Analysis of source regions for smoke events in Singapore for the 2009 El Nino burning season

Abstract

As part of the 7 SouthEast Asian Studies (7SEAS) program, a solar radiation and chemistry sampling site (“supersite”) was developed at the National University of Singapore (NUS) to monitor regional air quality. The first intensive operations period for this site occurred between August and October 2009, a period that coincided with a moderate El Nino event and enhanced tropical burning, particularly in peatlands. We use data from this period to analyze the transport of biomass burning emissions in the Maritime Continent (MC) to the NUS supersite. An overview of the aerosol environment is provided for Singapore, followed by more detailed discussion of four aerosol events. The 2009 burning season was similar to those described in previous analyses, which showed that fire activity begins in the western half of the MC in Sumatra and propagates eastward in time. Similarly, agricultural burning occurs first, generally followed by deforestation and peatland fires. Some of the biomass burning emissions make their way into the free troposphere, where they are transported regionally by the prevailing wind patterns. Our analyses show that the seasonal winds at 850 hPa (∼1500 m) shift transport patterns from source regions to the southwest of Singapore, to regions to the southeast over the course of the summer monsoon, patterns that allow Singapore to be impacted by peak burning regions in the MC. In contrast, winds at the surface are more typically from the south and southeast, demonstrating the prevalence of vertical wind shear over the region. As a result of the variable source regions influencing different levels of the atmosphere over Singapore, in-situ surface observations of aerosol mass concentrations are not always consistent with inferences of the presence of enhanced aerosol concentration from column optical depth. Our findings confirm the complexity of aerosol sources and transport over the MC, and the key role that biomass burning emissions play in influencing column aerosol optical depth and total particulate mass concentrations at the surface. The sea-level altitude of the NUS supersite means that non-local pollution transported above the boundary layer cannot be reliably sampled and characterized, but the combined effects of local emissions and downward-mixed, non-local pollutants in Singapore were consistently measured.