Heterogeneous SO₂ Oxidation in Sulfate Formation by Photolysis of Particulate Nitrate

Abstract

Heterogeneous oxidation of sulfur dioxide (SO 2 ) is suggested to be one of the most important pathways for sulfate formation during extreme haze events in China, yet the exact mechanism remains highly uncertain. We propose a much less explored pathway for aqueous-phase SO 2 oxidation for the formation of particulate sulfate by NO 2 and OH radicals produced from photolysis of particulate nitrate. Reactive uptake experiments with SO 2 and ammonium nitrate particles under ultraviolet irradiation show measured SO 2 uptake coefficients of ∼10 −5 . Model calculations of sulfate production rates, comparing known mechanisms of oxidation by O 3 , NO 2 , H 2 O 2 , and transition metal ions, and the nitrate photolysis mechanism suggest that the nitrate photolysis pathway could contribute significantly to the overall sulfate production at pH 4−6. This study provides new insight into the current debate about sulfate production pathways under typical haze conditions in China.

Aqueous phase oxidation of bisulfite influenced by nitrate photolysis

“…The pH of initial solution was 4.32 when the concentrations of NaHSO3 and NH4NO3 were both 30 mM. Hence, the formation of sulfate discussed before confirms the occurrence of nitrate photolysis under acidic conditions, as reported by earlier works (Gen et al, 2019b;Scharko et al, 2014;Benedict et al, 2017).…”

“…On the one hand, the inhibition of 2-propanol on sulfate yield in the presence and absence of O2 under light were similar ( Figure 5), demonstrating that O2 has little effect on the generation of •OH by nitrate photolysis. This is definitely distinct with previous report by Gen et al (2019b), who found that •OH is generated in the presence of O2. Furthermore, they claimed that the contribution of •OH pathway on the oxidation of dissolved SO2 was almost absent (<1%) under nitrate photolysis at 300 nm (Gen et al, 2019a).…”

“…Considering the insignificant increase of sulfate yield, experiments were conducted under 313 nm UV light (Figure 1 μM•min -1 ) compared with the low yield in dark. It may be attributed to the oxidation of HSO3by various oxidizing species produced by nitrate photolysis, such as •OH (Gen et al, 2019b;Mack and Bolton, 1999). •OH can be produced directly (R3) (Scharko et https://doi.org/10.5194/acp-2020-806 Preprint.…”

“…adsorbed HNO3 and gas-phase N2O and HONO under no light. And recently, Gen et al (2019b) proposed that nitrate photolysis at 250 nm is the major source of NO2 and •OH to oxidize dissolved SO2, in which •OH is believed to be formed when O2 exists, and the reaction of •OH with dissolved SO2 generates SO3 2-, which leads to the chain reaction involving SO5 -, HSO5and SO4to produce multiple SO4 2from each attack of •OH on dissolved SO2, providing a new insight for the heterogeneous aqueous phase oxidation of SO2 by nitrate photolysis. However, the impacts of various oxidizing species produced and/or initiated by nitrate photolysis under different conditions on the aqueous phase oxidation of SO2 remain unclear.…”

Sensitivity Analysis of the Surface Ozone and Fine Particulate Matter to Meteorological Parameters in China

“…But meanwhile SO 2production from heterogeneous reactions is reduced more when T is increased. Heterogeneous SO 2formation has been proposed as a major SO 4 2formation pathway during China haze events (Wang et al, 2014b;Gen et al, 2019;Huang et al, 2019;Li et al, 2019a) and in this study it accounts for up to ~75% of total SO 4 2production. The treatment of heterogeneous SO 4 2formation currently is modeled as a surface-controlled uptake process, in which the formation rate is determined by the aerosol surface area and the uptake coefficient of SO 2 on particle surface (Ying et al, 2014).…”