What air pollutant causes a reddish-brown haze in the sky?

The air pollutant that causes a reddish-brown haze in the sky is nitrogen dioxide (NO2). This gas is primarily produced from vehicle emissions and industrial processes. When NO2 interacts with sunlight, it can lead to the formation of ground-level ozone, contributing to smog and the characteristic reddish-brown color. This phenomenon is often observed in urban areas with high traffic and pollution levels.