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. 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. 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.