In order to better understand the molecular composition and sources of organic aerosols in Tianjin, a coastal megacity in North China, ambient fine aerosol (PM2.5) samples were collected on a day/night basis from November to December?2016 and from May to June?2017. The organic molecular composition of PM2.5 components, including aliphatic lipids (n-alkanes, fatty acids, and fatty alcohols), sugar compounds, and photooxidation products from isoprene, monoterpene, β-caryophyllene, naphthalene, and toluene, was analysed using gas chromatography–mass spectrometry. Fatty acids, fatty alcohols, and saccharides were identified as the most abundant organic compound classes among all of the tracers detected in this study during both seasons. High concentrations of most organics at night in winter may be attributed to intensive residential activities such as house heating as well as the low nocturnal boundary layer height. Based on tracer methods, the contributions of the sum of primary and secondary organic carbon (POC and SOC respectively) to aerosol organic carbon (OC) were 24.8% (daytime) and 27.6% (night-time) in winter and 38.9% (daytime) and 32.5% (night-time) in summer. In detail, POC derived from fungal spores, plant debris, and biomass burning accounted for 2.78%–31.6% (12.4%; please note that values displayed in parentheses in the following are average values) of OC during the daytime and 4.72%–45.9% (16.3%) at night in winter, and 1.28%–9.89% (5.24%) during the daytime and 2.08%–47.2% (10.6%) at night in summer. Biomass-burning-derived OC was the predominant source of POC in this study, especially at night (16.0±6.88% in winter and 9.62±8.73% in summer). Biogenic SOC from isoprene, α-∕β-pinene, and β-caryophyllene exhibited obvious seasonal and diurnal patterns, contributing 2.23±1.27% (2.30±1.35% during the daytime and 2.18±1.19% at night) and 8.60±4.02% (8.98±3.67% and 8.21±4.39%) to OC in winter and summer respectively. Isoprene and α-∕β-pinene SOC were obviously elevated in summer, especially during the daytime, mainly due to strong photooxidation. Anthropogenic SOC from toluene and naphthalene oxidation showed higher contributions to OC in summer (21.0±18.5%) than in winter (9.58±3.68%). In summer, toluene SOC was the dominant contributor to aerosol OC, and biomass burning OC also accounted for a high contribution to OC, especially at night-time; this indicates that land/sea breezes also play an important role in the aerosol chemistry of the coastal city of Tianjin in North China.
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