Efficient Conversion of NO to NO₂ on SO₂-Aged MgO under Atmospheric Conditions

Area of Science:

• Atmospheric Chemistry
• Environmental Science
• Surface Chemistry

Background:

• The nitrogen oxides (NOx) cycle, involving nitrogen monoxide (NO) and nitrogen dioxide (NO2), is crucial for tropospheric ozone (O3) formation and the atmosphere's oxidizing capacity.
• Heterogeneous oxidation of NO to NO2 on aerosols was traditionally considered negligible due to NO's low reactivity.
• Understanding atmospheric pollutant formation pathways is vital for air quality management.

Purpose of the Study:

• To investigate the role of sulfur dioxide (SO2) in the heterogeneous transformation of NO on magnesium oxide (MgO) particles.
• To elucidate the mechanism and active sites involved in NO oxidation on SO2-modified MgO.
• To assess the implications of this process for atmospheric oxidizing capacity and air pollution.

Main Methods:

• Experimental studies involving the reaction of NO with MgO particles aged with SO2 under ambient conditions.
• Measurement of NO uptake coefficients on fresh and aged MgO.
• Spectroscopic characterization (e.g., FTIR, XPS) to identify surface species and reaction intermediates.
• Density Functional Theory (DFT) calculations to model adsorption and reaction mechanisms.

Main Results:

• SO2 significantly enhances the heterogeneous transformation of NO to NO2 and HONO on MgO particles, increasing NO uptake coefficients by 2-3 orders of magnitude.
• Sulfates formed on the MgO surface after SO2 aging act as active sites for NO adsorption and oxidation.
• Spectroscopic and DFT studies identified a [SO4-NO] complex intermediate, leading to NO2 formation and subsequent reaction with sulfite to produce HONO.

Conclusions:

• SO2 promotes the heterogeneous oxidation of NO to NO2 and HONO on MgO, presenting a previously unrecognized pathway.
• This SO2-promoted process contributes to atmospheric oxidizing capacity and the formation of air pollution complexes, particularly in regions with high SO2 and NOx emissions.
• The findings necessitate a re-evaluation of the NO-NO2 cycle's heterogeneous components in atmospheric chemistry models.