Biomass burning aerosols and the low-visibility events in Southeast Asia

摘要

Fires including peatland burning in Southeast Asia have become a majorconcern to the general public as well as governments in the region. This isbecause aerosols emitted from such fires can cause persistent haze eventsunder certain weather conditions in downwind locations, degrading visibilityand causing human health issues. In order to improve our understanding of thespatiotemporal coverage and influence of biomass burning aerosols inSoutheast Asia, we have used surface visibility and particulate matterconcentration observations, supplemented by decade-long (2003 to 2014)simulations using the Weather Research and Forecasting (WRF) model with afire aerosol module, driven by high-resolution biomass burning emissioninventories. We find that in the past decade, fire aerosols are responsiblefor nearly all events with very low visibility ( 7 km). Fireaerosols alone are also responsible for a substantial fraction of low-visibility events (visibility   10 km) in the major metropolitanareas of Southeast Asia: up to 39 % in Bangkok, 36 % in Kuala Lumpur, and34 % in Singapore. Biomass burning in mainland Southeast Asia accounts forthe largest contribution to total fire-produced PM in Bangkok(99 %), while biomass burning in Sumatra is a major contributor tofire-produced PM in Kuala Lumpur (50 %) and Singapore (41 %).To examine the general situation across the region, we have further definedand derived a new integrated metric for 50 cities of the Association ofSoutheast Asian Nations (ASEAN): the haze exposure day (HED), which measuresthe annual exposure days of these cities to low visibility ( 10 km)caused by particulate matter pollution. It is shown that HEDs have increasedsteadily in the past decade across cities with both high and low populations.Fire events alone are found to be responsible for up to about half of thetotal HEDs. Our results suggest that in order to improve the overall airquality in Southeast Asia, mitigation policies targeting both biomass burningand fossil fuel burning sources need to be implemented.