Rapid sulfate formation from synergetic oxidation of SO2 by O3 and NO2 under ammonia-rich conditions: Implications for the explosive growth of atmospheric PM2.5 during haze events in China

Highlights: • Neutralization by NH₃ is the controlling factor for the efficient heterogeneous formation of sulfate. • O₃ and NO₂ mixture shows a strong synergetic effect on the heterogeneous oxidation of SO₂ only under NH₃-rich conditions. • The sharply rising ALWC caused by nitrate is responsible for the synergetic effect of mixed oxidants on sulfate formation. Extremely high levels of atmospheric sulfate aerosols have still frequently occurred in China especially in winter haze periods and often been underestimated by models due to some missing formation mechanisms. Here we investigated the heterogeneous reaction dynamics of SO₂ oxidation by the abundantly co-existing O₃ and NO₂ in the urban atmosphere of China by using a laboratory smog chamber simulation technique. Our results showed that with an increase of NH₃ concentrations from 0.05 ppm to 1.5 ppm, SO₂ oxidation by O₃ can be greatly promoted and lead to an exponential increase of diameter growth factor (GF) of particles in the chamber from 1.29 to 1.98 for NaCl seeds and from 1.20 to 1.60 for (NH₄)₂SO₄ seeds, along with an increasing uptake coefficient (γ) of SO₂ from 4.47 × 10⁻⁵ to 1.52 × 10⁻⁴ on NaCl seeds and from 2.32 × 10⁻⁵ to 5.74 × 10⁻⁵ on (NH₄)₂SO₄ seeds, respectively. The heterogeneous production of sulfate from oxidation of SO₂ under NH₃-rich conditions by O₃ and NO₂ mixture in the chamber was 2.0–3.5 times the sum of sulfate from SO₂ oxidations by O₃ and NO₂, suggesting a strongly synergetic effect of the mixed oxidants on the heterogeneous oxidation of SO₂, which can cause rapid formation of (NH₄)₂SO₄ and NH₄NO₃ and is responsible for the explosive growth of PM_2.5 in the winter haze period of China. Our chamber results further showed that such synergetic process is only efficient under NH₃-rich conditions, clearly indicating that the combined controls on O₃, NOx and NH₃ are necessary for further mitigating the PM_2.5 pollution in China.