This study investigates the chemical components of biomass burning (BB) aerosols obtained from Doi Ang Khang (DAK; near BB source) and Chiang Mai University (CMU; an urban location) over northern Southeast Asia in dry season (March to mid-April) 2014. PM 2.5 (particulate matter with an aerodynamic diameter less than or equal to 2.5 µm) samples were collected over a 24-h sampling period as a part of the Seven South East Asian Studies ) and CMU (90.7-93.1 µg m -3 ) were not significantly different (p > 0.05) and well correlated (r = 0.8), and likely originated from similar source origins. The number of fire hotspots was particularly high during 20-21 March (greater than 200) and, consequently, peaks of PM 2.5 were recorded at both sites. The most abundant elements at both sampling sites were K (49-50% of total elements), Al (26-31%), Mg (16%) and Zn (4-7%), whereas SO 4 2-(30-38% of total ions), NO 3 -(13-20%), Na + (16-20%) and NH 4 + (14-15%) were the most abundant ions. Concentrations of levoglucosan and K + (BB tracers) were well correlated (r = 0.5 for CMU and 0.7 for DAK) confirming that the PM 2.5 detected in these areas were mainly influenced by BB activity. Principal component analysis (PCA) revealed that BB, road traffic, agricultural activity and soil re-suspension were plausible sources of PM 2.5 over the study locations. Apart from local sources, the influence of long-range transport was also investigated by way of three-day backward trajectory analysis.

Trees have been recognized as air quality bioindicators, but they have still not been fully implemented in tropical areas. In this study, bark of Cassia fistula was used to inspect accumulation of air pollutants (metals) emitted from road traffic in the city of Chiang Mai, Thailand. The mean concentrations of metal accumulated on tree bark (ng/cm 2) in descending order were Al (1,238) > Fe (707) > Zn (162) >> Cu (21.1) >> Pb (6.37) > Cr (2.14). Correlations of Enrichment Factors: EF TS (metal concentrations on bark compared to those in soil) among metals were relatively strong (r > 0.6) meaning that they were probably generated from the same sources. Moreover, principal component analysis and cluster analysis of EF TS values revealed that Al and Fe were generated from soil resuspension that were attached on vehicle wheels and on road surfaces, while Cr, Cu, Pb

Abstract-Use of vegetation as a bioindicator of atmospheric heavy metal accumulation has received more attention worldwide due to the fact that this method has been found to be effective, cheap and simple to use. This study aims to find out the levels of heavy metals found in tree bark layers and to test the factors (exposed sides of tree to traffic and size of tree trunk) that affect atmospheric heavy metal accumulation in the bark of Cassia fistula, which is a common tree commonly found along the road sides and planted for shading and decoration. In order to provide the necessary information with regard to using a native tree species as a bioindicator, heavy metals emitted from road traffic including Cr, Cu, Fe, Ni, Pb and Zn were investigated. The results showed that only Cu, Fe and Zn were detected. Their concentrations were highest in the cork layer, decreased in the second (chlorenchyma), third (phloem) layers and increased in the innermost layer (vascular cambium). Heavy metals found on the outer most layer of bark definitely came from atmospheric pollution due to direct exposure to the environment. However, the concentrations in the vascular cambium were higher than in the chlorenchyma and phloem. This is probably due to some diffusion that took place in the xylem, which is connected to the cambium layer. Concerning the factors affecting the heavy metal accumulation in the tree bark, the size of Cassia fistula tree trunks (5 -30 cm) was not significantly correlated (p > 0.05) with the heavy metal concentrations found in the bark. This was also true of the sides of the exposure of the tree in relation to the traffic source. This means that the bark sampling can be done easily without any limitations with regard to the size of tree trunk and direction of exposure.Index Terms-Tree bark, heavy metal, bioindicator, Cassia fistula, atmospheric pollution.

The coronavirus disease 2019 (COVID-19) has been a global pandemic since early 2020. In Thailand, the number of cases increased exponentially from the middle of March 2020. In response, Thailand's government has imposed various pandemic prevention measures, such as a rigorous stay-at-home measure to reduce viral transmission between humans. Many human activities ceased, such as transportation, businesses, and services. This has been associated with reductions in air pollutant emissions. This study thus aimed to measure air quality in metropolitan, industrial, and suburban cities of in Thailand. Data on the hourly concentrations of six criteria pollutants were obtained from Thailand's Pollution Control Department from five stations during three measurement periods: pre-COVID, early COVID, and working-from-home (WFH) periods. The results indicated that vehicle-emitted pollutants had significantly decreased during WFH periods. Moreover, the air quality of all city types ostensibly improved (50%-70%) because of reduced transportation in Thailand's central and eastern provinces. However, results for Northern provinces were unclear because PM2.5 concentrations were still high during WFH period due to effects from open burning activity as well as meteorological condition.

Indoor air quality is associated with academic performance and harmful health effects on students and teachers who participate in the classroom. Outdoor sources always contribute to classroom air quality. This study aims to estimate the amounts of indoor and outdoor pollutants and the influence of outdoor sources on open-air classrooms in a school located in the city. A health risk assessment was applied to assess the non-carcinogenic risk to students and teachers from exposure to the pollutants in the classroom. The concentrations of indoor NO2 ranged between 46.40 and 77.83 µg/m3, which is about 0.8 times that of outdoor NO2. A strong correlation and a high indoor/outdoor (I/O) ratio (>0.5) without a source, indicated that indoor NO2 is significantly influenced by outdoor sources. The range of indoor PM2.5 concentrations was 1.66 to 31.52 µg/m3 which was influenced by meteorological conditions. The indoor PM2.5 concentrations were affected by both indoor and outdoor sources. Although the level of indoor air pollutants met the official standard, the young children were exposed to indoor air pollutants which were above the recommended limits to human health with regard to the hazard index (HI) of 1.12. Instant measures such as regularly cleaning the classrooms, zoning the students, and installation of solid and vegetation barriers are recommended to reduce the daily dose of pollutants affecting students in open-air classrooms.