Long-Term Analysis of PM2.5 Concentrations and Seasonal Variation in Bangkok and Chiang Mai, Thailand: Impacts of COVID-19 Lockdowns and Fire Control Measures

Abstract

An analysis of ten years of daily 24-h PM₂.₅ data (2015–2024) from Thailand’s Pollution Control Department at Bangkok (59T) and Chiang Mai (36T) reveals strong but distinct seasonality: Bangkok peaks in December–February with secondary formation and urban sources, whereas Chiang Mai exhibits pronounced February–April haze consistent with biomass burning. Two policy “experiments” were evaluated. During Bangkok’s COVID-19 Wave-1 (22 Mar–31 May 2020), the window mean declined from a 2017–2019 baseline of 20.00 to 17.01 µg m⁻³ (Δ = −2.99 µg m⁻³; −14.9%), a statistically significant reduction of small-to-moderate magnitude (permutation p = 0.0036; Cohen’s d = −0.39); two-way ANOVA showed a significant group effect, and daily exceedances of the 24-h standard (37.5 µg m⁻³) fell from 3.3% to 0%. In Chiang Mai, January–April 2022 (first year of burning control) decreased from 54.2 to 29.5 µg m⁻³ versus 2019 (Δ = −24.7 µg m⁻³; −45.5%), a large, statistically significant reduction (permutation p = 0.0001; Cohen’s d = −0.89), with significant group, month, and interaction terms; exceedances ≥37.5 µg m⁻³ dropped from 63.3% to 31.7%. NASA FIRMS hotspots concurrently decreased sharply in February–April 2022, corroborating reduced fire activity. Annual means show a steady decline in Bangkok (≈16.4 µg m⁻³ in 2024) and policy-sensitive variability in Chiang Mai; under the current Thai annual standard (15 µg m⁻³), neither city achieves compliance. The combined ground-satellite evidence indicates that sustained, region-specific measures—traffic/precursor controls in Bangkok and consistent burning management in the North—are required to meet tightened standards.