Partitioning of hydrogen peroxide in gas-liquid and gas-aerosol phases.

Hydrogen peroxide (H₂O₂) is a vital oxidant in the atmosphere and plays critical roles in the oxidation chemistry of both liquid and aerosol phases. The partitioning of H₂O₂ between the gas and liquid phases, or the aerosol phase, could affect its abundance in these condensed phases and eventually the formation of secondary components. However, the partitioning processes of H₂O₂ in gas-liquid and gas-aerosol phases are still unclear, especially in the ambient atmosphere. In this study, field observations of gas-, liquid-, and aerosolphase H₂O₂ were carried out in the urban atmosphere of Beijing during the summer and winter of 2018. The effective field-derived mean value of Henry's law constant (…, 2.1 x 10⁵Matm^-1) was 2.5 times of the theoretical value in pure water (…, 8.4 x 10[sup 4]M at m^-1) at 298 ± 2 K. The effective derived gas-aerosol partitioning coefficient (…, 3.8 x 10^-3 m³ ng^-1) was 4 orders of magnitude higher on average than the theoretical value (…, 2.8 x 10^-7 m³ ng^-1) at 270 ± 4 K. Beyond following Henry's law or Pankow's absorptive partitioning theory, the partitioning of H₂O₂ in the gas-liquid and gas-aerosol phases in the ambient atmosphere was also influenced by certain physical and chemical reactions. The average concentration of liquid-phase H₂O₂ in rainwater during summer was 44.12 ± 26.49 µM. In 69% of the collected rain samples, the measured level of H₂O₂ was greater than the predicted value in pure water calculated by Henry's law. In these samples, 41% of the measured H₂O₂ was from gas-phase partitioning, while most of the rest may be from residual H₂O₂ in raindrops. In winter, the level of aerosol-phase H₂O₂ was 0.093 ± 0.085 ng µg^-1, which was much higher than the predicted value based on Pankow's absorptive partitioning theory. The contribution of partitioning of the gas-phase H₂O₂ to the aerosol-phase H₂O₂ formation was negligible. The decomposition/hydrolysis rate of aerosol-phase organic peroxides could account for 11% - 74% of the consumption rate of aerosol-phase H₂O₂, and the value depended on the composition of organic peroxides in the aerosol particles. Furthermore, the heterogeneous uptake of HO₂ and H₂O₂ on aerosols contributed to 22% and 2% of the aerosol-phase H₂O₂ consumption, respectively. [ABSTRACT FROM AUTHOR]