Overview of ICARUS-A Curated, Open Access, Online Repository for Atmospheric Simulation Chamber Data.

Atmospheric simulation chambers continue to be indispensable tools for research in the atmospheric sciences. Insights from chamber studies are integrated into atmospheric chemical transport models, which are used for science-informed policy decisions. However, a centralized data management and access infrastructure for their scientific products had not been available in the United States and many parts of the world. ICARUS (Integrated Chamber Atmospheric data Repository for Unified Science) is an open access, searchable, web-based infrastructure for storing, sharing, discovering, and utilizing atmospheric chamber data [https://icarus.ucdavis.edu]. ICARUS has two parts: a data intake portal and a search and discovery portal. Data in ICARUS are curated, uniform, interactive, indexed on popular search engines, mirrored by other repositories, version-tracked, vocabulary-controlled, and citable. ICARUS hosts both legacy data and new data in compliance with open access data mandates. Targeted data discovery is available based on key experimental parameters, including organic reactants and mixtures that are managed using the PubChem chemical database, oxidant information, nitrogen oxide (NOx) content, alkylperoxy radical (RO2) fate, seed particle information, environmental conditions, and reaction categories. A discipline-specific repository such as ICARUS with high amounts of metadata works to support the evaluation and revision of atmospheric model mechanisms, intercomparison of data and models, and the development of new model frameworks that can have more predictive power in the current and future atmosphere. The open accessibility and interactive nature of ICARUS data may also be useful for teaching, data mining, and training machine learning models.

Contamination of tea leaves by anthraquinone: The atmosphere as a possible source.

The detection of anthraquinone in tea leaves has raised concerns due to a potential health risk associated with this species. This led the European Union to impose a maximum residue limit (MRL) of 0.02 mg/kg for anthraquinone in dried tea leaves. As atmospheric contamination has been identified as one of the possible sources of anthraquinone residue, this study investigates the contamination resulting from the deposition of atmospheric anthraquinone using a global chemical transport model that accounts for the emission, atmospheric transport, chemical transformation, and deposition of anthraquinone on the surface. The largest contribution to the global atmospheric budget of anthraquinone is from residential combustion followed by the secondary formation from oxidation of anthracene. Simulations suggest that atmospheric anthraquinone deposition could be a substantial source of the anthraquinone found on tea leaves in several tea-producing regions, especially near highly industrialized and populated areas of southern and eastern Asia. The high level of anthraquinone deposition in these areas may result in residues in tea products exceeding the EU MRL. Additional contamination could also result from local tea production operations.

Atmospheric Fate of a New Polyfluoroalkyl Building Block, C3F7OCHFCF2SCH2CH2OH.

Many per- and polyfluoroalkyl substances (PFAS) have been regulated or phased-out of usage due to concerns about persistence, bioaccumulation potential, and toxicity. We investigated the atmospheric fate of a new polyfluorinated alcohol 2-(1,1,2-trifluoro-2-heptafluoropropyloxy-ethylsulfanyl)-ethanol (C3F7OCHFCF2SCH2CH2OH, abbreviated FESOH) by assessing the kinetics and products of the gas-phase reaction of FESOH with chlorine atoms and hydroxyl radicals. Experiments performed in a stainless-steel chamber interfaced to an FTIR were used to determine reaction kinetics and gas-phase products. We report reaction rate constants of k(Cl + FESOH) = (1.5 ± 0.6) × 10-11 cm3 molecule-1 s-1 and k(OH + FESOH) = (4.2 ± 2.0) × 10-12 cm3 molecule-1 s-1. This leads to a calculated FESOH gas-phase lifetime of 2.8 ± 1.3 days with respect to reaction with OH, assuming [OH] = 106 molecule1 cm-3. Gas-phase products of FESOH oxidation included at least two aldehydes, likely C3F7OCHFCF2SCH2C(O)H and C3F7OCHFCF2SC(O)H, and secondary products including COF2, SO2 and C3F7OC(O)F. Additional gas-phase experiments performed in a Teflon chamber were used to assess aqueous products by collecting gaseous samples offline into an aqueous sink prior to analysis with ultrahigh performance liquid chromatography-tandem mass spectrometry, resulting in four acidic products: C3F7OCHFCF2SCH2C(O)OH, C3F7OCHFCF2S(O)(O)OH, C3F7OCHFC(O)OH, and perfluoropropanoic acid (C2F5C(O)OH).

Heterogeneous Nucleation Drives Particle Size Segregation in Sequential Ozone and Nitrate Radical Oxidation of Catechol.

Secondary organic aerosol formation via condensation of organic vapors onto existing aerosol transforms the chemical composition and size distribution of ambient aerosol, with implications for air quality and Earth's radiative balance. Gas-to-particle conversion is generally thought to occur on a continuum between equilibrium-driven partitioning of semivolatile molecules to the pre-existing mass size distribution and kinetic-driven condensation of low volatility molecules to the pre-existing surface area size distribution. However, we offer experimental evidence in contrast to this framework. When catechol is sequentially oxidized by O3 and NO3 in the presence of (NH4)2SO4 seed particles with a single size mode, we observe a bimodal organic aerosol mass size distribution with two size modes of distinct chemical composition with nitrocatechol from NO3 oxidation preferentially condensing onto the large end of the pre-existing size distribution (∼750 nm). A size-resolved chemistry and microphysics model reproduces the evolution of the two distinct organic aerosol size modes─heterogeneous nucleation to an independent, nitrocatechol-rich aerosol phase.

UV/Vis+ photochemistry database: Structure, content and applications.

The "science-softCon UV/Vis+ Photochemistry Database" (www.photochemistry.org) is a large and comprehensive collection of EUV-VUV-UV-Vis-NIR spectral data and other photochemical information assembled from published peer-reviewed papers. The database contains photochemical data including absorption, fluorescence, photoelectron, and circular and linear dichroism spectra, as well as quantum yields and photolysis related data that are critically needed in many scientific disciplines. This manuscript gives an outline regarding the structure and content of the "science-softCon UV/Vis+ Photochemistry Database". The accurate and reliable molecular level information provided in this database is fundamental in nature and helps in proceeding further to understand photon, electron and ion induced chemistry of molecules of interest not only in spectroscopy, astrochemistry, astrophysics, Earth and planetary sciences, environmental chemistry, plasma physics, combustion chemistry but also in applied fields such as medical diagnostics, pharmaceutical sciences, biochemistry, agriculture, and catalysis. In order to illustrate this, we illustrate the use of the UV/Vis+ Photochemistry Database in four different fields of scientific endeavor.

Atmospheric Acetaldehyde: Importance of Air-Sea Exchange and a Missing Source in the Remote Troposphere.

We report airborne measurements of acetaldehyde (CH3CHO) during the first and second deployments of the National Aeronautics and Space Administration (NASA) Atmospheric Tomography Mission (ATom). The budget of CH3CHO is examined using the Community Atmospheric Model with chemistry (CAM-chem), with a newly-developed online air-sea exchange module. The upper limit of the global ocean net emission of CH3CHO is estimated to be 34 Tg a-1 (42 Tg a-1 if considering bubble-mediated transfer), and the ocean impacts on tropospheric CH3CHO are mostly confined to the marine boundary layer. Our analysis suggests that there is an unaccounted CH3CHO source in the remote troposphere and that organic aerosols can only provide a fraction of this missing source. We propose that peroxyacetic acid (PAA) is an ideal indicator of the rapid CH3CHO production in the remote troposphere. The higher-than-expected CH3CHO measurements represent a missing sink of hydroxyl radicals (and halogen radical) in current chemistry-climate models.

The Essential Role for Laboratory Studies in Atmospheric Chemistry.

Laboratory studies of atmospheric chemistry characterize the nature of atmospherically relevant processes down to the molecular level, providing fundamental information used to assess how human activities drive environmental phenomena such as climate change, urban air pollution, ecosystem health, indoor air quality, and stratospheric ozone depletion. Laboratory studies have a central role in addressing the incomplete fundamental knowledge of atmospheric chemistry. This article highlights the evolving science needs for this community and emphasizes how our knowledge is far from complete, hindering our ability to predict the future state of our atmosphere and to respond to emerging global environmental change issues. Laboratory studies provide rich opportunities to expand our understanding of the atmosphere via collaborative research with the modeling and field measurement communities, and with neighboring disciplines.

Experimentally Determined Site-Specific Reactivity of the Gas-Phase OH and Cl + i-Butanol Reactions Between 251 and 340 K.

Product branching ratios for the gas-phase reactions of i-butanol, (CH3)2CHCH2OH, with OH radicals (251, 294, and 340 K) and Cl atoms (294 K) were quantified in an environmental chamber study and used to interpret i-butanol site-specific reactivity. i-Butyraldehyde, acetone, acetaldehyde, and formaldehyde were observed as major stable end products in both reaction systems with carbon mass balance indistinguishable from unity. Product branching ratios for OH oxidation were found to be temperature-dependent with the α, ß, and γ channels changing from 34 ± 6 to 47 ± 1%, from 58 ± 6 to 37 ± 9%, and from 8 ± 1 to 16 ± 4%, respectively, between 251 and 340 K. Recommended temperature-dependent site-specific modified Arrhenius expressions for the OH reaction rate coefficient are (cm3 molecule-1 s-1): kα(T) = 8.64 × 10-18 × T1.91exp(666/T); kß(T) = 5.15 × 10-19 × T2.04exp(1304/T); kγ(T) = 3.20 × 10-17 × T1.78exp(107/T); kOH(T) = 2.10 × 10-18 × T2exp(-23/T), where kTotal(T) = kα(T) + kß(T) + kγ(T) + kOH(T). The expressions were constrained using the product branching ratios measured in this study and previous total phenomenological rate coefficient measurements. The site-specific expressions compare reasonably well with recent theoretical work. It is shown that use of i-butanol would result in acetone as the dominant degradation product under most atmospheric conditions.

Rate constants and kinetic isotope effects for methoxy radical reacting with NO2 and O2.

Relative rate studies were carried out to determine the temperature dependent rate constant ratio k1/k2a: CH3O· + O2 → HCHO + HO2· and CH3O· + NO2 (+M) → CH3ONO2 (+M) over the temperature range 250­333 K in an environmental chamber at 700 Torr using Fourier transform infrared detection. Absolute rate constants k2 were determined using laser flash photolysis/laser-induced fluorescence under the same conditions. The analogous experiments were carried out for the reactions of the perdeuterated methoxy radical (CD3O·). Absolute rate constants k2 were in excellent agreement with the recommendations of the JPL Data Evaluation panel. The combined data (i.e., k1/k2 and k2) allow the determination of k1 as 1.3(­0.5)(+0.9) × 10(­14) exp[−(663 ± 144)/T] cm(3) s(­1), corresponding to 1.4 × 10(­15) cm(3) s(­1) at 298 K. The rate constant at 298 K is in excellent agreement with previous work, but the observed temperature dependence is less than was previously reported. The deuterium isotope effect, kH/kD, can be expressed in the Arrhenius form as k1/k3 = (1.7(­0.4)(+0.5)) exp((306 ± 70)/T). The deuterium isotope effect does not appear to be greatly influenced by tunneling, which is consistent with a previous theoretical work by Hu and Dibble. (Hu, H.; Dibble, T. S., J. Phys. Chem. A 2013, 117, 14230­14242.)

Laboratory studies of organic peroxy radical chemistry: an overview with emphasis on recent issues of atmospheric significance.

Organic peroxy radicals (often abbreviated RO(2)) play a central role in the chemistry of the Earth's lower atmosphere. Formed in the atmospheric oxidation of essentially every organic species emitted, their chemistry is part of the radical cycles that control the oxidative capacity of the atmosphere and lead to the formation of ozone, organic nitrates, organic acids, particulate matter and other so-called secondary pollutants. In this review, laboratory studies of this peroxy radical chemistry are detailed, as they pertain to the chemistry of the atmosphere. First, a brief discussion of methods used to detect the peroxy radicals in the laboratory is presented. Then, the basic reaction pathways - involving RO(2) unimolecular reactions and bimolecular reactions with atmospheric constituents such as NO, NO(2), NO(3), O(3), halogen oxides, HO(2), and other RO(2) species - are discussed. For each of these reaction pathways, basic reaction rates are presented, along with trends in reactivity with radical structure. Focus is placed on recent advances in detection methods and on recent advances in our understanding of radical cycling processes, particularly pertaining to the complex chemistry associated with the atmospheric oxidation of biogenic hydrocarbons.

Branching ratios for the reaction of selected carbonyl-containing peroxy radicals with hydroperoxy radicals.

An important chemical sink for organic peroxy radicals (RO(2)) in the troposphere is reaction with hydroperoxy radicals (HO(2)). Although this reaction is typically assumed to form hydroperoxides as the major products (R1a), acetyl peroxy radicals and acetonyl peroxy radicals have been shown to undergo other reactions (R1b) and (R1c) with substantial branching ratios: RO(2) + HO(2) → ROOH + O(2) (R1a), RO(2) + HO(2) → ROH + O(3) (R1b), RO(2) + HO(2) → RO + OH + O(2) (R1c). Theoretical work suggests that reactions (R1b) and (R1c) may be a general feature of acyl peroxy and α-carbonyl peroxy radicals. In this work, branching ratios for R1a-R1c were derived for six carbonyl-containing peroxy radicals: C(2)H(5)C(O)O(2), C(3)H(7)C(O)O(2), CH(3)C(O)CH(2)O(2), CH(3)C(O)CH(O(2))CH(3), CH(2)ClCH(O(2))C(O)CH(3), and CH(2)ClC(CH(3))(O(2))CHO. Branching ratios for reactions of Cl-atoms with butanal, butanone, methacrolein, and methyl vinyl ketone were also measured as a part of this work. Product yields were determined using a combination of long path Fourier transform infrared spectroscopy, high performance liquid chromatography with fluorescence detection, gas chromatography with flame ionization detection, and gas chromatography-mass spectrometry. The following branching ratios were determined: C(2)H(5)C(O)O(2), Y(R1a) = 0.35 ± 0.1, Y(R1b) = 0.25 ± 0.1, and Y(R1c) = 0.4 ± 0.1; C(3)H(7)C(O)O(2), Y(R1a) = 0.24 ± 0.15, Y(R1b) = 0.29 ± 0.1, and Y(R1c) = 0.47 ± 0.15; CH(3)C(O)CH(2)O(2), Y(R1a) = 0.75 ± 0.13, Y(R1b) = 0, and Y(R1c) = 0.25 ± 0.13; CH(3)C(O)CH(O(2))CH(3), Y(R1a) = 0.42 ± 0.1, Y(R1b) = 0, and Y(R1c) = 0.58 ± 0.1; CH(2)ClC(CH(3))(O(2))CHO, Y(R1a) = 0.2 ± 0.2, Y(R1b) = 0, and Y(R1c) = 0.8 ± 0.2; and CH(2)ClCH(O(2))C(O)CH(3), Y(R1a) = 0.2 ± 0.1, Y(R1b) = 0, and Y(R1c) = 0.8 ± 0.2. The results give insights into possible mechanisms for cycling of OH radicals in the atmosphere.

Temperature-dependent branching ratios of deuterated methoxy radicals (CH2DO•) reacting with O2.

The methoxy radical is an intermediate in the atmospheric oxidation of methane, and the branching ratio (k(1a)/k(1b)) (CH(2)DO• + O(2) → CHDO + HO(2) (1a) and CH(2)DO• + O(2) → CH(2)O + DO(2) (1b)) strongly influences the HD/H(2) ratio in the atmosphere, which is widely used to investigate the global cycling of molecular hydrogen. By using the FT-IR smog chamber technique, we measured the yields of CH(2)O and CHDO from the reaction at 250-333 K. Kinetic modeling was used to confirm the suppression of secondary chemistry. The resulting branching ratios are well fit by an Arrhenius expression: ln(k(1a)/k(1b)) = (416 ± 152)/T + (0.52 ± 0.53), which agrees with the room-temperature results reported in the only previous study. The present results will be used to test our theoretical understanding of the role of tunneling in the methoxy + O(2) reaction, which is the prototype for the entire class of alkoxy + O(2) reactions.

Spectroscopic and kinetic properties of HO(2) radicals and the enhancement of the HO(2) self reaction by CH(3)OH and H(2)O.

The line center absorption cross sections and the rate constants for self-reaction of hydroperoxy radicals (HO(2)) have been examined in the temperature range of 253-323 K using pulsed laser photolysis combined with tunable diode laser absorption in the near-IR region. The transition probed was in the 2nu(1) OH overtone transition at 1506.43 nm. The temperature dependence of the rate constant (k) for the HO(2) + HO(2) reaction was measured relative to the recommended value at 296 K, giving k = (3.95 +/- 0.45) x 10(-13) x exp[(439 +/- 39)/T] cm(3) molecule(-1) s(-1) at a total pressure of 30 Torr (N(2) + O(2)). After normalizing our determination and previous studies at low pressure, we recommend k = (2.45 +/- 0.50) x 10(-13) x exp[(565 +/- 130)/T] cm(3) molecule(-1) s(-1) (0 < P < 30 Torr, 95% confidence limits). The observed rate coefficient, k(obs), increases linearly with CH(3)OH concentration, and the enhancement coefficient (k'), defined by k(obs) = k + k'[CH(3)OH], is found to be (3.90 +/- 1.87) x 10(-35) x exp[(3849 +/- 135)/T] cm(6) molecule(-2) s(-1) at 30 Torr. The analogous water vapor enhancement coefficient (k'') is (1.16 +/- 0.58) x 10(-36) x exp[(4614 +/- 145)/T] cm(6) molecule(-2) s(-1). The pressure-broadened HO(2) absorption cross section is independent of temperature in the range studied. The line center absorption cross sections at 1506.43 nm, after correction for instrumental broadening, are (4.3 +/- 1.1) x 10(-19), (2.8 +/- 0.7) x 10(-19), and (2.0 +/- 0.5) x 10(-19) cm(2)/molecule at total pressures of 0, 30, and 60 Torr, respectively (95% confidence limits).

The atmospheric oxidation of diethyl ether: chemistry of the C2H5-O-CH(O.)CH3 radical between 218 and 335 K.

The products of the Cl atom initiated oxidation of diethyl ether (DEE) were investigated at atmospheric pressure over a range of temperatures (218-335 K) and O(2) partial pressures (50-700 Torr), both in the presence and absence of NO(x). The major products observed at 298 K and below were ethyl formate and ethyl acetate, which accounted for approximately equal to 60-80% of the reacted diethyl ether. In general, the yield of ethyl formate increased with increasing temperature, with decreasing O(2) partial pressure, and upon addition of NO to the reaction mixtures. The product yield data show that thermal decomposition reaction 3, CH(3)CH(2)-O-CH(O.)CH(3)--> CH(3)CH(2)-O-CH=O + CH(3), and reaction 6 with O(2), CH(3)CH(2)-O-CH(O.)CH(3) + O(2)--> CH(3)CH(2)-O-C(=O)CH(3) + HO(2) are competing fates of the CH(3)CH(2)-O-CH(O )CH(3) radical, with a best estimate of k3/k6 approximately equal to 6.9 x 10(24) exp(-3130/T). Thermal decomposition via C-H or C-O bond cleavage are at most minor contributors to the CH(3)CH(2)-O-CH(O.)CH(3) chemistry. The data also show that the CH(3)CH(2)-O-CH(O.)CH(3) radical is subject to a chemical activation effect. When produced from the exothermic reaction of the CH(3)CH(2)-O-CH(OO.)CH(3) radical with NO, prompt decomposition via both CH(3)- and probably H-elimination occur, with yields of about 40% and < or =15%, respectively. Finally, at temperatures slightly above ambient, evidence for a change in mechanism in the absence of NO(x), possibly due to chemistry involving the peroxy radical CH(3)CH(2)-O-CH(OO.)CH(3), is presented.

Atmospheric chemistry of hydrazoic acid (HN3): UV absorption spectrum, HO reaction rate, and reactions of the N3 radical.

Processes related to the tropospheric lifetime and fate of hydrazoic acid, HN3, have been studied. The ultraviolet absorption spectrum of HN3 is shown to possess a maximum near 262 nm with a tail extending to at least 360 nm. The photolysis quantum yield for HN3 is shown to be approximately 1 at 351 nm. Using the measured spectrum and assuming unity quantum yield throughout the actinic region, a diurnally averaged photolysis lifetime near the earth's surface of 2-3 days is estimated. Using a relative rate method, the rate coefficient for reaction of HO with HN3 was found to be (3.9 +/-0.8) x 10(-12) cm3 molecule(-1) s(-1), substantially larger than the only previous measurement. The atmospheric HN3 lifetime with respect to HO oxidation is thus about 2-3 days, assuming a diurnally averaged [HO] of 10(6) molecule cm(-3). Reactions of N3, the product of the reaction of HO with HN3, were studied in an environmental chamber using an FTIR spectrometer for end-product analysis. The N3 radical reacts efficiently with NO, producing N2O with 100% yield. Reaction of N3 with NO2 appears to generate both NO and N2O, although the rate coefficient for this reaction is slower than that for reaction with NO. No evidence for reaction of N3 with CO was observed, in contrast to previous literature data. Reaction of N3 with O2 was found to be extremely slow, k < 6 x 10(-20) cm3 molecule(-1) s(-1), although this upper limit does not necessarily rule out its occurrence in the atmosphere. Finally, the rate coefficient for reaction of Cl with HN3 was measured using a relative rate method, k = (1.0+/-0.2) x 10(-12) cm3 molecule(-1) s(-1).

Rates and mechanisms for the reactions of chlorine atoms with iodoethane and 2-iodopropane.

The reaction of Cl atoms with iodoethane has been studied via a combination of laser flash photolysis/resonance fluorescence (LFP-RF), environmental chamber/Fourier transform (FT)IR, and quantum chemical techniques. Above 330 K, the flash photolysis data indicate that the reaction proceeds predominantly via hydrogen abstraction. The following Arrhenius expressions (in units of cm3 molecule(-1) s(-1)) apply over the temperature range 334-434 K for reaction of Cl with CH3CH2I (k4(H)) and CD3CD2I (k4(D)): k4(H) = (6.53 +/- 3.40) x 10(-11) exp[-(428 +/- 206)/T] and k4(D) = (2.21 +/- 0.44) x 10(-11) exp[-(317 +/- 76)/T]. At room temperature and below, the reaction proceeds both via hydrogen abstraction and via reversible formation of an iodoethane/Cl adduct. Analysis of the LFP-RF data yields a binding enthalpy (0 K) for CD3CD2I x Cl of 57 +/- 10 kJ mol(-1). Calculations using density functional theory show that the adduct is characterized by a C-I-Cl bond angle of 84.5 degrees; theoretical binding enthalpies of 38.2 kJ/mol, G2'[ECP(S)], and 59.0 kJ mol(-1), B3LYP/ECP, are reasonably consistent with the experimentally derived result. Product studies conducted in the environmental chamber show that hydrogen abstraction from both the -CH2I and -CH3 groups occur to a significant extent and also provide evidence for a reaction of the CH3CH2I x Cl adduct with CH3CH2I, leading to CH3CH2Cl formation. Complementary environmental chamber studies of the reaction of Cl atoms with 2-iodopropane, CH3CHICH3, are also presented. As determined by relative rate methods, the reaction proceeds with an effective rate coefficient, k6, of (5.0 +/- 0.6) x 10(-11) cm3 molecule(-1) s(-1) at 298 K. Product studies indicate that this reaction also occurs via two abstraction channels (from the CH3 groups and from the -CHI- group) and via reversible adduct formation.