Can Online Aerosol Mass Spectrometry Analysis Classify Secondary Organic Aerosol (SOA) and Oxidized Primary Organic Aerosol (OPOA)? A Case Study of Laboratory and Field Studies of Indonesian Biomass Burning

Organic aerosol (OA) makes up a significant fraction of ambient particulate matter, including those emitted during the wildfire. Oxidized organic aerosol (OOA), obtained by factor analysis, dominates OA as the air mass ages. OOA includes secondary organic aerosol (SOA) and oxidized primary organic aerosol (OPOA). OPOA and SOA formations affect the OA scheme in the chemical models. Thus, their identification is essential. We aim to test the capability of the online aerosol mass spectrometry analysis to identify SOA and OPOA through laboratory studies. First, we separated particle- and gas-phase species emitted from the combustion of Indonesian peat and biomasses. Then, they were reacted in an oxidation reactor, where a time-of-flight aerosol chemical speciation monitor measured the oxidation products. Factor analysis was used to identify OPOA from the measured OA. We examined OPOA and SOA characteristics using the widely used fractional contributions of ion signal at mass-to-charge ratios (m/z's) 44 and 43 (f(44) vs f(43)) and m/z's 44 and 60 (f(44) vs f(60)). The OPOA and SOA were found to be indistinguishable. However, by examining all ions, particularly, the higher molecular weight species (m/z >= 100), we could differentiate the OPOA from some SOA. Additionally, OPOA and SOA mass spectra were compared to the ambient OOA factor observed in Singapore during the Indonesian wildfire in 2015. The comparison suggested that the observed OOA contained a mixture of SOA and OPOA. This study underlines the advantage of using detailed chemical information for characterizing OPOA and SOA in future development.