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Abstract. OH reactivity (OHR) is an important control on the oxidative capacity in the atmosphere but remains poorly constrained in many environments, such asremote, rural, and urban atmospheres, as well as laboratory experiment setups under low-NO conditions. For an improved understanding of OHR, itsevolution during oxidation of volatile organic compounds (VOCs) is a major aspect requiring better quantification. We use the fully explicitGenerator of Explicit Chemistry and Kinetics of Organics in the Atmosphere (GECKO-A) model to study the OHR evolution in the NO-free photooxidationof several VOCs, including decane (an alkane), m-xylene (an aromatic), and isoprene (an alkene). Oxidation progressively produces more saturated and functionalized species. Total organic OHR (including precursor and products, OHRVOC) first increases for decane (as functionalization increases OH rate coefficients) and m-xylene (as much more reactive oxygenated alkenes are formed). For isoprene, C=C bond consumption leads to a rapid drop in OHRVOC before significant production of the first main saturated multifunctional product, i.e., isoprene epoxydiol. The saturated multifunctional species in the oxidation of different precursors have similar average OHRVOC per C atom. The latter oxidation follows a similar course for different precursors, involving fragmentation of multifunctional species to eventual oxidation of C1 and C2 fragments to CO2, leading to a similar evolution of OHRVOC per C atom. An upper limit of the total OH consumption during complete oxidation to CO2 is roughly three per C atom. We also explore the trends in radical recycling ratios. We show that differences in the evolution of OHRVOC between the atmosphere and an environmental chamber, and between the atmosphere and an oxidation flow reactor (OFR), can be substantial, with the former being even larger, but these differences are often smaller than between precursors. The Teflon wall losses of oxygenated VOCs in chambers result in large deviations of OHRVOC from atmospheric conditions, especially for the oxidation of larger precursors, where multifunctional species may suffer substantial wall losses, resulting in significant underestimation of OHRVOC. For OFR, the deviations of OHRVOC evolution from the atmospheric case are mainly due to significant OHR contribution from RO2 and lack of efficient organic photolysis. The former can be avoided by lowering the UV lamp setting in OFR, while the latter is shown to be very difficult to avoid. However, the former may significantly offset the slowdown in fragmentation of multifunctional species due to lack of efficient organic photolysis. 

Environmental chambers have been playing a vital role in atmospheric chemistry research for seven decades. In last decade, oxidation flow reactors (OFR) have emerged as a promising alternative to chambers to study complex multigenerational chemistry. OFR can generate higher-than-ambient concentrations of oxidants via H 2 O, O 2 and O 3 photolysis by low-pressure-Hg-lamp emissions and reach hours to days of equivalent photochemical aging in just minutes of real time. The use of OFR for volatile-organic-compound (VOC) oxidation and secondary-organic-aerosol formation has grown very rapidly recently. However, the lack of detailed understanding of OFR photochemistry left room for speculation that OFR chemistry may be generally irrelevant to the troposphere, since its initial oxidant generation is similar to stratosphere. Recently, a series of studies have been conducted to address important open questions on OFR chemistry and to guide experimental design and interpretation. In this Review, we present a comprehensive picture connecting the chemistries of hydroxyl (OH) and hydroperoxy radicals, oxidized nitrogen species and organic peroxy radicals (RO 2 ) in OFR. Potential lack of tropospheric relevance associated with these chemistries, as well as the physical conditions resulting in it will also be reviewed. When atmospheric oxidation is dominated by OH, OFR conditions can often be similar to ambient conditions, as OH dominates against undesired non-OH effects. One key reason for tropospherically-irrelevant/undesired VOC fate is that under some conditions, OH is drastically reduced while non-tropospheric/undesired VOC reactants are not. The most frequent problems are running experiments with too high precursor concentrations, too high UV and/or too low humidity. On other hand, another cause of deviation from ambient chemistry in OFR is that some tropospherically-relevant non-OH chemistry ( e.g. VOC photolysis in UVA and UVB) is not sufficiently represented under some conditions. In addition, the fate of RO 2 produced from VOC oxidation can be kept relevant to the troposphere. However, in some cases RO 2 lifetime can be too short for atmospherically-relevant RO 2 chemistry, including its isomerization. OFR applications using only photolysis of injected O 3 to generate OH are less preferable than those using both 185 and 254 nm photons (without O 3 injection) for several reasons. When a relatively low equivalent photochemical age (<∼1 d) and high NO are needed, OH and NO generation by organic-nitrite photolysis in the UVA range is preferable. We also discuss how to achieve the atmospheric relevance for different purposes in OFR experimental planning. 

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 photolysis 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.

 

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, and 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. 

Abstract. Secondary organic aerosol derived from isopreneepoxydiols (IEPOX-SOA) is thought to contribute the dominant fraction oftotal isoprene SOA, but the current volatility-based lumped SOAparameterizations are not appropriate to represent the reactive uptake ofIEPOX onto acidified aerosols. A full explicit modeling of this chemistryis however computationally expensive owing to the many species and reactionstracked, which makes it difficult to include it in chemistry–climate modelsfor long-term studies. Here we present three simplified parameterizations(version 1.0) for IEPOX-SOA simulation, based on an approximateanalytical/fitting solution of the IEPOX-SOA yield and formation timescale.The yield and timescale can then be directly calculated using the globalmodel fields of oxidants, NO, aerosol pH and other key properties, and drydeposition rates. The advantage of the proposed parameterizations is thatthey do not require the simulation of the intermediates while retaining thekey physicochemical dependencies. We have implemented the newparameterizations into the GEOS-Chem v11-02-rc chemical transport model,which has two empirical treatments for isoprene SOA (the volatility-basis-set, VBS, approach and a fixed 3&thinsp;% yield parameterization), and comparedall of them to the case with detailed fully explicit chemistry. The bestparameterization (PAR3) captures the global tropospheric burden of IEPOX-SOAand its spatiotemporal distribution (R²=0.94) vs. thosesimulated by the full chemistry, while being more computationally efficient(∼5 times faster), and accurately captures the response tochanges in NO_x and SO₂ emissions. On the other hand, the constant3&thinsp;% yield that is now the default in GEOS-Chem deviates strongly (R²=0.66), as does the VBS (R²=0.47, 49&thinsp;% underestimation), withneither parameterization capturing the response to emission changes. Withthe advent of new mass spectrometry instrumentation, many detailed SOAmechanisms are being developed, which will challenge global and especiallyclimate models with their computational cost. The methods developed in thisstudy can be applied to other SOA pathways, which can allow includingaccurate SOA simulations in climate and global modeling studies in thefuture.