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  1. Free, publicly-accessible full text available July 5, 2023
  2. The role of hydroxyl radicals (OH) as a daytime oxidant is well established on a global scale. In specific source regions, such as the marine boundary layer and polluted coastal cities, other daytime oxidants, such as chlorine atoms (Cl) and even bromine atoms (Br), may compete with OH for the oxidation of volatile organic compounds (VOCs) and/or enhance the overall oxidation capacity of the atmosphere. However, the number of studies investigating halogen-initiated secondary organic aerosol (SOA) formation is extremely limited, resulting in large uncertainties in these oxidative aging processes. Here, we characterized the chemical composition and yield of laboratory SOA generated in an oxidation flow reactor (OFR) from the OH and Cl oxidation of n -dodecane ( n -C 12 ) and toluene, and the OH, Cl, and Br oxidation of isoprene and α-pinene. In the OFR, precursors were oxidized using integrated OH, Cl, and Br exposures ranging from 3.1 × 10 10 to 2.3 × 10 12 , 6.1 × 10 9 to 1.3× 10 12 and 3.2 × 10 10 to 9.7 × 10 12 molecules cm −3 s −1 , respectively. Like OH, Cl facilitated multistep SOA oxidative aging over the range of OFR conditions that weremore »studied. In contrast, the extent of Br-initiated SOA oxidative aging was limited. SOA elemental ratios and mass yields obtained in the OFR studies were comparable to those obtained from OH and Cl oxidation of the same precursors in environmental chamber studies. Overall, our results suggest that alkane, aromatic, and terpenoid SOA precursors are characterized by distinct OH- and halogen-initiated SOA yields, and that while Cl may enhance the SOA formation potential in regions influenced by biogenic and anthropogenic emissions, Br may have the opposite effect.« less
    Free, publicly-accessible full text available July 14, 2023
  3. Abstract Iodine is a reactive trace element in atmospheric chemistry that destroys ozone and nucleates particles. Iodine emissions have tripled since 1950 and are projected to keep increasing with rising O 3 surface concentrations. Although iodic acid (HIO 3 ) is widespread and forms particles more efficiently than sulfuric acid, its gas-phase formation mechanism remains unresolved. Here, in CLOUD atmospheric simulation chamber experiments that generate iodine radicals at atmospherically relevant rates, we show that iodooxy hypoiodite, IOIO, is efficiently converted into HIO 3 via reactions (R1) IOIO + O 3  → IOIO 4 and (R2) IOIO 4  + H 2 O → HIO 3  + HOI +  (1) O 2 . The laboratory-derived reaction rate coefficients are corroborated by theory and shown to explain field observations of daytime HIO 3 in the remote lower free troposphere. The mechanism provides a missing link between iodine sources and particle formation. Because particulate iodate is readily reduced, recycling iodine back into the gas phase, our results suggest a catalytic role of iodine in aerosol formation.
    Free, publicly-accessible full text available November 14, 2023
  4. Intense new particle formation events are regularly observed under highly polluted conditions, despite the high loss rates of nucleated clusters. Higher than expected cluster survival probability implies either ineffective scavenging by pre-existing particles or missing growth mechanisms. Here we present experiments performed in the CLOUD chamber at CERN showing particle formation from a mixture of anthropogenic vapours, under condensation sinks typical of haze conditions, up to 0.1 s −1 . We find that new particle formation rates substantially decrease at higher concentrations of pre-existing particles, demonstrating experimentally for the first time that molecular clusters are efficiently scavenged by larger sized particles. Additionally, we demonstrate that in the presence of supersaturated gas-phase nitric acid (HNO 3 ) and ammonia (NH 3 ), freshly nucleated particles can grow extremely rapidly, maintaining a high particle number concentration, even in the presence of a high condensation sink. Such high growth rates may explain the high survival probability of freshly formed particles under haze conditions. We identify under what typical urban conditions HNO 3 and NH 3 can be expected to contribute to particle survival during haze.
    Free, publicly-accessible full text available May 19, 2023
  5. Abstract New particle formation in the upper free troposphere is a major global source of cloud condensation nuclei (CCN) 1–4 . However, the precursor vapours that drive the process are not well understood. With experiments performed under upper tropospheric conditions in the CERN CLOUD chamber, we show that nitric acid, sulfuric acid and ammonia form particles synergistically, at rates that are orders of magnitude faster than those from any two of the three components. The importance of this mechanism depends on the availability of ammonia, which was previously thought to be efficiently scavenged by cloud droplets during convection. However, surprisingly high concentrations of ammonia and ammonium nitrate have recently been observed in the upper troposphere over the Asian monsoon region 5,6 . Once particles have formed, co-condensation of ammonia and abundant nitric acid alone is sufficient to drive rapid growth to CCN sizes with only trace sulfate. Moreover, our measurements show that these CCN are also highly efficient ice nucleating particles—comparable to desert dust. Our model simulations confirm that ammonia is efficiently convected aloft during the Asian monsoon, driving rapid, multi-acid HNO 3 –H 2 SO 4 –NH 3 nucleation in the upper troposphere and producing ice nucleating particles thatmore »spread across the mid-latitude Northern Hemisphere.« less
    Free, publicly-accessible full text available May 19, 2023
  6. Abstract. Delhi, India, is the second most populated city in the world and routinely experiences some of the highest particulate matter concentrations of any megacity on the planet, posing acute challenges to public health (World Health Organization, 2018). However, the current understanding of the sources and dynamics of PM pollution in Delhi is limited. Measurements at the Delhi Aerosol Supersite (DAS) provide long-term chemical characterization of ambient submicron aerosol in Delhi, with near-continuous online measurements of aerosol composition. Here we report on source apportionment based on positive matrix factorization (PMF), conducted on 15 months of highly time-resolved speciated submicron non-refractory PM1 (NR-PM1) between January 2017 and March 2018. We report on seasonal variability across four seasons of 2017 and interannual variability using data from the two winters and springs of 2017 and 2018. We show that a modified tracer-based organic component analysis provides an opportunity for a real-time source apportionment approach for organics in Delhi. Phase equilibrium modeling of aerosols using the extended aerosol inorganics model (E-AIM) predicts equilibrium gas-phase concentrations and allows evaluation of the importance of the ventilation coefficient (VC) and temperature in controlling primary and secondary organic aerosol. We also find that primary aerosol dominates severe air pollution episodes, and secondary aerosol dominates seasonal averages.
  7. 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
  8. Abstract. Chlorine-initiated oxidation of n-alkanes (C8−12) under high-nitrogen oxide conditions was investigated. Observed secondary organic aerosol yields (0.16 to 1.65) are higher than those for OH-initiated oxidation of C8−12 alkanes (0.04 to 0.35). A high-resolution time-of-flight chemical ionization mass spectrometer coupled to a Filter Inlet for Gases and AEROsols (FIGAERO–CIMS) was used to characterize the gas- and particle-phase molecular composition. Chlorinated organics were observed, which likely originated from chlorine addition to the double bond present on the heterogeneously produced dihydrofurans. A two-dimensional thermogram representation was developed to visualize the composition and relative volatility of organic aerosol components using unit-mass resolution data. Evidence of oligomer formation and thermal decomposition was observed. Aerosol yield and oligomer formation were suppressed under humid conditions (35% to 67% RH) relative to dry conditions (under 5% RH). The temperature at peak desorption signal, Tmax, a proxy for aerosol volatility, was shown to change with aerosol filter loading, which should be constrained when evaluating aerosol volatilities using the FIGAERO–CIMS. Results suggest that long-chain anthropogenic alkanes could contribute significantly to ambient aerosol loading over their atmospheric lifetime.