Ozone (O₃) is a reactive gas consisting of three oxygen atoms. Naturally occurring and also artificially produced, it exists in both the upper atmosphere (the stratosphere) and at ground level (the troposphere). Its impact on life depends on its altitude: while stratospheric ozone shields us from harmful UV rays, ground-level ozone can be a pollutant.
Tropospheric ozone, more commonly referred to as ground-level ozone, is produced through photochemical interactions between volatile organic compounds (VOCs) and nitrogen oxides (NOx)—two primary classes of atmospheric pollutants. Historically, these reactions were considered dependent on elevated temperatures and sunlight, leading to the characteristic peak in ambient ozone concentrations during summer months.
Nevertheless, research conducted over the last ten years has documented elevated ozone levels under specific cold-weather conditions as well. In select high-altitude locations across the western United States characterized by intense local VOC and NOx emissions, ozone formation has been observed even in the presence of snow cover and near-freezing temperatures. This ground-level ozone is a significant component of what is commonly perceived as "smog" or atmospheric haze—a condition that, while still most prevalent during summer, can now manifest throughout the entire year in certain southern and mountainous regions.
Despite the fact that a certain amount of stratospheric ozone is transported into the troposphere and that VOCs and NOx can be emitted from natural sources, the predominant contributor to ground-level ozone is the reaction of anthropogenic VOCs and NOx. Key sources of anthropogenic VOCs include chemical plants, gasoline dispensing stations, oil-based coatings, auto body shops, and commercial printing operations. Nitrogen oxides are predominantly generated through high-temperature combustion, with principal sources being electric utilities, industrial combustion units, and internal combustion engines in motor vehicles.
Ozone (O3) is formed through photochemical reactions where high-energy ultraviolet (UV) sunlight splits oxygen molecules (O2) into individual atoms, which then collide with other molecules. In the stratosphere, this creates the protective ozone layer, while in the troposphere, it is formed by sunlight reacting with pollutants like VOCs.
