Article pubs.acs.org/JPCA
Temperature-Dependent Henry’s Law Constants of Atmospheric Amines Chunbo Leng, J. Duncan Kish, Jason E. Roberts, Iman Dwebi, Nara Chon, and Yong Liu* Department of Chemistry, University of Colorado Denver, Denver, Colorado 80217, United States ABSTRACT: There has been growing interest in understanding atmospheric amines in the gas phase and their mass transfer to the aqueous phase because of their potential roles in cloud chemistry, secondary organic aerosol formation, and the fate of atmospheric organics. Temperature-dependent Henry’s law constants (KH) of atmospheric amines, a key parameter in atmospheric chemical transport models to account for mass transfer, are mostly unavailable. In this work, we investigated gas−liquid equilibria of five prevalent atmospheric amines, namely 1-propylamine, di-n-propylamine, trimethylamine, allylamine, and 4-methylmorpholine using bubble column technique. We reported effective KH, intrinsic KH, and gas phase diffusion coefficients of these species over a range of temperatures relevant to the lower atmosphere for the first time. The measured KH at 298 K and enthalpy of solution for 1-propylamine, di-n-propylamine, trimethylamine, allylamine, and 4-methylmorpholine are 61.4 ± 4.9 mol L−1 atm−1 and −49.0 ± 4.8 kJ mol−1; 14.5 ± 1.2 mol L−1 atm−1 and −72.5 ± 6.8 kJ mol−1; 8.9 ± 0.7 mol L−1 atm−1 and −49.6 ± 4.7 kJ mol−1; 103.5 ± 10.4 mol L−1 atm−1 and −42.7 ± 4.3 kJ mol−1; and 952.2 ± 114.3 mol L−1 atm−1 and −82.7 ± 9.7 kJ mol−1, respectively. In addition, we evaluated amines’ characteristic times to achieve gas−liquid equilibrium for partitioning between gas and aqueous phases. Results show gas−liquid equilibrium can be rapidly established at natural cloud droplets surface, but the characteristic times may be extended substantially at lower temperatures and pHs. Moreover, our findings imply that atmospheric amines are more likely to exist in cloud droplets, and ambient temperature, water content, and pH of aerosols play important roles in their partitioning.
1. INTRODUCTION Henry’s Law, which gives the relationship between solubility of a gas and its partial pressure above the liquid, plays a key role in controlling gas/liquid partitioning of a great number of atmospherically relevant organic compounds. As such, it impacts their participation in aerosol and cloud formation processes and in turn their contribution to various environmental issues including air pollution, climate change, and public health. There are various forms of Henry’s law constants; a commonly used form KH, also denoted as KH,cp, is defined as Caq/p (mol L−1 atm−1), where p is the partial pressure of the gaseous solute above the solution, and Caq is the concentration of the dissolved gas in the solution. Unfortunately, the current collection of Henry’s law constants (KH) available from direct measurements is far less than what are required to develop detailed chemical transport models. Moreover, temperaturedependent KH values of atmospherically relevant organic compounds are largely unavailable and they can only be estimated in atmospheric models.1,2 Because accurate knowledge of physical and thermodynamic properties is imperative to atmospheric simulation and air quality prediction, it becomes evident that there is a scientific need for the direct measurements of temperature-dependent KH values of atmospherically relevant organic compounds. Atmospheric amines are emitted from a variety of anthropogenic (automobiles, industries, combustion, animal husbandry, cooking, tobacco smoke, treatment of sewage and © 2015 American Chemical Society
waste, and CO2 capture), biogenic (ocean, biomass burning, and vegetation), and geologic sources.3 While in the atmosphere, amines can undergo various physical, chemical, and photochemical processes. For example, atmospheric amines may interact with atmospheric oxidants (OH, O3, NO3, and Cl etc.) to yield nitrogen-containing organic compounds,3 which can be condensed on preexisting aerosol particles or partition into cloudwater. They can further participate in heterogeneous and/or cloud chemistry with acidic or carbonyl species to form secondary organic aerosol (SOA) and “brown carbon”.3,4 Similar to ammonia, atmospheric amines are bases, some of which are actually more basic than ammonia due to the substitution by organic functional groups. Such unique acidneutralizing capacity allows atmospheric amines to participate in acid−base reaction in condensed phase and facilitate new aerosol particle formation by making strong hydrogen-bonded complexes with acids and stabilizing freshly nucleated nanoparticles.5,6 Comprehensive accounts of atmospheric sources, multiphase transformations, gas-to-particle conversion, surface deposition processes, and health effects of atmospheric amines can be found in recent review articles.3,4,7,8 Authors of these review articles agree that atmospheric amines play important roles in many environmental processes, however the extent to Received: May 30, 2015 Revised: July 4, 2015 Published: July 22, 2015 8884
DOI: 10.1021/acs.jpca.5b05174 J. Phys. Chem. A 2015, 119, 8884−8891
Article
The Journal of Physical Chemistry A
much smaller amount of 50 mL of water was used for 4methylmorphline to make equilibrium time more manageable. After 30−90 min equilibrium time, the pH of the dissolved amine solution in the second column was measured by a pH meter (Orion Star A111, Thermo Scientific). Then, two threeway valves were used to direct the carry gas (N2) either passing through the first column or bypass it. When the three-way valves were in the bypass position, amine solution was purged by N2 at various flow rates of 300−1000 mL min−1. Concentrations of amines in the headspace were monitored as a function of time, and mass transfer rate of amines from liquid phase to gaseous phase were utilized to derive Henry’s law constants based on the following equation13−15
which atmospheric amines impact our environment is still highly uncertain. This is largely due to the lack of thermodynamic and kinetic properties of atmospheric amines; as a result, they are usually not incorporated into atmospheric chemical transport models. As shown in Ge et al.’s recent review,7 only 65 out of 187 amine compounds have KH values reported at or near room temperature. Moreover, temperature dependence of KH values is available only for six amines. Because knowledge of KH is a prerequisite for an accurate representation of atmospheric amines and their behaviors, direct measurements of atmospheric amines’ temperaturedependent KH are much needed in order to understand their roles of in aerosol formation, and subsequently in atmospheric environment and climate system. In addition to the temperature effect, other factors can also influence KH. As atmospheric amines are weak bases, which can undergo aqueous dissociation equilibrium to form aminium ions, pH of the droplets is expected to influence their gas/liquid equilibrium. The pH of atmospheric aerosols can have a wide variation from slightly basic (pH ≈ 8) in fresh sea salt aerosol to very acidic (
