Ozone is one of the Bay Area's most stubborn air quality problems, and the region still does not fully meet the federal standard for it. Ozone near the ground is the main ingredient of summer smog. It follows rules that can seem backward at first, because cars and factories do not release it directly. Instead, it forms in the air when sunlight cooks other pollutants on hot days. As a result, ozone is often worst far from the busiest freeways. In the Bay Area, the worst readings come on hot summer afternoons in the inland valleys of the East Bay and South Bay. San Francisco rarely has a problem, because the sea breeze and the marine layer keep it cool and well ventilated.
How Ozone Forms
Ground-level ozone forms from two groups of pollutants that react in sunlight. The first is nitrogen oxides, which come mainly from vehicle exhaust, power plants and other burning of fuel. The second group, volatile organic compounds or VOCs, includes vapors from gasoline, paints, solvents and many household products, along with gases given off by some trees and plants.
When strong sunlight hits nitrogen dioxide, it breaks off a single oxygen atom. That atom quickly joins with an ordinary oxygen molecule to form ozone. VOCs keep the reaction going by turning nitrogen oxides back into the form that sunlight can break apart again. The process runs fastest when the sun is high, the air is hot and the wind is light.
Ground-level ozone is different from the ozone layer far above Earth, which blocks harmful ultraviolet light and protects life on the surface. The same gas that helps us 15 miles up harms our lungs when it forms in the air we breathe.
Because the reactions need sunlight and time, ozone builds through the day and usually peaks between about 1 and 5 in the afternoon. The pollutants released during the morning commute drift inland on the afternoon sea breeze while the chemistry runs. By the time the air reaches the inland valleys, it has had hours of sunshine to make ozone. That is why the highest levels show up downwind of the cities rather than in them.
Where Ozone Is Worst in the Bay Area
The Livermore Valley has long recorded the Bay Area's highest ozone levels, and it often exceeds the federal eight-hour standard on hot, calm summer days. The sea breeze carries pollution from the cities around the bay through the gaps in the East Bay hills and into the valley. There, the air sits under hot sun with hills on several sides, and ozone builds up. The Altamont Pass at the east end of the valley eventually lets some of that air move on toward the Central Valley.
Other inland areas see high ozone too. In Contra Costa County, Concord, Pittsburg and Antioch can reach high levels when the sea breeze is weak or the heat is extreme. In the South Bay, the southern Santa Clara Valley around Morgan Hill and Gilroy can do the same on hot, still days. Closer to the bay, places such as Santa Clara usually stay lower, because the sea breeze keeps the air moving.
Weather decides which days turn bad, and the worst ozone days come during heat waves, when temperatures soar and a ridge of high pressure weakens the sea breeze. Under those conditions, the inland valleys get more sunshine, more heat and less fresh air at the same time. Cool, foggy summers tend to bring fewer high-ozone days, while hot, still summers bring more.
Wildfire smoke can add to the ozone problem, since smoke carries the same kinds of gases that form ozone. A smoky summer day can bring high ozone along with high particle levels. The haze from smoke can also block some sunlight, which slows the chemistry near the fire itself. Farther downwind, though, smoke mixed with city pollution under strong sun can push ozone higher than either source would alone.
Deep Dive: Why Is Ozone Highest Downwind of the Cities?
Traffic makes the ingredients for ozone, but it also destroys ozone close to the source. Fresh exhaust contains nitric oxide, which reacts with ozone and removes it. As polluted air drifts inland and the chemistry runs in the sun, that balance shifts and ozone builds. That is why Livermore and the inland valleys see the worst levels.
How Can Traffic Remove Ozone Nearby?
Engines release nitrogen oxides mostly as nitric oxide. Nitric oxide reacts quickly with ozone, turning it back into ordinary oxygen and nitrogen dioxide. Chemists call this titration, and it happens near busy roads and in dense city centers, especially at night and in the early morning.
That is why ozone readings in downtown areas can drop overnight while they stay higher in the suburbs and hills. Over several hours of afternoon sunshine, though, the chemistry builds new ozone faster than titration removes it. By then, the sea breeze has carried the air inland.
Why Can Ozone Rise When Traffic Falls?
Ozone depends on the balance between nitrogen oxides and volatile organic compounds. Where nitrogen oxides are plentiful, the supply of volatile organic compounds limits ozone production. Chemists call that a VOC-limited area, and cutting nitrogen oxides there can actually raise ozone at first. Farther downwind, the air runs short of nitrogen oxides, and cutting them lowers ozone.
California's weekends offer a natural test. In a 2007 study in Atmospheric Chemistry and Physics, Jennifer Murphy and colleagues at UC Berkeley examined the weekend effect in and downwind of Sacramento. Nitrogen oxides were about 35 percent lower on weekends at all sites, while the volatile organic compounds changed by less than 10 percent.
In the Sacramento Valley, the highest 8-hour ozone was higher on weekends, and the standard was more likely to be exceeded. In the mountain counties downwind, exceedances were less likely on weekends. A companion paper described a shift from VOC-limited chemistry in the city to nitrogen-oxide-limited chemistry in the rural areas downwind.
The same logic applies in the Bay Area, where exhaust from the cities along the bay drifts toward the inland valleys. Lasting ozone cuts depend on reducing both kinds of pollution together. It is also why the Spare the Air program asks people to drive less on hot days, since exhaust feeds the ozone forming downwind that afternoon.
Ozone and Health
Ozone irritates the lungs and airways. Even a few hours of breathing high levels can cause coughing, a scratchy throat, chest tightness and shortness of breath. People with asthma may have more frequent attacks on high-ozone days. Athletes and outdoor workers who breathe hard during the afternoon peak can lose some lung function even when they feel fine. Over many years, high exposure is linked to lasting lung damage and to deaths from heart and lung disease.
On days when ozone or fine particles are forecast to reach unhealthy levels, the Bay Area Air Quality Management District issues a Spare the Air alert. In summer, these alerts ask people to drive less, carpool or use transit, and avoid gas-powered yard equipment. Each of those steps cuts the pollutants that form ozone.
The summer Spare the Air season runs from about May through October, when long days and heat make ozone most likely. Alerts usually go out the afternoon before a bad day, giving people time to change their plans. You can sign up for alerts by text, email or app, and local news stations announce them too.
On high-ozone days, plan hard exercise outdoors for before 10 a.m. or after 6 p.m., when levels are lower. Check the AQI at the monitoring station nearest you, since readings can differ a lot from one valley to the next. Coastal residents rarely face unhealthy ozone, because the sea breeze keeps coastal air clean even when the inland valleys reach the unhealthy range.
The long-term trend is good news. Ozone in the Bay Area has dropped a great deal since the 1970s and 1980s, thanks to cleaner cars, cleaner fuels and tighter rules on industry. The region now has far fewer high-ozone days than it once did. Hotter summers work against that progress, though, because heat speeds up the chemistry, so continued cuts in emissions will matter even more in the years ahead.
