4.2 Article

Insights into the formation of secondary organic carbon in the summertime in urban Shanghai

Journal

JOURNAL OF ENVIRONMENTAL SCIENCES
Volume 72, Issue -, Pages 118-132

Publisher

SCIENCE PRESS
DOI: 10.1016/j.jes.2017.12.018

Keywords

Secondary organic carbon; Secondary organic aerosol (SOA); Photochemical reaction; Aqueous phase reaction; Mixing state; Oxalate; Particle acidity

Funding

  1. National Natural Science Foundation of China [91544224, 21507010, 41775150]
  2. Ministry of Science & Technology of China [2012YQ220113-4]
  3. Changjiang Scholars program of the Chinese Ministry of Education

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To investigate formation mechanisms of secondary organic carbon (SOC) in Eastern China, measurements were conducted in an urban site in Shanghai in the summer of 2015. A period of high O-3 concentrations (daily peak > 120 ppb) was observed, during which daily maximum SOC concentrations exceeding 9.0 mu g/(C.m(3)). Diurnal variations of SOC concentration and SOC/organic carbon (OC) ratio exhibited both daytime and nighttime peaks. The SOC concentrations correlated well with O-x(= O-3 + NO2) and relative humidity in the daytime and nighttime, respectively, suggesting that secondary organic aerosol formation in Shanghai is driven by both photochemical production and aqueous phase reactions. Single particle mass spectrometry was used to examine the formation pathways of SOC. Along with the daytime increase of SOC, the number fraction of elemental carbon (EC) particles coated with OC quickly increased from 38.1% to 61.9% in the size range of 250-2000 nm, which was likely due to gas-to-particle partitioning of photochemically generated semi-volatile organic compounds onto EC particles. In the nighttime, particles rich in OC components were highly hygroscopic, and number fraction of these particles correlated well with relative humidity and SOC/OC nocturnal peaks. Meanwhile, as an aqueous-phase SOC tracer, particles that contained oxalate-Fe(III) complex also peaked at night. These observations suggested that aqueous-phase processes had an important contribution to the SOC nighttime formation. The influence of aerosol acidity on SOC formation was studied by both bulk and single particle level measurements, suggesting that the aqueous-phase formation of SOC was enhanced by particle acidity. (C) 2018 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. Published by Elsevier B.V.

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