Regional atmosphere aldehydes

This region of the spectrum around 300 nm is a crucial one for tropospheric photochemistiy in both clean and polluted atmospheres. As we have indicated earlier, it is here that species such as ozone and aldehydes photolyze to produce atoms and free radicals critical to the chemistry of the troposphere. 

Pena, R. M., S. Garcia, C. Herrero, M. Losada, A. Vazquez and T. Lucas (2002) Organic adds and aldehydes in rainwater in a northwest region of Spain. Atmospheric Environment 36, 5277-5288... 

Compounds that have a carbonyl moiety (group), C=0, on an end carbon (aldehydes) or middle carbon (ketones) are often the first species formed, other than unstable reaction intermediates, in the photochemical oxidation of atmospheric hydrocarbons. Aldehydes are important in atmospheric chemistry because they are second only to NO2 as atmospheric sources of free radicals produced by the absorption of light. This is because the carbonyl group is a chromophore, a molecular group that readily absorbs light and it absorbs well in the near-ultraviolet region of the spectrum to produce active species that can take part in atmospheric chemical processes. 

Nitrate radicals (NO3) are formed by the reaction of O3 and NO2 (Sect. 5.4.2) and play an important role in atmospheric chemistry at nighttime in polluted air. NO3 has an absorption spectrum in the visible region as seen in Sect. (4.2.4) so that daytime concentration is very low since it is easily photodecomposed by sun light. Simultaneously, since the reaction rate constant of NO3 with NO is large, it returns easily to NO2 by NO so that its concentration near NO sources is also very low. NO3 reacts with alkenes and aldehydes to form dinitrates and OH/HO2 radicals at nighttime. Rate constants of fundamental reactions of atmospheric NO3 and related N2O5 are cited in Table 5.6. 

The steady state concentrations of HCN would have depended on the pH and temperature of the early oceans and the input rate of HCN from atmospheric synthesis. Assuming favorable production rates, Miyakawa et al (30) estimated steady state concentrations of HCN of 2 x 10 M at pH 8 and 0°C in the primitive oceans. At 100° C and pH 8 the steady state concentration was estimated as 7 x 10 M. HCN hydrolyzes to formamide which then hydrolyzes to formic acid and ammonia. It has been estimated that oligomerization and hydrolysis compete at approximately 10 M concentrations of HCN at pH 9 (31), although it has been shown that adenine is still produced from solutions as dilute as 10 M (32). If the concentration of HCN were as low as estimated, it is possible that HCN tetramer formation may have occurred on the primitive Earth in eutectic solutions of HCN-H2O, which may have existed in the polar regions of an Earth of the present average temperature. High yields of the HCN tetramer have been reported by cooling dilute cyanide solutions to temperatures between -10° C and -30° C for a few months (31). Production of adenine by HCN polymerization is accelerated by the presence of formaldehyde and other aldehydes, which could have also been available in the prebiotic environment (29). 

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