The analysis of gaseous criteria pollutants in the Bangkok Metropolitan Region (BMR), Thailand, from 2010 to 2014 reveals that while the hourly concentrations of CO, SO₂ and NO₂ were mostly within the National Ambient Air Quality Standards (NAAQs) of Thailand, the hourly concentrations of O₃ frequently exceeded the standard. The results reveal that the problem of high O₃ concentration continuously persisted in this area. The O₃ photolytic rate constant (j₁) for BMR calculated based on assuming a photostationary state ranged from 0.008 to 0.013 s⁻¹, which is similar to the calculated j₁ using the NCAR TUV model (0.021±0.0024 s⁻¹). Interconversion between O₃, NO and NO₂ indicates that crossover points between the species occur when the concentration of NO_x ( = NO + NO₂) is ∼ 60 ppb. Under a low-NO_x regime ([NO_x] < 60 ppb), O₃ is the dominant species, while, under a high-NO_x regime ([NO_x] > 60 ppb), NO dominates. Linear regression analysis between the concentrations of O_x ( = O₃ + NO₂) and NO_x provides the role of local and regional contributions to O_x. During O₃ episodes ([O₃]_hourly > 100 ppb), the values of the local and regional contributions were nearly double of those during non-episodes. Ratio analysis suggests that the major contributors of primary pollutants over BMR are mobile sources. The air quality index (AQI) for BMR was predominantly good to moderate; however, unhealthy O₃ categories were observed during episode conditions in the region.

Ambient concentrations of both fine particulate matter (PM_2.5) and particulate matter with an aerodynamic diameter less than 10 micron (PM₁₀) were measured from 10 June 2015 to 13 July 2015 at three locations surrounding the Cheswick Power Plant, which is located between the boroughs of Springdale and Cheswick, Pennsylvania. The average concentrations of PM₁₀ observed during the periods were 20.5 ± 10.2 μg m⁻³ (Station 1), 16.1 ± 4.9 μg m⁻³ (Station 2) and 16.5 ± 7.1 μg m⁻³ (Station 3). The average concentrations of PM_2.5 observed at the stations were 9.1 ± 5.1 μg m⁻³ (Station 1), 0.2 ± 0.4 μg m⁻³ (Station 2) and 11.6 ± 4.8 μg m⁻³ (Station 3). In addition, concentrations of PM_2.5 measured by four Pennsylvania Department of Environmental Protection air quality monitors (all within a radius of 40 miles) were also analyzed. The observed average concentrations at these sites were 12.7 ± 6.9 μg m⁻³ (Beaver Falls), 11.2 ± 4.7 μg m⁻³ (Florence), 12.2 ± 5.3 μg m⁻³ (Greensburg) and 12.2 ± 5.5 μg m⁻³ (Washington). Elemental analysis for samples (blank – corrected) revealed the presence of metals that are present in coal (i.e., antimony, arsenic, beryllium, cadmium, chromium, cobalt, lead, manganese, mercury, nickel and selenium).

In Bangkok, the megacity of Thailand, concentrations of fine particulate matter (PM_2.5) have often exceeded the National Ambient Air Quality standards. During severe smog events over Bangkok, the air quality has exhibited moderate to unhealthy atmospheric conditions, according to the air quality index of the United States. To investigate the formation of secondary organic aerosols (SOA), a field campaign to estimate secondary organic carbon (SOC) in Bangkok using the EC tracer method was conducted in January 2021, when the concentrations of PM_2.5 were high. The monthly period was classified into three pollution groups, including high pollution, high PM, and low pollution events. The study showed that the correlations between PM_2.5 and O₃ were negative during both the daytime and night-time. The OC/EC ratios varied from 4.32 to 5.43, while the moderate OC/EC values implied that fossil fuel combustion was the major carbonaceous aerosol in Bangkok. The EC tracer-estimated SOC and POC showed that SOC contributed between 32.5 and 46.4% to OC, while the highest SOC contribution occurred during the low pollution event. The heightened formation of SOA during the low pollution event was perhaps owing to the levels of oxides of nitrogen (NO_x). Since Bangkok is more likely to have a NO_x-rich photochemical reaction regime, an increase in the NO_x level tended to decrease the SOA yield ([NO_x] was 21.6 ppb, 20.8 ppb, and 17.1 ppb during the high pollution, high PM, and low pollution events, respectively). Together with the high humidity and high light intensity during the low pollution event, the SOA formation was enhanced. Even though the driving factors of SOA formation over Bangkok remain unclear, the results of this study reveal the significance and urgency of local actions to reduce NO_x and O₃ towards more habitable and sustainable urban environments.

Biogenic volatile organic compounds (BVOC), which are mainly emitted from plants, are a major precursor for the formation of ground-level ozone (O₃) and secondary organic aerosols (SOA). In the northern region of Thailand, 63.8% of the land area is covered by forests. Herein we investigated the effects of biogenic volatile organic compounds (BVOC) emitted from plants and anthropogenic NO_x emissions on ground-level ozone (O₃) and fine particulate matters (PM_2.5) formation. The Weather Research and Forecasting Model coupled with Chemistry (WRF-Chem Model) was applied to simulate three scenarios including baseline, noBio and modiAntho simulations. The modeling results over the northern region of Thailand indicate that BVOC emissions over the northern region of Thailand contributed only 5.3%–5.6% of the total concentrations of PM_2.5 and BVOC had a direct relationship to glyoxal and SOA of glyoxal. The comparison between the observed and the modeled isoprene over the study site showed an underestimation (3- to 4-folds) of the simulated concentrations during the study period (June and November 2021). In June, decreases in anthropogenic NO_x emissions by 40% led to PM_2.5 reductions (5.3%), which corresponded to a zero BVOC emission scenario. While higher PM_2.5 reductions (5.6%) were found to be caused by anthropogenic NO_x reductions in November, small increases in PM_2.5 were observed over the area near a power plant located in Lampang Province. Therefore, both VOC and NO_x emission controls may be necessary for areas near the lignite mine and power plant. Since the areas within the vicinity of the power plant were under VOC-limited regimes, while the other areas were determined to be NO_x-limited.

Analysis of gaseous criteria pollutants in Bangkok Metropolitan Region (BMR), Thailand, during 2010–2014 reveals that the hourly concentrations of CO, SO₂ and NO₂ were mostly below the National Ambient Air Quality Standards (NAAQs) of Thailand. However, the hourly concentrations of O₃ exceeded the Thailand NAAQs. The maximum concentrations of O₃ ranged from 120–190 ppb. On average, the number of hourly O₃ exceedances ranged from 1–60 hours a year depending on monitoring station locations. The exceedances occurred during the summer and winter, dry seasons. Interconversion between O₃, NO and NO₂ indicates crossover points between species occur when the concentration of NO_x ([NO_x] = [NO]+[NO₂]) is ~ 60 ppb. However, when [NO_x] < 60 ppb, O₃ is the dominant species; conversely, NO dominates when [NO_x] > 60 ppb. The calculated photochemical reaction rate (the reaction between NO₂ with sunlight), during photostationary state ranges from 0.12 to 1.22 min⁻¹. Linear regression analysis between the concentrations of O_x ([O_x ] = [O₃]+[NO₂]) and NO_x provides the role of local and regional contributions to O_x . Both the local and regional O_x contributions enhance the concentration of O_x . Values of the local and regional O_x contributions during non-episode were ~ 44–54 ppb and ~ 0.13[NO_x ] to 0.33[NO_x ], respectively. Those values were about double during O₃ episodes ([O₃] > 100 ppb). Ratio analysis suggests that the major contributors of primary pollutants over BMR are mobile sources (CO/NO_x = 19.8). The Air Quality Index (AQI) for BMR was predominantly between good to moderate. Unhealthy O₃ categories were observed during episode conditions in the region.