The nitrate radical: Physics, chemistry, and the atmosphere

The late Archean atmosphere was probably rich in biologically generated CH4 and may well have contained a hydrocarbon haze layer similar to that observed today on Saturn's moon, Titan. Here we present a detailed model of the photochemistry of haze formation in the early atmosphere, and we examine the effects of such a haze layer on climate and ultraviolet radiation. We show that the thickness of the haze layer was limited by a negative feedback loop: A haze optical depth of more than ∼0.5 in the visible would have produced a strong “antigreenhouse effect,” thereby cooling the surface and slowing the rate at which CH4 was produced. Given this climatic constraint on its visible optical depth, the amount of UV shielding provided by the haze can be estimated from knowledge of the optical properties and size distribution of the haze particles. Contrary to previous studies [Sagan and Chyba, 1997], we find that when the finite size of the particles is taken into account, the amount of UV shielding provided by the haze is small. Thus NH3 should have been rapidly photolyzed and should not have been sufficiently abundant to augment the atmospheric greenhouse effect. We also examine the question of whether photosynthetically generated O2 could have accumulated beneath the haze layer. For the model parameters considered here, the answer is “no”: The upper limit on ground level O2 concentrations is ∼10−6 atm, and a more realistic estimate for pO2 during the late Archean is 10−8 atm. The stability of both O2 and NH3 is sensitive to the size distribution and optical properties of the haze particles, neither of which is well known. Further theoretical and laboratory work is needed to address these uncertainties.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2000JE001448

UV shielding of NH3 and O2 by organic hazes in the Archean atmosphere

A one-dimensional photochemical model is used to analyze the photochemistries of CH4 and HCN in the primitive terrestrial atmosphere. CH4, N2, and HCN photolysis are examined. The background atmosphere and boundary conditions applied in the analysis are described. The formation of HCN as a by-product of N2 and CH4 photolysis is investigated; the effects of photodissociation and rainfall on HCN is discussed. The low and high CH4 mixing ratios and radical densities are studied.

Photochemistry of methane and the formation of hydrocyanic acid (HCN) in the Earth's early atmosphere

Thermochemical and chemical kinetic data for fluorinated hydrocarbons

Kinetics of CH2OO Criegee intermediate reactions with SO2, NO2, NO, H2O and CH3CHO and CH2I radical reactions with NO2 are reported as a function of pressure at 295 K. Measurements were made under pseudo-first-order conditions using flash photolysis of CH2I2-O2-N2 gas mixtures in the presence of excess co-reagent combined with monitoring of HCHO reaction products by laser-induced fluorescence (LIF) spectroscopy and, for the reaction with SO2, direct detection of CH2OO by photoionisation mass spectrometry (PIMS). Rate coefficients for CH2OO + SO2 and CH2OO + NO2 are independent of pressure in the ranges studied and are (3.42 ± 0.42) × 10(-11) cm(3) s(-1) (measured between 1.5 and 450 Torr) and (1.5 ± 0.5) × 10(-12) cm(3) s(-1) (measured between 25 and 300 Torr), respectively. The rate coefficient for CH2OO + CH3CHO is pressure dependent, with the yield of HCHO decreasing with increasing pressure. Upper limits of 2 × 10(-13) cm(3) s(-1) and 9 × 10(-17) cm(3) s(-1) are placed on the rate coefficients for CH2OO + NO and CH2OO + H2O, respectively. The upper limit for the rate coefficient for CH2OO + H2O is significantly lower than has been reported previously, with consequences for modelling of atmospheric impacts of CH2OO chemistry.

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Kinetics of CH2OO reactions with SO2, NO2, NO, H2O and CH3CHO as a function of pressure

The reactions of S + OH SO + H (1) and SO + OH SO2 + H (2) were studied in a discharge flow reactor coupled to an EPR spectrometer. The rate constants obtained under the pseudo-first-order conditions with an excess of S or SO were found to be k1 = (6.6 ± 1.4) × 10−11 and k2 = (8.4 ± 1.5) × 10−11 at room temperature. Units are cm3/molec·sec. Besides no reactivity was observed between S and CO2 at 298 K and between CIO and SO2 up to 711 K.

Kinetic study of some elementary reactions of sulfur compounds including reactions of S and SO with OH radicals

The kinetics of the reactions of Cl atoms with HNO3, H2O2, and HO2 have been studied by the discharge flow–mass spectrometric technique. The following rate constants were obtained (in cm3 molecule−1⋅s−1): Cl+HNO3→HCl+NO3, k=1.5 ×10−11 exp(−8700/RT) in the range 298–633 K; Cl+H2O2 →HCl+HO2, k= (4.0±0.4) ×10−13 at 298 K; Cl+HO2→HCl +O2, k= (6.8±2.5) ×10−11 at 298 K. The stratospheric implication of these data is discussed, and it is estimated that the Cl+HO2 reaction could have a comparable effect to the Cl+CH4 reaction as a sink of chlorine atoms in the stratosphere.

Kinetic study of the reactions of Cl atoms with HNO3, H2O2, and HO2

EPR kinetic study of the reactions of F and Br atoms with H2CO

Detection of atoms by mass spectrometry has been used to study the reactions of hydrogen azide, HN3, with H atoms and active nitrogen, in a fast flow reactor at pressures of about 1 torr Stoichiometry and products of the H + HN3 reaction have been determined and the rate constant of the initial step, assumed to be H + HN3 NH2 + N2, was found to be 254 × 10−11 exp (−4600/RT) cm3 molecule−1 s−1, in the temperature range of 300–460K The formation of NH3 and H2 products has been discussed from the different secondary steps which may occur in the mechanism For the reaction of active nitrogen with HN3, evidence has been found for the participation of excited nitrogen molecules produced by a microwave discharge through molecular nitrogen The influence of excited nitrogen molecules has been reduced by lowering the gas flow velocity It was then possible to study the N + HN3 reaction for which the rate constant of the initial step was found to be 49 × 10−15 cm3 molecule−1 s−1 at room temperature Finally, the occurrence of these elementary reactions has been discussed in the mechanism of the decomposition flame of HN3

J. Combourieu

Papers

Kinetic study of some elementary reactions of sulfur compounds including reactions of S and SO with OH radicals

Kinetic study of the reactions of Cl atoms with HNO3, H2O2, and HO2

EPR kinetic study of the reactions of F and Br atoms with H2CO

The reactions of atomic hydrogen and active nitrogen with hydrogen azide

Determination of the rate constants of the reactions of N3 and NCl free radicals using a simulation technique