Reactive Uptake of Ammonia and Formation of Organic Nitrogen Species for Non-Liquid/Liquid Secondary Organic Material

Abstract:

Formation of ammonium and organic nitrogen (ON) species was studied for secondary organic material (SOM) of variable viscosity, ranging from non-liquid to liquid physical states. The SOM was produced as particles of 50 to 150 nm in diameter in aerosol form from six precursors, including three terpenoid and three aromatic species. The viscosity of the hygroscopic SOM was adjusted by exposure to relative humidity (RH) from <5% to >90% RH in steps of 10% at 293 ± 2 K. The aerosol was subsequently exposed to 5 ppm NH₃ for mean reaction times of 30, 370, or 5230 s. Ammonium and ON were characterized by high-resolution time-of-flight aerosol mass spectrometry (HR-ToF-AMS). The ammonium-to-organic ratio of mass concentrations (M_NH4/M_Org) in the particles increased monotonically from <5% RH to a limiting value at a threshold RH, implicating a switchover in the reaction kinetics from a system limited by diffusivity within the SOM for low RH to one limited by other factors, such as saturated uptake, at higher RH. Formation of ON was observed for aromatic-derived SOMs, but not significant for terpenoid-derived SOMs. For aromatic-derived SOMs, the ON-to-organic ratio of mass concentrations (M_ON/M_Org) was negligible for RH <20%, increased monotonically from 20% to 60% RH, and stayed constant for RH >60%. The threshold RH for the switchover from kinetically controlled regime to a non-kinetically-controlled one was thus different between formation of ammonium and ON. This difference suggests that water may play a role in the slow reactions of ON formation as a reactant or a catalyst, in addition to affecting the reactant diffusion as in the fast reaction of ammonium formation. The implication is that formation of ammonium salts and organic nitrogen species by certain SOMs should be treated separately in chemical transport models to reflect the different roles of water that may affect the phase state of the SOMs or may act as a reactant or a catalyst.