Particle-Phase Photoreactions of HULIS and TMIs Establish a Strong Source of H 2 O 2 and Particulate Sulfate in the Winter North China Plain.

During haze periods in the North China Plain, extremely high NO concentrations have been observed, commonly exceeding 1 ppbv, preventing the classical gas-phase H ₂ O ₂ formation through HO ₂ recombination. Surprisingly, H ₂ O ₂ mixing ratios of about 1 ppbv were observed repeatedly in winter 2017. Combined field observations and chamber experiments reveal a photochemical in-particle formation of H ₂ O ₂ , driven by transition metal ions (TMIs) and humic-like substances (HULIS). In chamber experiments, steady-state H ₂ O ₂ mixing ratios of 116 ± 83 pptv were observed upon the irradiation of TMI- and HULIS-containing particles. Correspondingly, H ₂ O ₂ formation rates of about 0.2 ppbv h ^-1 during the initial irradiation periods are consistent with the H ₂ O ₂ rates observed in the field. A novel chemical mechanism was developed explaining the in-particle H ₂ O ₂ formation through a sequence of elementary photochemical reactions involving HULIS and TMIs. Dedicated box model studies of measurement periods with relative humidity >50% and PM _2.5  ≥ 75 μg m ^-3 agree with the observed H ₂ O ₂ concentrations and time courses. The modeling results suggest about 90% of the particulate sulfate to be produced from the SO ₂ reaction with OH and HSO ₃ ^- oxidation by H ₂ O ₂ . Overall, under high pollution, the H ₂ O ₂ -caused sulfate formation rate is above 250 ng m ^-3 h ^-1 , contributing to the sulfate formation by more than 70%.