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1. Secondary organic aerosol from chlorine-initiated oxidation of isoprene

   https://doi.org/10.5194/acp-17-13491-2017  

   Wang, Dongyu S. ; Ruiz, Lea Hildebrandt ( January 2017 , Atmospheric Chemistry and Physics)

   Recent studies have found concentrations of reactive chlorine species to be higher than expected, suggesting that atmospheric chlorine chemistry is more extensive than previously thought. Chlorine radicals can interact with hydroperoxy (HO_x) radicals and nitrogen oxides (NO_x) to alter the oxidative capacity of the atmosphere. They are known to rapidly oxidize a wide range of volatile organic compounds (VOCs) found in the atmosphere, yet little is known about secondary organic aerosol (SOA) formation from chlorine-initiated photooxidation and its atmospheric implications. Environmental chamber experiments were carried out under low-NO_x conditions with isoprene and chlorine as primary VOC and oxidant sources. Upon complete isoprene consumption, observed SOA yields ranged from 7 to 36 %, decreasing with extended photooxidation and SOA aging. Formation of particulate organochloride was observed. A high-resolution time-of-flight chemical ionization mass spectrometer was used to determine the molecular composition of gas-phase species using iodide–water and hydronium–water cluster ionization. Multi-generational chemistry was observed, including ions consistent with hydroperoxides, chloroalkyl hydroperoxides, isoprene-derived epoxydiol (IEPOX), and hypochlorous acid (HOCl), evident of secondary OH production and resulting chemistry from Cl-initiated reactions. This is the first reported study of SOA formation from chlorine-initiated oxidation of isoprene. Results suggest that tropospheric chlorine chemistry could contribute significantly tomore »organic aerosol loading.« less
2. Nitrate radical generation via continuous generation of dinitrogen pentoxide in a laminar flow reactor coupled to an oxidation flow reactor

   https://doi.org/10.5194/amt-13-2397-2020  

   Lambe, Andrew T. ; Wood, Ezra C. ; Krechmer, Jordan E. ; Majluf, Francesca ; Williams, Leah R. ; Croteau, Philip L. ; Cirtog, Manuela ; Féron, Anaïs ; Petit, Jean-Eudes ; Albinet, Alexandre ; et al ( January 2020 , Atmospheric Measurement Techniques)

   Abstract. Oxidation flow reactors (OFRs) are an emerging tool for studying the formation and oxidative aging of organic aerosols and other applications.The majority of OFR studies to date have involved the generation of the hydroxyl radical (OH) to mimic daytime oxidative aging processes.In contrast, the use of the nitrate radical (NO3) in modern OFRs to mimic nighttime oxidative aging processes has been limited due to the complexity of conventional techniques that are used to generate NO3.Here, we present a new method that uses a laminar flow reactor (LFR) to continuously generate dinitrogen pentoxide (N2O5) in the gas phase at room temperature from the NO2 + O3 and NO2 + NO3 reactions.The N2O5 is then injected into a dark Potential Aerosol Mass (PAM) OFR and decomposes to generate NO3; hereafter, this method is referred to as “OFR-iN2O5” (where “i” stands for “injected”).To assess the applicability of the OFR-iN2O5 method towards different chemical systems, we present experimental and model characterization of the integrated NO3 exposure, NO3:O3, NO2:NO3, and NO2:O2 as a function of LFR and OFR conditions.These parameters were used to investigate the fate of representative organic peroxy radicals (RO2) and aromatic alkyl radicals generated from volatile organic compound (VOC) + NO3 reactions, andmore »VOCs that are reactive towards both O3 and NO3.Finally, we demonstrate the OFR-iN2O5 method by generating and characterizing secondary organic aerosol from the β-pinene + NO3 reaction.« less
3. Modeling Diurnal Variation of Biogenic SOA Formation via Multiphase Reaction of Biogenic Hydrocarbons

   https://doi.org/10.5194/acp-2022-327  

   Han, S. ; Jang, M. ( January 2022 , Atmospheric chemistry and physics discussion)

   The daytime oxidation of biogenic hydrocarbons is attributed to both OH radicals and O3, while nighttime chemistry is dominated by the reaction with O3 and NO3 radicals. Here, the diurnal pattern of Secondary Organic Aerosol (SOA) originating from biogenic hydrocarbons was intensively evaluated under varying environmental conditions (temperature, humidity, sunlight intensity, NOx levels, and seed conditions) by using the UNIfied Partitioning Aerosol phase Reaction (UNIPAR) model, which comprises multiphase gas-particle partitioning and in-particle chemistry. The oxidized products of three different hydrocarbons (isoprene, α-pinene, and β-caryophyllene) were predicted by using near explicit gas mechanisms for four different oxidation paths (OH, O3, NO3, and O(3P)) during day and night. The gas mechanisms implemented the Master Chemical Mechanism (MCM v3.3.1), the reactions that formed low volatility products via peroxy radical (RO2) autoxidation, and self- and cross-reactions of nitrate-origin RO2. In the model, oxygenated products were then classified into volatility-reactivity base lumping species, which were dynamically constructed under varying NOx levels and aging scales. To increase feasibility, the UNIPAR model that equipped mathematical equations for stoichiometric coefficients and physicochemical parameters of lumping species was integrated with the SAPRC gas mechanism. The predictability of the UNIPAR model was demonstrated by simulating chamber-generated SOA data undermore »varying environments day and night. Overall, the SOA simulation decoupled to each oxidation path indicated that the nighttime isoprene SOA formation was dominated by the NO3-driven oxidation, regardless of NOx levels. However, the oxidation path to produce the nighttime α-pinene SOA gradually transited from the NO3-initiated reaction to ozonolysis as NOx levels decreased. For daytime SOA formation, both isoprene and α-pinene were dominated by the OH-radical initiated oxidation. The contribution of the O(3P) path to all biogenic SOA formation was negligible in daytime. Sunlight during daytime promotes the decomposition of oxidized products via photolysis and thus, reduces SOA yields. Nighttime α-pinene SOA yields were significantly higher than daytime SOA yields, although the nighttime α-pinene SOA yields gradually decreased with decreasing NOx levels. For isoprene, nighttime chemistry yielded higher SOA mass than daytime at the higher NOx level (isoprene/NOx > 5 ppbC/ppb). The daytime isoprene oxidation at the low NOx level formed epoxy-diols that significantly contributed SOA formation via heterogeneous chemistry. For isoprene and α-pinene, daytime SOA yields gradually increased with decreasing NOx levels. The daytime SOA produced more highly oxidized multifunctional products and thus, it was generally more sensitive to the aqueous reactions than the nighttime SOA. β-Caryophyllene, which rapidly oxidized and produced SOA with high yields, showed a relatively small variation in SOA yields from changes in environmental conditions (i.e., NOx levels, seed conditions, and diurnal pattern), and its SOA formation was mainly attributed to ozonolysis day and night. To mimic the nighttime α-pinene SOA formation under the polluted urban atmosphere, α-pinene SOA formation was simulated in the presence of gasoline fuel. The simulation suggested the growth of α-pinene SOA in the presence of gasoline fuel gas by the enhancement of the ozonolysis path under the excess amount of ozone, which is typical in urban air. We concluded that the oxidation of the biogenic hydrocarbon with O3 or NO3 radicals is a source to produce a sizable amount of nocturnal SOA, despite of the low emission at night.« less
4. Organic peroxy radical chemistry in oxidation flow reactors and environmental chambers and their atmospheric relevance

   https://doi.org/10.5194/acp-19-813-2019  

   Peng, Zhe ; Lee-Taylor, Julia ; Orlando, John J. ; Tyndall, Geoffrey S. ; Jimenez, Jose L. ( January 2019 , Atmospheric Chemistry and Physics)

   Abstract. Oxidation flow reactors (OFRs) are a promising complement toenvironmental chambers for investigating atmospheric oxidation processes andsecondary aerosol formation. However, questions have been raised about howrepresentative the chemistry within OFRs is of that in the troposphere. Weinvestigate the fates of organic peroxy radicals (RO₂), which playa central role in atmospheric organic chemistry, in OFRs and environmentalchambers by chemical kinetic modeling and compare to a variety of ambientconditions to help define a range of atmospherically relevant OFR operatingconditions. For most types of RO₂, their bimolecular fates in OFRsare mainly RO₂+HO₂ and RO₂+NO, similar to chambers andatmospheric studies. For substituted primary RO₂ and acylRO₂, RO₂+RO₂ can make a significant contribution tothe fate of RO₂ in OFRs, chambers and the atmosphere, butRO₂+RO₂ in OFRs is in general somewhat less important than inthe atmosphere. At high NO, RO₂+NO dominates RO₂ fate inOFRs, as in the atmosphere. At a high UV lamp setting in OFRs,RO₂+OH can be a major RO₂ fate and RO₂isomerization can be negligible for common multifunctional RO₂,both of which deviate from common atmospheric conditions. In the OFR254operation mode (for which OH is generated only from the photolysismore »of addedO₃), we cannot identify any conditions that can simultaneouslyavoid significant organic photolysis at 254 nm and lead to RO₂lifetimes long enough (∼ 10 s) to allow atmospherically relevantRO₂ isomerization. In the OFR185 mode (for which OH is generatedfrom reactions initiated by 185 nm photons), high relative humidity, low UVintensity and low precursor concentrations are recommended for theatmospherically relevant gas-phase chemistry of both stable species andRO₂. These conditions ensure minor or negligible RO₂+OHand a relative importance of RO₂ isomerization in RO₂fate in OFRs within $\sim \times \mathrm{2}$ of that in the atmosphere. Under theseconditions, the photochemical age within OFR185 systems can reach a fewequivalent days at most, encompassing the typical ages for maximum secondaryorganic aerosol (SOA) production. A small increase in OFR temperature mayallow the relative importance of RO₂ isomerization to approach theambient values. To study the heterogeneous oxidation of SOA formed underatmospherically relevant OFR conditions, a different UV source with higherintensity is needed after the SOA formation stage, which can be done withanother reactor in series. Finally, we recommend evaluating the atmosphericrelevance of RO₂ chemistry by always reporting measured and/orestimated OH, HO₂, NO, NO₂ and OH reactivity (or at leastprecursor composition and concentration) in all chamber and flow reactorexperiments. An easy-to-use RO₂ fate estimator program is includedwith this paper to facilitate the investigation of this topic in futurestudies.

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5. Mechanistic study of the formation of ring-retaining and ring-opening products from the oxidation of aromatic compounds under urban atmospheric conditions

   https://doi.org/10.5194/acp-19-15117-2019  

   Zaytsev, Alexander ; Koss, Abigail R. ; Breitenlechner, Martin ; Krechmer, Jordan E. ; Nihill, Kevin J. ; Lim, Christopher Y. ; Rowe, James C. ; Cox, Joshua L. ; Moss, Joshua ; Roscioli, Joseph R. ; et al ( January 2019 , Atmospheric Chemistry and Physics)

   Abstract. Aromatic hydrocarbons make up a large fraction of anthropogenic volatile organic compounds and contribute significantly to the production of tropospheric ozone and secondary organic aerosol (SOA). Four toluene and four 1,2,4-trimethylbenzene (1,2,4-TMB) photooxidation experiments were performed in an environmental chamber under relevantpolluted conditions (NOx∼10 ppb). An extensive suite of instrumentation including two proton-transfer-reaction mass spectrometers (PTR-MS) and two chemical ionisation mass spectrometers (NH4+ CIMS and I− CIMS) allowed for quantification of reactive carbon in multiple generations of hydroxyl radical (OH)-initiated oxidation. Oxidation of both species produces ring-retaining products such as cresols, benzaldehydes, and bicyclic intermediate compounds, as well as ring-scission products such as epoxides and dicarbonyls. We show that the oxidation of bicyclic intermediate products leads to the formation of compounds with high oxygen content (an O:C ratio of up to 1.1). These compounds, previously identified as highly oxygenated molecules (HOMs), are produced by more than one pathway with differing numbers of reaction steps with OH, including both auto-oxidation and phenolic pathways. We report the elemental composition of these compounds formed under relevant urban high-NO conditions. We show that ring-retaining products for these two precursors are more diverse and abundant than predicted by current mechanisms. We present the speciated elemental composition of SOA for both precursorsmore »and confirm that highly oxygenated products make up a significant fraction of SOA. Ring-scission products are also detected in both the gas and particle phases, and their yields and speciation generally agree with the kinetic model prediction.« less