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Recurring transboundary haze from Indonesian wildfires in previous decades significantly elevated particulate matter (PM) concentrations in Southeast Asia. The elevated PM reduced visibility in Singapore, which is located around 200 km from the closest wildfires site in Indonesia. Light-absorbing brown carbon (BrC) constituents of organic aerosol (OA) have been shown to significantly absorb ultraviolet (UV) and visible light and thus impact visibility and radiative forcing. However, sources and light-absorbing properties of the haze particles are still less understood.

During that event on 10 to 31 October 2015, we conducted a real-time observation of non-refractory submicron PM (NR-PM1) in Singapore using an Aerodyne aerosol mass spectrometer. Simultaneously, we measured carbonaceous components, OA tracers, and BrC constituents from ambient fine PM (PM2.5) samples and laboratory Indonesian peat and biomass burning aerosols to support source apportionment of the online measurements.

The real-time analysis demonstrated that OA accounted for approximately 80 % of NR-PM1 mass during the wildfire haze period. Source apportionment analysis applied to the OA mass spectra using the multilinear-engine (ME-2) approach resulted in four factors: hydrocarbon-like OA (HOA), biomass burning OA (BBOA), peat burning OA (PBOA), and oxygenated OA (OOA). The OOA can be considered as a surrogate of both secondary organic aerosol (SOA) and oxidized primary organic aerosol (OPOA), while the other factors are considered as surrogates of primary organic aerosol (POA). The OOA accounted for approximately 50% of the total OA mass in NR-PM1 of the total OA mass. From the offline analysis, we identified 41 compounds that can potentially absorb near-UV and visible wavelengths, such as oxygenated?conjugated compounds, nitroaromatics, and S-containing compounds from the combusted Indonesian peat and biomasses. The BrC constituents contributes on average 24% of the total OA mass from laboratory biomass burning and 0.4% during the haze event. However, large uncertainties in mass closure remain because of the lack of authentic standards.

Our findings highlight the importance of atmospheric chemical processes, which likely include POA oxidation and SOA formation from oxidation of gaseous precursors, to the OOA concentration. Atmospheric processes could also affect the composition of haze particles that change their light-absorbing properties. As this research could not separately quantify the POA oxidation and SOA formation processes, future studies should attempt to investigate the contribution of gaseous precursor oxidation and POA aging to the OOA formation as well as BrC constituents in wildfire plumes.