What Is Smog? How LA's Photochemical Haze Forms

Los Angeles smog is not a single pollutant drifting in from somewhere else. It is photochemical smog, a mixture of ground-level ozone and other oxidants created when nitrogen oxides (NOx) and volatile organic compounds (VOCs), mostly from vehicle exhaust, fuels, and industrial sources, react with each other in strong sunlight. Nobody emits smog directly. Cars and equipment emit the raw ingredients, and sunlight does the rest, cooking them into ozone over the course of the day. That is why smog usually builds through the morning and peaks in the afternoon, well after the traffic that helped cause it has already thinned out.

What Is Smog?

Smog is a general term for visibly polluted air, but the word means something specific in Los Angeles. It originally described a literal blend of smoke and fog in industrial cities like London, where coal soot mixed with natural fog to form a thick, choking haze. Los Angeles smog is a different, more modern phenomenon: photochemical smog, first identified in the 1940s and formally explained by the California Institute of Technology biochemist Arie Haagen-Smit in the early 1950s. Haagen-Smit demonstrated that LA's haze and crop damage were not caused by industrial smoke alone but by a chemical reaction between vehicle and industrial emissions and sunlight, a discovery that reshaped how cities regulate air pollution.

One of the more counterintuitive parts of this story is that ozone concentrations can be high on days when the sky does not look dramatically brown or hazy. Ground-level ozone is a colorless gas. The visible haze people associate with smog usually comes from fine particles and other secondary pollutants that form alongside it, not from ozone itself. A clear-looking sky over Downtown Los Angeles can still carry an elevated ozone reading, which is one reason air quality has to be measured with instruments rather than judged by eye.

How Does Photochemical Smog Form?

Photochemical smog forms when nitrogen oxides and volatile organic compounds react in sunlight, producing ground-level ozone and other oxidants rather than being released as finished smog. Nitrogen oxides come mainly from combustion, tailpipes, diesel engines, power equipment, and industrial burners, while VOCs come from a wider mix of sources, gasoline vapor, solvents, paints, and other fuel and chemical products. On their own, neither group of gases is smog. It is the sunlight-driven chemistry between them, happening continuously in the atmosphere above the city, that converts these precursor gases into ozone and related oxidants over the course of a few hours.

This is why smog is described as a secondary pollutant. It is not measured at the tailpipe; it forms downwind in space and later in time, often peaking in the afternoon after the morning's emissions have had several hours of sunlight to react. The table below summarizes the main ingredients.

It also helps to separate this ground-level chemistry from the ozone layer people hear about in the context of global climate and UV protection. Stratospheric ozone sits many miles above the surface and shields the planet from ultraviolet radiation; it is considered beneficial. Ground-level ozone forms in the air we actually breathe and is a regulated pollutant precisely because it is harmful at that altitude. Same molecule, opposite role, depending on where in the atmosphere it sits.

Why Is Los Angeles So Prone to Smog?

Los Angeles combines abundant sunshine and a large volume of vehicle and industrial emissions with basin topography and weather patterns that can trap or transport that pollution rather than letting it disperse. The South Coast Air Basin sits in a bowl bounded by the Pacific Ocean on one side and the San Gabriel, San Bernardino, and Santa Ana Mountains on the others. Those mountains are exactly what block pollution from simply blowing through and out of the region the way it might over flatter terrain.

A semi-permanent zone of sinking air associated with the Eastern Pacific high-pressure system caps the basin with a temperature inversion for much of the warm season, warm air aloft sitting over cooler air near the surface. That inversion acts like a lid, limiting how high pollution can mix vertically and concentrating precursor emissions and ozone closer to the ground. The mechanics of that lid, and how it also shapes humidity and fog patterns, are covered in more depth in humidity and dew point in Los Angeles.

Daytime sea breezes complicate the picture further. Onshore flow can genuinely clean out the coast, but that same breeze carries precursor gases and ozone inland toward the San Gabriel Valley and beyond over the course of the afternoon, which is one reason inland communities often see worse ozone than the beaches that generated much of the original traffic. Urban heat also plays a role in how the basin holds and circulates this air; see the urban heat island effect in Los Angeles for how paved surfaces and dense development add to the mix. A fuller account of how the basin's geography, car dependence, and inversions combine into a persistent smog problem is covered in why Los Angeles is so smoggy.

When and Where Is LA Smog Usually Worst?

Ozone smog is most favored on sunny, hot, stagnant days and typically peaks in the afternoon in inland and downwind parts of the South Coast Air Basin, generally worsening from late spring through early fall when sunlight is strongest and the inversion is most persistent. According to South Coast AQMD and CARB monitoring, the coast tends to see the lowest ozone readings because onshore flow ventilates those areas first, while inland valleys, where the transported pollution has had more time to react and less wind to disperse it, tend to record the region's highest readings.

This is why a single coastal air quality reading cannot stand in for conditions in Pasadena or Woodland Hills. Ozone forms and accumulates downwind, so inland San Gabriel Valley and San Fernando Valley locations can register meaningfully worse air on the same afternoon that Long Beach or another coastal spot is enjoying a clean sea breeze. These patterns shift somewhat year to year with weather and emissions trends, so any specific ranking of which community is "worst" should be checked against a defined and current monitoring period rather than treated as permanent.

Is Smog Dangerous?

Ground-level ozone can irritate and damage the respiratory system, and the EPA ozone health summary identifies children, older adults, people with asthma or other lung disease, and people who are active outdoors as groups with greater cause for concern. Short-term exposure can aggravate breathing and worsen existing respiratory conditions, and repeated exposure over time is linked to broader respiratory harm. The AirNow Air Quality Index translates ozone and particle pollution levels into standard categories, from Good through Hazardous, along with recommended actions for each level, developed by the EPA in coordination with state and local agencies including South Coast AQMD.

Because ground-level ozone is invisible, individual risk cannot be judged from how hazy or clear the sky looks. A visually clear afternoon can still carry a Moderate or Unhealthy for Sensitive Groups ozone reading, particularly inland, while a slightly hazy coastal morning may reflect fog or marine moisture rather than pollution. The safest approach during warm, sunny, stagnant stretches is to check the current AQI and its dominant pollutant before planning strenuous outdoor activity, rather than relying on visibility alone.

Is the Haze Smog, Wildfire Smoke, or Marine Fog?

You cannot reliably identify smog, wildfire smoke, or marine fog by color alone, because ozone itself is invisible and visible haze can come from secondary particles, smoke, or plain water droplets. Photochemical smog is often ozone plus secondary particulate pollution built up over a sunny day. Wildfire smoke is a different animal, dominated by fine particulate matter (PM2.5) carried in from a burning source, which can spike air quality readings quickly regardless of sunlight or time of day.

Marine fog is neither: it is simply suspended water droplets from cool, moist ocean air condensing near the coast, more thoroughly explained in Los Angeles marine layer and gray mornings. Fog itself is made of water droplets rather than being an AQI pollutant, though those droplets can coexist with polluted air. Telling these three apart in the moment takes more than a glance at the sky, which is exactly the comparison WeatherEscape's smog vs. smoke vs. fog guide walks through in detail.

How Do You Track Smog and Ozone in LA?

The most reliable way to track smog in Los Angeles is to check the AQI's dominant pollutant, the nearest monitoring station, the observation's timestamp, and, when available, the day's ozone forecast, rather than relying on a single citywide haze label. South Coast AQMD operates the network of monitors across the South Coast Air Basin and issues forecasts and advisories. The California Air Resources Board ozone fact sheet explains the precursor chemistry, while AirNow aggregates official monitor data nationally and translates it into the standard AQI categories the EPA uses.

It also matters which pollutant is driving a given AQI reading. An ozone-dominated reading tends to rise through the afternoon on hot, sunny, stagnant days and fall off in the evening as sunlight fades, while a particle-dominated (PM2.5) reading, more typical of wildfire smoke or winter stagnation, can stay elevated around the clock. Knowing which pollutant is dominant on a given day tells you whether you are dealing with classic summer photochemical smog or something else entirely.

Smog in Los Angeles is ultimately a chemistry problem shaped by the basin's weather, not a fixed haze that just sits over the city. Before planning an afternoon hike, an outdoor event, or extended time outside anywhere from the coast to the inland valleys, check the current AQI and its dominant pollutant for your specific destination rather than judging conditions by what the sky looks like from somewhere else in the basin.