pH Dependence of Liquid–Liquid Phase Separation in Organic Aerosol

Abstract

Atmospheric aerosol particles influence climate through their direct and indirect effects. These impacts depend in part on the morphology of the particles, which is determined by their composition. The effect of pH on morphology was investigated using particles composed of 3-methylglutaric acid and ammonium sulfate by manipulating the starting pH of the bulk solution through the addition of aqueous sodium hydroxide. Efflorescence, deliquescence, phase separation, and mixing transitions were observed with optical microscopy. Due to changes in its protonation states, the solubility of the organic component increases with increasing pH, which shifts the location of the separation relative humidity (SRH) from 78.7% for the fully protonated acid to 63.9% for the fully deprotonated acid. Surprisingly, this shift in the SRH leads to hysteresis between the SRH and the mixing relative humidity (MRH). Particle pH has the greatest effect on phase transitions that require nucleus formation, that is, efflorescence and SRH.

Heterogeneous N2O5 Uptake During Winter: Aircraft Measurements During the 2015 WINTER Campaign and Critical Evaluation of Current Parameterizations

“…The change in correlation strength with increasing RH suggests that uptake is limited by the term Γ sol + Γ rxn ( and/or ) at low RH, but becomes limited by a different process, such as α as water availability increases (e.g., Bertram & Thornton, ; Thornton & Abbatt, ). The exact RH associated with this sensitivity change is thought to depend on the deliquescence point of each aerosol type and its propensity to form supersaturated liquids (e.g., Kane et al, ), which can be impacted by aerosol acidity and organics (e.g., Losey et al, ). In addition, several studies have observed the opposite, decreasing trend in γ (N 2 O 5 ) with RH on highly acidic sulfuric acid particles (Fried et al, ; Hallquist et al, ; Hu & Abbatt, ; Kane et al, ; Mozurkewich & Calvert, ), which supports an alternative, acid‐catalyzed mechanism discussed in section 4.2.5.…”

Aerosol Mixing State: Measurements, Modeling, and Impacts

“…Hundreds of studies using electron microscopy and X‐ray spectroscopy methods to investigate aerosol particles have been published over the years, ranging from studies characterizing the constituents of specific types of aerosol, such as marine, mineral, dust, and biomass (Buseck & Pósfai, ; Li et al, ; Pósfai et al, , ) to those focused specifically on characterizing per‐particle composition and morphology (Deboudt et al, ; Pósfai et al, ), phase transitions (Wise et al, ), hygroscopic behavior (Okada, ; Semeniuk et al, ), phase separation (Ault et al, ; Freedman, ; Losey et al, ; O'Brien et al, ; You et al, ), and other important aerosol physicochemical properties that impact mixing state. However, only in the past few years have these methods begun to be used to quantify aerosol mixing state, the examples of which are discussed later in section .…”

The sensitivity of PM<sub>2.5</sub> acidity to meteorological parameters and chemical composition changes: 10-year records from six Canadian monitoring sites

“…Few methods can directly measure the pH of aerosol (Rindelaub et al, 2016;Craig et al, 2018;Wei et al, 2018), so aerosol acidity is usually presented using indirect proxies such as ion balance (cationto-anion (Yao et al, 2006;Du et al, 2010;Zhou et al, 2017). However, these indirect methods can lead to substantial uncertainty in acidity assessment (Hennigan et al, 2015;Guo et al, 2016;Murphy et al, 2017). Calculation of pH through thermodynamic modelling such as E-AIM (Clegg et al, 1998) and ISORROPIA II (Fountoukis and Nenes, 2007), with the input of reliable chemical compositions of the aerosol and at least one semi-volatile gas and meteorological data, has been shown to be a more rigorous approach to calculate the pH of aerosol liquid water.…”

“…Calculation of pH through thermodynamic modelling such as E-AIM (Clegg et al, 1998) and ISORROPIA II (Fountoukis and Nenes, 2007), with the input of reliable chemical compositions of the aerosol and at least one semi-volatile gas and meteorological data, has been shown to be a more rigorous approach to calculate the pH of aerosol liquid water. Murphy et al (2017) and Song et al (2018) both showed that the constraint from phase partitioning of NH 3 /NH + 4 should be included in calculations using aerosol thermodynamic models to get reliable pH calculations, which indicates that NH 3 observation can greatly improve the reliability of the aerosol acidity assessment.…”