Slopes and intercepts from log-log correlations of gas/particle quotient and octanol-air partition coefficient (vapor-pressure) for semi-volatile organic compounds: I. Theoretical analysis

Gas/particle partitioning governs the transport and fate of semi-volatile organic compounds (SVOCs) released to the atmosphere. The partition quotient of SVOCs, K-P, is related to their subcooled liquid vapor pressure (logK(P) = m(p) logP(L) + b(p)) and to their octanol-air partition coefficient (logK(P) = m(o) logK(OA) + b(o)). Previous theory predicts that -m(p) and m(o) should be close to, or equal to 1 based on the assumption that gas- and particle-phases are at equilibrium in the atmosphere. Here, we develop analytical equations to calculate m(o) and b(o) as functions of logK(OA) and m(p) and b(p) as functions of logP(L). We find that experimental, analytical, or statistical artifacts and other reported factors are not the leading causes for deviations of the slopes, m(p) and m(o), from -1 and 1, respectively. Rather, it is the inherent parameter, K-OA, that determines m(o) and b(o), and equivalently, P-L is the major parameter determining m(p) and b(p), and such deviations are evidence that equilibrium is an inappropriate assumption. In contrast, the actual steady-state between gas and particle phases of SVOCs leads that their -m(p) and m(o) should range from 0 to 1, implying that equilibrium is a reasonable assumption only when -m(p) and m(o) are larger than 0.49. To illustrate these points, we provide a detailed discussion of the global atmospheric transport of polybrominated diphenyl ethers (PBDEs) with emphasis on Polar Regions where low air temperatures favor a special steady-state, where their slopes m(p) and m(o) can reach 0, indicating a constant value of logK(P) (-1.53). (C) 2020 Elsevier Ltd. All rights reserved.

Automated summary

Gas/particle partitioning plays a key role in the transport and fate of semi-volatile organic compounds in the atmosphere, with the equilibrium assumption being challenged by experimental findings. Analytical equations were developed to calculate parameters related to partitioning, revealing that deviations from theoretical predictions cannot be solely explained by experimental or statistical artifacts.