Particle-Phase Photoreactions of HULIS and TMIs Establish a Strong Source of H₂O₂ and Particulate Sulfate in the Winter North China Plain

Abstract

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%.

Acidity and the multiphase chemistry of atmospheric aqueous particles and clouds

“…In detail, the used H 2 O 2 and O 3 concentrations of 0.01 and 1 ppb, used by Cheng et al (2016) for urban haze conditions, are far too low. Recent measurements of H 2 O 2 and O 3 concentrations under haze conditions in the North China Plain (C. Fan et al, 2020;Ye et al, 2021) showed substantially higher values of about 0.5 and 10 ppb, respectively.…”

Ammonium nitrate promotes sulfate formation through uptake kinetic regime

“…However, a recent work found that it varied from 3.8 to 4.5 at RH > 30 % and showed a moderate acidity because of the thermodynamic equilibrium between NH + 4 and NH 3 (Ding et al, 2019). (3) The previous calculations were conducted using a high reaction rate constant of the NO 2 reaction with dissolved S(IV) (Clifton et al, 1988;Cheng et al, 2016), while a smaller value was reported in the more recent study (Spindler et al, 2003;Tilgner et al, 2021). (4) The relative importance of each path depends on the concentration of the relevant pollutants including H 2 O 2 and TMI (P. .…”

“…72 × 10 7 M −1 s −1 , and α = −0.74 (for pH ≤ 4.2) or k 4 = 2.51 × 10 13 M −1 s −1 , α = 0.67 (for pH > 4.2), and k 5 = (1.24-1.67) ×10 7 M −1 s −1 (for 5.3 ≤ pH ≤ 8.7; the linear interpolated values were used for pH between 5.3 and 8.7) at 298 K (Clifton et al, 1988;Tilgner et al, 2021;Liu et al, 2021 (Seinfeld and Pandis, 2006). H 2 O 2 concentrations were unavailable during our observations.…”

“…Modeling studies greatly underestimated (∼ 54 %) SO 2− 4 concentration in serve pollution events in Beijing if only considering gas-phase oxidation of SO 2 , while the normalized mean bias (NMB) decreased significantly after heterogeneous oxidation of SO 2 being considered (Zheng et al, 2015). Several heterogeneous and/or multiphase oxidation pathways, such as oxidation of SO 2 or sulfite by H 2 O 2 (Huang et al, 2015;Maaß et al, 1999;Ye et al, 2021;Liu et al, 2021), HONO (J. F. , and O 3 (Maahs, 1983) or photochemical oxidation of SO 2 (Yu et al, 2017;Xie et al, 2015), catalytic oxidation of SO 2 by transition metal ions (TMI) (Warneck, 2018;Martin and Good, 1991;Wang et al, 2021) and oxidation of SO 2 by NO 2 (He et al, 2014;Clifton et al, 1988;Wang et al, 2016;Cheng et al, 2016;Wu et al, 2019;Spindler et al, 2003) in aqueous phase and heterogeneous oxidation of SO 2 on black carbon (Zhao et al, 2017;Zhang et al, 2020;Yao et al, 2020), have been proposed based on field measurements and laboratory and modeling studies. However, the relative contribution of these pathways to the SO 2− 4 production is still controversial.…”

“…Gas-phase oxidation by OH could explain 33 %-36 % of SO 2− 4 production in the Beijing-Tianjin-Hebei region (L. , while it was negligible based on isotopic measurements (He et al, 2018) and OBM simulations (Xue et al, 2016). As for the oxidation of S(IV) species, which includes SO 2 , HSO − 3 and SO 2− 3 , in aqueous phase, oxidation by H 2 O 2 (T. Ye et al, 2021), NO 2 (J. F. Wang et al, 2016;Cheng et al, 2016), O 3 (Fang et al, 2019), or TMI (Mn 2+ ) (Wang et al, 2021) was proposed as the most important pathway by different researchers. However, the relative importance of these oxidation paths varied greatly among different researches.…”