Sheo S. Prasad

The lower ionosphere of Titan

Abstract Ionization of the atmosphere of Titan by galactic cosmic rays is a very significant process throughout the altitude range of 100 to 400 km. An approximate form of the Boltzmann equation for cosmic ray transport has been used to obtain local ionization rates. Models of both ion and neutral chemistry have been employed to compute electron and ion density profiles for three different values of the H 2 /CH 4 abundance ratio. The peak electron density is of the order 10 3 cm −3 . The most abundant positive ions are C 2 H 9 + and C 3 H 9 + , while the predicted densities of the negative ions H − and CH 3 − are very small ( −4 that of the positive ions). It is suggested that inclusion of the ion chemistry is important in the computation of the H and CH 3 density profiles in the lower ionosphere.

Cosmic ray synthesis of organic molecules in Titan's atmosphere

The possible synthesis of organic molecules by the absorption of galactic cosmic rays in an N2-CH4-H2 Titan model atmosphere has been studied. The cosmic-ray-induced ionization results in peak electron densities of 2000/cu cm, with NH(+), C3H9(+), and C4H9(+) being among the important positive ions. Details of the ion and neutral chemistry relevant to the production of organic molecules are discussed. The potential importance of N(2D) reactions with CH4 and H2 is also demonstrated. Although the integrated production rate of organic matter due to the absorption of the cosmic ray cascade is much less than that by solar ultraviolet radiation, the production of nitrogen-bearing organic molecules by cosmic rays may be greater.

Cosmic ray ionization of the Jovian atmosphere

Abstract An approximate form of the Boltzmann equation has been used to obtain local ionization rates due to the absorption of galactic cosmic rays in the Jovian atmosphere. It is shown that the muon flux component of the cosmic ray-induced cascade may be especially importannt in ionizing the atmosphere at levels where the total number density exceeds 10 19 cm −3 (well below the ionospheric layers produced by solar euv). A model containing both positive and negative ion reactions has been employed to compute equilibrium electron and ion number densities. Peak electron number densities on the order of 10 3 cm −3 may be expected even at relatively low magnetic latitudes. The dominant positive ions are NH 4 + and C n H m + cluster ions, with n ⩾ 2; it is suggested that the absorption of galactic cosmic ray energy at such relatively high pressures in the Jovian atmosphere (M ∾ 10 18 to 10 20 cm −3 ) and the subsequent chemical reactions may be instrumental in the local formation of complex hydrocarbons.

Potential atmospheric sources and sinks of nitrous oxide: 2. Possibilities from excited O2, “embryonic” O3, and optically pumped excited O3

Nitrous oxide (N2O) is an important constituent of the atmosphere because it is not only the dominant source of ozone (O3) destroying odd nitrogen in the stratosphere but also a greenhouse gas. Unfortunately, the classical chemistry of N2O has at least two problems, namely, (1) a possible source gap in the source-sink budget and (2) difficulties in explaining the observed heavy isotope enrichments. While the source gap can, in principle, be closed by sources of the classical type, the observed isotopic anomaly calls for atmospheric sources and sinks. This need motivated the present study, which has brought to light a totally unsuspected aspect of atmospheric chemistry, that is, short-lived (10 ps≤lifetime≤10 ns) excited species (e.g., O2(B 3Σ) and electronically excited O3) may be quite significant in the N2O photochemistry despite their relative insignificance in many other cases. Other specific findings of the present study are the following: (1) O2(B 3Σ), which is efficiently produced in the stratosphere by resonant absorption in the Schumann-Runge bands, is a possible source of N2O with a maximum strength of the order of 60 N2O cm−3 s−1 in the vicinity of 30 km. (2) The electronic energy in O2(A3Σ) is insufficient so that the potential reaction O2(A) + N2→ N2O + O is only marginally possible unless assisted by high-vibrational excitation (v≥6) in O2(A). This source, if it exists, may be significant only at higher altitudes around 50 km. (3) Bimolecular and possibly termolecular reactions of O2(b1Σ) have the potential to be sinks of N2O. (4) The O2(B3Σ) source, while insignificant for the source deficiency problem, may produce N2O with an isotopic enrichment close to the observations since its optical pumping of O2(B3Σ) is isotope sensitive. (5) The O2(B3Σ) source has another intriguing feature, namely, it maximizes in the same altitude region where UARS/cryogenic limb array etalon spectrometer (CLAES) and cryogenic whole air sampler (CWAS) observations show a fold in the N2O mixing ratios which is more pronounced than the same in CH4 and chlorofluorocarbons which have no atmospheric sources. (6) The O2-mediated production of N2O from O(1D) via the “embryonic” O3 is potentially more efficient in the atmosphere relative to the highly inefficient direct reaction N2+ O(1D)+M→N2O+M. The optical pumping of the ground state O3 to its electronically excited states may also lead to potentially substantial N2O production. The combined production of N2O from these two processes may approach 25% of the currently estimated microbiological production. (7) Since O3 is already isotopically enriched, it is quite possible that the N2O produced by the optically pumped excited O3 might also show isotopic enrichment. (8) If the current World Meteorological Organization position that the classical sources and sinks of N2O are in balance is accepted, then the new atmospheric sources discussed here suggest hitherto unrecognized, mainly biogenic, sinks of this species which significantly reduce the net emission of N2O from the soil and the aquatic environments. It is hoped that the discussions in this paper will create a greater appreciation of the problems with the classical N2O chemistry and the potentials of the new chemistry and will thereby stimulate further research. With this hope some suggestions for new laboratory and computational chemistry experiments are also made. In particular, new experiments to test the proposed N2O production mechanisms must avoid both the initial presence and the subsequent build up of O3 in the reaction chamber.

Atmospheric chemistry of the reaction ClO + O2 ↔ ClO · O2: Where it stands, what needs to be done, and why?

Possible existence and chemistry of ClO x O{sub 2} was originally proposed to explain the Norrish-Neville effect that O{sub 2} suppresses chlorine photosensitized loss of ozone. It was also thought that ClO x O{sub 2} might have some atmospheric chemistry significance. Recently, doubts have been cast on this proposal, because certain laboratory data seem to imply that the equilibrium constant of the title reaction is so small that ClO x O{sub 2} may be too unstable to matter. However, those data create only a superficial illusion to that effect, because on a closer analysis they do not disprove a moderately stable and chemically significant ClO x O{sub 2}. Furthermore, our state-of-the-science accurate computational chemistry calculations also suggest that ClO x O{sub 2} may be a weakly bound ClOOO radical with a reactive {sup 2}A ground electronic state. There is therefore a need to design and perform definitive experimental tests of the existence and chemistry of the ClO x O{sub 2} species, which we discuss and which have the potential to mediate the chlorine-catalyzed stratospheric ozone depletion.

Potential atmospheric sources and sinks of nitrous oxide: 3. Consistency with the observed distributions of the mixing ratios

This paper discusses the idea that the potential atmospheric sources and sinks of nitrous oxide (N2O) discussed by Prasad [1994, this issue] may not be inconsistent with the observed vertical profiles of the N2O mixing ratios. For this purpose the vertical distributions of N2O mixing ratios were calculated in one-dimensional model atmosphere with and without the new sources and sinks. It was found that the inclusion of the new sources and sinks in the model atmosphere predicts very nearly the same mixing ratios as are predicted by the use of only the classical chemistry of N2O. This means that these sources and sinks are in accord with atmospheric observations also, given the very good agreement between the classical chemistry predictions and the observations. The important issue of the observed correlations of the N2O mixing ratios with those of other long-lived tracers such as CH4 and CF2Cl2 is also discussed. The key point is that while similar chemistries (i.e., similar nature and spatial distributions of sources and sinks) ensure good cross-species correlations, the reverse is not necessarily true. It is therefore possible that N2O may be correlated with CH4 or chlorofluorocarbons despite differences in their chemistry suggested by the potential new sources and sinks of N2O.

Asymmetrical ClO3: its possible formation from ClO and O2 and its possible reactions

Abstract An analysis of recent accurate experimental studies of Cl2-photosensitized O3 decomposition, in which O3 disappearance and OClO formation were directly monitored, suggests the possibility that the suppression of the quantum yield in the presence of O2 may be due to the formation of asymmetrical chlorine trioxide (ClO·O2). Other intermediaries, such as Cl2O2, which may also form in the system are not thought to explain the observations. In addition to its capacity to oxidize, which it shares with other peroxo compounds, asymmetrical ClO3 appears to undergo an interesting class of reactions in which the loosely bound O2 adduct is relatively easily displaced by reactive atoms and radicals such as chlorine.

Some aspects of the stratospheric Cl-ClO-Cl cycle: Possible roles of ClO*, ClNO3 and HOCl

Abstract Depending on such factors as (a) the probabilities of exciting the various vibrational states in ClO formed in the reaction of Cl with O 3 , (b) the radiative lifetime of ClO * , (c) Δ H ƒ (ClO 3 ), and (d) the rate coeffic`ient of the relevant three-body reaction, the production of ClO 3 via the reaction ClO * +O 2 +M→ClO 3 +M may be quite substantial in the stratosphere. The significance of this result lies in the subsequent elimination (from the stratosphere) of ClO 3 and its associated chlorine atom as HClO 4 , in the manner recently suggested by Samonaitis and Heicklen. In the stratosphere, ClO 3 most probably photodissociates primarily into OClO and O. Upon photodissociation, OClO may also yield atomic oxygen. Thus the formation of ClO 3 from ClO * and O 2 , and the above-mentioned photodissociation steps constitute an interesting, indirect mechanism of O 2 dissociation into two odd oxygen species. Other aspects of ClO * chemistry, applicable in stratospheric conditions, also deserve attention in view of Nicholls recent interpretation of the Umkehr measurements by Brewer et al . The reactions of ClO with HO 2 , and NO 2 , possess the potential of significantly obstructing the completion of the C1-ClO-Cl cycle, at least in the region below 35 km. An accurate and critical study of the chemistry of oxyacids, higher oxides, and nitrates of chlorine in the stratospheric environment is needed. Obviously, this is only a partial list of the difficult problems associated with a proper understanding of stratospheric chlorine chemistry which appears to be far more complex than what is implied in the literature. (See also notes added in proof stage.)

O2⋅N2 photochemistry in the present and Precambrian atmosphere

The absorption of radiation [λ200–260 nm] by the O2⋅N2 collision complex produces NOX with an average yield of (0.03±0.008) odd-nitrogen per absorbed photon. This process is also a new source of isotope enriched odd-nitrogen in the atmospheres.

A new model of N2O quantum yield in the UV photolysis of O3/O2/N2 mixtures: Contributions of electronically excited O3 and O3⋅N2

Electronically excited O3 and O3⋅N2 dimer are proposed as contributors to the N2O quantum yield, φN2O, following UV photolysis of O3 in O3/O2/N2 mixture. At 100 to 900 torr N2 pressures, φN2O is dominated by the electronically excited O3. In this pressure regime φN2O in the 310⩽λ⩽340 nm region could, potentially, exceed φN2O in the λ<310 nm region. The “classical” O(1D), N2 association predominates above 10 atm. The O3⋅N2+hν may dominate at high pressures (≳500 atm) if the temperature is very low (≲50 K). The atmospheric importance of N2O production via the classical mechanism is well known to be insignificant. In contrast, the production from excited O3 appears to have the potential to significantly affect our current understandings of stratospheric coupled NOx–O3 chemistry and the climatologically important N2O source-sink budget. It is therefore critical to determine the wavelength variation of φN2O in the 310⩽λ⩽340 nm region by gas phase experiments. Theoretical studies are needed to understand, at th...