Maho Matsumura*, Kokoha Tsukamoto
Ritsumeikan High School
*Email:
Introduction
In Japan, the concentrations of nitrogen and sulfur dioxides have declined since the 1970s due to environmental regulations and increasing public awareness. However, the concentrations of photochemical oxidants have continued to increase gradually since the 1980s, contributing to persistent photochemical smog. Photochemical smog forms when ultraviolet radiation from sunlight initiates photochemical reactions involving nitrogen oxides and hydrocarbons from vehicle and industrial emissions. Due to the complexity of atmospheric composition, it is difficult to isolate specific reaction processes through field observations alone. Smog chambers are commonly used in laboratory studies to model and analyze individual reactions under controlled conditions.
In this study, we used a round-bottom flask as a simplified smog chamber to simulate reactions between volatile organic compounds (VOCs) and nitrogen dioxide (NO2). Trace amounts of ozone and aldehydes were observed as secondary products.
The study aimed to accomplish the following goals:
・Examine the conditions for generating simulated photochemical smog.
・Confirm and quantify ozone generation.
・Detect and quantify aldehydes as secondary products.
・Isolate and understand specific atmospheric reactions through experimental modelling.
While many educational resources address atmospheric chemistry, most focus on simulations or data interpretation. Few allow students to observe actual photochemical reactions. To address this gap, we developed a hands-on experiment using accessible materials to visualize smog formation.
We compared three VOCs, limonene, isoprene, and butane, chosen for their different carbon bond structures. Limonene and isoprene contain double bonds, while butane contains only single bonds. This allows us to examine how molecular unsaturation affects reactivity. Since ozone and aldehydes are major secondary pollutants in photochemical smog, their quantification also helps assess the environmental impact of each compound.
Experimental Method
Smog Generation with Different VOCs under UV Irradiation
Conditions for generating simulated photochemical smog: A 3 cm × 5 cm piece of plastic bottle was placed in the large suction filtration bottle. NO2 was generated by burning 14.3 mg of lead(II) nitrate in the test tube. 5 mL of NO2 was drawn using a 100 mL glass syringe and diluted in the atmosphere. This NO2 was injected into the 200 mL round bottom flask. One drop (about 0.05 mL) of either limonene, butane, or isoprene was placed inside the flask, and the flask was sealed with a silicone rubber stopper. These round bottom flasks were placed in a UV irradiation box and exposed to ultraviolet light. After that, the laser light was attached to the side of the flask.
Quantitative Detection of Ozone and Aldehydes
Detection and quantification of ozone: 2.0 mL of 1.0% potassium iodide solution was added to the flask containing the generated gas. The color of these solutions turned light brown. This indicated the presence of ozone. After that, 0.5 mL of the 1% starch solution was added to the flask, and the color of the solutions turned light purple.
Detection and quantification of aldehydes: We utilized the reduction of Fehling’s solution and the generation of tetraamine copper(II) ions. First, the Fehling solution was prepared. Then, about 1 mL of this solution was added to flasks containing either ozone or aldehyde. The flasks were then covered with a silicone rubber stopper and shaken to mix. These solutions were transferred to a test tube and heated with a burner to produce the copper(I) oxide (Cu2O). These solutions were filtered using a decompression device, a suction filtration bottle, and a glass filter. Nitric acid was added to the copper(II) ions and ammonia to make a complex ion. The amount of the aldehyde inside the flask was investigated by using a spectrophotometer.
Results & Discussion
Smog Generation with Different VOCs under UV Irradiation
There was a thin haze in the round bottom flask, so it was thought that simulated photochemical smog was generated. To determine if smog was generated, a laser light was attached to the side of the flask. The Tyndall effect, which is the scattering of a beam of light by a medium containing suspended particles, occurred (Figure 1). This confirmed that simulated photochemical smog was generated.
Figure 1: Tyndall effect observed in the flask
Table 1 shows the results after ultraviolet radiation was used to irradiate nitrogen dioxide diluted by the air and the volatile organic compound (VOC) in the flask. The presence of butane-induced smog was not clearly confirmed. However, there was a Tyndall effect in the flasks that contained isoprene and limonene for more than 30 seconds. The reason for this was that isoprene and limonene have double bonds, but butane has a single bond, making it less reactive.
Table 1. Results for each VOC after UV irradiation
Quantitative Detection of Ozone and Aldehydes
The presence of ozone generated by photochemical reactions was confirmed using a potassium iodide solution. In addition, using the reduction reaction of Fehling’s solution and the formation of tetraamminecopper(II) ions, a calibration curve was drawn, enabling the number of aldehydes generated in the simulated atmosphere to be estimated (Figure 2).
Figure 2. The standard curve using Cu₂O
Conclusions
This study demonstrated that photochemical smog can be generated in a controlled laboratory environment using a round-bottom flask and ultraviolet irradiation. The Tyndall effect confirmed the formation of aerosol-like particles, and the results varied depending on the structure of the VOCs used. Compounds with double bonds, such as isoprene and limonene, produced stronger Tyndall effects than saturated hydrocarbons like butane.
In addition, the presence of ozone was detected by potassium iodide-starch colorimetric reactions, and aldehydes were quantified using a copper(II)-ammine complex. These methods proved effective in identifying secondary pollutants typically found in atmospheric photochemical smog.
The experimental approach used here is simple and reproducible, making it a suitable model for science education and further research into atmospheric chemistry.
References
Akimoto, H., Hoshino, M., Inoue, G., Sakamaki, F., Washida, N., & Okuda, M. (1979). Design and characterization of the evaluable and bakeable photochemical smog chamber. Environmental Science & Technology, 17(4), 471–475. https://doi.org/10.1021/es60152a014
Hathaway, B. J., & Tomlinson, A. A. G. (1970). Copper(II) ammonia complexes. Coordination Chemistry Reviews, 5(1), 1–43. https://doi.org/10.1016/S0010-8545(00)80073-9
Kitada, T., & Hiraoka, M. (1977). Photochemical smog reaction relating to aldehyde. Journal of the Japan Society of Air Pollution, 12(1), 26–37. https://doi.org/10.11298/taiki1966.12.26
