PM2.5 was continuously collected in Ho Chi Minh City (HCMC), Vietnam, during the period from September 2019 to August 2020, which included the period of socioeconomic suppression caused by restrictions imposed in the face of the coronavirus disease of 2019. The concentrations of PM2.5 mass, water-soluble ions (WSIs), organic carbon (OC), elemental carbon (EC), and water-soluble organic carbon (WSOC) were determined to evaluate the seasonal variations in PM2.5, the effect of socioeconomic suppression on PM2.5, and potential PM2.5 sources in HCMC. The PM2.5 mass concentration during the sampling period was 28.44 ± 11.55 μg m−3 (average ± standard deviation). OC, EC, and total WSIs accounted for 30.7 ± 6.6%, 9.7 ± 2.9%, and 24.9 ± 6.6% of the PM2.5 mass, respectively. WSOC contributed 46.4 ± 10.1% to OC mass. NO3−, SO42−, and NH4+ were the dominant species in WSIs (72.7 ± 17.7% of the total WSIs’ mass). The concentrations of PM2.5 mass and total WSIs during the rainy season were lower than those during the dry season, whereas the concentrations of carbonaceous species during the rainy season were higher. The concentrations of PM2.5 mass and chemical species during the socioeconomic suppression period significantly decreased by 45%–61% compared to the values before this period. The OC/EC ratio (3.28 ± 0.61) and char-EC/soot-EC (4.88 ± 2.72) suggested that biomass burning, coal combustion, vehicle emissions, cooking activities are major PM2.5 sources in HCMC. Furthermore, the results of a concentration-weighted trajectory analysis suggested that the geological sources of PM2.5 were in the local areas of HCMC and the northeast provinces of Vietnam (where coal-fired power plants are located).

Airborne microplastics (AMPs) undergo ultraviolet (UV)-driven physicochemical aging during atmospheric transport, influencing cloud processes, greenhouse-gas release, and potential respiratory health impacts. Quantifying this transformation is particularly challenging for particles smaller than 10 µm and for polymers such as polyethylene terephthalate (PET), whose intrinsic ester carbonyl band obscures newly formed acid carbonyls in conventional infrared analyses. Here, we develop a µFTIR attenuated total reflection (µFTIR-ATR) imaging method combined with a fourth-derivative oxidation index (carbonyl ratio at 1701/1716 cm⁻¹) that resolves these overlapping bands and enables sensitive, quantitative evaluation of PET surface oxidation. The approach automates detection, identification, and oxidation analysis of particles down to ~2 µm. Laboratory UV irradiation experiments show a systematic increase in this derivative-based oxidation index with exposure dose. Application to ambient PET collected from Mt. Fuji, Tokyo, Osaka (Japan), and Siem Reap (Cambodia) reveals clear regional differences corresponding to local UV-A environments: PET from Siem Reap exhibited the highest oxidation, whereas particles from the Japanese sites showed moderate but variable aging. These results demonstrate that derivative-based µFTIR-ATR imaging provides a practical and highly sensitive tool for quantifying photo-oxidative degradation in fine AMPs and highlight the value of chemical-aging metrics for interpreting atmospheric processing and transport pathways.

Airborne microplastics (AMPs) undergo ultraviolet (UV)–driven physicochemical aging during atmospheric transport, influencing cloud processes, greenhouse-gas release, and potential respiratory health impacts. Quantifying this transformation is particularly challenging for particles smaller than 10 µm and for polymers such as polyethylene terephthalate (PET), whose intrinsic ester carbonyl band obscures newly formed acid carbonyls in conventional infrared analyses. Here, we develop a µFTIR attenuated total reflection (µFTIR-ATR) imaging method combined with a fourth-derivative oxidation index (carbonyl ratio at 1701/1716 cm⁻¹) that resolves these overlapping bands and enables sensitive, quantitative evaluation of PET surface oxidation. The approach automates detection, identification, and oxidation analysis of particles down to ~2 µm. Laboratory UV irradiation experiments show a systematic increase in this derivative-based oxidation index with exposure dose. Application to ambient PET collected from Mt. Fuji, Tokyo, Osaka (Japan), and Siem Reap (Cambodia) reveals clear regional differences corresponding to local UV-A environments: PET from Siem Reap exhibited the highest oxidation, whereas particles from the Japanese sites showed moderate but variable aging. These results demonstrate that derivative-based µFTIR-ATR imaging provides a practical and highly sensitive tool for quantifying photo-oxidative degradation in fine airborne microplastics and highlight the value of chemical-aging metrics for interpreting atmospheric processing and transport pathways.

While biomass burning (BB) significantly affects air quality in Southeast Asia, its contribution to PM_2.5 in Ho Chi Minh City (HCMC), Vietnam's most populous city, remains poorly understood. This study aimed to identify the major BB sources contributing to PM_2.5 in urban HCMC, with particular attention to the potential underestimation caused by chemical degradation of levoglucosan (Lev), a widely used BB tracer. PM_2.5 samples were collected from October to December 2021 and analyzed for their chemical composition, including Lev, mannosan (Man), and potassium from BB (K_BB ⁺). The average PM_2.5 mass concentration was 26.6 ± 8.8 μg m^-3 and was higher during the dry season than the rainy season due to reduced wet deposition. Notably, evaluation of BB tracers indicated substantial chemical degradation of Lev, with an estimated loss of approximately 88%. After correcting for this degradation, the reconstructed Lev concentration without chemical loss (Lev_no‑chem) reached 1766 ± 637 ng m^-3. Diagnostic ratios of Lev/Man, Lev_no‑chem/K_BB ⁺, and OC/Lev_no‑chem were 16.7 ± 2.6, 1.76 ± 0.39, and 4.54 ± 0.74, respectively. Importantly, source identification based on the uncorrected Lev would have suggested crop residue and grass burning as the dominant BB sources. In contrast, the degradation-corrected ratios consistently indicate that hardwood and hardwood charcoal combustion from cooking are the predominant BB sources in HCMC. Weighted concentration weighted trajectory analysis further revealed that these emissions are primarily associated with local urban activities rather than long-range transport. These findings demonstrate that accounting for Lev degradation is essential for accurate BB source identification in Southeast Asia.