Year-round modeling of sulfate aerosol over Asia through updates of aqueous-phase oxidation and gas-phase reactions with stabilized Criegee intermediates

Sulfate aerosol (SO42-) is a main component of particulate matter in Asian countries. Accurate numerical modeling is important for appropriate policy-making aimed at reducing SO42- concentrations. The modeling performance for SO42- is superior among aerosol components, however, current models underestimate SO(4)(2- )concentrations during winter over Asia. Previous studies have proposed a heterogeneous process for winter haze events, but these kinds of studies are limited spatially and temporally because they cover only mainland China in winter. Underestimation has also been reported in other regions where the aerosol concentration is much lower than in China. In this study, the aqueous- and gas-phase oxidation processes in the current modeling were reconsidered, and their roles were evaluated over Asia using a year-round simulation. The existing aqueousphase oxidation of O-2 with Fe and Mn as catalysts was refined, and oxidation with NO2 was added due to the neutralized atmosphere over Asia. For gas-phase oxidation, three stabilized Criegee intermediates (SCIs; formaldehyde oxide (CH2OO), acetaldehyde oxide (CH3CHOO), and propionaldehyde oxide ((CH3)(2)COO)) were introduced. Considering the uncertainty of the reaction of CH2OO with water, the upper and lower limits of the rate constant were applied. The updated oxidation processes led to an increase in the modeled SO42- concentration. The model performance over Asia in winter was effectively improved, and the updates did not degrade the model performance in other seasons. The improvements in biases were approximately 3% during winter, whereas the deterioration in biases were within 1% in spring to summer and 2% in autumn, when the lower limit of the CH2OO rate constant with water was used. The role of SCIs depended strongly on the rate constants of the reaction of CH2OO with water. The simulated concentration of SCIs agreed well with the estimated levels, and the lower limit of the rate constant of CH2OO with water fitted within the estimated SCIs concentration levels. SCIs may play an important role in SO42- production over Asia in winter, especially the downwind region of China. The approach taken in this study has the potential to improve modeling performance in other regions as well.

Automated summary

The study reevaluated gas-phase and aqueous-phase oxidation processes, which led to an increase in simulated SO42- concentration. The model performance in winter over Asia was effectively improved by the updates, without degrading the model performance in other seasons.