Formation of Light-Absorbing Organosulfates during Evaporation of Secondary Organic Material Extracts in the Presence of Sulfuric Acid

Organic aerosols affect the climate by scattering or absorbing incoming solar radiation. Secondary organic material (SOM), which represents the major chemical constituent of atmospheric aerosol particles, is produced by the oxidation of atmospheric volatile organic compounds (VOCs). SOM in clouds, fogs, and aerosols undergoes concentration/dilution cycles due to the evaporation/condensation of water droplets. These physical processes could lead to the chemical processing of SOM and the formation of new, light-absorbing compounds. In this study, model SOM was generated through smog chamber photooxidation and flow tube ozonolysis of various atmospherically relevant anthropogenic and biogenic VOCs, including toluene (TOL), D-limonene (LIM), alpha-pinene (APIN), beta-pinene (BPIN), and isoprene (ISO). Collected SOM was extracted in water, and the solutions were acidified with sulfuric acid to pH 2 and dried to simulate the evaporation of acidic particles containing SOM. Significant changes in mass absorption coefficients (MACs) were observed after the evaporation and redissolution of SOM in the presence of sulfuric acid. At visible wavelengths, the MAC values of most SOM increased after the evaporation, with the fractional increase being the largest for LIM/O-3 SOM at 400 nm (fractional increase of 65.0). Exceptions to evaporation increasing MAC values in the presence of sulfuric acid were ISO/OH and TOL/OH/NOx. Light-absorbing species in LIM/O-3 SOM were chromatographically separated and detected using a photodiode array detector and a high-resolution electrospray ionization mass spectrometer. The increase in MAC was accompanied by the appearance of more than 300 organosulfate peaks. Five potential brown carbon (BrC) chromophores in LIM/O-3 SOM were separated and assigned chemical formulas, including C10H16SO6, C10H14SO6, C10H16SO5, C11H16SO7, and C11H18SO8. This study suggests that evaporation-driven processes may occur in the atmosphere, substantially modifying the molecular composition and optical properties of SOM. The evaporation of filter extracts from the field or laboratory could similarly produce organosulfates as artifacts if the extract is sufficiently acidic before the evaporation. We recommend that complete drying of particulate matter filter extracts should be avoided in future work.