Atmospheric particulate matter characterization by Fourier transform infrared spectroscopy: a review of statistical calibration strategies for carbonaceous aerosol quantification in US measurement networks

Atmospheric particulate matter (PM) is a complex mixture of many different substances and requires a suite of instruments for chemical characterization. Fourier transform infrared (FT-IR) spectroscopy is a technique that can provide quantification of multiple species provided that accurate calibration models can be constructed to interpret the acquired spectra. In this capacity, FT-IR spectroscopy has enjoyed a long history in monitoring gas-phase constituents in the atmosphere and in stack emissions. However, application to PM poses a different set of challenges as the condensed-phase spectrum has broad, overlapping absorption peaks and contributions of scattering to the mid-infrared spectrum. Past approaches have used laboratory standards to build calibration models for prediction of inorganic substances or organic functional groups and predict their concentration in atmospheric PM mixtures by extrapolation. In this work, we review recent studies pursuing an alternate strategy, which is to build statistical calibration models for mid-IR spectra of PM using collocated ambient measurements. Focusing on calibrations with organic carbon (OC) and elemental carbon (EC) reported from thermal-optical reflectance (TOR), this synthesis serves to consolidate our knowledge for extending FT-IR spectroscopy to provide TOR-equivalent OC and EC measurements to new PM samples when TOR measurements are not available. We summarize methods for model specification, calibration sample selection, and model evaluation for these substances at several sites in two US national monitoring networks: seven sites in the Interagency Monitoring of Protected Visual Environments (IMPROVE) network for the year 2011 and 10 sites in the Chemical Speciation Network (CSN) for the year 2013. We then describe application of the model in an operational context for the IMPROVE network for samples collected in 2013 at six of the same sites as in 2011 and 11 additional sites. In addition to extending the evaluation to samples from a different year and different sites, we describe strategies for error anticipation due to precision and biases from the calibration model to assess model applicability for new spectra a priori. We conclude with a discussion regarding past work and future strategies for recalibration. In addition to targeting numerical accuracy, we encourage model interpretation to facilitate understanding of the underlying structural composition related to operationally defined quantities of TOR OC and EC from the vibrational modes in mid-IR deemed most informative for calibration. The paper is structured such that the life cycle of a statistical calibration model for FT-IR spectroscopy can be envisioned for any substance with IR-active vibrational modes, and more generally for instruments requiring ambient calibrations

Water Activity from Equilibrium Molecular Dynamics Simulations and Kirkwood-Buff Theory

Water activity and related thermodynamic properties are calculated for several aqueous solutions using equilibrium molecular dynamics in conjunction with the recent extension of the Kirkwood-Buff (KB) theory for closed systems. The general applicability of this method is evaluated on aqueous mixtures of ethanol, glyoxal, malonic acid, and NaCl, which represent different types of condensed-phase interactions. Solution microstructures are analyzed using KB integrals and cluster analysis to identify molecular associations due to hydrophobic interactions, hydrate formation, hydrogen bonding, or electrostatic forces affecting solution nonideality in the different systems. Activity estimation by this implementation-subvolume-KB molecular dynamics, or SKBMD, simulation-agrees well with experimental measurements and UNIFAC calculations over a wide range of nonideality, with the exception of the malonic acid/water system. Systematic deviations for this system are attributed to the deficiency of the standard OPLS force field, and are partially remediated with a Non-Bonded FIX (NBFIX) correction to reduce its extensive hydrogen-bonded clustering. Comparison of water and solute excess chemical potentials against other molecular simulation techniques for NaCl/water mixtures shows the SKBMD method to be competitive in performance with those requiring additional external constraints or computational complexity. Equilibrium molecular dynamics and KB theory can therefore be suitable for estimation of solution properties and testing the suitability of force fields, though strongly associating components leading to large and long-lived molecular clusters (either in reality or as a result of a bias in atom-atom potentials) can lead to inefficient sampling and higher estimation errors

Molecular Structure Inhibiting Synergism in Charged Surfactant Mixtures: An Atomistic Molecular Dynamics Simulation Study

Synergistic and nonsynergistic surfactant-water mixtures of sodium dodecyl sulfate (SDS), lauryl betaine (C12B), and cocoamidopropyl betaine (CAPB) systems are studied using molecular simulation to understand the role of interactions among headgroups, tailgroups, and water on, structural and thermodynamic properties at the air water interface. SDS is an anionic surfactant, while C12B and CAPB are zwitterionic; CAPB differs from C12B by an amide group in the tail. While the lowest surface tensions at high surface concentrations in the SDS C12B synergistic system could not be reproduced by simulation, estimated partitioning between surface and bulk shows trends consistent with synergism. Structural analysis shows the influence of the SDS headgroup pulling C12B to the surface, resulting in closely packed structures compared to their respective homomolecular-surfactant systems. The SDS CAPB system, on the other hand, is nonsynergistic when the surfactants are mixed on account of the tilted structure of the CAPB tail. The translational excess entropy due to the tailgroup interactions discriminates between the synergistic and nonsynergistic systems. The implications of such interactions on surfactant effects in complex, multicomponent atmospheric aerosols are discussed

Molecular simulations of interfacial systems: challenges, applications and future perspectives

none6sinoneLbadaoui-Darvas, Mária; Garberoglio, Giovanni; Karadima, Katerina S.; Cordeiro, M. Natália D. S.; Nenes, Athanasios; Takahama, SatoshiLbadaoui-Darvas, Mária; Garberoglio, Giovanni; Karadima, Katerina S.; Cordeiro, M. Natália D. S.; Nenes, Athanasios; Takahama, Satosh