Air quality in Monterey County

How is the air quality in Monterey?

Monterey is located in Central California along the southern coast of the Monterey Bay, approximately 120 miles south of San Francisco. It is the largest city on the Monterey Peninsula, which includes the cities of Seaside, Carmel, and Pacific Grove as well as the unincorporated community of Pebble Beach.

According to IQAir air quality data as well as the United States Environmental Protection Agency’s (EPA) Air Quality Index (AQI), Monterey air quality is generally deemed “good” on average (AQI ranging from 0 to 50) In 2020, only September and October averaged “moderate” air quality (AQI 51 to 100), with air pollution levels posing elevated risks of adverse health effects, especially to sensitive groups such as children and the elderly. These months suffered from frequent wildfire smoke pollution as a result of California’s record-breaking wildfire season that year.

According to annual State of the Air Report by the American Lung Association (AMA), Monterey County has acceptable levels of ground-level ozone, but unacceptable levels of short-term fine particle pollution, or PM2.5 (smaller than 2.5 microns in diameter). Monterey’s failing grade for short-term PM2.5 is due to 9 days between 2016 and 2018 with PM2.5 levels that were “Unhealthy for Sensitive Groups” (orange) or “Unhealthy” (red).¹ The federal target is no more than 3.2 unhealthy pollution days annually – thus, instances of short-term PM2.5 pollution days occurred nearly 3 times more than the mandated limit.

State standards set by the California Air Resources Board (CARB) additionally place Monterey County in nonattainment for coarse particulate matter, or PM10, as well as ozone. Monterey PM10 levels exceeded the 24-hour standard of 50 μg/m³ for a total of 11 days during 2010. Since then, average maximum 24-hour concentration levels have ranged between 57 and 95 μg/m³.² PM10 describes coarse airborne particulate matter smaller than 10 microns in diameter, such as dust, pollen and mold. Breathing in PM10 can cause short-term airway irritation and contribute to asthma and chronic obstructive pulmonary disease (COPD) in the long term.³

In addition to PM10, ozone levels in Monterey have exceeded maximum recommended concentration levels for 1-hour ozone (9 parts per billion, or ppb) and 8-hour ozone (70 ppb) several times in recent years. Both standards must be met under CARB regulations to be in attainment status. While Monterey remains in nonattainment for ozone, long-term trends have shown a reduction in days when ozone exceeded the state or the federal standard.⁴

Where does air pollution in Monterey come from?

Air pollution in Monterey comes from several sources:

• mobile sources, including vehicle emissions
• prescribed burning and wildfires
• manufacturing and industrial activity, including oil refineries and power plants
• agriculture, such as from wineries

Monterey is part of the North Central Coast Air Basin (NCCAB) and is monitored by the Monterey Bay Air Resources District (MBARD). The NCCAB covers a land area of 5,159 square miles along the California Central Coast region that includes Santa Cruz, Scotts Valley, and Hollister, among other cities.⁵ These cities share the same general topography and meteorological patterns. As a result, their emissions and weather conditions often influence overall air quality in the basin.⁶

Ozone

Monterey ground-level ozone is formed when precursor emissions like nitrogen oxides (NOx) and volatile organic compounds (VOCs) from motor vehicles, stationary fuel combustion, and industry meet UV radiation from sunlight.⁷When this happens, chemical reactions occur and harmful ozone is formed.

The Pacific High-Pressure Cell can exacerbate ozone formation by creating temperature inversions. These happen when warm air in the upper atmosphere traps cooler air lower in the atmosphere like a lid, preventing precursor emissions in the lower atmosphere from moving into the upper atmosphere and thus causing ozone levels to accumulate near the ground level.

Persistent winds can also transport ozone from other cities in the NCCAB or urban areas north of Monterey Bay. Ozone transport is estimated to impact Monterey ozone exceedances significantly. Air quality sensors have traced the source of precursor emissions as far away as San Jose and San Francisco (areas more known for their traffic congestion).⁸

As a highly reactive gas, ground-level ozone can trigger a variety of respiratory symptoms, such as chest pain, coughing, and airway irritation. When paired with respiratory conditions, such as bronchitis, emphysema, and asthma, breathing ozone can lead to more severe symptoms and increased mortality from these conditions.

Particulate Matter (PM)

Particulate matter (PM) pollution comprises respirable particles much smaller than the width of a human hair. The most dangerous particulate pollutants range in size from 10 microns in diameter to as small as 0.003 microns (ultrafine particles).

PM sources generally include:

• motor vehicles
• dust from construction operations
• mobile and stationary construction equipment
• exposed soil eroded by wind (often originating from agricultural fields)
• reactive organic gases (ROGs) from architectural coatings and asphalt paving9
• chemical reactions between gases in the atmosphere

Particulate pollution is common in Monterey during the fall season. PM2.5 levels in the region often reach their highest levels during September and October as a result of increased wildfires during this period. In 2020, PM2.5 in Pacific Grove peaked at 20.8 μg/m³ in September and 13.2 μg/m³ in October. For reference, the World Health Organization (WHO) recommends an annual PM2.5 concentration of 10 μg/m³ or less, maintaining that any level of PM2.5 exposure may have health consequences.¹⁰

Short-term exposure to particulate pollution has been associated primarily with worsening symptoms and pathology of respiratory diseases, including asthma and chronic obstructive pulmonary disease (COPD), that can lead to increased risk of hospitalization and emergency room visits.

A unique source of Monterey air pollution is its booming wine industry, originating from nearly 100 wineries spanning 40,000 grape vineyard acres.¹¹ There are five industrial-sized wineries in Monterey that collectively produce over 150 tons of VOC emissions each year from the fermentation, aging, bottling, and storage processes. The state-mandated cap for VOCs in Monterey is 25 tons per year per facility, though MBARD has set a tighter cap on Monterey at 137 pounds per day (10 tons per year) for seasonal producers, such as wineries.¹² VOCs are precursors to ozone, and those from wineries include carbon dioxide (CO₂), ethanol, and sulfur dioxide (SO₂). Monterey VOC emissions from wineries are seasonal, occurring in late summer and early fall during peak ozone season.

Why is there smoke in Monterey?

Smoke in Monterey is caused primarily by wildfires along the California west coast and can contribute to elevated air quality index (AQI) levels. Since 1999, Monterey County has experienced 15 large wildfires burning 300 or more acres each.¹³ Wildfires in Los Padres National Forest in the northern part of Monterey County have historically been a major source of smoke in Monterey.

Wildfire smoke contains significant levels of ozone precursors as well as PM2.5 and PM10 pollution. These pollutants can irritate the lungs, nose, throat, and eyes as well as contribute to premature death. Fine particulate matter (PM2.5) from wildfire smoke is the biggest health threat to those who breathe it, and can be especially harmful to sensitive groups, such as children, the elderly, and people with heart and lung conditions.¹⁴

Wildfire smoke can travel far from its source. Fires as far away as Oregon and the Pacific Northwest have impacted air quality in Monterey.¹⁵ The length of time a fire burns as well as its intensity both impact the likelihood of the long-distance transport of smoke. The Pacific High-Pressure Cell also allows emissions to travel farther by trapping the smoke in the troposphere and channeling it through wind currents into more populated areas.

As hotter, drier conditions brought about by anthropogenic (human) sources of climate change continue to impact the severity and frequency of this trend, the California Department of Forestry and Fire Prevention (CAL FIRE) has increasingly used low-intensity prescribed burning to clear and reduce the growth of vegetation that provides fuel for wildfires.¹⁶

By maintaining forest floors and slowing or reversing the pace of climate change through reduced and halted carbon emissions, it is also possible to reverse the growing wildfire trend.

Why is there Spare the Air Day in Monterey?

Spare the Air Day in Monterey was started by MBARD to regulate air quality beyond state and national guidelines as well as to encourage residents to take measures to reduce pollution-emitting activities during winter and fire seasons.¹⁷ Like the Bay Area Air Quality Management District (BAAQMD) program, MBARD issues an alert to residents in Monterey, San Benito and Santa Cruz counties when AQI levels are expected to be above “100”, indicating air quality considered “unhealthy.” Unlike the BAAQMD program, which makes certain activities like burning wood illegal during a Spare the Air alert, recommended behaviors that may reduce Monterey air pollution are instead voluntary in the MBARD program.¹⁸

During a Spare the Air Day in Monterey, check online health recommendations for how to reduce exposure to harmful air pollutants. Some health-guarding precautions include:

• wearing a pollution mask outdoors
• avoiding strenuous outdoor activity
• sealing indoor spaces by closing windows and doors
• running an air purifier indoors (if available)

When is air pollution the worst in Monterey?

Monterey air pollution is typically worst from late summer to November due to:

• increased particulate emissions from wildfires
• higher concentrations of ground-level ozone resulting from elevated temperatures
• weather conditions that restrict dispersion

However, different times of year are worse than others for certain pollutants in Monterey. In general, the following periods are the historically worst times of the year for air pollution in Monterey:

• Ozone: June through September
• PM10: October through November
• PM2.5: August through November¹⁹

To understand when pollution levels are or will be their highest, use the Monterey air pollution map and forecast here to plan outdoor activities for the healthiest times. Even when pollution levels are unhealthy, it is possible to reduce one’s pollution exposure. Follow health recommendations on this page for health advice based on real-time conditions.

+ Article Resources

[1] American Lung Association (ALA). (2020). State of the Air.
[2] California Air Resources Board (CARB). (2019). Air quality trends summary.
[3] California Air Resources Board (CARB). (2020). Inhalable particulate matter and health (PM2.5 and PM10).
[4] Monterey Bay Air Resources District (MBARD). (2017). Air quality management plan.
[5] County of Santa Cruz. (2020). Air quality.
[6] Monterey Bay Unified Air Pollution Control District. (2008). CEQA air quality guidelines.
[7] United States Environmental Protection Agency (EPA). (2020). Ground-level ozone basics.
[8] Monterey Bay Unified Air Pollution Control District. (2008). 2008 air quality management plan.
[9] United States Environmental Protection Agency (EPA). (2020). Particulate matter (PM) basics.
[10] World Health Organization. (2005). Air quality guidelines – global update 2005.
[11] Monterey Wine Country. (2020). About Monterey wine country.
[12] Monterey Bay Air Resources District (MBARD). (2020). Advisory committee meeting minutes from February 6, 2020.
[13] Monterey County Office of Emergency Services. (2020). Wildfire.
[14] Environmental Protection Agency (EPA). (2020). How smoke from fires can affect your health.
[15] Monterey Bay Air Resources District (MBARD). (2020). Wildfire smoke information and resources.
[16] California Air Resources Board (CARB). (2020). Prescribed burning.
[17] Hagemann H. (2018, November 4). Knowing when to spare the air. Monterey Herald.
[18] Wright T. (2019, October 31). Monterey Bay Air Resources District creates Spare the Air program. Monterey Herald. ().
[19] California Air Resources Board (CARB). (2019). Top 4 measurements and days above the standard.