Marcel Nicolet

Bio: Marcel Nicolet is an academic researcher from Pennsylvania State University. The author has contributed to research in topics: Photodissociation & Stratosphere. The author has an hindex of 20, co-authored 33 publications receiving 1788 citations.

The photochemistry of atmospheric water vapor

Abstract: Solar radiation dissociates water vapor into hydrogen atoms and hydroxyl radicles Hydrogen and hydrogen peroxide molecules, and perhydroxyl radicles, are also produced as a result of subsequent chemical reactions with the allotropic forms of oxygen The rate of the oxidizing processes falls off more rapidly with increase of altitude than does that of the reducing processes, and the hydrogen compounds are almost completely broken down at about the 90-km level (or even lower) There is a continual escape of the hydrogen atoms into interplanetary space; but the liberated oxygen atoms remain in the atmosphere, and the number that must thus have been added in geological time seems to be comparable with the number now present Consideration of the general equilibrium reveals several features of interest, such as, for example, the existence of a thin layer of molecular hydrogen In spite of the prominence of the Meinel bands, the concentration of hydroxyl radicles is quite small It is thought that these radicles are excited during, rather than after, their formation The mechanism proposed is two body collisions between hydrogen atoms and ozone molecules

Nitrogen oxides in the chemosphere

Abstract: A study is made of the various reactions in which nitrogen oxides are involved in the chemosphere. The hydrogen compounds do not play an important role, and it is found that the essential aeronomic reactions depend on ozone and atomic oxygen. Thus, the ratio of nitrogen dioxide to nitric oxide can be determined. The absolute values of the NO2 and NO concentrations depend on the dissociation of molecular nitrogen in the chemosphere. The chemical conditions cannot be applied in the mesosphere, since the lifetime of NO is relatively long, and a downward transport is involved. Very special assumptions about chemical reactions would be necessary to reconcile the photochemical picture and the observational results. The introduction of ionic reactions, considered in an accompanying paper, will lead to a correct interpretation.

Ionospheric processes and nitric oxide

Abstract: Analysis of ionic processes in the ionosphere leads to the conclusion that nitric oxide and its ion are produced by a reaction between nitrogen molecules and molecular oxygen ions. Such a process implies a substantial increase of nitric oxide in the E layer to a value greater than the photochemical concentration. In the lower D region, N2+ and O2+ ions produced by cosmic rays are transformed into NO+ ions. A quantitative estimate shows that these conclusions are consistent with observational data in the chemosphere and ionosphere and also suggests the explanation of the night airglow continuum.

The influence of nitrogen oxides on the atmospheric ozone content

Abstract: The probable importance of NO and NO2 in controlling the ozone concentrations and production rates in the stratosphere is pointed out. Observations on and determinations of nitric acid concentrations in the stratosphere by Murcray et al. (1968) and Rhine et al. (1969) support the high NO and NO2 concentrations indicated by Bates/Hays (1967). Some processes which may lead to production of nitric acid are discussed. The importance of O (1S), possibly produced in the ozone photolysis below 2340 A, on the ozone photochemistry is mentioned.

Rapid calculation of radiative heating rates and photodissociation rates in inhomogeneous multiple scattering atmospheres

Abstract: The solution of the generalized two-stream approximation for radiative transfer in homogeneous multiple scattering atmospheres is extended to vertically inhomogeneous atmospheres in a manner which is numerically stable and computationally efficient. It is shown that solar energy deposition rates, photolysis rates, and infrared cooling rates all may be calculated with the simple modifications of a single algorithm. The accuracy of the algorithm is generally better than 10 percent, so that other uncertainties, such as in absorption coefficients, may often dominate the error in calculation of the quantities of interest to atmospheric studies.

Reduction of stratospheric ozone by nitrogen oxide catalysts from supersonic transport exhaust.

Abstract: Although a great deal of attention has been given to the role of water vapor from supersonic transport (SST) exhaust in the stratosphere, oxides of nitrogen from SST exhaust pose a much greater threat to the ozone shield than does an increase in water. The projected increase in stratospheric oxides of nitrogen could reduce the ozone shield by about a factor of 2, thus permitting the harsh radiation below 300 nanometers to permeate the lower atmosphere.

The effect of breaking gravity waves on the dynamics and chemical composition of the mesosphere and lower thermosphere

Abstract: The influence of breaking gravity waves on the dynamics and chemical composition of the 60- to 110-km region has been investigated with a two-dimensional dynamical/chemical model that includes a parameterization of gravity wave drag and diffusion. The momentum deposited by breaking waves at mesospheric altitudes reverses the zonal winds, drives a strong mean meridional circulation, and produces a very cold summer and warm winter mesopause, in general agreement with observations. The seasonal variations of the computed eddy diffusion coefficient are consistent with the behavior of mesospheric turbulence inferred from MST radar echoes. In particular, it is found that eddy diffusion is strong in summer and winter but much weaker at the equinoxes and that this seasonal behavior has important consequences for the distribution of chemical species. Comparison between computed atomic oxygen and ozone, and the abundances of these constituents inferred from the 557.7-nm and 1.27-μm airglow emissions, reveals excellent agreement. The consistency between model results and these diverse types of observations lends strong support to the hypothesis that gravity waves play a very important role in determining the zonally averaged structure of the mesosphere and lower thermosphere.

Photodissociation in the atmosphere: 1. Actinic flux and the effects of ground reflections and clouds

Abstract: The actinic flux must be distinguished from other radiometric quantities such as the irradiance. This distinction shows that (1) a fraction of the recent atmospheric chemistry literature contains improperly calculated rates of photodissociation, and (2) photodissociation rate coefficients inside clouds, and particularly inside cloud droplets, can frequently exceed the clear-sky values, in contrast to current usage in cloud chemistry models. Both of these findings are traceable to the ∼2 cos θ factor incurred in the actinic flux (but not in the irradiance) when the solar beam is scattered and diffused into nearly isotropic light.