5:15 PM - 7:15 PM
[AAS11-P16] Impact of formaldehyde oxide from CH2I2 on atmospheric chemistry
Criegee intermediates (CIs) are a group of carbonyl species that have been attracting a growing amount of attention in recent years. CIs are particularly important as a source of aerosols formed by the reaction of CIs and air pollutants such as SO2, NO2, etc., which contribute air pollutions and climate change. Formaldehyde oxide (CH2OO) is conventionally believed to be generated by the ozonolysis of olefins from both anthropogenic and biogenic sources. However, by combining elementary reactions, CH2OO should be formed through reaction pathways derived from NO2 and diiodomethane (CH2I2), which is mainly emitted by the biogenic synthesis in ocean. In this study, we determined the branching ratios of a series of related elementary reactions derived from NO2/CH2I2 system that leads to the formation of CH2OO. The whole elementary reactions were incorporated into a global chemical transport model to determine their contribution to the ambient concentration of CH2OO.
Quantum chemical calculations (QCC) and transition state theory (TST) were conducted to derive the rate coefficients of the reactions of CH2I2 with O(3P) and OH. Gaussian 16 was used for QCC. Geometrical optimization and rovibrational frequencies were calculated at the M06-2X/aug-cc-pVTZ-pp level of theory while detailed single point energy calculations were performed with CCSD(T)/aug-cc-pVTZ-pp level of theory. TST calculations were conducted by the Gaussian Post Processor (GPOP).
GEOS-Chem was used for global chemical transport modelling coupled with MERRA2 as the meteorological input. The calculation period was set from July 1, 2018 to December 31, 2019 of which first half of the period was treated as the spin-up period and remained entire 2019 was treated as the analysis period. Approximately 20 CH2I2-related reactions and ~100 CI reactions were incorporated to the gas-phase chemical mechanism of GEOS-Chem.
The results showed that the increase in CH2OO concentrations in coastal areas were estimated to be maximally 1.0×10-3 molecule cm-3 with the concentrations of CH2I2 to be ~1.0×1016 molecule cm-3. The maximum increase ratio of CH2OO was about 0.4% of the total CH2OO production. The NOx emissions from vessels contributed to O(3P) formation, and which consumed CH2I2, forming CH2OO. The generated CH2OO on ocean is rapidly consumed by water dimers forming hydroxymethyl hydroperoxide (HMHP), subsequently producing organic acid and secondary aerosol. Although these chemical species have been linked to phytotoxicity and acid rain, their environmental impact on the ocean has not been confirmed. Based on the above, it can be concluded that the environmental impact of CH2OO due to new formation origins is small. In addition, the rate coefficients of the branching reactions of CH2I2 + O(3P) were extremely low (5.00 × 10-15 and 1.36 × 10-44 cm3 molecule-1 s-1), and therefore, the contribution of those reactions to CH2OO formation would be negligibly small: the reaction rate coefficient for the main reaction contributing to the formation of CH2OO was 7.36×10-11 cm3 molecule-1 s-1. The CH2I2 + OH branching reaction is currently under consideration using Variational TST (VTST), and the results will be shown in the presentation.
Quantum chemical calculations (QCC) and transition state theory (TST) were conducted to derive the rate coefficients of the reactions of CH2I2 with O(3P) and OH. Gaussian 16 was used for QCC. Geometrical optimization and rovibrational frequencies were calculated at the M06-2X/aug-cc-pVTZ-pp level of theory while detailed single point energy calculations were performed with CCSD(T)/aug-cc-pVTZ-pp level of theory. TST calculations were conducted by the Gaussian Post Processor (GPOP).
GEOS-Chem was used for global chemical transport modelling coupled with MERRA2 as the meteorological input. The calculation period was set from July 1, 2018 to December 31, 2019 of which first half of the period was treated as the spin-up period and remained entire 2019 was treated as the analysis period. Approximately 20 CH2I2-related reactions and ~100 CI reactions were incorporated to the gas-phase chemical mechanism of GEOS-Chem.
The results showed that the increase in CH2OO concentrations in coastal areas were estimated to be maximally 1.0×10-3 molecule cm-3 with the concentrations of CH2I2 to be ~1.0×1016 molecule cm-3. The maximum increase ratio of CH2OO was about 0.4% of the total CH2OO production. The NOx emissions from vessels contributed to O(3P) formation, and which consumed CH2I2, forming CH2OO. The generated CH2OO on ocean is rapidly consumed by water dimers forming hydroxymethyl hydroperoxide (HMHP), subsequently producing organic acid and secondary aerosol. Although these chemical species have been linked to phytotoxicity and acid rain, their environmental impact on the ocean has not been confirmed. Based on the above, it can be concluded that the environmental impact of CH2OO due to new formation origins is small. In addition, the rate coefficients of the branching reactions of CH2I2 + O(3P) were extremely low (5.00 × 10-15 and 1.36 × 10-44 cm3 molecule-1 s-1), and therefore, the contribution of those reactions to CH2OO formation would be negligibly small: the reaction rate coefficient for the main reaction contributing to the formation of CH2OO was 7.36×10-11 cm3 molecule-1 s-1. The CH2I2 + OH branching reaction is currently under consideration using Variational TST (VTST), and the results will be shown in the presentation.