Aqueous Photochemistry of Secondary Organic Aerosol of α-Pinene and α-Humulene in the Presence of Hydrogen Peroxide or Inorganic Salts

The effect of common atmospheric solutes on aqueous-phase aging of secondary organic aerosol (SOA) was explored under irradiated and dark conditions. SOA particles were produced from dark ozonolysis of alpha-pinene or alpha-humulene in a photochemical smog chamber, collected on filters, and extracted in either pure water or aqueous solutions containing 0.010 mM H2O2, 0.15 mM NaNO3, or 0.15 mM NH4NO3. These aqueous samples were either irradiated for up to 4 h to simulate photochemical aqueous aging by sunlight or kept in the dark for the same amount of time to simulate nighttime aqueous chemistry of SOA. The chemical composition of SOA was monitored over time using direct infusion electrospray ionization high-resolution mass spectrometry. The presence of salts accelerated the loss of high-molecular-weight conditions, making the dissolved SOA compounds smaller and more volatile. These effects of atmospheric salts have important implications for understanding SOA evolution in cloud and fogwater.