A chemistry transport model study for source contribution of PM2.5 and its implications for Southeast Asia regional haze management policy

Abstract PM 2.5 pollution is a persistent issue in Southeast Asia. In response, the Association of Southeast Asian Nations (ASEAN) launched the Second Hazefree Roadmap (2023-2030) to alleviate this issue. This roadmap can be an important institutional measure to reduce PM 2.5 pollution regionally; however, it largely centres on biomass burning and does not comprehensively tackle other sources. This study aims to quantify the contributions of three key sectors—biomass burning, transport, and industry—to PM 2.5 pollution. In addition, for the focus of Thailand, the study assesses transboundary PM 2.5 pollution coming into Thailand. We employ the GEOS-Chem chemistry transport model, a widely used global model for source contribution and transboundary pollution studies. The model utilizes global 4°x5° simulations and nested 0.25°x0.3125° simulations with a domain covering Southeast Asia (91.0°E to 127.0°E and 10°S to 29°N). Our Yindings show that Bangkok, Hanoi, and Jakarta exceed PM 2.5 thresholds, primarily due to industrial and transport emissions. In Bangkok’s dry season, average PM2.5 was 50.87 μg/m 3 , with industry contributing 27.04% (13.75 μg/m 3 ), transport 17.80% (9.06 μg/m 3 ), and biomass burning 14.34% (7.29 μg/m 3 ). Among the mentioned contributions, transboundary contribution accounts for 11.43% (5.81 μg/m 3 ) for biomass burning, 8.49% (4.32 μg/m 3 ) for industrial sector, and 2.63% (1.34 μg/m 3 ) for transport sector. In Hanoi, PM2.5 peaked at 98.26 μg/m 3 in the dry season, with 21.63% (21.26 μg/m 3 ) from industrial sector, 8.12% (7.98 μg/m 3 ) from transport sector, and 1.28%(1.26 μg/m 3 ) from biomass burning. Jakarta saw elevated PM2.5 (48.68 μg/m 3 ) in the dry season (June-July), with transport accounting for 20% (9.56 μg/m 3 ), industrial sector at 17% (8.23 μg/m3), and biomass burning showed minimal contribution. Conversely, cities like Phnom Penh, Vientiane, and Kuala Lumpur, which remained below the WHO Interim Target 3 of 37.5 μg/m 3 (24-hour average), showed lower industrial contributions and more variation from transport and biomass burning. These Yindings underscore the need for a broader strategy targeting additional sectors beyond biomass burning to meet WHO air quality goals.