Abstract. Particulate matter (PM) pollution in China is an emerging environmental issue which policy makers and public have increasingly paid attention to. In order to investigate the characteristics, sources, and chemical processes of PM pollution in Guangzhou, a field measurement was conducted from 20 November 2017 to 5 January 2018, with a Time-of-Flight Aerosol Chemical Speciation Monitor (ToF-ACSM) and other collocated instruments. Mass concentrations of non-refractory submicron particulate matters (NR-PM1) measured by the ToF-ACSM were correlated well with those of PM2.5 or PM1.1 measured by filter-based methods. The organic mass fraction increased from 45 % to 53 % when the air switched from non-pollution periods to pollution episodes, indicating significant roles of organic aerosols (OA) during the whole study. Based on the mass spectra measured by the TOF-ACSM, Positive Matrix Factorization (PMF) with multilinear engine (ME-2) algorithm was performed to deconvolve OA into four factors, including hydrocarbon-like OA (HOA, 12 %), cooking OA (COA, 18 %), semi-volatile oxygenated OA (SVOOA, 30 %), and low-volatility oxygenated OA (LVOOA, 40 %). Furthermore, we found that SVOOA and nitrate were significantly contributed from local traffic emissions while sulfate and LVOOA were mostly attributed to regional pollutants. Comparisons between this work and other previous studies in China show that SOA fraction in total OA increases spatially across China from the North to the South. Two distinctly opposite trends for NR-PM1 formation were observed during non-pollution period and pollution EPs. The ratio of secondary PM (SPM = SVOOA + LVOOA + sulfate + nitrate + ammonium) to primary PM (PPM = HOA + COA + chloride), together with peroxy radicals RO2* and ozone, increased with increasing NR-PM1 concentration during non-pollution period, while an opposite trend of these three quantities was observed during pollution EPs. Furthermore, oxidation degrees of both OA and SOA were investigated using the f44/f43 space and the results show that at least two OOA factors are needed to cover a large range of f44 and f43 in Guangzhou. Comparisons between our results and other laboratory studies imply that volatile organic compounds (VOCs) from traffic emissions in particular from diesel combustion and aromatic compounds are most possible SOA precursors in Guangzhou. Peroxy radical RO2* was used as a tracer for SOA formed through gas phase oxidation. For non-pollution period, SOA concentration was reasonably correlated with RO2* concentration during both daytime and nighttime, suggesting that gas phase oxidation was primarily responsible for SOA formation. For pollution EPs, when NR-PM1 mass concentration was divided into six segments, in each segment except for the lowest one, SOA concentration was correlated moderately with RO2* concentration, suggesting that gas phase oxidation still plays important roles in SOA formation. In addition, the slopes of linear regressions for the above correlations increased with increasing NR-PM1 mass concentration, probably representing enhanced gas-to-particle partitioning under high NR-PM1 concentration. The intercepts of the above linear regressions, which correspond to the extent of other mechanisms (i.e., heterogeneous and multiphase reactions), increased with increasing NR-PM1 mass concentration. Our results suggest that while gas phase oxidation contributes predominantly to SOA formation during non-pollution periods, other mechanisms such as heterogeneous and multiphase reactions play more important roles in SOA formation during pollution EPs than gas phase oxidation.

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