Ultraviolet Solar Radiation Related to Mesospheric Processes

Observation of the onset of a blue jet into the stratosphere

Blue jets are lightning-like, atmospheric electric discharges of several hundred millisecond duration that fan into cones as they propagate from the top of thunderclouds into the stratosphere¹. They are thought to initiate in an electric breakdown between the positively charged upper region of a cloud and a layer of negative charge at the cloud boundary and in the air above. The breakdown forms a leader that transitions into streamers² when propagating upwards³. However, the properties of the leader, and the altitude to which it extends above the clouds, are not well characterized⁴. Blue millisecond flashes in cloud tops^5,6 have previously been associated with narrow bipolar events^7,8, which are 10- to 30-microsecond pulses in wideband electric field records, accompanied by bursts of intense radiation at 3 to 300 megahertz from discharges with short (inferred) channel lengths (less than one kilometre)^9-11. Here we report spectral measurements from the International Space Station, which offers an unimpeded view of thunderclouds, with 10-microsecond temporal resolution. We observe five intense, approximately 10-microsecond blue flashes from a thunderstorm cell. One flash initiates a pulsating blue jet to the stratopause (the interface between the stratosphere and the ionosphere). The observed flashes were accompanied by 'elves'¹² in the ionosphere. Emissions from lightning leaders in the red spectral band are faint and localized, suggesting that the flashes and the jet are streamer ionization waves, and that the leader elements at their origin are short and localized. We propose that the microsecond flashes are the optical equivalent of negative narrow bipolar events observed in radio waves. These are known to initiate lightning within the cloud and to the ground, and blue lightning into the stratosphere, as reported here.

The early faint sun paradox: organic shielding of ultraviolet-labile greenhouse gases

Atmospheric mixing ratios of approximately 10(-5 +/- 1) for ammonia on the early Earth would have been sufficient, through the resulting greenhouse warming, to counteract the temperature effects of the faint early sun. One argument against such model atmospheres has been the short time scale for ammonia photodissociation by solar ultraviolet light. Here it is shown that ultraviolet absorption by steady-state amounts of high-altitude organic solids produced from methane photolysis may have shielded ammonia sufficiently that ammonia resupply rates were able to maintain surface temperatures above freezing.

Photochemistry of CO and H2O: analysis of laboratory experiments and applications to the prebiotic Earth's atmosphere

The role photochemical reactions in the early Earth's atmosphere played in the prebiotic synthesis of simple organic molecules was examined. We have extended an earlier calculation of formaldehyde production rates to more reduced carbon species, such as methanol, methane, and acetaldehyde. We have simulated the experimental results of Bar-Nun and Chang (1983) as an acid in the construction of our photochemical scheme and as a way of validating our model. Our results indicate that some fraction of CO2 and H2 present in the primitive atmosphere could have been converted to simple organic molecules. The exact amount is dependent on the partial pressure of CO2 and H2 in the atmosphere and on what assumptions are made concerning the shape of the absorption spectra of CO2 and H2O. In particular, the results are most sensitive to the presence or absence of absorption at wavelengths longward of 2000 angstroms. We also find that small quantities of CH4 could have been present in the prebiotic Earth's atmosphere as the result of the photoreduction of CO.

Hydrogen and deuterium loss from the terrestrial atmosphere: a quantitative assessment of nonthermal escape fluxes

A comprehensive one-dimensional photochemical model extending from the middle atmosphere (50 km) to the exobase (432 km) has been used to study the escape of hydrogen and deuterium from the Earth's atmosphere. The model incorporates recent advances in chemical kinetics as well as atmospheric observations by satellites, especially the Atmosphere Explorer C satellite. The results suggest: (1) the escape fluxes of both H and D are limited by the upward transport of total hydrogen and total deuterium at the homopause (this result is known as Hunten's limiting flux theorem); (2) about one fourth of total hydrogen escape is thermal, the rest being nonthermal; (3) escape of D is nonthermal; and (4) charge exchange and polar wind are important mechanisms for the nonthermal escape of H and D, but other nonthermal mechanisms may be required. The efficiency to escape from the terrestrial atmosphere for D is 0.74 of the efficiency for H. If the difference between the D/H ratio measured in deep-sea tholeiite glass and that of standard sea water, delta D = -77%, were caused by the escape of H and D, we estimate that as much water as the equivalent of 36% of the present ocean might have been lost in the past.

Oxidant abundances in rainwater and the evolution of atmospheric oxygen

A one-dimensional photochemical model has been used to estimate the flux of dissolved hydrogen peroxide (H2O2) and of other soluble species in rainwater as a function of atmospheric oxygen level. H2O2 should have replaced O2 as the dominant oxidant in rainwater at oxygen levels below 10(-3)-10(-2) times the present atmospheric level (PAL). The exact value of pO2 at which H2O2 becomes more important than O2 depends on the abundance of trace gases such as CO, CH4, and NO. H2O2 was probably an important oxidant even in an O2-free atmosphere, provided that CO2 levels were significant higher than today's. In model atmospheres containing free O2 the concentration of photochemically produced oxidants generally exceeds that of photochemically produced reductants. The oxidizing power of rainwater is therefore greater than that due to dissolved molecular O2 alone. The difference is small at present but becomes important at O2 levels less than 10(-3) PAL. At O2 levels between 10(-4) and 10(-5) PAL the oxidizing power of rainwater is almost independent of pO2. Precambrian soils in which a part or all of the Fe2+ in their source rocks has been oxidized to Fe3+ could therefore have developed in the presence of an atmosphere with very low values of pO2. On the other hand, the upper limit for pO2 during early and mid-Precambrian time suggested by the incomplete oxidation of FeO in soils developed on basaltic rocks is affected only slightly by the presence of photochemical products in rainwater.

The prebiological paleoatmosphere: stability and composition

In the past, it was generally assumed that the early atmosphere of the Earth contained appreciable quantities of methane (CH4) and ammonia (NH3). This was the type of atmosphere believed to be the most suitable environment for chemical evolution, the nonbiological formation of complex organic molecules, the precursors of living systems. Photochemical considerations suggest that a CH4-NH3 dominated early atmosphere was probably very short-lived, if it ever existed at all. Instead, an early atmosphere of carbon dioxide (CO2) and nitrogen (N2) is favored by photochemical as well as geological and geochemical considerations. Photochemical calculations also indicate that the total oxygen column density of the prebiological paleoatmosphere did not exceed 10(-7) of the present atmospheric level.

The photochemistry of the paleoatmosphere

The ideas of Harold Urey on the origin and evolution of the atmosphere have dominated thinking in this area for 3 decades. Recent progress in this area is reviewed, with particular emphasis on photochemical modeling studies of atmospheric evolution. Research into the paleoatmosphere can be divided into 3 distinct areas: (1) The photochemistry/chemistry of the prebiological paleoatmosphere, (2) the evolution of oxygen and the transition to an oxidizing atmosphere, and (3) the origin and evolution of ozone. Photochemical calculations indicate that the stability of a heavily reducing paleoatmosphere of CH4--NH3 was extremely shortlived, if such a prebiological atmosphere ever existed at all. A more mildly reducing early atmosphere of CO2--N2 is favored by photochemical considerations. Recent calculations of O2 in the prebiological paleoatmosphere vary from less than 10(-14) of present atmospheric level (PAL) to 10(-1) PAL. Clearly, additional work is indicated. The evolution of O3 as a function of O2 level has been investigated with increasingly detailed photochemical models that have included the photochemistry/chemistry of the oxygen, hydrogen, nitrogen, carbon, and chlorine species, as well as the effects of eddy transport, the rainout of water-soluble species, dry deposition and lightning as a source of trace atmospheric gases.