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1. 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 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.

    

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2. Secondary organic aerosol formation from the β-pinene+NO₃ system: effect of humidity and peroxy radical fate

   https://doi.org/10.5194/acp-15-7497-2015  

   Boyd, C. M. ; Sanchez, J. ; Xu, L. ; Eugene, A. J. ; Nah, T. ; Tuet, W. Y. ; Guzman, M. I. ; Ng, N. L. ( January 2015 , Atmospheric Chemistry and Physics)

   Abstract. The formation of secondary organic aerosol (SOA) from the oxidation of β-pinene via nitrate radicals is investigated in the Georgia Tech Environmental Chamber (GTEC) facility. Aerosol yields are determined for experiments performed under both dry (relative humidity (RH) < 2 %) and humid (RH = 50 % and RH = 70 %) conditions. To probe the effects of peroxy radical (RO₂) fate on aerosol formation, "RO₂ + NO₃ dominant" and "RO₂ + HO₂ dominant" experiments are performed. Gas-phase organic nitrate species (with molecular weights of 215, 229, 231, and 245 amu, which likely correspond to molecular formulas of C₁₀H₁₇NO₄, C₁₀H₁₅NO₅, C₁₀H₁₇NO₅, and C₁₀H₁₅NO₆, respectively) are detected by chemical ionization mass spectrometry (CIMS) and their formation mechanisms are proposed. The NO⁺ (at m/z 30) and NO₂⁺ (at m/z 46) ions contribute about 11 % to the combined organics and nitrate signals in the typical aerosol mass spectrum, with the NO⁺ : NO₂⁺ ratio ranging from 4.8 to 10.2 in all experiments conducted. The SOA yields in the "RO₂ + NO₃ dominant" and "RO₂ + HO₂ dominant" experiments are comparable. For a wide range of organic mass loadings (5.1–216.1 μg m^&minus;3), the aerosol mass yield is calculated to be 27.0–104.1 %. Although humidity does not appear to affect SOA yields, there is evidence of particle-phase hydrolysis of organic nitrates, which are estimated to compose 45–74 % of the organic aerosol. The extent of organic nitrate hydrolysis is significantly lower than that observed in previous studies on photooxidation of volatile organic compounds in the presence of NO_x. It is estimated that about 90 and 10 % of the organic nitrates formed from the β-pinene+NO₃ reaction are primary organic nitrates and tertiary organic nitrates, respectively. While the primary organic nitrates do not appear to hydrolyze, the tertiary organic nitrates undergo hydrolysis with a lifetime of 3–4.5 h. Results from this laboratory chamber study provide the fundamental data to evaluate the contributions of monoterpene + NO₃ reaction to ambient organic aerosol measured in the southeastern United States, including the Southern Oxidant and Aerosol Study (SOAS) and the Southeastern Center for Air Pollution and Epidemiology (SCAPE) study.

    

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3. A simplified parameterization of isoprene-epoxydiol-derived secondary organic aerosol (IEPOX-SOA) for global chemistry and climate models: a case study with GEOS-Chem v11-02-rc

   https://doi.org/10.5194/gmd-12-2983-2019  

   Jo, Duseong S. ; Hodzic, Alma ; Emmons, Louisa K. ; Marais, Eloise A. ; Peng, Zhe ; Nault, Benjamin A. ; Hu, Weiwei ; Campuzano-Jost, Pedro ; Jimenez, Jose L. ( January 2019 , Geoscientific Model Development)

   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.

    

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4. Long-term lidar observations of the gravity wave activity near the mesopause at Arecibo

   https://doi.org/10.5194/acp-19-3207-2019  

   Yue, Xianchang ; Friedman, Jonathan S. ; Zhou, Qihou ; Wu, Xiongbin ; Lautenbach, Jens ( January 2019 , Atmospheric Chemistry and Physics)

   Abstract. Using 11-year-long K Doppler lidar observations of temperatureprofiles in the mesosphere and lower thermosphere (MLT) between 85 and100&thinsp;km, conducted at the Arecibo Observatory, Puerto Rico(18.35^∘&thinsp;N, 66.75^∘&thinsp;W), seasonalvariations of mean temperature, the squared Brunt–Väisäläfrequency, N², and the gravity wave potential energy (GWPE) are estimated in a compositeyear. The following unique features are obtained. (1) The mean temperaturestructure shows similar characteristics to an earlier report based on a smallerdataset. (2) Temperature inversion layers (TILs) occur at 94–96&thinsp;km inspring, at ∼92&thinsp;km in summer, and at ∼91&thinsp;km in early autumn.(3) The first complete range-resolved climatology of GWPE derived from temperature data in the tropical MLT exhibits analtitude-dependent combination of annual oscillation (AO) and semiannualoscillation (SAO). The maximum occurs in spring and the minimum in summer, and asecond maximum is in autumn and a second minimum in winter. (4) The GWPE perunit volume reduces below ∼97&thinsp;km altitude in all seasons. Thereduction of GWPE is significant at and below the TILs but becomes faintabove; this provides strong support for the mechanism that the formation ofupper mesospheric TILs is mainly due to the reduction of GWPE. The climatologyof GWPE shows an indeed pronounced altitudinal and temporal correlation withthe wind field in the tropical mesopause region published in the literature.This suggests the GW activity in the tropical mesopause region should bemanifested mainly by the filtering effect of the critical level of the localbackground wind and the energy conversion due to local dynamical instability.

    

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5. Contributions of biomass-burning, urban, and biogenic emissions to the concentrations and light-absorbing properties of particulate matter in central Amazonia during the dry season

   https://doi.org/10.5194/acp-19-7973-2019  

   de Sá, Suzane S. ; Rizzo, Luciana V. ; Palm, Brett B. ; Campuzano-Jost, Pedro ; Day, Douglas A. ; Yee, Lindsay D. ; Wernis, Rebecca ; Isaacman-VanWertz, Gabriel ; Brito, Joel ; Carbone, Samara ; et al ( January 2019 , Atmospheric Chemistry and Physics)

   Abstract. Urbanization and deforestation have important impacts on atmosphericparticulate matter (PM) over Amazonia. This study presents observations andanalysis of PM₁ concentration, composition, and opticalproperties in central Amazonia during the dry season, focusing on theanthropogenic impacts. The primary study site was located 70&thinsp;km downwind ofManaus, a city of over 2 million people in Brazil, as part of theGoAmazon2014/5 experiment. A high-resolution time-of-flight aerosol massspectrometer (AMS) provided data on PM₁ composition, and aethalometermeasurements were used to derive the absorption coefficient b_abs,BrC ofbrown carbon (BrC) at 370&thinsp;nm. Non-refractory PM₁ mass concentrationsaveraged 12.2&thinsp;µg&thinsp;m⁻³ at the primary study site, dominated byorganics (83&thinsp;%), followed by sulfate (11&thinsp;%). A decrease inb_abs,BrC was observed as the mass concentration of nitrogen-containingorganic compounds decreased and the organic PM₁ O:C ratio increased,suggesting atmospheric bleaching of the BrC components. The organic PM₁was separated into six different classes by positive-matrix factorization(PMF), and the mass absorption efficiency E_abs associated with eachfactor was estimated through multivariate linear regression ofb_abs,BrC on the factor loadings. The largest E_abs values wereassociated with urban (2.04±0.14&thinsp;m²&thinsp;g⁻¹) and biomass-burning(0.82±0.04 to 1.50±0.07&thinsp;m²&thinsp;g⁻¹) sources. Together, these sources contributed at least 80&thinsp;% ofb_abs,BrC while accounting for 30&thinsp;% to 40&thinsp;% of the organic PM₁ massconcentration. In addition, a comparison of organic PM₁ compositionbetween wet and dry seasons revealed that only part of the 9-foldincrease in mass concentration between the seasons can be attributed tobiomass burning. Biomass-burning factor loadings increased by 30-fold,elevating its relative contribution to organic PM₁ from about 10&thinsp;% inthe wet season to 30&thinsp;% in the dry season. However, most of the PM₁mass (&gt;60&thinsp;%) in both seasons was accounted for by biogenicsecondary organic sources, which in turn showed an 8-fold seasonalincrease in factor loadings. A combination of decreased wet deposition andincreased emissions and oxidant concentrations, as well as a positivefeedback on larger mass concentrations are thought to play a role in theobserved increases. Furthermore, fuzzy c-means clustering identified threeclusters, namely “baseline”, “event”, and “urban” to representdifferent pollution influences during the dry season. The baseline cluster,representing the dry season background, was associated with a mean massconcentration of 9±3&thinsp;µg&thinsp;m⁻³. This concentration increasedon average by 3&thinsp;µg&thinsp;m⁻³ for both the urban and the event clusters.The event cluster, representing an increased influence of biomass burningand long-range transport of African volcanic emissions, was characterized byremarkably high sulfate concentrations. The urban cluster, representing theinfluence of Manaus emissions on top of the baseline, was characterized byan organic PM₁ composition that differed from the other two clusters.The differences discussed suggest a shift in oxidation pathways as well asan accelerated oxidation cycle due to urban emissions, in agreement withfindings for the wet season.

    

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