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

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, freebase 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; alpha) 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 beta) a mostly organic phase that is less polar than an alpha 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., (NH4+)(1-y) (AmH+)(y)HSO4(s)- where y << 1. Neglecting dissolution in solid salts, and considering only partitioning to liquid phases, the overall gas/particle partitioning constant is K-p,K-tot(m(3) mu g(-1)) = c(p,tot)/c(g) = Sigma(theta)f(theta)K(p,fb)(theta)/alpha(theta)(fb). The quantity c(p,tot) (mu g mu g(-1)) is the total Am concentration (Am + AmH+) in the PM as summed over all phases using the index theta (= w, alpha, beta); c(g) is the gas-phase concentration of Am; f(theta) is the mass fraction of the total PM that is the theta phase; K-p,fb(theta) is the gas/particle partitioning constant for the free-base (Am) form to the theta phase; and theta < alpha(theta)(fb) < 1 is the fraction of the amine in the theta phase that is in the free-base form. To date, most treatments of the partitioning of amines to PM have only considered contributions to K-p,K-tot from absorption into a mostly water phase, according to the term f(w)K(p,fb)(w)/alpha(w)(fb). However, unless the PM contains little or no organic-phase material, the alpha and/or beta terms are likely to also be relevant. The Am form of a low MW amine will in general have reasonable affinities for both alpha and beta type phases, so in general K-p,fb(w), K-p,fb(alpha) and K-p,fb(beta) will all be roughly similar in magnitude. And, with significant water uptake into an alpha phase certain to occur at moderate to high RH values, solvation of ions will often be possible in an alpha 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, alpha(w)(fb) and alpha(alpha)(fb) can be similar in magnitude, making K-p,fb(alpha)/alpha(alpha)(fb) likely to be generally comparable to K-p,fb(w)/alpha(w)(fb). In a beta phase, ion solvation will not be as good, so that for acidic aerosol alpha(beta)(fb) will generally be closer to one than the other two alpha(fb) values, making K-p,fb(beta)/alpha(beta)(fb) smaller than both K-p,fb(w)/alpha(w)(fb) and K-p,fb(alpha)/alpha(alpha)(fb) 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 ail, values in multiphase alpha(fb) 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. (C) 2015 Elsevier Ltd. All rights reserved.