Transboundary Transport of Biomass Burning Emissions in Southeast Asia and Contribution to Local Air Quality During the 2006 Fire Event

Abstract:

Smoke from biomass and peat burning has a notable impact on ambient air quality and climate in the Southeast Asia (SEA) region. We modeled the largest fire-induced haze episode in the past decade (2006) that originated in Indonesia using WRF-Chem. Our study addressed 3 research questions: (1) Can the WRF-Chem model reproduce observations of both aerosol and CO concentrations in this complex region? (2) What is the evolution in the chemical composition of the aerosol fire plume during its atmospheric transport? and (3) What is the relative contribution of these fires to air quality in the urbanized area of the city-state of Singapore?

To test model performance, we used three independent datasets for comparison (PM10 in Singapore, CO measurements in Sumatra, and AOD column observations from 4 satellite-based sensors). We found reasonable agreement of the model runs with ground-based measurements of both CO and PM10. However, the comparison with AOD was less favorable and indicated the model underestimated AOD. In the past, modeling studies using only AOD as a constraint have often boosted fire emissions to get a better agreement with observations. In our case, this approach would seriously deteriorate the difference with ground-based observations.

Finally, our results show that about 21% of the total mass loading of ambient PM10 during the July–October study period in Singapore was due to the influence of biomass and peat burning in Sumatra, with an increased contribution during high burning periods. The composition of this biomass burning plume was largely dominated by primary organic carbon. In total, our model results indicated that during 35 days aerosol concentrations in Singapore were above the threshold of 50 μg m⁻³ day⁻¹ (WHO threshold). During 17 days this deterioration was due to Indonesian fires, based on the difference between the simulations with and without fires. Local air pollution in combination with recirculation of air masses was probably the main cause of the lack of good air quality during the other 18 days. This finding is mostly due to the relatively fine scale of our model set-up; coarser resolution models would have likely attributed the increased aerosol loads to burning in Borneo from which plumes were advected close to Singapore, but did not contribute to local pollution according to our model.