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.
more »
« less
Dimensionality-reduction techniques for complex mass spectrometric datasets: application to laboratory atmospheric organic oxidation experiments
Abstract. Oxidation of organic compounds in the atmosphere produces an immenselycomplex mixture of product species, posing a challenge for both theirmeasurement in laboratory studies and their inclusion in air quality andclimate models. Mass spectrometry techniques can measure thousands of thesespecies, giving insight into these chemical processes, but the datasetsthemselves are highly complex. Data reduction techniques that groupcompounds in a chemically and kinetically meaningful way provide a route tosimplify the chemistry of these systems but have not been systematicallyinvestigated. Here we evaluate three approaches to reducing thedimensionality of oxidation systems measured in an environmental chamber:positive matrix factorization (PMF), hierarchical clustering analysis (HCA),and a parameterization to describe kinetics in terms of multigenerationalchemistry (gamma kinetics parameterization, GKP). The evaluation isimplemented by means of two datasets: synthetic data consisting of athree-generation oxidation system with known rate constants, generationnumbers, and chemical pathways; and the measured products of OH-initiatedoxidation of a substituted aromatic compound in a chamber experiment. Wefind that PMF accounts for changes in the average composition of allproducts during specific periods of time but does not sort compounds intogenerations or by another reproducible chemical process. HCA, on the otherhand, can identify major groups of ions and patterns of behavior andmaintains bulk chemical properties like carbon oxidation state that can beuseful for modeling. The continuum of kinetic behavior observed in a typicalchamber experiment can be parameterized by fitting species' time traces tothe GKP, which approximates the chemistry as a linear, first-order kineticsystem. The fitted parameters for each species are the number of reaction stepswith OH needed to produce the species (the generation) and an effectivekinetic rate constant that describes the formation and loss rates of thespecies. The thousands of species detected in a typical laboratory chamberexperiment can be organized into a much smaller number (10–30) of groups,each of which has a characteristic chemical composition and kinetic behavior.This quantitative relationship between chemical and kinetic characteristics,and the significant reduction in the complexity of the system, provides anapproach to understanding broad patterns of behavior in oxidation systemsand could be exploited for mechanism development and atmospheric chemistrymodeling.
more »
« less
- Award ID(s):
- 1638672
- NSF-PAR ID:
- 10188520
- Date Published:
- Journal Name:
- Atmospheric Chemistry and Physics
- Volume:
- 20
- Issue:
- 2
- ISSN:
- 1680-7324
- Page Range / eLocation ID:
- 1021 to 1041
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Chemical mechanisms play an important role in simulating the atmospheric chemistry of volatile organic compound oxidation. Comparison of mechanism simulations with laboratory chamber data tests our level of understanding of the prevailing chemistry as well as the dynamic processes occurring in the chamber itself. α-Pinene photooxidation is a well-studied system experimentally, for which detailed chemical mechanisms have been formulated. Here, we present the results of simulating low-NO α-pinene photooxidation experiments conducted in the Caltech chamber with the Generator for Explicit Chemistry and Kinetics of Organics in the Atmosphere (GECKO-A) under varying concentrations of seed particles and OH levels. Unexpectedly, experiments conducted at low and high OH levels yield the same secondary organic aerosol (SOA) growth, whereas GECKO-A predicts greater SOA growth under high OH levels. SOA formation in the chamber is a result of a competition among the rates of gas-phase oxidation to low-volatility products, wall deposition of these products, and condensation into the aerosol phase. Various processes – such as photolysis of condensed-phase products, particle-phase dimerization, and peroxy radical autoxidation – are explored to rationalize the observations. In order to explain the observed similar SOA growth at different OH levels, we conclude that vapor wall loss in the Caltech chamber is likely of order 10−5 s−1, consistent with previous experimental measurements in that chamber. We find that GECKO-A tends to overpredict the contribution to SOA of later-generation oxidation products under high-OH conditions. Moreover, we propose that autoxidation may alternatively resolve some or all of the measurement–model discrepancy, but this hypothesis cannot be confirmed until more explicit mechanisms are established for α-pinene autoxidation. The key role of the interplay among oxidation rate, product volatility, and vapor–wall deposition in chamber experiments is illustrated.more » « less
-
null (Ed.)Abstract. Reactions of the hydroxyl (OH) and peroxy (HO2 and RO2) radicals playa central role in the chemistry of the atmosphere. In addition to controlling the lifetimes ofmany trace gases important to issues of global climate change, OH radical reactionsinitiate the oxidation of volatile organic compounds (VOCs) which can lead to the production ofozone and secondary organic aerosols in the atmosphere. Previous measurements of these radicalsin forest environments characterized by high mixing ratios of isoprene and low mixing ratios ofnitrogen oxides (NOx) (typically less than 1–2 ppb) have shown seriousdiscrepancies with modeled concentrations. These results bring into question our understanding ofthe atmospheric chemistry of isoprene and other biogenic VOCs under low NOxconditions. During the summer of 2015, OH and HO2 radical concentrations, as well as totalOH reactivity, were measured using laser-induced fluorescence–fluorescence assay by gasexpansion (LIF-FAGE) techniques as part of the Indiana Radical Reactivity and Ozone productioN InterComparison (IRRONIC). This campaign took place in a forested area near Indiana University's Bloomington campus which is characterized by high mixing ratios of isoprene (average daily maximum ofapproximately 4 ppb at 28 ∘C) and low mixing ratios of NO (diurnal averageof approximately 170 ppt). Supporting measurements of photolysis rates, VOCs,NOx, and other species were used to constrain a zero-dimensional box model basedon the Regional Atmospheric Chemistry Mechanism (RACM2) and the Master Chemical Mechanism (MCM 3.2),including versions of the Leuven isoprene mechanism (LIM1) for HOx regeneration(RACM2-LIM1 and MCM 3.3.1). Using an OH chemical scavenger technique, the study revealed thepresence of an interference with the LIF-FAGE measurements of OH that increased with bothambient concentrations of ozone and temperature with an average daytime maximum equivalentOH concentration of approximately 5×106 cm−3. Subtraction of theinterference resulted in measured OH concentrations of approximately4×106 cm−3 (average daytime maximum) that were in better agreement with modelpredictions although the models underestimated the measurements in the evening. The addition ofversions of the LIM1 mechanism increased the base RACM2 and MCM 3.2 modeled OH concentrationsby approximately 20 % and 13 %, respectively, with the RACM2-LIM1 mechanism providing thebest agreement with the measured concentrations, predicting maximum daily OH concentrationsto within 30 % of the measured concentrations. Measurements of HO2 concentrationsduring the campaign (approximately a 1×109 cm−3 average daytime maximum)included a fraction of isoprene-based peroxy radicals(HO2*=HO2+αRO2) and were found to agree with modelpredictions to within 10 %–30 %. On average, the measured reactivity was consistent with thatcalculated from measured OH sinks to within 20 %, with modeled oxidation productsaccounting for the missing reactivity, however significant missing reactivity (approximately40 % of the total measured reactivity) was observed on some days.more » « less
-
more » « less
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
-
Abstract. Acid-catalyzed multiphase chemistry of epoxydiols formed from isopreneoxidation yields the most abundant organosulfates (i.e., methyltetrolsulfates) detected in atmospheric fine aerosols in the boundary layer. Thispotentially determines the physicochemical properties of fine aerosols inisoprene-rich regions. However, chemical stability of these organosulfatesremains unclear. As a result, we investigate the heterogeneous oxidation ofaerosols consisting of potassium 3-methyltetrol sulfate ester(C5H11SO7K) by gas-phase hydroxyl (OH) radicals at a relativehumidity (RH) of 70.8 %. Real-time molecular composition of the aerosolsis obtained by using a Direct Analysis in Real Time (DART) ionization sourcecoupled to a high-resolution mass spectrometer. Aerosol mass spectra revealthat 3-methyltetrol sulfate ester can be detected as its anionic form(C5H11SO7-) via direct ionization in the negativeionization mode. Kinetic measurements reveal that the effective heterogeneousOH rate constant is measured to be 4.74±0.2×10-13 cm3 molecule−1 s−1 with a chemical lifetime against OHoxidation of 16.2±0.3 days, assuming an OH radical concentration of1.5×106 molecules cm−3. Comparison of this lifetime withthose against other aerosol removal processes, such as dry and wetdeposition, suggests that 3-methyltetrol sulfate ester is likely to bechemically stable over atmospheric timescales. Aerosol mass spectra only showan increase in the intensity of bisulfate ion (HSO4-) afteroxidation, suggesting the importance of fragmentation processes. Overall,potassium 3-methyltetrol sulfate ester likely decomposes to form volatilefragmentation products and aqueous-phase sulfate radial anion(SO4⚫-). SO4⚫- subsequently undergoesintermolecular hydrogen abstraction to form HSO4-. These processesappear to explain the compositional evolution of 3-methyltetrol sulfate esterduring heterogeneous OH oxidation.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.5194/acp-20-1021-2020
