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Photochemistry of multifunctional aromatics like vanillin can depend strongly on wavelength, initial concentration, and ionic strength. Using several narrow-band LEDs we present wavelength resolved quantum yields for loss of vanillin. 

Atmospheric oxidation of monoterpenes (C₁₀H₁₆) contributes to ambient particle number and mass concentrations due, in part, to the resulting peroxy radicals undergoing auto-oxidation to low-volatility highly oxygenated molecules (HOMs). 

Abstract. Highly oxygenated organic molecules (HOMs) from α-pinene ozonolysis have been shown to be significant contributors to secondary organic aerosol (SOA), yet our mechanistic understanding of how the peroxy-radical-driven autoxidation leads to their formation in this system is still limited. The involved isomerisation reactions such as H-atom abstractions followed by O2 additions can take place on sub-second timescales in short-lived intermediates, making the process challenging to study. Similarly, while the end-products and sometimes radical intermediates can be observed using mass spectrometry, their structures remain elusive. Therefore, we propose a method utilising selective deuterations for unveiling the mechanisms of autoxidation, where the HOM products can be used to infer which C atoms have taken part in the isomerisation reactions. This relies on the fact that if a C−D bond is broken due to an abstraction by a peroxy group forming a −OOD hydroperoxide, the D atom will become labile and able to be exchanged with a hydrogen atom in water vapour (H2O), effectively leading to loss of the D atom from the molecule. In this study, we test the applicability of this method using three differently deuterated versions of α-pinene with the newly developed chemical ionisation Orbitrap (CI-Orbitrap) mass spectrometer to inspect the oxidation products. The high mass-resolving power of the Orbitrap is critical, as it allows the unambiguous separation of molecules with a D atom (mD=2.0141) from those with two H atoms (mH2=2.0157). We found that the method worked well, and we could deduce that two of the three tested compounds had lost D atoms during oxidation, suggesting that those deuterated positions were actively involved in the autoxidation process. Surprisingly, the deuterations were not observed to decrease HOM molar yields, as would have been expected due to kinetic isotope effects. This may be an indication that the relevant H (or D) abstractions were fast enough that no competing pathways were of relevance despite slower abstraction rates of the D atom. We show that selective deuteration can be a very useful method for studying autoxidation on a molecular level and likely is not limited to the system of α-pinene ozonolysis tested here. 

The main nucleating vapor in the atmosphere is thought to be sulfuric acid (H₂SO₄), stabilized by ammonia (NH₃). However, in marine and polar regions, NH₃is generally low, and H₂SO₄is frequently found together with iodine oxoacids [HIO_x, i.e., iodic acid (HIO₃) and iodous acid (HIO₂)]. In experiments performed with the CERN CLOUD (Cosmics Leaving OUtdoor Droplets) chamber, we investigated the interplay of H₂SO₄and HIO_xduring atmospheric particle nucleation. We found that HIO_xgreatly enhances H₂SO₄(-NH₃) nucleation through two different interactions. First, HIO₃strongly binds with H₂SO₄in charged clusters so they drive particle nucleation synergistically. Second, HIO₂substitutes for NH₃, forming strongly bound H₂SO₄-HIO₂acid-base pairs in molecular clusters. Global observations imply that HIO_xis enhancing H₂SO₄(-NH₃) nucleation rates 10- to 10,000-fold in marine and polar regions. 

Abstract. Iodine species are important in the marine atmosphere foroxidation and new-particle formation. Understanding iodine chemistry andiodine new-particle formation requires high time resolution, highsensitivity, and simultaneous measurements of many iodine species. Here, wedescribe the application of a bromide chemical ionization mass spectrometer(Br-CIMS) to this task. During the iodine oxidation experiments in theCosmics Leaving OUtdoor Droplets (CLOUD) chamber, we have measured gas-phaseiodine species and sulfuric acid using two Br-CIMS, one coupled to aMulti-scheme chemical IONization inlet (Br-MION-CIMS) and the other to aFilter Inlet for Gasses and AEROsols inlet (Br-FIGAERO-CIMS). From offlinecalibrations and intercomparisons with other instruments, we havequantified the sensitivities of the Br-MION-CIMS to HOI, I2, andH2SO4 and obtained detection limits of 5.8 × 106,3.8 × 105, and 2.0 × 105 molec. cm−3,respectively, for a 2 min integration time. From binding energycalculations, we estimate the detection limit for HIO3 to be1.2 × 105 molec. cm−3, based on an assumption of maximumsensitivity. Detection limits in the Br-FIGAERO-CIMS are around 1 order ofmagnitude higher than those in the Br-MION-CIMS; for example, the detectionlimits for HOI and HIO3 are 3.3 × 107 and 5.1 × 106 molec. cm−3, respectively. Our comparisons of the performanceof the MION inlet and the FIGAERO inlet show that bromide chemicalionization mass spectrometers using either atmospheric pressure or reducedpressure interfaces are well-matched to measuring iodine species andsulfuric acid in marine environments.