Particle Size Dependence and Phase Partitioning of Dimethylamine and Diethylamine in an Urban Atmosphere across Seasons

Alkyl amines are N-containing bases that impact the formation and composition of atmospheric particles despite their trace abundance in the atmosphere. Amine concentrations strongly depend on local sources due to their relatively short atmospheric lifetimes, and given their temperature and pH-dependent aqueous solubility, simultaneous measurements in both gas and particle phases are essential for understanding their atmospheric fate. From November 2022 to May 2024, size-resolved particulate amines were measured across the Greater Toronto Area using a 10-stage micro-orifice uniform deposition impactor (MOUDI) in 51 independent multiday observation periods. For 18 of those periods, a denuder was used to simultaneously collect gaseous ammonia and amines. Dimethylamine (DMA) and diethylamine (DEA) were the only amines consistently measured above the limit of detection by ion chromatography. Concentrations of particulate DMAH⁺ and DEAH⁺ exhibited stronger spatial and temporal variations than ammonium (NH₄⁺). Between 79 and 97% of total diethylamine was in the particle phase, significantly higher than the particulate fraction of dimethylamine (39–95%) and ammonia (NH₃) (2–25%). The phase partitioning of ammonia and amines in the atmosphere predicted by the E-AIM model was reasonably consistent with the observed results. Particulate amine/ammonia ratios were higher in the smallest particles, and on some sampling dates, the DEAH⁺/ DMAH⁺ ratios clearly decreased with size over the 0.18–1 μm range. Estimating the loss rates to gas and aqueous phase oxidation of the amines indicates the importance of investigating their phase partitioning for a better understanding of their atmospheric fates.