Ozone Concentrations

How is the fight against smog going?

Ozone Concentrations

Ozone concentrations refer to the quantity of ozone (O3) molecules in the air. In the Bay Area, ozone is measured by numerous monitoring stations that record hourly concentrations at each site. These hourly data are used to calculate eight-hour peak ozone levels. Ultimately, peak ozone levels on the fourth-worst day of the year are used to determine compliance with national standards and serve as the most common indicator of ozone performance.

Ground-level ozone, commonly known as smog, is formed when emissions of ozone precursors from power plants, factories, cars and other sources react with sunlight in the atmosphere. While declining emissions of ozone precursors in recent years has reduced smog in many communities across the country, current ozone concentrations still affect public health, particularly on hot summer days. Reducing smog has been one of the Bay Area’s major public policy priorities for many decades.

Regional Performance
Even with significant population and economic growth, Bay Area ozone concentrations have declined over the past half-century.

Peak Bay Area ozone concentration levels are well below those first recorded when air quality monitoring began more than 50 years ago. Thanks in part to cleaner cars and trucks, ozone concentrations today are at least one-third lower than 1970s levels. These improved conditions have come about despite growth in population, vehicles and miles traveled in the Bay Area. Since the early 2000s, Bay Area ozone concentrations have consistently been one-third lower than in the 1970s. At the same time, however, ozone concentrations can vary from year to year depending on weather conditions. In the last two years, the weather was unfavorable and as a result concentrations in the region’s worst affected locations ticked upward. Based on these recent data, the Bay Area exceeds the new, more stringent federal standards for ozone.

Local Focus
Ozone readings vary widely across the region, with inland areas showing the highest levels of this pollutant.

Due to prevailing winds and weather conditions, ozone from motor vehicle emissions and industrial sources in the center of the region accumulates in inland communities. Eastern Contra Costa, eastern Alameda, and southern Santa Clara counties are particularly affected by ozone. Conditions continue to be especially high in Livermore and San Martin, for example. In contrast, the region’s second- and third-largest cities, San Francisco and Oakland, had the lowest concentrations of ozone. Given that the majority of the region’s population lives in the urban core of the region, this means that ozone exposure is well within the federal standards for most Bay Area residents.

Ozone Concentrations

Select sensors on the map to see detailed historical ozone concentrations.
National Context
Despite significant improvements in air quality over the past half-century, Los Angeles still has the highest ozone levels of any major U.S. metro area.

California’s stringent motor vehicle emissions standards have played a key role in reducing ozone levels in our state over the past half-century. Although ozone levels vary annually based on weather conditions, Bay Area ozone levels have generally been lower than most major metro areas in recent years. While emissions standards have greatly improved conditions in the City of Angels as well, Los Angeles still has the highest ozone concentrations of any major American metropolis. This is a result of its mountainous terrain, which trap air pollution in the Los Angeles basin. This topography contributes to the creation of smog, but at levels significantly lower than in decades past.


Bay Area Air Quality Management District: Air Quality Sensor Data (1970-2016)

Environmental Protection Agency: Air Quality System (2013-2016)

Image: Flickr (Creative Commons license), Photographer: Joshua Aaron, https://www.flickr.com/photos/38324365@N00/314894944

Methodology Notes: 

Data is collected at specific sensor sites operated by the Air District that capture the concentration of ozone in the air; sensor locations may change over the years and thus only minor relocations are shown as the same station in this analysis (e.g., sensor moved across the street from its prior location). Data are used to calculate the eight-hour peak concentration for ozone. The fourth-highest day of the year is identified and then reported as a three-year rolling average to minimize the impact of short-term variations in weather conditions in any given year. This average is compared to the national air quality standard to determine compliance. The site with the highest ozone level may shift from year to year depending on which sensor location had the highest levels of ozone over that particular time period.

For current air quality data, refer to the Bay Area Air Quality Management District’s website: http://www.baaqmd.gov/about-air-quality/current-air-quality