Uncovering the effect of fires in Jambi, Riau, and South Sumatra on PM2.5 concentration levels in Greater Kuala Lumpur during September 2019 transboundary haze pollution

The Greater Kuala Lumpur (GKL) is the most urbanised region in Malaysia that experiences persistently high levels of PM_2.5. Transboundary haze caused by biomass burnings in Sumatra, Indonesia, exacerbates the PM_2.5 concentration levels in GKL, negatively impacting the public's health and the socioeconomic environment. We aim to investigate the specific influence of fires induced by biomass burnings in Sumatra provinces, namely Jambi, Riau, and South Sumatra, on PM_2.5 concentration levels in GKL during the transboundary haze of September 2019. Our research addresses four key objectives: i)analysing PM_2.5 pollution level during 2019 in GKL at Bangi (BG), Batu Muda (BM), Cheras (CS), Klang (KG), Kuala Selangor (KS), Putrajaya (PA), Petaling Jaya (PJ), and Shah Alam (SA); ii)estimating PM_2.5 emissions from biomass burnings in Sumatra in 2019; iii)determining smoke pathways during biomass burning events in September 2019; and iv)estimating the contribution of Riau, Jambi and South Sumatra provinces towards PM_2.5 concentration load in GKL during September 2019. Our analysis revealed that in September 2019, 80% to 100% of days in GKL exceeded the Malaysian Air Quality Standard (MAQS) and the World Health Organization (WHO) daily guideline for PM_2.5 levels. We utilised the Global Fire Assimilation System (GFASv1.2) biomass emission inventory to estimate the total PM_2.5 mass emitted from biomass burning events within our domain, latitude 5^oS to 10^oN and longitude 95^oE to 110^oE. Our estimation showed that 1.78 teragrams (Tg) of PM_2.5 mass was emitted from biomass burnings within the study region in 2019, with 55% (0.90Tg) of these emissions occurring in September. Spearman's analysis demonstrated a strong positive correlation (ρ = 0.747, p < 0.001) between PM_2.5 mass emissions from Sumatran biomass burnings and elevated PM_2.5 concentration levels in GKL in September 2019. Emissions from Jambi, Riau and South Sumatra provinces accounted for approximately 94% of the total PM_2.5 mass emitted during September 2019. Using the Numerical Atmospheric-Dispersion Modelling Environment (NAME) backward run, we observed that the southwestern air pathway influenced the transport of smoke-induced by biomass burning from Jambi, Riau, and South Sumatra towards GKL throughout September 2019. By integrating the NAME backward run with GFASv1.2, we simulated the PM_2.5 concentrations in GKL that originated from biomass burnings in these three provinces. The NAME-GFAS model exhibited a slight underestimation (mean bias: -3 µgm^-3 to -12 µgm^-3) compared to biomass-induced  PM_2.5 concentration levels in GKL at the eight locations in GKL in 2019. Notably, the model and observations demonstrated good agreement at these locations for September (correlation coefficient: 0.62– 0.70). Our model predicts that fires from Riau and Jambi provinces collectively account for 97% of  PM_2.5 concentration levels in GKL during transboundary haze. Of these, fires from Riau dominated PM_2.5 concentration levels in KG (56%), KS(51%) and BG (56%). While Jambi contributed mostly to BM(57%),PJ(51%),CS(55%) and PA (44%). SA has equal contributions from Riau and Jambi. South Sumatra consistently contributed between 1-3% at the respective stations. These findings stress the urgency of considering the effect of geographical morphology in addressing elevated PM_2.5 levels during transboundary haze events in GKL.