Theoretical investigation of the relative impacts of water and ammonia on the tropospheric conversion of N2O5 to HNO3

Reaction of ammonia with N₂O₅, without and with the assistance of water, in the troposphere has been investigated by electronic structure and chemical kinetic calculations. The whole process has been compared against the hydrolysis reaction, uncatalyzed as well as water and ammonia catalyzed. A comparative study between hydrolysis and ammonolysis based on relative rates has been extensively carried out. The analysis reveals that ammonolysis has negligible practical atmospheric implication compared to hydrolysis. The former could have a significant contribution in tropospheric HNO₃ formation only at 0 km altitude under two conditions: either on a local scale, where ammonia concentration could reach around a thousand times its global average value, or under very low humidity and at a lower temperature. Relative rate studies also suggest that the catalytic effect of both ammonia and water is negligibly small in determining the atmospheric fate of N₂O₅via gas phase hydrolysis and ammonolysis.

100 Years of Progress in Gas-Phase Atmospheric Chemistry Research

Remarkable progress has occurred over the last 100 years in our understanding of atmospheric chemical composition, stratospheric and tropospheric chemistry, urban air pollution, acid rain, and the formation of airborne particles from gas-phase chemistry. Much of this progress was associated with the developing understanding of the formation and role of ozone and of the oxides of nitrogen, NO and NO2, in the stratosphere and troposphere. The chemistry of the stratosphere, emerging from the pioneering work of Chapman in 1931, was followed by the discovery of catalytic ozone cycles, ozone destruction by chlorofluorocarbons, and the polar ozone holes, work honored by the 1995 Nobel Prize in Chemistry awarded to Crutzen, Rowland, and Molina. Foundations for the modern understanding of tropospheric chemistry were laid in the 1950s and 1960s, stimulated by the eye-stinging smog in Los Angeles. The importance of the hydroxyl (OH) radical and its relationship to the oxides of nitrogen (NO and NO2) emerged. The chemical processes leading to acid rain were elucidated. The atmosphere contains an immense number of gas-phase organic compounds, a result of emissions from plants and animals, natural and anthropogenic combustion processes, emissions from oceans, and from the atmospheric oxidation of organics emitted into the atmosphere. Organic atmospheric particulate matter arises largely as gas-phase organic compounds undergo oxidation to yield low-volatility products that condense into the particle phase. A hundred years ago, quantitative theories of chemical reaction rates were nonexistent. Today, comprehensive computer codes are available for performing detailed calculations of chemical reaction rates and mechanisms for atmospheric reactions. Understanding the future role of atmospheric chemistry in climate change and, in turn, the impact of climate change on atmospheric chemistry, will be critical to developing effective policies to protect the planet.

The Reaction Rate Constant of Chlorine Nitrate Hydrolysis

The first-order rate constant for the decomposition of chlorine nitrate (ClONO2) by water in a cyclic 1:3 complex at stratospheric temperatures is shown to be close to the values for the hydrolysis rate coefficient of chlorine nitrate on an ice surface determined in the laboratory. On the other hand the rate constants calculated for the cyclic 1:1 and 1:2 complexes are much lower than the experimental results. From the mechanistic point of view the reaction is found to be similar to a SN2 mechanism and coupled with water-mediated proton transfer in accordance with the intriguing findings of Bianco and Hynes [R. Bianco, J. T. Hynes. J. Phys. Chem. A 1998, 102, 309-314]. The function of additional water molecules is to act as a catalyst, that is, to accelerate the hydrolysis process. Quantum-mechanical tunneling is negligible above 125 K in the 1:3 complex and above 175 K in the 1:2 complex. At temperatures below these limits all involved protons tunnel through the barrier at energies at least 5 kcalmol(-1) below the barrier-top in a concerted, but asynchronous manner.