Atmospheric aerosol layers over Bangkok Metropolitan Region from CALIPSO observations

Atmospheric Research 127 (2013) 1–7

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Atmospheric aerosol layers over Bangkok Metropolitan Region from CALIPSO observations Arika Bridhikitti ⁎ Faculty of Environment and Resource Studies, Mahasarakham University, Muang District, Maha Sarakham, 44000, Thailand

a r t i c l e

i n f o

Article history: Received 14 June 2012 Received in revised form 31 January 2013 Accepted 8 February 2013 Keywords: CALIPSO Bangkok air pollution Long-range transported air pollution Aerosol optical depth Mixing height

a b s t r a c t Previous studies suggested that aerosol optical depth (AOD) from the Earth Observing System satellite retrievals could be used for inference of ground-level air quality in various locations. This application may be appropriate if pollution in elevated atmospheric layers is insignificant. This study investigated the significance of elevated air pollution layers over the Bangkok Metropolitan Region (BMR) from all available aerosol layer scenes taken from Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) for years 2007 to 2011. The results show that biomass burning smoke layers alone were the most frequently observed. The smoke layers accounted for high AOD variations and increased AOD levels. In the dry seasons, the smoke layers alone with high AOD levels were likely brought to the BMR via northeasterly to easterly prevailing winds and found at altitudes above the typical BMR mixing heights of approximately 0.7 to 1.5 km. The smoke should be attributed to biomass burning emissions outside the BMR. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Traffic-related air pollution potentially causes adverse health effects to people living in Bangkok, Thailand (Jinsart et al., 2002; Ruchirawat et al., 2005; Vichit-Vadakan et al., 2008). These effects have been related to high ambient particulate matter, especially at roadsides where the level of the 24-h average PM10 often exceeds the Thailand's National Ambient Air Quality Standard of 120 μg m−3 (Jinsart et al., 2002; Vichit-Vadakan et al., 2008). Inference of air quality from satellite retrievals might be useful for Bangkok Metropolitan Region (BMR) air pollution management. This application has been recommended for many locations. Wang and Christopher (2003) examined the relationship between an hourly fine particulate matter concentration and the aerosol optical depth (AOD) retrieved from Moderate Resolution Imaging Spectroradiometer (MODIS, Levy et al., 2007) at many populous locations in Alabama, USA. They suggest that MODIS AOD can be used to estimate air quality categories, ranging from good to hazardous conditions, ⁎ Tel.: +66 4375433x6622; fax: +66 43742135. E-mail address: [email protected]. 0169-8095/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.atmosres.2013.02.008

with an accuracy of more than 90% in cloud free conditions (Wang and Christopher, 2003). This MODIS instrument is onboard Earth Observing System (EOS) Terra and Aqua satellites. The satellites have one to two day global coverage which could provide near-daily variations of atmospheric column aerosol abundance. A similar application of MODIS AOD was used for Texas (USA) air quality monitoring as well (Hutchison, 2003). In larger scale, a team of US researchers developed an application called Infusing Satellite Data into Environmental Applications (IDEA, http://www.star.nesdis. noaa.gov/smcd/spb/aq/) to generate daily maps of AOD from MODIS and Aerosol and Smoke Product from Geostationary Operational Environmental Satellite (Hoff and Christopher, 2009). Ratios between the AOD and the measured particulate matter with size less than 2.5 μm for over 500 air quality monitoring stations have been used for the estimation of ground-based particulate matter concentration across the USA (Hoff and Christopher, 2009). The satellite-derived fine aerosol concentration map could be useful for epidemiologic and health impact studies (van Donkelaar et al., 2010). Correlations between the MODIS AOD and ground-based aerosol concentration are location-dependent (Engel-Cox et

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al., 2004; Hoff and Christopher, 2009). A high correlation was observed at the cities in Alabama because most of the aerosols there are in well-mixed lower boundary layers during the satellite overpass time (Wang and Christopher, 2003). In the case of Beijing, China, long-range transport of Gobi desert dust is significant and it causes poor correlation between MODIS AOD and ground-level aerosol concentration (Li et al., 2005). Negative correlation between MODIS AOD and ground-measured fine particulate matter concentration observed in the western United States is also possibly due to the presence of fire plumes or Asian dust in elevated layers (Engel-Cox et al., 2004). Elevated aerosol layers were observed in many locations, such as anthropogenic European Haze and Saharan dust over Germany (Mattis et al., 2004) and “Asian brown cloud” over Indian subcontinent (Quinn and Bates, 2003). Inference of the BMR air quality from the EOS satellite retrievals could be more reliable if long-range transported air pollution in elevated layers was insignificant. Currently, there are very few studies on contributions of the long-range transported air pollutants to air quality in the BMR. Bridhikitti and Overcamp (2011) classified SE Asian aerosols based on their optical properties measured using Cimel sun/sky-radiometers. The data was acquired from the Aerosol Robotic Network (AERONET, Holben et al., 1998) at four SE Asian stations. The result indicates potential long-range transport of air masses with high-level aerosol from inland eastern China to the stations at northern Vietnam and at northeastern Thailand during early dry seasons from September to December. However, this long-range transport aerosol was not dominant at the central Thailand station, located about 50 km from Bangkok to the West (Bridhikitti and Overcamp, 2011). Nevertheless, there are some indications showing high possibility that the long-range transported air masses has been polluting the BMR lower boundary layer. First, a relatively high level of carbon monoxide was observed at Srinakarin, Kanjanaburi (174 km to the NW from Bangkok) when the NE monsoon air masses predominate in the early dry seasons (Pochanart et al., 2003). Another indication is based on cluster analysis of air trajectories associated with the BMR metrological characteristics in the work of Pongkiatkul and Oanh (2007). They show that the relatively high groundlevel particulate matter at the BMR was likely found with air masses passing over populated areas, including in southeastern China and northern Thailand (Pongkiatkul and Oanh, 2007). The long-range transport of polluted air masses to the BMR in the work of Pongkiatkul and Oanh (2007), however, was related to air quality near the ground. This study further investigated contributions of long-range transported aerosols in elevated layers on total aerosol budget in the BMR atmospheric column. Aerosol products from Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) were used for the investigation. They were also used to determine afternoon mixing heights of the BMR boundary layer. The mixing height and vertical aerosol profiles are useful for simulations of air pollution dispersion in the BMR lower atmosphere and could be further used to improve relationships between satellite-based AOD and ground-level aerosol concentration (Gupta et al., 2006; Hoff and Christopher, 2009; Emili et al., 2010; Wang et al., 2010).

2. Methodology 2.1. Site description The BMR includes Bangkok, the capital of Thailand, and its five adjacent provinces: Nakhon Pathom, Pathum Thani, Nonthaburi, Samut Prakan and Samut Sakhon (see Fig. 1). These have industrial interconnection and high population influx. The BMR has a tropical-monsoon climate having three main seasons: wet from June to September (JJAS) with strong southwesterly prevailing winds, local dry summer from February to May (FMAM) with weaker southerly to southwesterly prevailing winds and local dry winter from October to January (ONDJ) with northeasterly prevailing winds. 2.2. Data description 2.2.1. Aerosol layers from Cloud–Aerosol Lidar in Infrared Pathfinder Satellite Observations (CALIPSO) satellite CALIPSO is a part of the NASA afternoon constellation satellites, having a 705-km sun-synchronous near polar orbit. It provides information at the same ground position every 16 days (Winker et al., 2007), 1 to 2 scenes over BMR per month. CALIPSO carries a polarization-sensitive active lidar, called the Cloud–Aerosol Lidar with Orthogonal Polarization, which makes backscatter measurement at 532 and 1064 nm. This instrument is used to determine profiles of atmospheric aerosols and clouds (Omar et al., 2009). All available CALIPSO scenes over the BMR (13.51–14.1°N and 100.15–100.79°E, shown in Fig. 1) from years 2007 to 2011, total 101 scenes, were used for the analysis and these were taken during 1:00 and 2:30 pm local time. In this study, level 2 version 3 aerosol products with 5-km ground horizontal resolution were used. These are aerosol layer top and base altitudes, attenuated backscattering at 532 nm (γ′), volume depolarization ratio (δv) and total aerosol optical depth (AOD) at 532 nm. They were acquired through web-based CALIPSO search and subset tool developed by the Atmospheric Science Data Center (ASDC) at NASA Langley Research Center. Different aerosol subtypes were distinguished using the algorithm described in the work of Omar et al. (2009), considering γ′, δv and layer base altitude. There are six CALIPSO aerosol subtypes: biomass burning smoke (δv ≤ 0.075, γ′ > 0.0005, base altitude > 80 km), polluted continental aerosol (δv ≤ 0.075, γ′ > 0.0005, base altitude ≤ 80 km), polluted dust (0.075 b δv ≤ 0.2), clean continent aerosol (δv ≤ 0.075, γ′ ≤ 0.0005), dust (δv > 0.2), and clean marine aerosol (Fig. 2 in Omar et al., 2009). The aerosol classification scheme requires the presence of clean marine aerosol over the ocean (Omar et al., 2009), which is outside the studied boundary. Thus, clean marine aerosol is not determined by this analysis. Most of the aerosol subtypes were not significantly sensitive to 25% changes in the threshold values of γ′, δv and the base altitude, except for the dust which were misclassified to the polluted dust in some cases (Omar et al., 2009). The polluted continent aerosol defined in the CALIPSO aerosol algorithm is found near the Earth's surface. Their potential sources include local industrial, urban and biomass burning emissions (Omar et al., 2009). To be consistent with this algorithm, the polluted continent aerosol in this study had been hypothesized having their origins inside the BMR

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Fig. 1. Map of the Bangkok Metropolitan Region, Thailand and a spatial boundary for CALIPSO data selection in this study.

and their base layers were set to less than 80 m above the ground, about 20-stories building height. The aerosols with similar light scattering and absorption properties to this polluted continent aerosol, but having higher base layers, were assigned to biomass burning smoke (Omar et al., 2009). The polluted dust is altitude independent and expected a mixture of polluted continent aerosol and dusts, such as from soil, road and desert (Omar et al., 2009). It is noted that the CALIPSO aerosol products used in this study could be associated with some uncertainties. The CALIPSO aerosol top and base altitudes were validated by Kim et al. (2008) using ground-based lidar at a Seoul station for three days and nights. They found that the CALIPSO aerosol layer altitudes were reliable under cloud-free and semi-transparent cirrus cloud conditions but limited under thick clouds (Kim et al., 2008). Comparison between AODs from CALIPSO (at 532 nm) and Moderate Resolution Imaging Spectrophotometer (MODIS) onboard Aqua satellite (at 550 nm) in the work of Kittaka et al. (2011) showed that their regional–seasonal biases were within the range of the expected MODIS uncertainties over land and ocean (Kittaka et al., 2011). Cloud–aerosol discrimination (CAD) score indicates a confidential level for the cloud–aerosol discrimination algorithm. The higher confidence that the classification is correct is with the |CAD| score closer to 100 (Yang et

al., 2012). In this study, 41.4%, 39.2% and 9.1% of total BMR aerosol layer observations were with |CAD| score of 100, 99 and 90–98, respectively. Zero CAD score was found in 6.9% of the BMR observations and they were mainly having aerosol top layers of greater than 7 km. These observations, therefore, were excluded from the analysis. 2.2.2. Air back trajectory analysis For each CALIPSO aerosol layer scene over the BMR, sevenday air back trajectory analysis was conducted using Hybrid Single Particle Lagrangian Integrated Trajectory Model (HYSPLIT) to evaluate potential aerosol sources. The model was run interactively through NOAA's Air Resources Laboratory (ARL) website, http://ready.arl.noaa.gov/HYSPLIT.php. Meteorological data chosen for the analysis were from the archive reanalysis project under joint venture between the National Centers for Environmental Prediction (NCEP) and the National Center for Atmospheric Research (NCAR). 3. Results and discussion 3.1. Significances of elevated-layer aerosols in the BMR atmosphere Based on the available CALIPSO aerosol layer scenes taken over the BMR from years 2007 to 2011, 60% (975 aerosol

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were likely observed with AOD b 0.25, the higher AOD values suggest that high atmospheric aerosol episode over the BMR is likely from the smoke in elevated layers.

layers) of aerosols were biomass burning smoke (observed in elevated layers), 19% were polluted dust and 16% were polluted continent aerosol (observed near the ground). From Table 1, polluted continent aerosol layers alone were likely observed with low AOD (b 0.25). This accounted for 7.4%, 17.4% and 9.6% of total seasonal observations in the local dry summer (FMAM), the wet (JJAS) and the local dry winter (ONDJ) seasons, respectively. Similarly, polluted dust layers alone were likely observed with AOD b 0.25, and they were found at 7.7%, 11.1% and 8.8%, respectively. Observing the low AOD, however, was less frequent in the local dry winter seasons, when many observations of the polluted continent aerosol (7.7%) and polluted dust layers were found with AOD > 0.25 (7.0%). Biomass burning smoke layers alone were the most frequently observed above the BMR atmosphere. They accounted for 56.2%, 46.7% and 38.4% of total seasonal aerosol layer observations in FMAM, JJAS and ONDJ, respectively. The biomass burning smoke layers were observed in a wide range of AOD levels; whereas the polluted continent aerosol and polluted dust layers were frequently observed with low AOD. Furthermore, the observations with AOD > 1 in local dry summer season, FMAM, (see Table 1) were more frequently found with the smoke layers alone (6.9%) than with other aerosol layer patterns (0.3–1.1%). These indicate that biomass burning smoke in the elevated layers above the BMR is contributing to high AOD values and large AOD variations here. For the observations with the smoke layers on the top of the polluted continent aerosol layers, their AOD values were more frequently observed in the range of 0.25 to 0.5 than in AOD b 0.25, especially in the dry seasons (8.8% for AOD of 0.25–0.5 and 2.7% for AOD b 0.25). The presences of smoke layers on the top of the polluted dust layers with the higher AOD were also evident in the local dry winter seasons. Since the polluted dust or the polluted continent aerosol layers

3.2. Altitudes of the aerosol layers and afternoon mixing heights Fig. 2 shows frequency distributions of top layer altitudes for biomass burning smoke, polluted continent aerosol and polluted dust observed over the BMR. The top layer altitudes were divided into 35 height intervals with 200-m step starting from 0 to 7 km. The results show that the polluted continent aerosol had top layers at approximately (25th: 50th:75th percentiles) 0.7:1.3:1.5, >1.0:1.1:1.3 and 0.7:1.1:1.5 km for FMAM, JJAS and ONDJ, respectively. This polluted continental aerosol found over the BMR should mainly be a mixture of fine aerosols from automobile emissions and biomass burnings (Chuersuwan et al., 2008). Since the polluted continent aerosol likely has ground origins, its top layers could be indicative of afternoon mixing heights of the BMR boundary layer. These estimated mixing heights from CALIPSO aerosol layer observations are consistent with the previous estimated mixing heights over Bangkok from dry adiabatic method using the data from radiosonde measurements in the work of Saensorn (2009). His estimated mixing heights were 1.0, 1.3 and 1.4 km at 1:00 pm local time in 19, 20 and 21 February 2008, respectively (Saensorn, 2009). In addition, the CALIPSO-derived mixing heights are in agreement with the maximum daytime mixing height of approximately 2 km, estimated for BMR using mesoscale air quality model simulations by Oanh and Zhang (2004). Biomass burning smoke top layers were at approximately (25th: 50th:75th percentiles) 1.7:2.3:2.7, 1.3:1.5:1.7 and 1.1:1.5:1.9 km for FMAM, JJAS and ONDJ, respectively. Polluted dust top layers were randomly distributed over wide ranges, from 0.2 to 4 km, with their 25th: 50th: 75th percentile values

Table 1 Percent frequencies of aerosol optical depth (AOD at 532 nm) levels observed with different aerosol layer patterns over the BMR from CALISO observations for years 2007 to 2011.

Aerosol vertical profile characteristics

AOD

AOD

AOD

b0.25 0.25–0.5 0.5–0.75 0.75–1 >1

b0.25 0.25–0.5 0.5–0.75 0.75–1 >1

b0.25 0.25–0.5 0.5–0.75 0.75–1 >1

Local dry summer season, FMAM (363)a

Wet season, JJAS (144)

Local dry winter season, ONDJ (342)

Biomass burning 22.9% 13.2% aerosol layers alone Polluted continent 7.4% aerosol layers alone Polluted dust layers 7.7% alone 2.7% 8.8% Biomass burning aerosol layers on the top of polluted continent aerosol layers 0.8% 1.4% Polluted dust layers on the top of polluted continent aerosol layers 3.8% 3.8% Biomass burning aerosol layers on the top of polluted dust layers a

9.9%

3.3%

6.9% 37.6% 4.9%

1.4%

2.1%

0.7% 18.4% 9.6%

7.3%

2.3%

0.8%

9.6% 4.4%

0.6%

1.5%

1.2%

8.8% 2.6%

1.7%

1.2%

1.5%

4.2% 5.5%

1.7% 4.7%

2.9%

0.6%

6.9%

0.6% 1.5%

1.1%

5.5%

0.9% 3.5%

0.3% 17.4% 0.3% 11.1% 1.4% 0.8%

0.3%

2.7%

1.1%

Number in parenthesis is number of observation. Blank cell means no observation.

0.7%

0.7%

0.3%

0.9%

2.0%

1.7%

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Fig. 2. Frequency distributions of aerosol top layer level over the BMR from CALIPSO observations. Number in parenthesis is number of observation.

of 0.7:1.7:2.1, 1.3:1.5:2.1 and 1.1:1.5:1.9 km, respectively. Comparing to the mixing heights indicated from the polluted continent aerosol top layers, the smoke top layers were more frequently observed at higher altitudes, especially in the local dry summer (FMAM). 3.3. Potential sources of the elevated-layer aerosols Results from air trajectory analyses show that 46% and 50% of the summertime (FMAM) observations for biomass burning smoke and polluted dust, respectively, were with prevailing easterly winds having their early trajectories over the South China Sea and the eastern part of SE Asian mainland, including major agricultural areas in the Mekong River Delta, in Vietnam and Cambodia. In the dry winter season (ONDJ), both biomass burning smoke and polluted dust were frequently (60% and 55%, respectively) brought to the BMR via prevailing northeasterly or easterly winds, having their early trajectories over the Southern China and the Northeastern-Eastern part of SE Asia, primarily covered by rice paddies (see one case of the smoke plumes on December 12th, 2007 in the daytime at 1.3 km above BMR ground level in Fig. 3a). Agricultural biomass open burnings are widely practiced over the region in the dry seasons. In one case during November 25th to December 1st, 2007 (see Fig. 3a-3), high number of active fires was found over the Southeastern China. Bridhikitti and Overcamp (2011) also found urban/industrial-like aerosol from mid-September to December at many SE Asian sites and they hypothesize that the aerosol could be from the urbanized areas in the Eastern China. Based on the air back trajectories, active fire observations and the related literature, biomass burning smoke and polluted dust observed in the elevated layer over the BMR in the dry seasons should be attributed to biomass burnings and emissions from urbanized areas in the Eastern China. In the wet seasons (JJAS), the smoke and polluted dust did not likely contribute to high aerosol level in the BMR (see

Table 1) and 60% and 64%, respectively, of the aerosol layers had their back trajectories over the Southern Indian subcontinent and the Northern Indian Ocean before reaching the BMR through prevailing southwesterly winds. Fig. 3b shows one case of the elevated smoke layers over BMR on July 5th, 2011 in the daytime and their corresponding back trajectories over the Northern Indian Ocean and active fires over the Southern India and Sri Lanka. The smoke, therefore, should be partly attributed to agricultural burnings and fossil fuel combustions in the Southern Indian subcontinent (Ramanathan et al., 2001). Polluted dust characterizes a high volume depolarization ratio (>0.2), indicating nonspherical shape. In the CALIPSO aerosol retrieval algorithm, the polluted dust has been hypothesized being mixtures of biomass burning smoke and dust, having nonspherical properties (Omar et al., 2009). Deserts, however, should not be a potential source because there were only five and nine percent of the polluted dust observations having their previous trajectories over the Gobi (in Mongolia) and the Thar (between India and Pakistan boundary) deserts, respectively. The dust found closer to the ground could be local soil and construction dusts (Wimolwattanapun et al., 2011). These dusts, primarily in coarse fraction (Wimolwattanapun et al., 2011), should not be responsible for polluted dust layers observed in the elevated layers above the mixing heights. Such the polluted dust appeared above the BMR ground on July 5th, 2011 (shown in Fig. 3b) at 1.8 km and higher. This study hypothesizes that natural clean sea salt, exhibiting a nonspherical cubic shape (Chamaillard et al., 2003), could be seen as dust here as well. This dust-like sea salt is commonly seen in the BMR atmosphere, and it has been brought to the BMR from the Gulf of Thailand by either synoptic sea–land or larger scale air mass circulations for most of the year (Chueinta et al., 2000). 4. Conclusion This study investigated contributions of long-range transported aerosols in elevated layers on total aerosol budget in the

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Fig. 3. CALIPSO aerosol subtypes (1 = clean marine, 2 = dust, 3 = polluted continental, 4 = clean continental, 5 = polluted dust, 6 = smoke, N/A = not applicable) for the daytime on December 1st, 2007 (a-1) and the daytime on July 5th, 2011 (b-1) and their corresponding HYSPLIT back trajectories at 13.99°N 100.42°E (a-2) and at 14.06°N 100.43°E (b-2), respectively. The composite MODIS active fires over the SE Asia and the Southeastern China (a-3) and over the Southern India and Sri Lanka (b-3) were in the corresponding periods. The CALIPSO aerosol subtype images were acquired from CALIPSO product website (http:// www-calipso.larc.nasa.gov/products/). The HYSPLIT results were run and acquired through NOAA's Air Resources Laboratory (ARL) website, http://ready.arl.noaa. gov/HYSPLIT.php. The MODIS active fires for the areas of interest were obtained through the FIRMS MODIS Archive Download service (http://firms.modaps. eosdis.nasa.gov/download/). Note: 94% and 92% of |CAD| score at the pointed locations in a-1 and b-1, respectively, are 100 or 99.

Bangkok Metropolitan Region (BMR) atmospheric column. Aerosol layer scenes and their corresponding aerosol optical depth (AOD) taken from Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) for years 2007 to 2011 were used for the investigation. Different aerosol types were categorized based on the CALIPSO aerosol subtyping algorithm in the work of Omar et al. (2009). The results show that biomass burning smoke layers alone (observed in elevated layers) were the most frequent among aerosol layer patterns. Polluted continent aerosol and polluted dust layers were likely observed with low aerosol optical depth (AOD b 0.25) from local dry summer to wet period covering February and August. The smoke layers accounted for high AOD variations and increase AOD levels from those with the presences of polluted continent aerosol layers alone. Afternoon mixing heights of the BMR boundary layer were estimated from top layers of the polluted continent aerosol, having ground origins. The mixing heights were approximately (25th: 50th:75th percentiles) 0.7:1.3:1.5, >1.0:1.1:1.3, and

0.7:1.1:1.5 km for FMAM, JJAS, and ONDJ, respectively. Biomass burning smoke top layers were frequently observed above the mixing heights, especially in the local dry summer season (median values = 2.3 km). Seven-day air back trajectory analyses were conducted to investigate potential sources of the polluted air masses in the elevated layers over the BMR. The majority of the smoke and polluted dust observations in dry summer and winter seasons were with prevailing northeasterly or easterly winds, which have their trajectories over the Southeastern China and major SE Asian agricultural fields. These aerosols may be attributed to biomass open burnings. Dust seen as the polluted dust observed closer to the ground could be local soil and/or construction dusts and those observed in the elevated layers may be sea salt. Acknowledgment The author acknowledges anonymous reviewers for their valuable suggestions.

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