Synergistic effect of nitrate-doped TiO2 aerosols on the fast photochemical oxidation of formaldehyde

Wei Wei Xu,Jing Shang,Chun Xiang Ye,Christian George,Tong Zhu

doi:10.1038/s41598-017-01396-x

Abstract

The uptake of formaldehyde (HCHO) on mineral dust affects its budget as well as particle properties, yet the process has not yet been fully investigate. Here, TiO2 and nitrate-doped TiO2 aerosols were used as proxies for mineral dust, and the uptake of HCHO was explored in a chamber under both dark and illuminated conditions. The uptake loss of HCHO on UV-illuminated aerosols is 2–9 times faster than its gaseous photolysis in our experimental system. The uptake coefficient in the range of 0.43–1.68 × 10−7 is 1–2 orders of magnitude higher than previous reports on model mineral dust particles. The reaction rate exhibits a Langmuir-Hinshelwood-type dependence on nitrate content and relative humidity, suggesting the competitive role of nitrate salts, water vapor and HCHO on the TiO2 surface. The reaction produces carbon dioxide as the main product and gaseous formic acid as an important intermediate. The hydroxyl radical produced on illuminated TiO2 primarily drives the fast oxidation of HCHO. The nitrate radical arising from the TiO2-catalyzed photoreaction of nitrate synergistically promotes the oxidation process. This study suggests a novel oxidation route for HCHO in the atmosphere, taking into account high abundance of both mineral dust and anthropogenic TiO2 aerosols.

Highlights

Formaldehyde (HCHO) is the most abundant carbonyl compound in the atmosphere; its presence affects both the radical budget and secondary aerosol formation[1, 2]
The results show that UV-illuminated pure TiO2 or mineral dust greatly enhanced the uptake of O3, NOx (NO + NO2), SO2, volatile organic compounds (VOCs), compared to that under dark conditions[27,28,29,30,31,32,33,34,35,36]
We demonstrate that the oxidation of HCHO is significantly photo-enhanced, with a synergistic effect from the photoreaction of nitrate on the aerosol

Summary

Introduction

Formaldehyde (HCHO) is the most abundant carbonyl compound in the atmosphere; its presence affects both the radical budget and secondary aerosol formation[1, 2]. The uptake coefficient of HCHO on various aerosol components, including sulfuric acid, organic matters, and mineral dust, has been measured[11, 12, 15,16,17]. The results show that UV-illuminated pure TiO2 or mineral dust greatly enhanced the uptake of O3, NOx (NO + NO2), SO2, volatile organic compounds (VOCs), compared to that under dark conditions[27,28,29,30,31,32,33,34,35,36]. Under a typical flux of 103–104 photons·cm−2 · nm−1 between 300–390 nm in the solar radiation[37], excited TiO2 is theoretically able to generate photoactive species that drive rapid redox reactions of HCHO on mineral dust aerosols and photocatalytic anthropogenic surfaces. The uptake of HCHO and formation of products were monitored as the chamber was illuminated under environment-relevant UV radiation in the region of 300–420 nm

Methods

Results

Conclusion