Phase considerations in the gas/particle partitioning of organic amines in the atmosphere

Abstract Amines in the atmosphere are of interest because of their likely role in new particle formation, and because of anthropogenic emissions of amines at post-combustion carbon capture (PCCC) facilities. A conceptual framework for considering the partitioning of a monobasic amine (Am = unprotonated, “free-base form”) from the gas phase to atmospheric particulate matter (PM) is presented for cases when the PM may be composed of multiple liquid phases. Three types of liquid phases are considered as being individually or simultaneously possible for absorptive uptake of atmospheric amines: w) a mostly water phase; α) a mostly (by mass) organic phase that has at least some polarity (e.g., predominantly secondary organic aerosol (SOA), may contain significant water on a mole fraction basis); and β) a mostly organic phase that is less polar than an α phase (e.g., predominantly primary organic aerosol (POA), containing little water). That one or more salts may contain the aminium ion AmH+ (formed by protonation of Am) is subject to the fact that the trace levels of individual amines in the atmosphere make formation of a pure solid such as AmHHSO4(s) exceedingly unlikely: when solid salts of AmH+ are indeed present, by far the most likely form is as a solid solution, e.g., ( NH 4 + ) 1 − y ( AmH + ) y HSO 4 ( s ) − where y K p,tot ( m 3 μg − 1 ) = c p,tot / c g = ∑ θ f θ K p,fb θ / α fb θ . The quantity cp,tot (μg μg−1) is the total Am concentration (Am + AmH+) in the PM as summed over all phases using the index θ (= w, α, β); cg is the gas-phase concentration of Am; f θ is the mass fraction of the total PM that is the θ phase; K p,fb θ is the gas/particle partitioning constant for the free-base (Am) form to the θ phase; and 0 α fb θ f w K p,fb w / α fb w . However, unless the PM contains little or no organic-phase material, the α and/or β terms are likely to also be relevant. The Am form of a low MW amine will in general have reasonable affinities for both α and β type phases, so in general K p,fb w , K p,fb α , and K p,fb β will all be roughly similar in magnitude. And, with significant water uptake into an α phase certain to occur at moderate to high RH values, solvation of ions will often be possible in an α phase. This will assist protonation of Am to AmH+ (as is known to occur for nicotine in tobacco smoke PM). The overall result is that to a first approximation, α fb w and α fb α can be similar in magnitude, making K p,fb α / α fb α likely to be generally comparable to K p,fb w / α fb w . In a β phase, ion solvation will not be as good, so that for acidic aerosol α fb β will generally be closer to one than the other two α fb values, making K p,fb β / α fb β smaller than both K p,fb w / α fb w and K p,fb α / α fb α . Overall, modeling of amine behavior in the atmosphere should include consideration of partitioning into organic PM. Unfortunately, this will be more difficult than water-phase only modeling because prediction of α fb values in multiphase PM will be greatly complicated by the needs to: 1) have estimated values of acidity constants in mostly organic phases of variable composition; and 2) allow distribution of chemicals over multiple liquid phases.