Properties and Behavior of α-Pinene Secondary Organic Aerosol Particles Exposed to Ammonia at Low relative Humidity

Description

Heterogeneous chemistry and aging of secondary organic aerosol (SOA) particles can alter their physical and chemical properties, including particle composition and morphology. Ammonia, one of the most common atmospheric reactive constituents, can react with SOA particles, changing their properties and behavior. Ammonia was shown to readily react with α-pinene SOA at high relative humidity (RH), producing a number of nitrogen-containing organic molecules that contribute to formation of light-absorbing brown carbon. At low RH, NH3 uptake by α-pinene SOA particles appears to be limited to the particle surface, which suggests that the reacted particles might not be homogeneous and have complex morphology. We will present the results of a recent study, in which we characterized the changes in size, composition, morphology, density, viscosity, and evaporation dynamics of size-selected α-pinene SOA particles that were exposed to ammonia at low (<5%) RH. We find that a small amount of NH3 diffuses and reacts throughout the particles bulk, while most of the ammoniated products result from the reaction of NH3 with carboxylic acids on the particle surface, leading to a slight increase in particle size. We show that the reaction products form a semi-volatile, solid coating that is a few nanometers thick. As it forms, this crust limits additional ammonia from diffusing into the particle. The ammoniated products also affect particles evaporation kinetics, more so for the smaller particles that contain higher fraction of ammoniated products and have higher density. Moreover, this solid coating prevents coagulating particles from coalescing for over two days. However, when the gas phase is diluted, this semi-volatile coating evaporates in minutes, which is ensued by rapid coalescence. These findings have important implications for a recently proposed method to determine SOA viscosity based on particle coalescence time. If particles are assumed to be morphologically homogeneous, the observed coalescence time for the SOA particles after the reaction with ammonia yields viscosity greater than 4x1010 Pa∙sec, which is more than two orders of magnitude higher than the viscosity of ~108 Pa∙sec that we and others previously calculated for α-pinene SOA under dry conditions. Our data clearly show that the ammoniated particles have core-shell morphology, with a solid shell that is only 4 nm thick on top of less viscous SOA particle.