The Reactivity of Toluene-Derived Secondary Organic Material with Ammonia and the Influence of Water Vapor

The atmospheric reactions of secondary organic material (SOM) with gaseous reactants alter its composition and properties, which can further impact the Earth system. To investigate how water content and precursor affect the reactivity of SOM, the reaction between toluene-derived SOM and ammonia for variable relative humidity (RH) was investigated. A Fourier transform infrared spectrometer was used to monitor the absorbance change of the functional groups as a function of exposure time. There was a fast response to water vapor compared with a gradual spectral variation associated with ammonia uptake. When RH is higher than 25 ± 5%, the spectral changes across 1500–1900 cm^–1 showed a decreasing trend for carboxylic acids and an increasing trend for carboxylates, suggesting a neutralization reaction by ammonia uptake. The observed increasing trend for the region of 1270–1360 cm^–1 might be associated with amines and suggests the formation of organonitrogen compounds for the toluene-derived SOM aging by ammonia at high RH. The corresponding intensity change of C–O groups (1000–1260 cm^–1) with the increased liquid water content as RH increases at the first 6 min suggested that the possible chemical reactions, such as hydrolysis of acetals and hemiacetals to aldehydes and alcohols or esters to carboxylic acids and alcohols, might change the diffusivity of particles and affect the ammonia uptake. The threshold point of ammonia uptake at 30% RH was consistent with a more significant absorbance change of liquid water content and C–O groups at RH ≥ 35 ± 5%. For comparison between anthropogenic and biogenic precursor gases, an isoprene-derived SOM film was also studied. It was more volatile and reactive to ammonia than the toluene-derived SOM. This result implies that the diffusion of ammonia was faster inside isoprene-derived SOM. Overall, the chemical reactions of SOM particles during their atmospheric residence time are precursor- and RH-dependent, which may alter the current understanding of their impact on the Earth system.