Model Evaluation of New Techniques for Maintaining High-NO Conditions in Oxidation Flow Reactors for the Study of OH-Initiated Atmospheric Chemistry

Oxidation flow reactors (OFRs) efficiently produce OH radicals using low-pressure Hg-lamp emissions at λ = 254 nm (OFR254) or both λ = 185 and 254 nm (OFR185). OFRs under most conditions are limited to studying low-NO chemistry (where RO₂ + HO₂ dominates RO₂ fate), even though substantial amounts of initial NO may be injected. This is due to very fast NO oxidation by high concentrations of OH, HO₂, and O₃. In this study, we model new techniques for maintaining high-NO conditions in OFRs, that is, continuous NO addition along the length of the reactor in OFR185 (OFR185-cNO), recently proposed injection of N₂O at the entrance of the reactor in OFR254 (OFR254-iN₂O), and an extension of that idea to OFR185 (OFR185-iN₂O). For these techniques, we evaluate (1) fraction of conditions dominated by RO₂ + NO while avoiding significant nontropospheric photolysis and (2) fraction of conditions where reactions of precursors with OH dominate over unwanted reactions with NO₃. OFR185-iN₂O is the most practical for general high-NO experiments because it represents the best compromise between experimental complexity and performance upon proper usage. Short lamp distances are recommended for OFR185-iN₂O to ensure a relatively uniform radiation field. OFR185-iN₂O with low O₂ or using Hg lamps with higher 185 nm-to-254 nm ratio can improve performance. OFR185-iN₂O experiments should generally be conducted at higher relative humidity, higher UV, lower concentration of non-NO_<i>y</i> external OH reactants, and percent-level N₂O. OFR185-cNO and OFR185-iN₂O at optimal NO precursor injection rate (∼2 ppb/s) or concentration (∼3%) would have satisfactory performance in typical field studies where ambient air is oxidized. Exposure estimation equations are provided to aid experimental planning. This work enables improved high-NO OFR experimental design and interpretation

Model Evaluation of New Techniques for Maintaining High-NO Conditions in Oxidation Flow Reactors for the Study of OH-Initiated Atmospheric Chemistry

Oxidation flow reactors (OFRs) efficiently produce OH radicals using low-pressure Hg-lamp emissions at λ = 254 nm (OFR254) or both λ = 185 and 254 nm (OFR185). OFRs under most conditions are limited to studying low-NO chemistry (where RO₂ + HO₂ dominates RO₂ fate), even though substantial amounts of initial NO may be injected. This is due to very fast NO oxidation by high concentrations of OH, HO₂, and O₃. In this study, we model new techniques for maintaining high-NO conditions in OFRs, that is, continuous NO addition along the length of the reactor in OFR185 (OFR185-cNO), recently proposed injection of N₂O at the entrance of the reactor in OFR254 (OFR254-iN₂O), and an extension of that idea to OFR185 (OFR185-iN₂O). For these techniques, we evaluate (1) fraction of conditions dominated by RO₂ + NO while avoiding significant nontropospheric photolysis and (2) fraction of conditions where reactions of precursors with OH dominate over unwanted reactions with NO₃. OFR185-iN₂O is the most practical for general high-NO experiments because it represents the best compromise between experimental complexity and performance upon proper usage. Short lamp distances are recommended for OFR185-iN₂O to ensure a relatively uniform radiation field. OFR185-iN₂O with low O₂ or using Hg lamps with higher 185 nm-to-254 nm ratio can improve performance. OFR185-iN₂O experiments should generally be conducted at higher relative humidity, higher UV, lower concentration of non-NO_<i>y</i> external OH reactants, and percent-level N₂O. OFR185-cNO and OFR185-iN₂O at optimal NO precursor injection rate (∼2 ppb/s) or concentration (∼3%) would have satisfactory performance in typical field studies where ambient air is oxidized. Exposure estimation equations are provided to aid experimental planning. This work enables improved high-NO OFR experimental design and interpretation

Model Evaluation of New Techniques for Maintaining High-NO Conditions in Oxidation Flow Reactors for the Study of OH-Initiated Atmospheric Chemistry

Oxidation flow reactors (OFRs) efficiently produce OH radicals using low-pressure Hg-lamp emissions at λ = 254 nm (OFR254) or both λ = 185 and 254 nm (OFR185). OFRs under most conditions are limited to studying low-NO chemistry (where RO₂ + HO₂ dominates RO₂ fate), even though substantial amounts of initial NO may be injected. This is due to very fast NO oxidation by high concentrations of OH, HO₂, and O₃. In this study, we model new techniques for maintaining high-NO conditions in OFRs, that is, continuous NO addition along the length of the reactor in OFR185 (OFR185-cNO), recently proposed injection of N₂O at the entrance of the reactor in OFR254 (OFR254-iN₂O), and an extension of that idea to OFR185 (OFR185-iN₂O). For these techniques, we evaluate (1) fraction of conditions dominated by RO₂ + NO while avoiding significant nontropospheric photolysis and (2) fraction of conditions where reactions of precursors with OH dominate over unwanted reactions with NO₃. OFR185-iN₂O is the most practical for general high-NO experiments because it represents the best compromise between experimental complexity and performance upon proper usage. Short lamp distances are recommended for OFR185-iN₂O to ensure a relatively uniform radiation field. OFR185-iN₂O with low O₂ or using Hg lamps with higher 185 nm-to-254 nm ratio can improve performance. OFR185-iN₂O experiments should generally be conducted at higher relative humidity, higher UV, lower concentration of non-NO_<i>y</i> external OH reactants, and percent-level N₂O. OFR185-cNO and OFR185-iN₂O at optimal NO precursor injection rate (∼2 ppb/s) or concentration (∼3%) would have satisfactory performance in typical field studies where ambient air is oxidized. Exposure estimation equations are provided to aid experimental planning. This work enables improved high-NO OFR experimental design and interpretation

Solvents Effects on Charge Transfer from Quantum Dots

To predict and understand the performance of nanodevices in different environments, the influence of the solvent must be explicitly understood. In this Communication, this important but largely unexplored question is addressed through a comparison of quantum dot charge transfer processes occurring in both liquid phase and in vacuum. By comparing solution phase transient absorption spectroscopy and gas-phase photoelectron spectroscopy, we show that hexane, a common nonpolar solvent for quantum dots, has negligible influence on charge transfer dynamics. Our experimental results, supported by insights from theory, indicate that the reorganization energy of nonpolar solvents plays a minimal role in the energy landscape of charge transfer in quantum dot devices. Thus, this study demonstrates that measurements conducted in nonpolar solvents can indeed provide insight into nanodevice performance in a wide variety of environments

Laser Ablation-Aerosol Mass Spectrometry-Chemical Ionization Mass Spectrometry for Ambient Surface Imaging

Mass spectrometry imaging is becoming an increasingly common analytical technique due to its ability to provide spatially resolved chemical information. Here, we report a novel imaging approach combining laser ablation with two mass spectrometric techniques, aerosol mass spectrometry and chemical ionization mass spectrometry, separately and in parallel. Both mass spectrometric methods provide the fast response, rapid data acquisition, low detection limits, and high-resolution peak separation desirable for imaging complex samples. Additionally, the two techniques provide complementary information with aerosol mass spectrometry providing near universal detection of all aerosol molecules and chemical ionization mass spectrometry with a heated inlet providing molecular-level detail of both gases and aerosols. The two techniques operate with atmospheric pressure interfaces and require no matrix addition for ionization, allowing for samples to be investigated in their native state under ambient pressure conditions. We demonstrate the ability of laser ablation-aerosol mass spectrometry-chemical ionization mass spectrometry (LA-AMS-CIMS) to create 2D images of both standard compounds and complex mixtures. The results suggest that LA-AMS-CIMS, particularly when combined with advanced data analysis methods, could have broad applications in mass spectrometry imaging applications

Modeling the Radical Chemistry in an Oxidation Flow Reactor: Radical Formation and Recycling, Sensitivities, and the OH Exposure Estimation Equation

Oxidation flow reactors (OFRs) containing low-pressure mercury (Hg) lamps that emit UV light at both 185 and 254 nm (“OFR185”) to generate OH radicals and O₃ are used in many areas of atmospheric science and in pollution control devices. The widely used potential aerosol mass (PAM) OFR was designed for studies on the formation and oxidation of secondary organic aerosols (SOA), allowing for a wide range of oxidant exposures and short experiment duration with reduced wall loss effects. Although fundamental photochemical and kinetic data applicable to these reactors are available, the radical chemistry and its sensitivities have not been modeled in detail before; thus, experimental verification of our understanding of this chemistry has been very limited. To better understand the chemistry in the OFR185, a model has been developed to simulate the formation, recycling, and destruction of radicals and to allow the quantification of OH exposure (OH_exp) in the reactor and its sensitivities. The model outputs of OH_exp were evaluated against laboratory calibration experiments by estimating OH_exp from trace gas removal and were shown to agree within a factor of 2. A sensitivity study was performed to characterize the dependence of the OH_exp, HO₂/OH ratio, and O₃ and H₂O₂ output concentrations on reactor parameters. OH_exp is strongly affected by the UV photon flux, absolute humidity, reactor residence time, and the OH reactivity (OHR) of the sampled air, and more weakly by pressure and temperature. OH_exp can be strongly suppressed by high OHR, especially under low UV light conditions. A OH_exp estimation equation as a function of easily measurable quantities was shown to reproduce model results within 10% (average absolute value of the relative errors) over the whole operating range of the reactor. OH_exp from the estimation equation was compared with measurements in several field campaigns and shows agreement within a factor of 3. The improved understanding of the OFR185 and quantification of OH_exp resulting from this work further establish the usefulness of such reactors for research studies, especially where quantifying the oxidation exposure is important

Materials Properties and Solvated Electron Dynamics of Isolated Nanoparticles and Nanodroplets Probed with Ultrafast Extreme Ultraviolet Beams

We present ultrafast photoemission measurements of isolated nanoparticles in vacuum using extreme ultraviolet (EUV) light produced through high harmonic generation. Surface-selective static EUV photoemission measurements were performed on nanoparticles with a wide array of compositions, ranging from ionic crystals to nanodroplets of organic material. We find that the total photoelectron yield varies greatly with nanoparticle composition and provides insight into material properties such as the electron mean free path and effective mass. Additionally, we conduct time-resolved photoelectron yield measurements of isolated oleylamine nanodroplets, observing that EUV photons can create solvated electrons in liquid nanodroplets. Using photoemission from a time-delayed 790 nm pulse, we observe that a solvated electron is produced in an excited state and subsequently relaxes to its ground state with a lifetime of 151 ± 31 fs. This work demonstrates that femotosecond EUV photoemission is a versatile surface-sensitive probe of the properties and ultrafast dynamics of isolated nanoparticles

Mapping Nanoscale Absorption of Femtosecond Laser Pulses Using Plasma Explosion Imaging

We make direct observations of localized light absorption in a single nanostructure irradiated by a strong femtosecond laser field, by developing and applying a technique that we refer to as plasma explosion imaging. By imaging the photoion momentum distribution resulting from plasma formation in a laser-irradiated nanostructure, we map the spatial location of the highly localized plasma and thereby image the nanoscale light absorption. Our method probes individual, isolated nanoparticles in vacuum, which allows us to observe how small variations in the composition, shape, and orientation of the nanostructures lead to vastly different light absorption. Here, we study four different nanoparticle samples with overall dimensions of ∼100 nm and find that each sample exhibits distinct light absorption mechanisms despite their similar size. Specifically, we observe subwavelength focusing in single NaCl crystals, symmetric absorption in TiO₂ aggregates, surface enhancement in dielectric particles containing a single gold nanoparticle, and interparticle hot spots in dielectric particles containing multiple smaller gold nanoparticles. These observations demonstrate how plasma explosion imaging directly reveals the diverse ways in which nanoparticles respond to strong laser fields, a process that is notoriously challenging to model because of the rapid evolution of materials properties that takes place on the femtosecond time scale as a solid nanostructure is transformed into a dense plasma

Impact of Thermal Decomposition on Thermal Desorption Instruments: Advantage of Thermogram Analysis for Quantifying Volatility Distributions of Organic Species

We present results from a high-resolution chemical ionization time-of-flight mass spectrometer (HRToF-CIMS), operated with two different thermal desorption inlets, designed to characterize the gas and aerosol composition. Data from two field campaigns at forested sites are shown. Particle volatility distributions are estimated using three different methods: thermograms, elemental formulas, and measured partitioning. Thermogram-based results are consistent with those from an aerosol mass spectrometer (AMS) with a thermal denuder, implying that thermal desorption is reproducible across very different experimental setups. Estimated volatilities from the detected elemental formulas are much higher than from thermograms since many of the detected species are thermal decomposition products rather than actual SOA molecules. We show that up to 65% of citric acid decomposes substantially in the FIGAERO–CIMS, with ∼20% of its mass detected as gas-phase CO₂, CO, and H₂O. Once thermal decomposition effects on the detected formulas are taken into account, formula-derived volatilities can be reconciled with the thermogram method. The volatility distribution estimated from partitioning measurements is very narrow, likely due to signal-to-noise limits in the measurements. Our findings indicate that many commonly used thermal desorption methods might lead to inaccurate results when estimating volatilities from observed ion formulas found in SOA. The volatility distributions from the thermogram method are likely the closest to the real distributions