Diverse chemical pathways in the troposphere convert sulphur and nitrogen oxides and organic compounds into acids, involving the gas phase, the liquid phase (cloud, fog and rain water) and, possibly, certain suspended aerosols. The rates of acid generation are critically affected by the extent of generation of the oxidizing species and the kinetics of the reactions. Precipitation in the eastern United States shows a strong seasonal variation in deposition of sulphates in contrast to nitrates. Computer simulations can now rationalize the observed seasonal trends. Recent tropospheric measurements of gaseous hydrogen peroxide show that this gas is a major oxidant leading to sulphuric acid generation in cloud water.

Chemical mechanisms of acid generation in the troposphere

A rapid increase in the concentration of hydrogen peroxide was observed when samples of natural surface and ground water from various locations in the United States were exposed to sunlight. The hydrogen peroxide is photochemically generated from organic constituents present in the water; humic materials are believed to be the primary agent producing the peroxide. Studies with superoxide dismutase suggest that the superoxide anion is the precursor of the peroxide.

https://science.sciencemag.org/content/sci/220/4598/711.full.pdf

Photochemical formation of hydrogen peroxide in surface and ground waters exposed to sunlight.

The coupled gas- and aqueous-phase photochemistry of a stratiform cloud in a remote region of the marine atmosphere is investigated with a time-dependent box model. Both scavenging of ambient acidic aerosols and gases as well as aqueous-phase chemical reactions within droplets are found to be important sources of acidity to cloud water and can lead to pH levels in cloud water in the remote marine atmosphere well below 5.6. The major sources of acidity via aqueous-phase chemical reactions are the generation of sulfuric acid from dissolved SO2 and the generation of formic acid from dissolved formaldehyde. In both cases, aqueous-phase free radicals can play a significant role either directly by oxidizing dissolved SO2 and HCHO or indirectly by producing the aqueous-phase oxidant H2O2. The rate of SO2 conversion to sulfuric acid is sensitive to a variety of parameters including the accommodation or sticking coefficient for SO2, H2O2, HO2, and OH, the liquid water content, and the ambient levels of SO2, HNO3, and other acidic or basic gases. Because high levels of SO2 tend to deplete cloud water of H2O3, the possibility exists that the pH of precipitation in polluted regions will respond nonlinearly to reduced SO2 emissions.

The photochemistry of a remote marine stratiform cloud

Calculations are presented that simulate the free radical chemistries of the gas phase and aqueous phase within a warm cloud during midday. It is demonstrated that in the presence of midday solar fluxes, the heterogeneous scavenging of OH and HO2 from the gas phase by cloud droplets can represent a major source of free radicals to cloud water, provided the accommodation or sticking coefficient for these species impinging upon water droplets is not less than 0.0001. The aqueous-phase of HO2 radicals are found to be converted to H2O2 by aqueous-phase chemical reactions at a rate that suggests that this mechanism could produce a significant fraction of the H2O2 found in cloud droplets. The rapid oxidation of sulfur species dissolved in cloudwater by this free-radical-produced H2O2 as well as by aqueous-phase OH radicals could conceivably have a significant impact upon the chemical composition of rain.

The free radical chemistry of cloud droplets and its impact upon the composition of rain

An automated analytical technique for the determination of hydrogen peroxide (H/sub 2/O/sub 2/) in the liquid phase has been developed. The chemistry of this technique is based on the reaction of H/sub 2/O/sub 2/ with horseradish peroxidase and p-hydroxyphenylacetic acid (POPHA). The resulting reaction forms the fluorescent dimer of POPHA. By use of conventional fluorescence detection techniques a detection limit of 1.2 x 10/sup -8/ M (0.4 ppbm) H/sup 2/O/sup 2/ is obtained for a 1.5-mL aqueous sample. The coefficient of variation is 0.66% at 1.6 x 10/sup -6/ M (53 ppbm). The analytical chemical reaction responds stoichiometrically to both H/sup 2/O/sup 2/ and organic hydroperioxides. To discriminate H/sup 2/O/sup 2/ from organic hydroperioxides, a novel dual-channel chemical flow system has been devised to separately determine total hydroperioxides and organic hydroperioxides. The concentration of H/sup 2/O/sup 2/ is determined by the difference between these two measurements. The system has been tested extensively for potential interferences commonly found in environmental aqueous samples, and none has been observed. 17 references, 4 figures, 2 tables.

Automated fluorimetric method for hydrogen peroxide in atmospheric precipitation

Measurements of H2O2 in rainwater collected in Miami, Florida, and the Bahama Islands area indicate the presence of H2O2 concentration levels ranging from 100,000 to 700,000 M No systematic trends in H2O2 concentration were observed during an individual storm, in marked contrast to the behavior of other anions for example, NO3(-), SO4(-2), and Cl(-) The data suggest that a substantial fraction of the H2O2 found in precipitation is generated by aqueous-phase reactions within the cloudwater rather than via rainout and washout of gaseous H2O2

/pdf/h2o2-levels-in-rainwater-collected-in-south-florida-and-the-3ykk5k6vcg.pdf

R. Zika

Papers

H2O2 levels in rainwater collected in south Florida and the Bahama Islands