Visibility, aerosol optical depth, and low-visibility events in Bangkok during the dry season and associated local weather and synoptic patterns.

Visibility and aerosol optical depth (AOD) characterization, and their relationship with PM10 and local and synoptic meteorology, were studied for January-March in 2014 and 2015 over Bangkok. Visibility degradation intensifies in the dry season as compared to the wet season due to increase in PM10 and unfavorable meteorological conditions. The average visibility is lower in January and February as compared to the other months. Relatively higher AOD in March despite lower PM10 is attributed to the synergetic effect of moderate relative humidity, secondary aerosols, elevated aerosol layer due to summertime convection, and biomass burning. Larger variability in visibility and PM10 in winter months is due to more synoptic weather fluctuations while AOD shows similar variability for all months attributed partly to fires. Higher PM10 and moderate-to-high relative humidity cause lower visibility in the morning while it improves in afternoon as PM10 and relative humidity decrease. AOD is higher in the afternoon as compared to that in the morning and evening as it is less sensitive to diurnal change in aerosols and meteorology at the surface level. Visibility and AOD relationships with PM10 are dependent on relative humidity. Weaker winds lead to lower visibility, higher PM10, and higher AOD irrespective of wind direction. Stronger winds improve visibility and decrease PM10 for all directions while AOD is higher for all directions except eastern and northeastern. The back-trajectory results show that the transport of pollutant and moist air is coupled with the synoptic weather and influence visibility and AOD. Two low-visibility events were investigated. The first event is potentially caused by the combined effect of local emissions and their accumulation due to stagnant weather conditions, secondary aerosols, and forest fires in the nearby regions. The second event can be attributed to the local emission and fires in the nearby area with hygroscopic growth of aerosols due to moist air from the Gulf of Thailand. Based on these findings, some policy implications have also been given.

Impact of transport of fine and ultrafine particles from open biomass burning on air quality during 2019 Bangkok haze episode.

Transboundary and domestic aerosol transport during 2018-2019 affecting Bangkok air quality has been investigated. Physicochemical characteristics of size-segregated ambient particles down to nano-particles collected during 2017 non-haze and 2018-2019 haze periods were analyzed. The average PM2.5 concentrations at KU and KMUTNB sites in Bangkok, Thailand during the haze periods were about 4 times higher than in non-haze periods. The highest average organic carbon and elemental carbon concentrations were 4.6 ± 2.1 µg/m3 and 1.0 ± 0.4 µg/m3, respectively, in PM0.5-1.0 range at KU site. The values of OC/EC and char-EC/soot-EC ratios in accumulation mode particles suggested the significant influence of biomass burning, while the nuclei and coarse mode particles were from mixed sources. PAH concentrations during 2018-2019 haze period at KU and KMUTNB were 3.4 ± 0.9 ng/m3 and 1.8 ± 0.2 ng/m3, respectively. The PAH diagnostic ratio of PM2.5 also suggested the main contributions were from biomass combustion. This is supported by the 48-hrs backward trajectory simulation. The higher PM2.5 concentrations during 2018-2019 haze period are also associated with the meteorological conditions that induce thermal inversions and weak winds in the morning and evening. Average values of benzo(a)pyrene toxic equivalency quotient during haze period were about 3-6 times higher than during non-haze period. This should raise a concern of potential human health risk in Bangkok and vicinity exposing to fine and ultrafine particulate matters in addition to regular exposure to traffic emission.

The influence of the open burning of agricultural biomass and forest fires in Thailand on the carbonaceous components in size-fractionated particles.

Size-segregated ambient particles down to particles smaller than 0.1 µm (PM0.1) were collected during the year 2014-2015 using cascade air samplers with a PM0.1 stage, at two cities in Thailand, Bangkok and Chiang Mai. Their characteristics and seasonal behavior were evaluated based on the thermal/optical reflectance (IMPROVE_TOR) method. Diagnostic indices for their emission sources and the black carbon (BC) concentration were assessed using an aethalometer and related to the monthly emission inventory (EI) of particle-bound BC and organic carbon (OC) in order to investigate the contribution of agricultural activities and forest fires as well as agro-industries in Thailand. Monthly provincial EIs were evaluated based on the number of agricultural crops produced corresponding to field residue burning and the use of residues as fuel in agro-industries, and also on the number of hot spots from satellite images corresponding to the areas burned by forest fires. The ratio of char-EC/soot-EC describing the relative influence of biomass combustion to diesel emission was found to be in agreement with the EI of BC from biomass burning in the size range <1 µm. This was especially true for PM0.1, which usually tends to be indicative of diesel exhaust particles, and was shown to be very sensitive to the EI of biomass burning. In Chiang Mai, the northern part of Thailand, the forest fires located upwind of the monitoring site were found to be the largest contributor while the carbon behavior at the site in Bangkok was better accounted for by the EI of provinces in central Thailand including Bangkok and its surrounding provinces, where the burning of crop residues and the cultivation of sugarcane for sugar production are significant factors. This suggests that the influence of transportation of polluted air masses is important on a multi-provincial scale (100-200 km) in Thailand.