Gas-phase Atmospheric Chemistry Research Articles

Graph characterization of higher-order structure in atmospheric chemical reaction mechanisms

Abstract Atmospheric chemical reactions play an important role in air quality and climate change. While the structure and dynamics of individual chemical reactions are fairly well understood, the emergent properties of the entire atmospheric chemical system, which can involve many different species that participate in many different reactions, are not well described. In this work, we leverage graph-theoretic techniques to characterize patterns of interaction (“motifs”) in three different representations of gas-phase atmospheric chemistry, termed “chemical mechanisms.” These widely used mechanisms, the master chemical mechanism, the GEOS-Chem mechanism, and the Super-Fast mechanism, vary dramatically in scale and application, but they all generally aim to simulate the abundance and variability of chemical species in the atmosphere. This motif analysis quantifies the fundamental patterns of interaction within the mechanisms, which are directly related to their construction. For example, the gas-phase chemistry in the very small Super-Fast mechanism is entirely composed of bimolecular reactions, and its motif distribution matches that of an individual bimolecular reaction well. The larger and more complex mechanisms show emergent motif distributions that differ strongly from any specific reaction type, consistent with their complexity. The proposed motif analysis demonstrates that while these mechanisms all have a similar design goal, their higher-order structure of interactions differs strongly and thus provides a novel set of tools for exploring differences across chemical mechanisms.

Impacts of ocean biogeochemistry on atmospheric chemistry

Ocean biogeochemistry involves the production and consumption of an array of organic compounds and halogenated trace gases that influence the composition and reactivity of the atmosphere, air quality, and the climate system. Some of these molecules affect tropospheric ozone and secondary aerosol formation and impact the atmospheric oxidation capacity on both regional and global scales. Other emissions undergo transport to the stratosphere, where they contribute to the halogen burden and influence ozone. The oceans also comprise a major sink for highly soluble or reactive atmospheric gases. These issues are an active area of research by the SOLAS (Surface Ocean Lower Atmosphere) community. This article provides a status report on progress over the past decade, unresolved issues, and future research directions to understand the influence of ocean biogeochemistry on gas-phase atmospheric chemistry. Common challenges across the subject area involve establishing the role that biology plays in controlling the emissions of gases to the atmosphere and the inclusion of such complex processes, for example involving the sea surface microlayer, in large-scale global models.

The Caltech Photooxidation Flow Tube reactor: design, fluid dynamics and characterization

Abstract. Flow tube reactors are widely employed to study gas-phase atmospheric chemistry and secondary organic aerosol (SOA) formation. The development of a new laminar-flow tube reactor, the Caltech Photooxidation Flow Tube (CPOT), intended for the study of gas-phase atmospheric chemistry and SOA formation, is reported here. The present work addresses the reactor design based on fluid dynamical characterization and the fundamental behavior of vapor molecules and particles in the reactor. The design of the inlet to the reactor, based on computational fluid dynamics (CFD) simulations, comprises a static mixer and a conical diffuser to facilitate development of a characteristic laminar flow profile. To assess the extent to which the actual performance adheres to the theoretical CFD model, residence time distribution (RTD) experiments are reported with vapor molecules (O3) and submicrometer ammonium sulfate particles. As confirmed by the CFD prediction, the presence of a slight deviation from strictly isothermal conditions leads to secondary flows in the reactor that produce deviations from the ideal parabolic laminar flow. The characterization experiments, in conjunction with theory, provide a basis for interpretation of atmospheric chemistry and SOA studies to follow. A 1-D photochemical model within an axially dispersed plug flow reactor (AD-PFR) framework is formulated to evaluate the oxidation level in the reactor. The simulation indicates that the OH concentration is uniform along the reactor, and an OH exposure (OHexp) ranging from ∼ 109 to ∼ 1012 molecules cm−3 s can be achieved from photolysis of H2O2. A method to calculate OHexp with a consideration for the axial dispersion in the present photochemical system is developed.

Comparison of Peroxy Radical Concentrations at Several Contrasting Sites

Abstract The importance of the peroxy radicals in gas phase atmospheric chemistry has prompted the development of a number of methods for their measurement. These have resulted in several reports of H02 and R02 levels around the globe. The chemical amplifier has been used for measurements in seven intensive measurement campaigns over the last few years. The measured levels tell a great deal about the tropospheric chemistry of these radicals and also tell much about our understanding of the processes that create and destroy them. The results of these measured levels at sites that differ greatly in oxidant precursor concentration as well as photochemical activity are compared. A steady-state model has been developed to help explain the radical observations, and its use is discussed.

Effects of turbulence on gas-phase atmospheric chemistry: Calculation of the relationship between time scales for diffusion and chemical reaction

Non-uniform mixing of gas-phase trace species may limit the accuracy of the predictions of Eulerian transport/transformation models if the chemical reactions are rapid enough to be diffusion limited. If a reaction is diffusion limited, its average reaction rate might not be accurately represented by those models that assume instantaneous uniform mixing. One possible consequence of this artificial dilution is the overprediction of ozone and hydroxyl radicals. We have determined which reactions in the Regional Acid Deposition Model Gas-Phase Chemical Mechanism (Stockwell et al., 1990) are diffusion limited for a typical atmospheric condition through the calculation of Damkohler numbers. Damkohler numbers are defined to be the ratio of the diffusion mixing time to the chemical reaction time for a given chemical reaction (McRae et al., 1982; Hill, 1976). The reactions of hydroxyl radicals and the reactions of peroxy radicals with NO are diffusion limited under typical atmospheric conditions. Both sets of reactions are especially significant because NOx and organic species strongly affect ozone and hydroxyl radical concentrations. It is suggested that Damkohler numbers could be used to help determine the placement of Eulerian model boundaries and to determine model grid structure.

Gas-phase atmospheric chemistry of selected thiocarbamates

The kinetics and product of the gas-phase reactions of OH radicals, NO 3 radicals, and O 3 with the thiocarbamates S-methyl N,N-dimethylthiocarbamate (MDTC), S-ethyl N,N-dipropylthiocarbamate (EPTC), and S-ethyl N-cyclohexyl-N-ethylthiocarbamate (cycloate) have been studied at 298±2 K and ∼735 Torr air

Gas-phase atmospheric chemistry of 1- and 2-nitronaphthalene and 1,4-naphthoquinone

The gas-phase photolysis of 1- and 2-nitronaphthalene and the reactions of these nitroarenes with OH and NO 3 radicals, N 2O 5 and O 3 have been investigated at 298±2 K and 1 atm total pressure of air. Since 1,4-naphthoquinone was observed as a photolysis product of 1-nitronaphthalene in air, its photolysis and reactions with OH and NO 3 radicals, N 2O 5 and O 3 were also studied. Using relative rate methods for the determination of rate constants for the reactions with OH radicals and NO 3 radicals and/or N 2O 5, and rate constants of k(OH+cyclohexane) = 7.49 × 10 −12cm 3molecule −1 s −1 and k( NO 3 + propene) = 9.4 × 10 15 cm 3 molecule −1s −1 and the equilibrium constant for the NO 3 + NO 2 ahN 2 O 5 reactions of K = 2.3 × 10 −11 cm 3molecule −1, the rate constants obtained for these reactions were (in cm 3molecule −1s −1 units): for reaction with the OH radical; 1-nitronaphthalene, (5.4±1.8)× 10 −12; 2-nitronaphthalene, (5.6±0.9)× 10 −12; 1,4-naphthoquinone, (3.1±1.2) × 10 −12; for reaction with the NO 3 radical; 1-nitronaphthalene, ≤7.2× 10 −15; 2-nitronaphthalene, ≤7.2×10 −15; 1,4-naphthoquinone, <1 × 10 −15; for reaction with N 2O 5; 1-nitronaphthalene, (1.3±0.7) × 10 −18; 2-nitronaphthalene, (1.2±0.5) ×10 −18; 1,4-naphthoquinone, <4×10 −19; for reaction with O 3; 1-nitronaphthalene, include the uncertainties in the rate constants for the reference reactions or in the eq <6×10 −19; 2-nitronaphthalene, <6×10 −19; 1,4-naphthoquinone, <2×10 −19 (where the cited error limits do notuilibrium constant K). The photolysis reactions were investigated under blacklamp irradiation and under natural solar irradiation. With solar irradiation at an NO 2 photolysis rate of J NO 2 = 4.3 × 10 −3s −1, the outdoor photolysis rates were (in s −1 units): 1-nitronaphthalene, (1.37±0.10) × 10 −4; 2-nitronaphthalene, (1.06±0.08)× 10 −41,4-naphthoquinone, ~ 9 × 10 −5. These data show that under atmospheric conditions the dominant loss process of 1- and 2-nitronaphthalene and 1,4-naphthoquinone is photolysis, with calculated photolysis lifetimes of ~2 h for all three compounds. The OH radical reactions are calculated to be an order of magnitude less important than photolysis as an atmospheric loss process, with the N 2O 5, NO 3 radical and O 3 reactions being of negligible importance.