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Secondary organic aerosol(SOA)is an important component of fine particulatematter in the atmosphere.Acetylene is the lightest one of non-methane hydrocarbons.The mainproduct of acetylene oxidation,glyoxal,is an important precursor of SOA.The heterogeneousphotochemical oxidation of acetylene was investigated under different relative humidity(RH)conditions in the presence of nitrogen oxide and sodium chlorine in an indoor smog chamber.Through the experiments we found that SOA could be rarely formed at <22%RH in the presenceof NaCl seed particles,and that SOA began to be formed when RH exceeded 29%in the presenceof NaCl,which shows the importance of RH in the formation of SOA.The yield of SOA fromC2H2 was 0.2%at 51%RH,which was increased by a factor of 17.5 as RH reached 83%.Thegeometric mean diameter of the particles was increased by a factor of 1.17,1.22,1.28 and 1.51 atthe RH of 51%,63%,74%and 83%after the 4 hrs of the experiment,respectively,indicating thatthe growth of the particle size also increased with increasing RH.The analysis of SOA withFourier transform infrared(FTIR)spectrometer indicated that the generated particles from C2H2contained the functional groups of-OH,C=O,C-O-C,and C-C-OH,for which the strengthincreases with increasing RH.