Hydrogen peroxide (H₂O₂) is one of the important trace species which play significant roles in the chemical cycles in the atmosphere. Atmospheric H₂O₂ is formed by the combination of two hydroperoxy radicals (HO₂), which are the direct precursors of H₂O₂.:

The hydroperoxy radical is generated from both ozone photolysis and through the photochemical degradation of hydrocarbons. Therefore, gaseous H₂O₂ concentration is affected by the atmospheric pollutants and the meteorological conditions (solar radiation, temperature and humidity). Watanaba and Tanaka demonstrate, in their paper, the ground-level measurements of gaseous H₂O₂ concentration made in the city of Nagoya located in central Japan, from January in 1993 to August in 1994. Gaseous H₂O₂ was sampled by cold-trap method, and hydrogen peroxide concentration was determined by a method using fluorometry.

The first evidence that has been shown is a diurnal variation. The mean gaseous H₂O₂ concentration in the daytime is found significantly higher than the night-time concentration. The maximum gaseous concentration appeared in the afternoon and minimum concentration appeared at mid-night. The gradual increase in concentration during the morning to the afternoon seems to reflect photochemical production.

The maximum H₂O₂ concentrations were observed a few hours after the peak of O₃ concentration, when NO_x concentrations were lowest. Hydrogen peroxide is mainly formed by the recombination of HO₂ radicals as shown in reaction (1). This reaction is affected by NO concentration. Reaction of NO with H₂O₂ competes with reaction (1):

Apart this diurnal variation, hydrogen peroxide concentrations show clear seasonal variation, with the highest concentrations in summer, and the lowest concentrations from fall to winter. Strong solar radiation and warm temperature cause active photochemical reactions during the summer, and therefore an increase in H₂O₂ concentration. The measured H₂O₂ concentrations in the city of Nagoya were about the same as the concentrations measured in Los Angeles and the eastern United States, but were higher than those in England. As England is located at a high latitude, photochemical reaction seems to be suppressed.

To find what is controlling the gaseous H₂O₂ concentration in the urban atmosphere, measured data were compared with atmospheric pollutants and meteorological conditions. Because of the unavailability of the atmospheric pollution data in the cold season, only the data in the warm season (March to September 1993) were used.

The results obtained from the daytime data indicate that H₂O₂ concentration tends to increase with both solar radiation and concentration of O₃. There is a weak negative relationship between NO_x and H₂O₂ concentrations. Reaction of NO with HO₂ reduces HO₂ concentration. In addition, reaction of NO₂ with HO produces nitric acid. As a result, HOx, a precursor of H₂O₂, is consumed by NO_x. Therefore, in the regions of large NO_x emission, H₂O₂ concentrations are expected to be lower.

Hydrogen peroxide concentrations poorly correlate with non-methane hydrocarbon (NHMC) and carbon monoxide (CO) concentrations. NHMC and CO enhance the production of HO₂ radical and H₂O₂. Hence, the production of H₂O₂ is accelerated by an increase in NHMC and CO concentrations if other environmental conditions are constant. The reason for poor correlation between concentrations of H₂O₂ and NHMC lies in the fact that NHMC and CO concentrations are negatively correlated with solar radiation.

Hydrogen peroxide concentration at night-time is well correlated with wind speed. A high level of nighttime H₂O₂ is observed when the atmospheric stability in the nocturnal boundary layer breaks and resulting vertical mixing occurs. Such a condition is accompanied by high wind speeds. This is the reason why the ground-level H₂O₂ concentration at night-time is dependent on wind speed.

Reference

Watanabe, Koichi and Hiroshi Tanaka, 1995: Measurement of Gaseous Hydrogen Peroxide (H₂O2) Concentrations in the Urban Atmosphere Journal of the Meteorological Society of Japan, 13(5), 839-847.