Smog forms when pollutants from vehicle exhaust, power plants, and industrial activity react with sunlight in the lower atmosphere. The two key ingredients are nitrogen oxides and volatile organic compounds (VOCs), which undergo a chain of chemical reactions driven by ultraviolet radiation to produce ground-level ozone and other toxic secondary pollutants. This photochemical process is the most common type of smog in modern cities, though a second type, sulfurous smog, forms through a different mechanism involving coal smoke and humidity.
The Two Types of Smog
There are two distinct forms. Photochemical smog is the brown haze common in sunny cities like Los Angeles, Mexico City, and Beijing. It requires sunlight to form and peaks during warm months. Sulfurous smog, sometimes called “London smog,” results from high concentrations of sulfur oxides released by burning sulfur-heavy fossil fuels, particularly coal. Sulfurous smog is made worse by dampness and high levels of suspended particles in the air. While sulfurous smog was the dominant form during the industrial era, photochemical smog is the primary concern today.
How Photochemical Smog Forms Step by Step
The process starts with primary pollutants, meaning chemicals released directly into the air. Cars, trucks, and industrial facilities emit nitric oxide and VOCs. VOCs come from a wide range of sources: vehicle exhaust, solvent-based paints, printing inks, petroleum products, and even some consumer goods. Nitric oxide reacts with oxygen in the atmosphere to form nitrogen dioxide, a reddish-brown gas that gives smog its characteristic color.
When ultraviolet light from the sun hits nitrogen dioxide, it splits the molecule apart, releasing a single oxygen atom. That free oxygen atom quickly bonds with an ordinary oxygen molecule to create ozone. Ozone high in the stratosphere protects us from UV radiation, but at ground level it’s a harmful pollutant that irritates airways and damages lungs.
This is where VOCs make the problem much worse. Without them, the cycle would partly cancel itself out because ozone would react with nitric oxide and break back down. But VOCs interrupt that cleanup process. They react with hydroxyl radicals in the air to form highly reactive molecules called peroxy radicals. These radicals convert nitric oxide into nitrogen dioxide without consuming ozone in the process. The result is a one-way buildup: more nitrogen dioxide keeps getting split by sunlight, producing more and more ozone with no chemical brake to slow it down.
The same chain of reactions also produces peroxyacetyl nitrates, a family of compounds that cause the stinging eye irritation associated with smog. These form when nitrogen dioxide reacts with organic radicals derived from hydrocarbons like propene, butene, and common aldehydes. Ozone and peroxyacetyl nitrates are both secondary pollutants, meaning they aren’t emitted directly by any source. They only exist because of chemical reactions that happen in the atmosphere itself.
Why Smog Is Worse at Certain Times of Day
Because sunlight drives the key reactions, smog doesn’t form evenly throughout the day. Morning rush-hour traffic floods the air with nitrogen oxides and VOCs. As the sun climbs higher and UV intensity increases, those raw ingredients get “cooked” into ozone and particulates. Ground-level ozone concentrations typically peak in the early-to-mid afternoon, hours after the initial burst of tailpipe emissions. By evening, as sunlight fades, the photochemical reactions slow and ozone levels drop.
This lag between emission and smog formation is why air quality warnings often target the afternoon, not the morning commute. It also means that smog can drift downwind, sometimes affecting suburbs and rural areas far from the original pollution sources.
How Geography and Weather Trap Smog
Certain cities suffer far worse smog than others, even with similar traffic levels, because of geography. Cities sitting in valleys or basins surrounded by mountains are especially vulnerable. The terrain blocks horizontal wind flow, preventing polluted air from dispersing. Research on Lanzhou, a valley city in northwest China, found that the basin shape delayed the development of normal daytime air mixing by one to two hours compared to open terrain, giving pollutants more time to accumulate near ground level.
Temperature inversions make the problem even worse. Normally, warm air near the ground rises and carries pollutants upward, where they disperse. But during an inversion, a layer of warm air sits above cooler air near the surface, acting like a lid. The cooler, polluted air is trapped underneath with nowhere to go. These inversions are especially common during high-pressure winter weather, when the ground loses heat rapidly at night and the air closest to the surface becomes very cold. Each day the inversion persists, pollution from traffic and industry keeps accumulating in the same stagnant air layer.
Primary vs. Secondary Pollutants in Smog
Understanding the difference between these two categories helps clarify why smog is so difficult to control. Primary pollutants like nitrogen oxides, carbon monoxide, sulfur oxides, and particulate soot come straight out of tailpipes and smokestacks. They can be reduced directly through cleaner fuels, catalytic converters, and emissions regulations.
Secondary pollutants like ground-level ozone and peroxyacetyl nitrates don’t come from any single source. They form in the open air from complex reactions among multiple precursors. You can’t filter ozone out of a tailpipe because it doesn’t exist there yet. Reducing secondary pollutants requires cutting emissions of their precursors, particularly VOCs and nitrogen oxides, which is why regulations target both categories simultaneously. Even modest reductions in one precursor can disrupt the chain of reactions enough to lower peak ozone levels significantly.
Health and Environmental Effects
Ground-level ozone is the most harmful component of smog for human health. It penetrates deep into the lungs, triggering inflammation that worsens asthma, reduces lung function, and increases the risk of respiratory infections. Peroxyacetyl nitrates cause eye irritation and can also damage plant tissue, reducing crop yields. Fine particulate matter (PM2.5), another product of smog chemistry, is small enough to enter the bloodstream through the lungs. The World Health Organization set its recommended annual PM2.5 exposure limit at just 5 micrograms per cubic meter in 2021, down from 10 micrograms in its previous guidelines, reflecting growing evidence that even low levels cause harm.
Smog also damages ecosystems. Ground-level ozone impairs photosynthesis in plants, weakening forests and reducing agricultural productivity. The fine particles in smog reduce visibility, creating the hazy conditions that define a smoggy day, and settle onto soil and water, altering nutrient balances in sensitive habitats.