Abstract. The relationship between hygroscopic properties and chemical characteristics of Indonesian biomass burning (BB) particles, which are dominantly generated from peatland fires, was investigated using a humidified tandem differential mobility analyzer. In addition to peat, acacia (a popular species at plantation) and fern (a pioneering species after disturbance by fire) were used for experiments. Fresh Indonesian peat burning particles are almost non-hygroscopic (mean hygroscopicity parameter, κ < 0.06) due to predominant contribution of water-insoluble organics. The range of κ spans from 0.02 to 0.04 (dry diameter = 100 nm, hereinafter) for Riau peat burning particles, while that for Central Kalimantan ranges from 0.05 to 0.06. Fern combustion particles are more hygroscopic (κ = 0.08), whereas the acacia burning particles have a mediate κ value (0.04). These results suggest that κ is significantly dependent on biomass types. This variance in κ is partially determined by fractions of water-soluble organic carbon (WSOC), as demonstrated by a correlation analysis (R = 0.65). κ of water-soluble organic matter is also quantified, incorporating the 1-octanolwater partitioning method. κ values for the water extracts are high, especially for peat burning particles (A 0 (a whole part of the water-soluble fraction): κ = 0.18, A 1 (highly watersoluble fraction): κ = 0.30). This result stresses the importance of both the WSOC fraction and κ of the water-soluble fraction in determining the hygroscopicity of organic aerosol particles. Values of κ correlate positively (R = 0.89) with the fraction of m/z 44 ion signal quantified using a mass spectrometric technique, demonstrating the importance of highly oxygenated organic compounds to the water uptake by Indonesian BB particles. These results provide an experimentally validated reference for hygroscopicity of organicsdominated particles, thus contributing to more accurate estimation of environmental and climatic impacts driven by Indonesian BB particles on both regional and global scales.

Abstract. Recurring transboundary haze from Indonesian wildfires in previous decades significantly elevated particulate matter (PM) concentrations in Southeast Asia. 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 characterized carbonaceous components and organic aerosol (OA) tracers from fine PM (PM2.5) samples 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, while POA subtypes from wildfires (BBOA and PBOA) contributed to approximately 30 % of the total OA mass. 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. As this research could not separately quantify the POA oxidation and SOA formation processes, further studies should attempt to investigate the contribution of gaseous precursor oxidation and POA aging to the OOA formation in wildfire plumes.

Polarity distribution of water-soluble organic matter (WSOM) is an important factor in determining the hygroscopic and cloud nucleation abilities of organic aerosol particles. We applied a novel framework to quantitatively classify WSOM based on the 1octanol−water partition coefficient (K OW ), which often serves as a proxy of polarity. In this study, WSOM was generated in a laboratory biomass-burning experiment by smoldering of Indonesian peat and vegetation samples. The fractionated WSOM was analyzed using a UV−visible spectrophotometer, spectrofluorometer, and time-offlight aerosol chemical speciation monitor. Several deconvolution methods, including positive matrix factorization, parallel factor analysis, and least-squares analysis, were applied to the measured spectra, resulting in three classes of WSOM. The highly polar fraction of WSOM, which predominantly exists in the range of log K OW < 0, is highly oxygenated and exhibits similar optical properties as those of light-absorbing humic-like substances (HULIS, termed after the humic substances due to the similarity in chemical characteristics). WSOM in the least-polar fraction, which mainly distributes in log K OW > 1, mostly consists of hydrocarbon-like and high molecular weight species. In between the mostand least-polar fraction, WSOM in the marginally polar fraction likely contains aromatic compounds. The analyses have also suggested the existence of HULIS with different polarities. Comparison with previous studies indicates that only WSOM in the highly polar fraction (log K OW < 0) likely contributes to water uptake.