Secondary organic aerosol formation from ethyne in the presence of NaCl in a smog chamber

The heterogeneous photochemical oxidation of ethyne was investigated under different relative humidity (RH) conditions in the presence of nitrogen oxides and sodium chloride in a self-made indoor smog chamber. The purpose was to study the influence of RH on the formation of secondary organic aerosol (SOA) from C2H2. Through the experiments, we found that SOA was rarely formed at < 22% RH in the presence of NaCl seed particles, and that SOA began to be formed at >= 29% RHin the presence of NaCl, which shows the importance ofRHin the formation of SOA. The yield of SOA (Y-SOA) from C2H2 was 0.2% at 51% RH, and increased by a factor of 17.5 as RH reached 83 %. The SOA yield increased with increasing RH. The geometric mean diameter of the particles increased by a factor of 1.17, 1.22, 1.28 and 1.51 at a RH of 51, 63, 74 and 83% respectively at the end of the experiment, indicating that the growth of the particle size also increased with increasing RH. Analysis of the SOA with Fourier-transform infrared (FTIR) spectrometry indicated that the particles generated from C2H2 contained the functional groups -OH, C=O, C-O-C and C-C-OH, for whose absorption peaks increase with increasing RH.