Appearance of strong absorbers and fluorophores in limonene‐O 3 secondary organic aerosol due to NH 4 + ‐mediated chemical aging over long time scales

This study investigated long‐term chemical aging of model biogenic secondary organic aerosol (SOA) prepared from the ozonolysis of terpenes. Techniques including electrospray ionization mass spectrometry (ESI‐MS), UV‐visible spectroscopy, Fourier transform infrared (FTIR) spectroscopy, NMR, and three‐dimensional fluorescence were used to probe the changes in chemical composition of SOA collected by impaction on substrates and also of aqueous extracts of SOA. The addition of ammonium ions or amino acids to limonene SOA reproducibly produced orange‐colored species that strongly absorbed visible radiation and fluoresced at UV and visible wavelengths. Simultaneous addition of H 2 SO 4 to the SOA aqueous extracts inhibited this color transformation. These observations suggest the existence of aging processes leading to heavily conjugated molecules containing organic nitrogen. The presence of nitrogen in the chromophores was confirmed by the dependence of the absorption and fluorescence spectra on the amino acids added. In contrast to the strong change in the absorption and fluorescence spectra, there was no significant change in the ESI‐MS, FTIR, and NMR spectra, suggesting that the chromophores were minor species in the aged SOA. Aqueous extracts of aged limonene + NH 4 + SOA were characterized by an effective base‐e absorption coefficient of ∼3 L g −1 cm −1 at 500 nm. Assuming particulate matter concentrations typical of polluted rural air gives an upper limit of 0.2 M m −1 for the aerosol absorption coefficient due to the aged limonene oxidation products. Biogenic SOA can therefore become weakly absorbing if they undergo aging in the presence of NH 4 + ‐containing aerosol.