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  1. Isoprene (C5H8) is the largest non-methane volatile organic compound emitted into the atmosphere. Isoprene reacts rapidly with ambient hydroxyl radicals (OH) and subsequent addition of O2 results in the formation alkyl peroxy (RO2) radicals. The fate of the initially formed RO2 radicals has been the focus of continuing theoretical and experimental research. Under pristine conditions where bimolecular reactions of RO2 are limited, the thermodynamically favored RO2 undergoes an intramolecular H-shift followed by reaction with O2 and elimination of HO2 to yield 4-hydroperoxy aldehyde (4-HPALD, C5H8O3), predicted to account for up to 13% of first-generation isoprene photochemical oxidation products. Mass spectrometric evidence has been reported for 4-HPALD, but lack of an authentic standard has precluded definitive confirmation of both the structure of 4-HPALD and its origin as a first-generation product of OH oxidation of isoprene. We report the synthesis and characterization of 4-HPALD and establish that it is a major product of isoprene oxidation. Synthetic 4-HPALD is isolated as the peroxyhemiacetal. As expected for the 4-hydroperoxy aldehyde, 1H NMR spectra show no evidence for equilibration with the carbonyl form, even in protic solvents, and gas-phase chemical analysis by CIMS also shows only a single form. OH oxidation of isoprene in anmore »oxidation flow reactor coupled to an ion mobility source with an HR-CIMS detector unequivocally demonstrates 4-HPALD (and likely also 1-HPALD) as isoprene oxidation products. Although HPALDs have been discounted as significant contributors to SOA, oxidation of 4-HPALD in a potential aerosol mass (PAM) reactor in the presence of ozone and OH indicates 4-HPALD rapidly undergoes autooxidation reactions forming low-volatility particulate products. We have confirmed highly oxygenated compounds with compositions C5H8O6 and C5H10O6 likely from OH oxidation, and C5H10O7 and C5H10O8 compounds likely products of ozonolysis. The PAM oxidation experiment further demonstrates that the highly oxygenated, low-volatility products efficiently nucleate particles.« less
    Free, publicly-accessible full text available October 1, 2023
  2. Abstract. Atmospheric aerosols are a significant public health hazard and havesubstantial impacts on the climate. Secondary organic aerosols (SOAs) havebeen shown to phase separate into a highly viscous organic outer layersurrounding an aqueous core. This phase separation can decrease thepartitioning of semi-volatile and low-volatile species to the organic phaseand alter the extent of acid-catalyzed reactions in the aqueous core. A newalgorithm that can determine SOA phase separation based on their glasstransition temperature (Tg), oxygen to carbon (O:C) ratio and organic massto sulfate ratio, and meteorological conditions was implemented into theCommunity Multiscale Air Quality Modeling (CMAQ) system version 5.2.1 andwas used to simulate the conditions in the continental United States for thesummer of 2013. SOA formed at the ground/surface level was predicted to bephase separated with core–shell morphology, i.e., aqueous inorganic coresurrounded by organic coating 65.4 % of the time during the 2013 SouthernOxidant and Aerosol Study (SOAS) on average in the isoprene-rich southeasternUnited States. Our estimate is in proximity to the previously reported∼70 % in literature. The phase states of organic coatingsswitched between semi-solid and liquid states, depending on theenvironmental conditions. The semi-solid shell occurring with lower aerosolliquid water content (western United States and at higher altitudes) has aviscosity that was predicted tomore »be 102–1012 Pa s, whichresulted in organic mass being decreased due to diffusion limitation.Organic aerosol was primarily liquid where aerosol liquid water was dominant(eastern United States and at the surface), with a viscosity <102 Pa s.Phase separation while in a liquid phase state, i.e.,liquid–liquid phase separation (LLPS), also reduces reactive uptake ratesrelative to homogeneous internally mixed liquid morphology but was lowerthan aerosols with a thick viscous organic shell. The sensitivity casesperformed with different phase-separation parameterization and dissolutionrate of isoprene epoxydiol (IEPOX) into the particle phase in CMAQ can havevarying impact on fine particulate matter (PM2.5) organic mass, interms of bias and error compared to field data collected during the 2013 SOAS.This highlights the need to better constrain the parameters thatgovern phase state and morphology of SOA, as well as expand mechanisticrepresentation of multiphase chemistry for non-IEPOX SOA formation in modelsaided by novel experimental insights.« less
  3. Aerosol particles negatively affect human health while also having climatic relevance due to, for example, their ability to act as cloud condensation nuclei. Ultrafine particles (diameter D p < 100 nm) typically comprise the largest fraction of the total number concentration, however, their chemical characterization is difficult because of their low mass. Using an extractive electrospray time-of-flight mass spectrometer (EESI-TOF), we characterize the molecular composition of freshly nucleated particles from naphthalene and β-caryophyllene oxidation products at the CLOUD chamber at CERN. We perform a detailed intercomparison of the organic aerosol chemical composition measured by the EESI-TOF and an iodide adduct chemical ionization mass spectrometer equipped with a filter inlet for gases and aerosols (FIGAERO-I-CIMS). We also use an aerosol growth model based on the condensation of organic vapors to show that the chemical composition measured by the EESI-TOF is consistent with the expected condensed oxidation products. This agreement could be further improved by constraining the EESI-TOF compound-specific sensitivity or considering condensed-phase processes. Our results show that the EESI-TOF can obtain the chemical composition of particles as small as 20 nm in diameter with mass loadings as low as hundreds of ng m −3 in real time. This was until nowmore »difficult to achieve, as other online instruments are often limited by size cutoffs, ionization/thermal fragmentation and/or semi-continuous sampling. Using real-time simultaneous gas- and particle-phase data, we discuss the condensation of naphthalene oxidation products on a molecular level.« less
  4. Abstract. We present a comprehensive simulation of tropospheric chlorinewithin the GEOS-Chem global 3-D model of oxidant–aerosol–halogen atmosphericchemistry. The simulation includes explicit accounting of chloridemobilization from sea salt aerosol by acid displacement of HCl and by otherheterogeneous processes. Additional small sources of tropospheric chlorine(combustion, organochlorines, transport from stratosphere) are also included.Reactive gas-phase chlorine Cl*, including Cl, ClO, Cl2, BrCl, ICl,HOCl, ClNO3, ClNO2, and minor species, is produced by theHCl+OH reaction and by heterogeneous conversion of sea salt aerosolchloride to BrCl, ClNO2, Cl2, and ICl. The modelsuccessfully simulates the observed mixing ratios of HCl in marine air(highest at northern midlatitudes) and the associated HNO3decrease from acid displacement. It captures the high ClNO2 mixingratios observed in continental surface air at night and attributes thechlorine to HCl volatilized from sea salt aerosol and transported inlandfollowing uptake by fine aerosol. The model successfully simulates thevertical profiles of HCl measured from aircraft, where enhancements in thecontinental boundary layer can again be largely explained by transport inlandof the marine source. It does not reproduce the boundary layer Cl2mixing ratios measured in the WINTER aircraft campaign (1–5 ppt in thedaytime, low at night); the model is too high at night, which could be due touncertainty in the rate ofmore »the ClNO2+Cl- reaction, but we haveno explanation for the high observed Cl2 in daytime. The globalmean tropospheric concentration of Cl atoms in the model is 620 cm−3and contributes 1.0 % of the global oxidation of methane, 20 % ofethane, 14 % of propane, and 4 % of methanol. Chlorine chemistryincreases global mean tropospheric BrO by 85 %, mainly through theHOBr+Cl- reaction, and decreases global burdens of troposphericozone by 7 % and OH by 3 % through the associated bromine radicalchemistry. ClNO2 chemistry drives increases in ozone of up to8 ppb over polluted continents in winter.« less