LAUSR

Abstract Secondary Organic Aerosol (SOA) formation during the photolysis and OH-photooxidation of α-methylstyrene was investigated using a simulation chamber at atmospheric pressure and room temperature (296 ± 1) K. α-Methylstyrene concentration was followed by gas chromatography with a mass spectrometric detector (GC–MS) and the aerosol production was monitored using a Fast Mobility Particle Sizer (FMPS). The effect of varying α-methylstyrene (0.5 ppm - 2 ppm) and NOx (0.5 ppm - 1.0 ppm) concentrations on SOA formation was explored, as was the effect of the relative humidity (RH) (5–50%) and the presence of inorganic seed particles. Results indicate that SOA yields increase at higher α-methylstyrene concentrations; low NOx and high RH conditions favour more rapid aerosol formation and a higher aerosol yield; SOA formation is independent of seed surface area, within the studied range, for both inorganic seed particles ((NH4)2SO4 and CaCl2). An off-line chemical analysis using a filter/denuder sampling system and GC–MS confirms acetophenone as the main gas phase product in both processes, photolysis and photooxidation. For the first time, SOA composition in α-methylstyrene degradation was analysed, observing acetophenone and acetol as products present in the formed aerosol in presence and absence of NOx.