Dialkylsulfate formation in sulfuric acid-seeded secondary organic aerosol produced using an outdoor chamber under natural sunlight

Secondary organic aerosols (SOA) were produced by the photooxidation of the volatile organic hydrocarbons (VOCs) isoprene, alpha-pinene and toluene, in the presence of excess amounts of sulfuric acid seed aerosol with varying NOx concentrations using a large, outdoor smog chamber. Aerosol acidity ([ H+], mu mol m(-3)) was measured using colorimetry integrated with a reflectance UV-visible spectrometer (C-RUV). The C-RUV technique measures aerosol acidity changes through the neutralisation of sulfuric acid with ammonia and the formation of dialkylsulfate, a diester of sulfuric acid. The concentration (mu mol m(-3)) of dialkylsulfate in aerosol was estimated using the difference in [ H+] obtained from C-RUV and particle-into-liquid-sampler ion chromatography (PILS-IC). The yield of dialkylsulfate (YdiOS) was defined as the dialkylsulfate concentration normalised by the concentrations of both the ammonium-free sulfate ([SO42-](free) = [SO42-] - 0.5 [ NH4+]) and organic carbon. The highestY(diOS) appeared in isopreneSOA and the lowestY(diOS) in alpha-pinene SOA. Under our experimental conditions, more than 50% of the total sulfates in sulfuric acid-seeded isoprene SOA were dialkylsulfates. For all SOA, higher YdiOS was observed under higher NOx conditions (VOC (ppb C)/NO (ppb) < 15). Among the major functional groups (-COOH, -CO-H, -CHO and -ONO2) predicted to be present using a simple absorptive partitioning model of organic products in the multiphase system (gas, organic aerosol and inorganic aerosol), the concentrations of -CO-H, -CHO and -ONO2 groups were found to be correlated with Y-diOS. In particular, a strong correlation was observed between Y-diOS and the concentration of alcohol functional groups.