Aqueous phase oxidation of bisulfite influenced by nitrate photolysis

Abstract. Nitrate aerosol is ubiquitous in the atmosphere, and it can exit in both solid aerosol particles and fog and cloud droplets. Nitrate in the aqueous and particulate phase can undergo photolysis to produce oxidizing active radicals, which will inevitably affect various atmospheric chemical processes. However, the role of nitrate aerosols in these atmospheric photochemical processes remains unclear. In this study, the effects of nitrate photolysis on the aqueous phase oxidation of bisulfite under different conditions were investigated. Results show that nitrate photolysis can significantly promote the oxidation of bisulfite to sulfate. It is found that pH plays a significant role in the reaction, and ammonium sulfate has significant impacts on regulating the pH of solution and the enhancement of sulfate production. We also found an apparent synergism among halogen chemistry, nitrate and its photochemistry and S(IV) aqueous oxidation, especially the oxidation of halide ions by the nitrate photolysis and by the intermediate peroxymonosulfuric acid (HSO5−) produced by the free radical chain oxidation of S(IV) in acidic solution leads to the coupling of the redox cycle of halogen with the oxidation of bisulfite, which promotes the continuous aqueous oxidation of bisulfite and the formation of sulfate. In addition, it is also found that O2 is of great significance on nitrate photolysis for the conversion of HSO3−, and H2O2 generation during the nitrate photolysis is verified. These results provide a new insight into the heterogeneous aqueous phase oxidation pathways and mechanisms of SO2 in cloud and fog droplets and haze particles.


The photon fluxes (Iλ) in the sample chamber was measured by using 500 μM 2NB of the same volume as the NH4NO3 samples as a chemical actinometer. For the low light-absorbing conditions of our actinometry, 2-nitrobenzaldehyde undergoes firstorder photodegradation such that: (Galbavy et al., 2010) ln ( where [2NB]t and [2NB]0 are the concentration at illumination times t and zero, respectively. The measured rate constant for 2NB loss (j2NB, λ) is related to photon fluxes where ε 2NB,λ Φ 2NB,λ is the product of the molar absorptivity and quantum efficiency of 2NB (640 M -1 cm -1 at 313nm (Anastasio et al., 1994)) and l is the effective path length of the sample(cm). 2NB was determined using high performance liquid chromatography (HPLC, Ultimate 3000, Thermo Fisher Scientific) with a diode array detector (λ = 355 nm); C-18 Beta Basic reverse-phase column (4.6 mm × 250 mm, 5 μm bead); Column temperature 25 ℃. The eluent was 70% acetonitrile/30% H2O, run at a flow rate of 0.8 mL· min -1 .
As shown in Figure S2, the average value of j2NB,313 in our experiments was 2.86×10 -
Calculation of ·OH quantum yield. As mentioned above, only a small fraction of this incident light was absorbed by the NH4NO3 solutions in our experiments. Under these low light-absorbing conditions the rate of •OH formation can be expressed as (Chu and Anastasio, 2003) where ε 3 − ,λ is the molar absorptivity of nitrate (5.3 M -1 cm -1 at 313nm), , is the quantum yield of •OH from nitrate photolysis, and [ 3 − ] is the molar concentration 7 of nitrate. Rearranging eq S7 to solve for Iλ and substituting that into eq S9 produces an expression for the quantum yield of •OH.
Methods. Hydrogen peroxide in products was measured by titanium salt UV spectrophotometry using an UV-6300 double beam spectrophotometer (Shanghai mapada Instrument Co., Ltd). A stable yellow complex with a characteristic absorption wavelength forms when hydrogen peroxide reacts with trivalent titanium ion. At this wavelength, the concentration of H2O2 is directly proportional to absorbance.
Through the pre-experiment, experimental conditions were established as follows: