An Adiabatic-Expansion-Induced Perturbation Study on Gas-Aerosol Partitioning in Ambient Air – Dimethylamine and Trimethylamine (1)

Eulerian observations of chemical species at fixed positions in a flow field are known to violate conservation laws, while the observations tracking moving air parcels are practically unfeasible. Eulerian observations often cause positive correlations between reactants and products in the atmosphere, which are frequently misinterpreted as evidence of the related chemical conversion. This dilemma has motivated innovative trials. The perturbation technique, widely used in mathematical and physical studies, offers a potential solution. Combining Eulerian observations with perturbation techniques may compensate the weakness, making this approach particularly valuable for studying the gas-aerosol partitioning of semi-volatile particulate species in ambient air. As an example, we examined this combination through an adiabatic-expansion-induced perturbation study on the gas-aerosol partitioning of dimethylamine (DMA) and trimethylamine (TMA) in ambient air. Eulerian observations of chemical species in size-segregated atmospheric particles ranging from 10 μm to 0.056 μm, coupled with downstream adiabatic-expansion-induced perturbation observations, were performed in coastal and marine atmospheres using a commercial sampler (Nano MOUDI-II, MSP, US), followed by offline chemical analysis. The results revealed that particulate DMA generally tended to evaporate in ambient air during the observational periods, while enhanced adiabatic-expansion-induced perturbations occasionally led to the co-formation of DMAHNO3 and NH4NO3. However, gaseous TMA apparently underwent gas-particle condensation to reach equilibrium in ambient air, with adiabatic-expansion-induced perturbation resulting in the formation of non-ionized TMA particulates. Thermodynamic analysis further supported that the observed particulate TMA was primarily determined by the equilibrium of gaseous TMA with non-ionized particulate TMA, rather than ionic TMAH+.