A study of the N2O5-SO2-O3 reaction system

Area of Science:

• Atmospheric Chemistry
• Chemical Kinetics
• Spectroscopy

Background:

• Understanding the chemical reactions of nitrogen oxides and sulfur oxides is crucial for air quality assessment.
• Dinitrogen pentoxide (N2O5) and sulfur dioxide (SO2) are components of polluted atmospheres, influencing atmospheric chemistry.
• The role of N2O5 and its reaction products in atmospheric processes requires detailed kinetic and spectroscopic investigation.

Purpose of the Study:

• To investigate the reaction rates and products of gaseous N2O5 with SO2, with and without ozone (O3).
• To determine the significance of N2O5 and SO2 reactions as removal pathways in polluted atmospheres.
• To assess the contribution of N2O5 photochemical decomposition to atmospheric concentrations of N2O5 and NO3 radicals.

Main Methods:

• Infrared spectroscopy was employed to monitor chemical changes in gas mixtures.
• Kinetic analysis was used to estimate upper limits for rate constants of key reactions.
• Near-ultraviolet absorption spectrum of N2O5 was measured.

Main Results:

• Sulfur trioxide (SO3) was not detected as a product, indicating minimal SO2 removal via reactions with N2O5 or NO3 radicals.
• Upper limits for rate constants k(NO3 + SO2) and k(N2O5 + SO2) were determined to be ≤ 4.2 L mol⁻¹s⁻¹ and ≤ 2.5 x 10⁻² L mol⁻¹s⁻¹, respectively.
• Photochemical decomposition of N2O5 in urban atmospheres was found to be insignificant.
• Gaseous SO3 and nitrogen dioxide (NO2) rapidly formed a nonvolatile white solid adduct, suggesting a potential role in urban aerosol formation.

Conclusions:

• The reactions between N2O5 and SO2 are not significant removal mechanisms for SO2 in sunlit, polluted urban atmospheres.
• Photochemical degradation of N2O5 does not substantially impact N2O5 and NO3 concentrations in the urban atmosphere.
• The rapid formation of a solid adduct from SO3 and NO2 warrants further investigation regarding its contribution to urban aerosol.