Hydrogen peroxide, tropospheric radical

The hydroxyl radical is normally present only in low concentrations in the troposphere, as it reacts with further ozone to form the hydroperoxy radical HOO- which in turn gives hydrogen peroxide H202. Ozone, the hydroxyl radical, and hydrogen peroxide are the main oxidizing species in the troposphere, from the standpoint of environmental chemistry. The hydroxyl radical in particular performs an important function as a natural cleansing agent for the atmosphere.26 In elevated concentrations, however,... 

Hydrogen peroxide is an interesting molecule from both structural and chemical point of view. It is chemically the smallest molecule showing internal rotation. It is an important constituent of troposphere and stratosphere, the recombination ofthe two HO2 radicals being the main cause otTLC formation in atmosphere. It is related to acid rain formation by the oxidation of SO2 by H2O2 either in gas phase or in a water droplet [1-4]. Techniques for the detection of H2O2 can be... 

Although the gas phase provides major pathway for hydroxyl radical and hydrogen peroxide production in the atmosphere, there is overwhelming evidence [158-168] that aqueous phases in the troposphere also provides a significant medium for the photolytic production of these important oxidants. 

The nitrate radical (NO3) which is present in the troposphere primarily during nighttime is also a powerful oxidant, and reacts efficiently with many organic compounds (nonmethane hydrocarbons, DMS, etc.). Hydrogen peroxide (H2O2) is a major oxidant for SO2 inside water droplets, and contributes to the formation of sulfate aerosols. 

In continental clouds, hydrogen peroxide is the most important oxidant of suhur dioxide dissolved in the aqueous phase, contributing about 80% to the total oxidation rate. Ozone and peroxynitric acid oxidize up to 10% each, and the gas-phase reaction of SO2 with OH radicals adds about 3%. Clouds are estimated to occupy about 15% of the airspace in the lower troposphere. In-cloud reactions thus oxidize 70-80% of SO2 in the troposphere, the remaining 20-30% of SO2 is oxidized in the gas-phase by reaction with OH radicals in cloud-free air. 

Hydrogen peroxide H2O2 is formed by the radical termination reaction HO2 + HO2, and exists in the troposphere generally at the mixing ratio in order of ppbv. Since H2O2 is water soluble, it is removed by the dissolution into cloud and fog water, while photolytic reaction is another important removal process. Methyl hydroperoxide CH3OOH also exists in the whole region of the troposphere in natural atmosphere as an oxidation product of methane. Its photolytic reaction is important as its removal process, and also as a radical source in the upper troposphere. 

Most of our previous studies have been devoted to reactive oxygen species (ROS) at the air-water interface because such species are ubiquitous and play a crucial role in atmospheric chemistry, in environmental processes, water treatment technologies and biochemical reactions. The complex chemistries associated to these species, and their interconnection across different reaction media, have recently been reviewed [53]. The stability of ozone, molecular oxygen, hydrogen peroxide, hydroxyl and hydroperoxyl radicals, and other related compounds at the air-water interface had been established through classical molecular dynamics simulations [54—56]. Those studies, in particular, suggested that many of the compounds could accumulate at the surface of cloud water droplets, influencing in this way the overall chemistry of the troposphere. Recenfly, combined QM/MM MD simulations have confirmed the marked aflSnity of ROS species such as HO2 [27] and ozone [30] for the air-water interface. 

The temperature and density structure of the troposphere, along with the concentrations of major constituents, are well documented and altitude profiles have been measured over a wide range of seasons and latitudes for the minor species water, carbon dioxide, and ozone. A few profiles are available for carbon monoxide, nitrous oxide, methane, and molecular hydrogen, while only surface or low-altitude measurements have been made for nitric oxide, nitrogen dioxide, ammonia, sulfur dioxide, hydrogen sulfide, and nonmethane hydrocarbons. No direct measurements of nitric acid and formaldehyde are available, though indirect information does exist. The concentrations of a number of other important species, such as peroxides and oxy and peroxy radicals, have never been determined. Therefore, while considerable information concerning trace constituent concentrations is available, the picture is far from complete. 

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