Dominant contribution of oxygenated organic aerosol to haze particles from real-time observation in Singapore during an Indonesian wildfire event in 2015

Abstract

15Abstract. Recurring transboundary haze from Indonesian wildfires in previous decades significantly elevated particulate matter (PM) concentrations in Southeast Asia. During that event on October 10 to 31, 2015, we conducted a real-time observation of non-refractory submicron PM (NR-PM 1 ) in Singapore using an Aerodyne aerosol mass spectrometer.Simultaneously, we characterized carbonaceous components and organic aerosol (OA) tracers from fine PM (PM 2.5 ) samples to support source apportionment of the online measurements. The real-time analysis demonstrated that OA accounted for 20 approximately 80% of NR-PM 1 mass during the wildfire haze period. Source apportionment analysis applied to the OA mass spectra using 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-PM 1 , 25 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 precursors oxidation and POA aging to the OOA formation in wildfire plumes. 30

Secondary aerosol formation promotes water uptake by organic-rich wildfire haze particles in equatorial Asia

“…The total mass concentrations of aerosol particles quantified by the filter samples and the ToF-ACSM also correlated well (R 2 = 0.96). The mass loading of the PM 2.5 filter samples was approximately 30 % higher than that of the ToF-ACSM results, likely because of the difference in particle size range and the lack of EC content for the ToF-ACSM measurements (Budisulistiorini et al, 2018). Figure 3c shows the mean mass spectra of organics averaged over the observation period.…”

“…The ToF-ACSM sampled particles that were desiccated by a Nafion tubing. The organic mass spectra measured by the ToF-ACSM were analyzed in detail using the multilinear engine (ME-2 solver) software (Canonaco et al, 2013). Four specific types of OA were identified: hydrocarbon-like OA (HOA), peat burning OA (PBOA), non-peat biomass burning OA (briefly BBOA), and oxygenated OA (OOA).…”

“…The PBOA and BBOA factors were attributed to the long-range transport from Indonesian peatland fires, and the two OA factors were well separated. Details about the ToF-ACSM measurements and data analysis are provided in Budisulistiorini et al (2018). PM 2.5 filter samples for chemical analysis were also collected using filter holders (BGI Inc.).…”

Revised estimate of particulate emissions from Indonesian peat fires in 2015

“…From Singapore, there are also measurements of non-refractory, composition resolved sub-micron PM from an Aerosol Chemical Speciation Monitor (ACSM) (Budisulistiorini et al, 2018). We summed the chemically-resolved masses to give PM1.…”

Responses to the referee's comments

“…The density of oxygenated organics was assumed to be 1.50 g cm −3 (as summarized in Table 1), which is a typical value for carboxylic and multifunctional organic acids (Saxena et al, 1995;Peng et al, 2001;Gysel et al, 2004;Carrico et al, 2010;Ogawa et al, 2016). According to the reported average density for common dry smoke particles (i.e., 1.20-1.40 g cm −3 ; Reid et al, 2005), and considering the dominance of non-refractory organic material in fresh Indonesian peatland burning particles (Budisulistiorini et al, 2017), the densities of PBOA and non-peat BBOA were assumed to be 1.10 and 1.20 g cm −3 , respectively. Detailed information on the parameters utilized for the κ org calculation is provided in Table 1.…”