Article pubs.acs.org/JPCA
Heterogeneous Reaction of NO2 on α‑Al2O3 in the Dark and Simulated Sunlight Chun Guan, Xinling Li,* Yueqi Luo, and Zhen Huang Key Laboratory of Power Machinery and Engineering, Ministry of Education, Shanghai Jiao Tong University, Dongchuan Road No. 800, Minhang District, Shanghai 200240, China ABSTRACT: Although there have been a number of publications focused on heterogeneous of NO2 on mineral particles, most of these studies were focused on β-Al2O3 and performed in the dark. Less was known about the reaction process of NO2 on α-Al2O3, especially the effect of sunlight factor. The heterogeneous reaction between NO2 and α-Al2O3 was investigated by using diffuse reflectance infrared Fourier transform spectrometry. The effects of NO2 and O2 concentrations as well as simulated sunlight were examined, and the reaction mechanism including the consumption of surface OH groups, oxidation process of nitrites, and the formation of water was also discussed in detail. It was observed that the formation rates of nitrates and nitrites were sensitive to NO2 concentrations and O2 concentrations. Nitrite was identified to be an intermediate production and disappeared very soon as [NO2] was up to 4.035 × 1015 molecules/cm3. Light played an important role in the changes of the electronic configuration of mineral dust, such as electronic donating ability, surface OH groups orientation, as well as the conversion efficiency between proton acid and nonproton acid, all of which could significantly enhance the heterogeneous reaction process. The reaction order for NO2 and O2 was determined to be 0.960 ± 0.111 and 0.620 ± 0.028, respectively. The uptake coefficient of NO2, which dominated the first step of the heterogeneous reaction, was calculated by the infrared absorbance with the use of ion chromatography and determined to be 9.9 × 10−10 in the dark and varied from 2.54 to 3.33 × 10−9 under simulated sunlight from 0.45 to 1.35 mW/cm2. It was also found that γNO2 was independent of [NO2] and sunlight increased the uptake coefficient by three times, indicating that the heterogeneous reaction between NO2 and α-Al2O3 was enhanced under sunlight.
1. INTRODUCTION NO2 is a significant component of air pollution and plays an important role in atmospheric chemical reactions due to its high chemical reactivity.1 It is also regarded as a predecessor that can contribute to the formation of acid rain and lead to stratospheric ozone depletion.2−5 Although the concentrations of NOx in the air photochemical smog are lower as 70 part per billion (ppb), more and more research indicates that heterogeneous reaction between gas pollutant and particulates turns out to be of great significance. At present, one of the major reactions of NO2 is the interaction with OH groups from carbon particulates or water, resulting in the formation of HONO6−9 and production of nitric acid.10 Another is heterogeneous process on mineral aerosol, including the uptake of NO2 on Al2O3, NaCl, and dust particles, followed by the formation of adsorbed products such as surface nitrates and nitrites.11−13 Mineral aerosol, regarded as one of the largest mass fractions of the global aerosol, constitutes 36% of total primary aerosol emission.14,15 Usually, it consists of windblown soil and is produced mainly in the arid areas of the earth, particularly in the great deserts, but can be transported over long distances of up to 5000 km, especially for the smaller fractions
