Experimental Study On The Secondary Organic Aerosol Formation By Oxidation Of α-pinene

Secondary organic aerosol (SOA) has been an issue of enormous interest in recent years because of the possible impacts of SOA on the earth's radiative balance linked to climate change, visibility degradation, and health effects. In this study, SOA formation potential ofα-pinene, one of the most abundant and reactively biogenic VOCs, was investigated at Tsinghua Indoor Chamber Facility. The photo-oxidation has been initiated by irradiation ofα-pinene/NOx mixture, and ozonolysis has been initiated by injection of ozone orα-pinene under dark condition. Methanol was also injected as OH radical scavenger for ozonolysis experiments. A Scanning Mobility Particle Sizer system (3936, TSI) and a Condensation Particle Counter (3010, TSI) were used to study the SOA formation and a gas chromatograph (GC) equipped with a DB-5 column and a flame ionization detector (FID) was used to measureα-pinene, and the concentration of NO, NO_x-NO and O₃ were recorded simultaneously.For the experiments without seed aerosol, the SOA yield can be regressed by two-product model, and the particle size distribution imply that the SOA were initially formed by homogenous nucleation of products with lower volatility, and the following growth of particle size was due to the gas/particle partitioning process. For the photo-oxidation, the presence of dry ammonia sulfate seed aerosol increased the SOA yield significantly, and a strong linear relationship (r²=0.96) between SOA yield enhancement (ΔY*) and surface concentration of seed aerosol (PMi,s)has been found, denoting that the PMi,s is the control factor for SOA yield enhancement. And the possible reason for the enhancement is due to acid-catalyzed heterogeneous reactions of pinonaldehyde to form oligomer. For ozonolysis, the SOA yield has not been changed by inorganic seed aerosol, because the products are mainly carboxylic acids, which has much lower saturation vapor pressure and thus easily undergo homogeneous nucleation or gas-particle partitioning to be in the aerosol phase.Box simulation was conducted using CHEMKIN software, mechanism analysis with box simulation denoted that, the HONO off-gassing rate are 1.65×10^-5 cm³molecule^-1s^-1.By using the chamber characterization experiments and one box simulation, a method to evaluate the background HONO formation of smog chamber has been established.