Physical Properties of Tropospheric Aerosols from a Biomass Burning Episode in Equatorial Southeast Asia

The present study investigates some of the physical properties of remotely sensed aerosols that were emitted from an intense biomass burning episode over equatorial Southeast Asia during August 2005. A total of 1245 active fire counts were detected in Sumatera, Indonesia by the MODIS Aqua satellite from 8 to 14 August 2005, and consequently, increased the amount of tropospheric oceanic aerosol mass concentrations and aerosol optical depth (AOD) to more than 23 mu g cm(-2) and 0.8, respectively over Peninsular Malaysia. This is in contrast to lower AOD values over the uninhabited neighbouring areas such as the Bay of Bengal or the equatorial Indian Ocean. Coarser and lower mass concentrations of aerosols were detected over the equatorial Indian Ocean, in contrast to the area east of Sumatera, including Peninsular Malaysia and the southern South China Sea, where the finer sized aerosols originated mainly from the biomass burning activities. Here, the effective radii of fine sized aerosols ranged from 0.1 to 0.4 mu m compared to the effective radii of coarse aerosols that were typically of size 1 mu m over the equatorial Indian Ocean. Forward air trajectories showed that the fine aerosols originated from the burned biomass in Sumatera were transported downstream to Peninsular Malaysia by the prevailing southwest monsoon. The near-stagnant low level conditions that occurred from 9 to 11 August 2005 exacerbated the poor dispersion of the aerosols as corroborated by the extremely high ground level PM10 concentrations recorded on the western coast of Peninsular Malaysia. Time series of the AOD showed that the elevated turbidity of the atmosphere associated with the suspended aerosols from the intense biomass burning activities were transient in nature, which lasted for approximately a few days before the aerosols were subsequently removed from the atmosphere.