Electrospray surface-enhanced Raman spectroscopy (ES-SERS) for studying organic coatings of atmospheric aerosol particles

Heterogeneous reactions between atmospheric aerosol particles and gaseous pollutants, such as those forming brown carbon (BrC), represent an important mechanism. These reactions alter the particle chemical compositions and aerosol-climate interactions. While most studies assume homogeneous particle compositions, organic coatings can be formed on solid or highly viscous particles due to heterogeneous reactions but the underlying mechanism is relatively less examined. We used electrospray surface-enhanced Raman spectroscopy (ES-SERS) to directly probe the formation of BrC coatings on methylaminium sulfate, nitrate, and chloride particles from heterogeneous reactions with gas-phase glyoxal. To create BrC coatings on particle surfaces, heterogeneous reactions were performed under low relative humidity (RH) conditions (i.e., 10 or 30% RH). The reacted particles fluoresced when irradiated at 532 nm in normal Raman analysis, indirectly suggesting the presence of light-absorbing species in them. Further ES-SERS analyses showed Raman bands of 1,3-dimethylimidazole, one of the major known products of reactions of glyoxal with methylaminium, from all the reacted particles at 30% RH. However, only methylaminium sulfate particles showed the formation of BrC coatings at 10% RH. We speculate that methylaminium sulfate particles may have more surface adsorbed water (SAW) than the other particle samples to initiate the formation of BrC coatings detectable by ES-SERS. The present study highlights the surface sensitivity of ES-SERS as well as the potential importance of SAW in heterogeneous reactions of atmospheric particles with gaseous pollutants.