Enhanced sulfate formation during China's severe winter haze episode in January 2013 missing from current models

A regional haze with daily PM_2.5 (fine particulate matters with diameters less than 2.5 µm) exceeding 500 µg/m³ lasted for several days in January 2013 over North China, offering an opportunity to evaluate models. Observations show that inorganic aerosols (sulfate, nitrate, and ammonium) are the largest contributor to PM_2.5 during the haze period, while sulfate shows the largest enhancement ratio of 5.4 from the clean to haze period. The nested-grid GEOS-Chem model reproduces the distribution of PM_2.5 and simulates up to 364 µg/m³ of daily maximum PM_2.5. Yet on average, the model is a factor of 3 and 4 lower in PM_2.5 and fails to capture the large sulfate enhancement from the clean to haze period. A doubling of SO₂ emissions over North China, along with daily meteorology corrections, would be required to reconcile model results with surface SO₂ observations, but it is not sufficient to explain the model discrepancy in sulfate. Heterogeneous uptake of SO₂ on deliquesced aerosols is proposed as an additional source of sulfate under high-relative humidity conditions during the haze period. Parameterizing this process in the model improves the simulated spatial distribution and results in a 70% increase of sulfate enhancement ratio and a 120% increase in sulfate fraction in PM_2.5. Combined adjustments in emissions, meteorology, and sulfate chemistry lead to higher sulfate by a factor of 3 and 50% higher PM_2.5, significantly reducing the model's low bias during the haze.