameter of 2.5 mm) at a flow rate of 16.7 L min−1 to discard coarse particles. Samples were collected on a 1.13 cm2 quartz filter for 40 min, and then the mass concentrations of EC and OC were quantified by the thermal optical transmittance (TOT) method based on the National Institute for Occupational Safety and Health (NIOSH) protocol [Birch and Cary, 1996]. The zero level concentration of EC was measured by a particle filter placed upstream of the denuder. The average zero level was 0.07 mg m−3 during the measurement periods and the detection limit defined as ±3s of a blank sample was 0.03 mg m−3. Further details of this analyzer and protocols used for the analyses of EC and OC aerosols were presented by Han et al. [2009]. [8] The optical measurements of BC were performed by the Continuous Soot Monitoring System (COSMOS, Kanomax, Osaka, Japan), for which the sampling and optical detection parts are based on the combination of those of a Particle Soot Absorption Photometer (PSAP) and Aethalometer [Miyazaki et al., 2008]. The operating wavelength of 565 nm in the COSMOS is the same as used in the PSAP. Air samples were drawn in by an internally mounted pump, and aerosols are collected on a quartz fiber filter at a flow rate of 0.7 L min−1. The sample collecting spot area of COSMOS was 18.1 mm2, which is smaller compared to the PSAP (19.6 mm2). The filter material, quartz fiber filter (Pallflex E70‐2075W) used in COSMOS is the same as those used in the PSAP and Aethalometer. The COSMOS system automatically advances a roll of filter tape (40 mm wide) for 20 mm depending on a preset value of filter transmittance. The criterion of transmittance was usually set to 0.7 for the present study. This is one of the most significant advantages of the COSMOS compared to the PSAP, as it enables unattended operation. A stainless steel (SS) tube with an outer diameter of 3/8 inch and wall thickness of 0.049 inch was used for sample inlet. The inlet line was heated to 400°C in order to effectively volatilize the nonrefractory components of aerosol [Kondo et al., 2006]. The COSMOS measures the absorption coefficient (b0) which is determined by the following equation: b0 ðÞ ¼ ð A=V Þ ln½ItDt =It ;
ð1Þ
where A is the area of the sample spot, V is the volume of air sample during a period of Dt and It−Dt and It are the average transmittances. The b0 is corrected for the effects of multiple scattering as
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babs ðÞ ¼ ff il b0 ðÞ;
ð2Þ
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4 ppbv and 20 ppbv, respectively, at a mixing ratio of 400 ppbv using the data integrated for 1 min. The measurements of CO could not be performed continuously due to technical reasons, therefore we have a limited database for discussing the temporal variation of CO. [11] The measurements of meteorological parameters (ambient pressure, temperature, relative humidity (RH), wind speed, wind direction, precipitation, solar radiation, etc.) were performed using automated sensors (WXT‐510, Vaisala, Finland) at the observation site [Miyazaki et al., 2009]. The data were recorded at a time resolution of 10 min but statistics calculated for 1 h and longer periods are used in the present study.
3. Measurement Site and Emissions Sources
Figure 2. Map of Bangkok and network of roads (taken from Wikimapia http://wikimapia.org); the triangle shows the measurement site at AIT in Bangkok. where ffil is the correction factor for absorption by multiple scattering in the filter medium [Bond et al., 1999; Virkkula et al., 2005]. Based on the simultaneous measurements of the optical absorption using the heated inlet line and EC the estimated mass absorption cross section (Cabs) was stable between 9.6 m2 g−1 in the wet season and 10.6 m2 g−1 in the dry season in Bangkok [Kondo et al., 2009]. In the present study a Cabs value of 10 m2 g−1 was used to calculate the mass concentration of BC. [9 ] For ambient observations of BC in Tokyo, Kondo et al. [2011] compared the measurements obtained using laser incandescence (single particle soot photometer, (SP2)), refraction (refractory mass method (RMM)), thermal optical transmittance (TOT), and light absorption (COSMOS). The excellent agreement between RMM and TOT (MTOT = 0.96 Mref + 0.11 (mgC m−3), r2 = 0.88) and COSMOS versus SP2 (MCOSMOS = 0.99 MSP2 − 0.02 (mgC m−3), r2 = 0.97) was reported. Based on the observations at 6 different sites in Asia, including the present data at Bangkok, Kondo et al. [2009, 2011] have reported very stable and good agreement between a heated COSMOS and EC. The good agreement between the thermal and optical methods has also been reported in several of previous studies [e.g., Venkatachari et al., 2006; Sahu et al., 2009; Miyazaki et al., 2008]. Therefore, for the present study, we have used BC instead of EC throughout the manuscript. [10] A nondispersive infrared (NDIR) gas analyzer (Model 48C, Thermo Environmental Instruments, USA) was used for the measurement of carbon monoxide (CO) with an integration time of 1 min. The ambient air samples were dried using an electric cooler to reduce interference from water vapor. The background signal (zero level) was routinely measured every 2 h by supplying purified air into the sample line. The zero air was generated by passing ambient air through a column filled with hopcalite (mixture of manganese dioxide and copper oxide). The calibration of the analyzer was performed by supplying a standard of 5 ppmv of CO in air (manufactured by the Nissan‐Tanaka Corporation, Japan). The overall precision and accuracy were estimated to be
[12] Bangkok, the capital city of Thailand, is situated in the central part of Thailand, in the Chao Phraya River Basin of immediate proximity to the Gulf of Thailand. Economically, Bangkok is one of the most important cities in Southeast Asia (SEA). The Bangkok Metropolitan Region (BMR) covers an area of 7,761.5 km2 and had a registered population of 11,971,000 as of January 2008. The city often faces serious traffic congestion due to both public and private vehicles there are estimated to be more than 5.4 million vehicles running on the roads of Bangkok city [Department of Land Transport (DLT), 2008]. New vehicles registrations in the city have increased by 39% compared to that in the year 2002 (details given at http://apps.dlt.go.th/statistics_web/statistics.html). [13] The measurement site on the campus of the Asian Institute of Technology (AIT, 14.08°N, 100.62°E), which is in the Pathumthani and is located ∼40 km north from the Bangkok city center along the west side of the Phaholyothin road (see Figure 2). This road is important for the northeastern sector connected to Bangkok city. The traffic consists of both gasoline‐ and diesel‐fuelled vehicles. The traffic volume along the Phaholyothin highway does not show significant day‐to‐day variability during the weekday however, it is less on the weekend [Leong et al., 2002]. The other major sources of pollutants are small industrial estates located ∼6 km north of the site, intensive construction activities about 500 m to the north of the site and other day‐ to‐day activities in this academic center. Additional information about the measurement site and traffic in Bangkok can be found elsewhere [Kim Oanh et al., 2000; Leong et al., 2002]. [14] The emissions from vehicular traffic contribute up to 80% of NOx (=NO + NO2), 75% of CO, 54% of particulate matter, and most of the volatile organic compounds (VOCs) in the BMR [Bangkok Metropolitan Administration, 2001]. Among the countries in SEA, the number of registered vehicles was highest in Thailand, while the amount of fuel and biomass burned was second after Indonesia [Streets et al., 2003]. The open biomass burning, mainly agricultural waste field burning, surrounding the site is significant source of air pollution in the dry season [Chuersuwan et al., 2008]. Pathumthani, where the site is located, is one of the largest rice growing provinces in Thailand [Office of Agriculture Economics, 2007] with an estimate emission from the agroresidue field burning, mainly rice straw, in tones per year in 2007 of 1470 PM2.5, 90 BC and 530 OC [Kanabkaew and Kim Oanh, 2011].
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Figure 3. Time series plots of meteorological parameters and fire count data during April 2007 to March 2008. [15] During the dry season, most of harvested paddy (>90%) is burned and the burning were particularly extensive during December‐February period [Tipayarom and Kim Oanh, 2007]. The open burning of rice straw in the Pathumthani region has important implications for the air quality of Bangkok as these sources are located upwind of the BMR. The emissions from vehicular traffic and industrial activity can be assumed to be fairly constant throughout the year, as they do not depend on season. On the other hand, the activities of biomass burning and forest fire exhibit strong seasonality.
4. General Circulation and Variations in Meteorological Parameters [16] There are three main seasons, namely wet (May‐ October), dry (November‐February), and hot (March‐April), in Thailand [Pochanart et al., 2003]. The wet season prevails due to southwesterly (SW) wind flow, and the dry season is due to northeasterly (NE) wind flow. The SW wind flow is associated with the northward movement of the intertropical convergence zone (ITCZ) across Thailand which brings cleaner marine air from the southern Indian Ocean. During the dry season, the long‐range transport of continental air from different regions of East Asia takes place due to the southward movement of the ITCZ. The
observation site is influenced by mixed air masses (marine and continental) during the hot season. [17] The time series variations of surface level wind speed, wind direction, RH, pressure, and temperature based on daily data and total rainfall data observed between April 2007 and March 2008 are shown in Figure 3. During the months of May to October, episodes of rainfall and high RH (>60%) were very frequent. Therefore this period is also known as the wet season in Thailand. The wind speed varied between 0.5 m s−1 and 2.0 m s−1, including several episodes of stronger winds, particularly in the month of August. From the months of November to February rainfall was rare therefore, this period is also known as the dry season. The time series variation of surface temperature shows an increasing trend from March to April therefore, this period is known as the hot season. The measurements during the dry and hot seasons were influenced by a fairly stable wind flow of ∼1 m s−1 from the east‐southeast (E‐SE) direction. The surface level pressure varied between the ranges of 1000– 1010 hPa and 1010–1015 hPa during the wet and dry seasons, respectively. The day‐to‐day variations in temperature and pressure were anticorrelated, which can be clearly seen during the episodes of strong winds from NE direction. [18] Almost all the meteorological parameters show systematic diurnal dependencies, with stronger amplitudes of
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Figure 4. The daily means and monthly variations of concentrations of (a) BC, (b) OC, and (c) OC/BC ratio. In the box‐whisker plots the horizontal solid lines represent 10th, 25th, 50th, 75th, and 90th percentiles while dashed lines give the means obtained using hourly data. variation in the dry season and weaker during the wet season. Generally, the levels of RH were elevated during the nighttime, while lower values were observed during the daytime. The diurnal variations of RH were in the ranges of 43–83%, 53–83%, and 40–76% during the hot, wet, and dry seasons, respectively. Variations in the wind speed show cycles opposite to that of RH, i.e., lower values during the nighttime and higher values during the daytime. The diurnal ranges of wind speed were 0.5–1.1 m s−1, 0.5–1.3 m s−1 and 0.5–1.1 m s−1 during the hot, wet, and dry seasons, respectively. The surface level pressure shows a peak between 0900 and 1200 LT and a minimum between 1700 and 1900 LT. The diurnal variation of surface temperature was similar to that of wind speed, exhibiting a minimum during 0600–0900 LT and a maximum during 1400–1700 LT. Since the observation site is not very far from the Gulf of Thailand, the systematic diurnal changes observed in the meteorological parameters could partly be attributed to the presence of land‐sea breeze circulation. The relations of the concentrations of BC and OC aerosols with the meteorological parameters are also discussed in section 5.4. Based on the analysis of meteorological data reordered at different locations in the BMR during January 2002 to December 2004, the daily average depths of the planetary boundary layer (PBL) were 860 m, 990 m and 940 m for the wet, dry and hot seasons, respectively [Pongkiatkul and Kim Oanh, 2007].
5. Temporal Variations of BC and OC Aerosols 5.1. Seasonal Variations [19] As shown in Figure 4, time series plots of the concentrations of BC and OC and the OC/BC ratio show signifi-
cant day‐to‐day variations. However, not shown in Figure 4, the hourly data of BC, OC, and OC/BC ratio were in the ranges of 0.06–28 mg m−3, 0.06–50 mg m−3, and 0.2–27 mg mg−1, respectively. Such large variations in the hourly data were not just random but constitute very systematic diurnal variations in Bangkok. The diurnal features observed in the concentrations of aerosols and impacts of the meteorological parameters are discussed in section 5.2. The daily averaged concentrations of BC and OC and the OC/BC ratio fall in the ranges of 0.9– 13.7 mg m−3, 2.01–32.5 mg m−3, and 0.85–5.4 mg mg−1, respectively. Overall, these short‐term variations in the concentrations of aerosols were significant throughout the year, and these were particularly strong during the dry and hot seasons. [20] Box‐whisker plots representing the monthly statistics of BC, OC, and OC/BC ratio are also shown in Figure 4. The concentrations of both BC and OC show clear seasonality, as their levels were low during the wet season and high during the dry season. The average concentrations of BC and OC were 3.0 ± 1.2 mg m−3 and 5.3 ± 2.0 mg m−3 in the wet season, while these were 4.3 ± 1.3 mg m−3 and 13.1 ± 5.8 mg m−3, respectively, during the dry season. The ratio of OC/BC shows a slightly different seasonality, with a lowest value of 1.9 ± 1.3 mg mg−1 in the wet season and highest of 3.3 ± 1.5 mg mg−1 during the hot season. In the hot season, though the activities of biomass burning were highest near the observation site, yet the levels of aerosols were moderate (see Table 1). The concentrations of both BC and OC aerosols show minima in the month of August and maxima in January. The monthly average OC/BC ratio shows a minimum of 1.7 mg mg−1 in the month of August and a maximum of
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Table 1. Monthly Statistics of BC, OC, and OC/BC Ratio Measured at AIT, Bangkok in Thailanda Month
Season
Apr 2007 May 2007 Jun 2007 Jul 2007 Aug 2007 Sep 2007 Oct 2007 Nov 2007 Dec 2007 Jan 2008 Feb 2008 Mar 2008
Hot Wet Wet Wet Wet Wet Wet Dry Dry Dry Dry Hot
BC (mg m−3) 3.7 2.8 3.2 2.9 2.4 2.8 3.2 3.9 5.1 5.6 4.3 3.0
(3.8 (3.1 (3.3 (3.0 (2.7 (3.0 (3.2 (4.5 (5.2 (5.7 (4.7 (3.9
± ± ± ± ± ± ± ± ± ± ± ±
OC (mg m−3)
1.2) 8.0 (9.1 ± 2.8) 1.1) 4.1 (4.5 ± 1.5) 1.2) 4.1 (4.2 ± 1.1) 1.6) 4.1 (4.5 ± 1.5) 1.2) 3.7 (3.9 ± 0.8) 1.1) 4.3 (4.7 ± 1.5) 1.0) 4.8 (5.2 ± 1.9) 1.8) 7.2 (8.4 ± 2.8) 1.6) 10.8 (11 ± 3.0) 2.3) 17.4 (17.2 ± 6.4) 1.3) 14.6 (14.7 ± 5.1) 2.4) 9.4 (13.1 ± 7.5)
OC/BC (mg/mg) 2.9 1.6 1.7 1.9 2.0 2.0 1.9 2.0 2.4 3.6 3.6 3.8
(3.0 (2.0 (1.7 (2.4 (1.9 (2.0 (2.0 (2.1 (2.6 (3.5 (3.6 (3.9
± ± ± ± ± ± ± ± ± ± ± ±
0.8) 1.4) 0.4) 1.5) 0.4) 0.6) 0.6) 0.3) 0.6) 0.7) 0.9) 0.6)
a Statistics are median values with mean plus or minus standard deviation given in parentheses.
3.9 mg mg−1 in March. The highest value of OC/BC observed in March coincides with the largest numbers of fire count (hot spot) over SEA and regions surrounding the observation site (see Figure 3). Agreeing to this observation, higher SOC formation has been reported as also highest ozone (O3) is observed in the BMR during March‐April period [Nghiem and Kim Oanh, 2008]. In section 5.3, the back trajectory and fire count data have been analyzed to explain the major
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causes of seasonality observed in the concentrations of aerosols. 5.2. Diurnal Variations [21] The diurnal variations of BC and OC for the different seasons are shown in Figure 5. The concentration of BC shows significant diurnal variability throughout the year. There are two prominent peaks of BC concentration a primary peak at around 0700 LT and a secondary peak at 2000 LT. The concentration of BC was observed to be lowest in the afternoon hours. The variations of BC were within the ranges of 1.4–5.3 mg m−3, 2.3–8.3 mg m−3, and 2.3–6.5 mg m−3 during the wet, dry, and hot seasons, respectively. The concentration of OC exhibits somewhat similar diurnal variation to that of BC in the wet and dry seasons. On the other hand, the concentration of OC shows less pronounced variation in the hot season. The diurnal variations of OC were within the ranges of 3.5–6.3 mg m−3, 8.5–17.5 mg m−3, and 9.5–12 mg m−3 during the wet, dry, and hot seasons, respectively. [22] The local time dependencies of emission and meteorological parameters can be important factors controlling the diurnal distribution of primary pollutants in urban areas. However, the contributions of different factors cannot be separated in a strict sense. The traffic volume on the Phaholyothin road peaked during the morning (0700–0900 LT) and evening (1600–1800 LT) hours, while it was moderate
Figure 5. Diurnal variations of concentrations of BC and OC, OC/BC ratio, and meteorological parameters during the wet (May‐September), dry (November‐February), and hot (March‐April) seasons. 6 of 14
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Figure 6. Back trajectories and hot spot map for different seasons of observations during April 2007 to March 2008. in the afternoon and lowest during the midnight and early morning [Kim Oanh et al., 2000; Leong et al., 2002]. The morning rush hour coincided with the period of stagnant airflow (
