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. Particulate matter (PM) pollution in China is an emergingenvironmental issue which policy makers and the public have increasingly paidattention to. In order to investigate the characteristics, sources, andchemical processes of PM pollution in Guangzhou, field measurements wereconducted from 20 November 2017 to 5 January 2018, with a time-of-flightaerosol chemical speciation monitor (ToF-ACSM) and other collocatedinstruments. Mass concentrations of non-refractory submicron particulatematter (NR-PM 1 ) measured by the ToF-ACSM correlated well withthose of PM 2.5 or PM 1.1 measured by filter-based methods. Theorganic mass fraction increased from 45 % to 53 % when the air switchedfrom non-pollution periods to pollution episodes (EPs), indicating significantroles of organic aerosols (OAs) during the whole study. Based on the massspectra measured by the ToF-ACSM, positive matrix factorization (PMF) withthe multilinear engine (ME-2) algorithm was performed to deconvolve OA into fourfactors, including hydrocarbon-like OA (HOA, 12 %), cooking OA (COA,18 %), semi-volatile oxygenated OA (SVOOA, 30 %), and low-volatilityoxygenated OA (LVOOA, 40 %). Furthermore, we found that SVOOA and nitratewere significantly contributed from local traffic emissions while sulfateand LVOOA were mostly attributed to regional pollutants. Comparisons betweenthis work and other previous studies in China show that secondary organic aerosol (SOA) fraction intotal OA increases spatially across China from the north to the south. Two distinctly opposite trends for NR-PM 1 formation were observedduring non-pollution periods 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 RO 2 ∗ and ozone, increased with increasing NR-PM 1 concentration duringnon-pollution periods, while an opposite trend of these three quantities wasobserved during pollution EPs. Furthermore, oxidation degrees of both OA andSOA were investigated using the f44∕f43 space and the results showthat at least two OOA factors are needed to cover a large range of f44 and f43 in Guangzhou. Comparisons between our results and otherlaboratory studies imply that volatile organic compounds (VOCs) from trafficemissions, in particular from diesel combustion and aromatic compounds, are themost likely SOA precursors in Guangzhou. Peroxy radical RO 2 ∗ was used as a tracer for SOA formed through gas-phase oxidation. Fornon-pollution periods, SOA concentration was reasonably correlated with RO 2 ∗ concentration during both daytime and nighttime, suggesting thatgas-phase oxidation was primarily responsible for SOA formation. However,there was no correlation between SOA and RO 2 ∗ in pollution EPs,suggesting a dramatically changed mechanism for SOA formation. This conclusioncan also be supported by different features of SOA in a van Krevelen diagrambetween non-pollution periods and pollution EPs. Furthermore, for pollutionEPs, when NR-PM 1 mass concentration was divided into six segments, ineach segment except for the lowest one SOA concentration was correlatedmoderately with RO 2 ∗ concentration, suggesting that gas-phase oxidationstill plays important roles in SOA formation. The intercepts of the abovelinear regressions, which likely correspond to the extent of othermechanisms (i.e., heterogeneous and multiphase reactions), increase withincreasing NR-PM 1 mass concentration. Our results suggest that whilegas-phase oxidation contributes predominantly to SOA formation duringnon-pollution periods, other mechanisms such as heterogeneous and multiphasereactions play more important roles in SOA formation during pollution EPsthan gas-phase oxidation.

atmospheric chemistry and physics

Characterization of submicron particles by time-of-flight aerosol chemical speciation monitor (ToF-ACSM) during wintertime: aerosol composition, sources, and chemical processes in Guangzhou, China