Air quality in Davis

How is the air quality in Davis, CA?

The city of Davis is located in Northern California, approximately 11 miles west of Sacramento and 70 miles northeast of San Francisco. Davis is the largest city by population in Yolo County and is considered part of the Sacramento Valley, a region prone to strong and frequent inversion layers during the summer.¹ These inversions can negatively impact air quality in Davis and other areas of the Sacramento Valley and become even more problematic during the fall, when lighter winds fail to disperse airborne pollutants at significant volumes.

In addition to inversions, Davis air quality is impacted by its close proximity to Sacramento, the state capital of California with a population roughly seven times larger than that of Davis. Sacramento’s larger population means more traffic and greater emissions of harmful pollutants that find their way through the atmosphere to its smaller neighbor to the west.

The American Lung Association State of the Air (SOTA) report presents air quality averages, grades, and rankings over a series of rolling three-year periods, for three key pollution measures (high ozone days, 24-hour particle pollution, and annual particle pollution).² The most recent SOTA report revealed that Yolo County’s air quality was relatively poor: two key measures, 24-hour PM2.5 and daily ozone, were both given C grades, while the third measure, annual PM2.5, received a PASS grade.

A closer look at the numbers reveals both positive and negative results.

On the positive side, the longer-term trend for Yolo County ozone levels has been quite good, staying at or below the pass/fail line during nearly the entire past decade.

However, the county’s air quality needs improvement when it comes to both particle pollution metrics, as they have exhibited troubling trends.

First, there was a rise in 24-hour particle pollution since the 2014-2016 measurement period. After reaching a low of 0 high-particle days for three successive periods, the number rose to 0.8 days in the 2015-2017 period, then up to 2 days for both 2016-2018 and 2017-2019.

Second, while the county’s annual particle pollution level of 9.8 μg/m³ was good enough to receive a passing grade (since it was below the pass/fail cutoff of 12 μg/m³), average concentrations increased during each of the three most recent measurement periods, reflecting the rise in short-term particle pollution. From its low of 6.6 μg/m³ in the 2014-2016 period, the number rose to 7.5 μg/m³ in 2015-2017, then jumped to 9.3 μg/m³ in 2016-2018, followed by another increase to 9.8 μg/m³ in 2017-2019.

Davis’s air quality index (AQI) for 2018 averaged 119 – worse than the national AQI of 74, with the city experiencing AQI spikes in late fall that doubled the average reading.³

Why is Davis air quality bad today?

A number of factors contribute to the current air quality in Davis, but air quality in the city is generally considered bad when the area experiences elevated levels of either ozone or particle pollution (especially smaller PM2.5 particles). When both levels are elevated, air quality is especially dangerous, and outdoor activities should be reduced or avoided altogether.

One of the biggest drivers of air pollution in Davis is seasonality. Summer months mean more smoke from wildfires as well as lighter winds and very low rainfall levels, all of which can increase harmful PM2.5 and ozone (which forms more readily when temperatures rise and winds are absent or stagnant). During winter, the situation reverses, with more wind, rain, and lower temperatures; these changes mean fewer (or no) wildfires, which tends to reduce the amount of smoke and ash in the air, reducing PM2.5 and air pollution in Davis overall. However, winter can also increase the amount of wood burning in area homes, which can cause smoke and ash levels to rise.

Air quality in Davis may also suffer when reduced air pollution dispersion occurs due to inversions in the Sacramento Valley. Inversions are especially bad for air quality because air gets trapped near ground level and cannot rise in the normal process of dispersion, during which pollutants get cleared out. In the Sacramento Valley, these negative inversion effects are most troublesome during the fall, when relatively weak winds fail to offer enough dispersion. Along with fall’s elevated temperatures as summer heat wanes, and fall tends to be when Davis air quality is most affected.

Yolo County air pollution has increasingly worsened in recent years. With Davis being the county’s most populous city, it is important to track the city’s air quality if you plan to be outside for long periods of time. Please use this page to quickly and comprehensively track air quality in Davis. To find out the current air quality outlook, go to the top of this page and look under “Forecast,” where you will find data such as the Davis air quality index (AQI) for today and every day in a ten-day window.

On days when pollution levels in Davis rise to unhealthy levels, those who are sensitive to particle pollution should either remain indoors or curtail outdoor activities.

Where does Davis air pollution come from?

Several factors can cause the air in Davis to become polluted.

The increasing number of Northern California wildfires, caused in part by increasingly hotter summers, is a major factor that can negatively impact Davis air pollution levels. These fires can cause a rapid rise in unhealthy particle pollution levels caused by smoke and ash in the air.

Another factor is the air quality in neighboring Sacramento, which exerts a big influence over the state of Davis’s air. Sacramento, a mere 11 miles away, has a population roughly seven times that of Davis (approximately half a million in comparison to about 70,000 in Davis).^4,5 Increased traffic and industrial activity causing greater emissions in Sacramento can drastically affect the air in Davis. Sacramento County has received even worse grades in previous SOTA reports (F for both ozone and 24-hour particle pollution) than those of Yolo County, and the city’s influence on Davis air quality tends to be negative.⁶

While the increasing number of wildfires during hotter months is one of the primary causes of dangerous pollution levels in Davis and surrounding areas, wood burning can be a major contributor to air pollution during colder months, when a large number of Northern California wood stoves and fireplaces are active and cause smoke and ash levels to rise.

Has air quality in Davis improved?

Ozone pollution in Davis has improved dramatically since 2000. However, annual and 24-hour particle pollution have been trending in a negative direction in recent years.

Annual particle pollution (the annual average concentration) had been decreasing steadily since it reached a high of 10.5 μg/m³ in the 2000-2002 period. After that period, the metric reached and maintained its lowest levels (between 6.6 and 7.2 μg/m³) for several consecutive periods (from 2009-2011 to 2014-2016). Then, the trend reversed, with annual particle pollution numbers rising to 7.5 μg/m³ in 2015-2017, then 9.3 μg/m³ in 2016-2018, and up to 9.8 μg/m³ in 2017-2019 period. The most recent elevated number is still below the pass/fail cutoff of 12 μg/m³, but the upward trend for annual particle pollution in Yolo County should be concerning to county officials, as action will be needed in order to return to the lows of the previous decade.

A relatively long period of low numbers was documented for 24-hour particle pollution between the 2008-2010 and 2014-2016 measurement periods (just 0.3 days for four consecutive periods, followed by an all-time low of 0 days for three straight periods). Then, the numbers started rising, just as the annual particle pollution numbers did: up to 0.8 days in 2015-2017, followed by an increase to 2 days in 2016-2018, where it remained for the 2017-2019 period.

In short, both measures of Yolo County particle pollution have worsened during recent years, which means that air quality in Davis has almost certainly suffered as well.

While Yolo County particle pollution has generally taken a turn for the worse in recent years, ozone levels (as measured by the annual weighted average number of High Ozone Days) have decreased dramatically since 1998-2000, when the county saw its highest reading (26.2 days). The High Ozone Days measure has steadily decreased since this all-time high until finally reaching and then passing below the pass/fail cutoff of 3.2 days. Furthermore, the county has maintained these low ozone numbers for nearly a decade: in 2009-2011, the number of High Ozone Days finally dropped close to the cutoff (4.3 days versus the 3.2 days cutoff), then met the cutoff by 2012-2014, and has stayed at or below the cutoff in every period since.

What is Davis, California doing about air pollution?

Yolo County, where Davis is located, received a C grade for both ozone and 24-hour particle pollution during the most recent measurement period. Positive action is needed in order to combat and reverse this troubling trend toward more polluted air in the Davis region.

The University of California at Davis spearheaded an initiative to report on how California can decarbonize transportation by the year 2045.⁷ Such decarbonization, if done properly, could have a major positive impact not only on air quality in Davis but also in the entire state, as vehicle emissions are a major source of harmful airborne pollutants throughout California.

The study’s equity aspects identified policy options with the goal of making sure benefits gained from the transition to low-carbon transport will first go to communities in need and avoid prolonging past inequities caused in disadvantaged communities by California’s transportation choices.

In parallel with the above initiative, the city of Davis issued a ban on both gas-powered and electric leaf blowers in an effort to combat poor air quality, implemented with a unanimous vote by the Davis City Council.⁸ The Chair of the city’s Natural Resources Commission pointed out the need to avoid extra stresses on the area’s air given the unprecedented amounts of smoke and ash caused by recent wildfires. This indicates that local officials recognize the dangerous state of the air in Davis and the factors causing it. While the ordinance was only temporary, the city is not ruling out taking longer-term, more permanent action.

+ Article Resources

[1] Lorenzen A. (1974). Climate of the Sacramento Valley air basin. California Air Resources Board: Division of Technical Services.
[2] American Lung Association. (2020). Yolo County. State of the Air.
[3] Davis, California (CA) – profile. (2021). City Data.
[4] Sacramento, California population 2021 (demographics, maps, graphs). (2021). World Population Review.
[5] Davis, California population 2021 (Demographics, Maps, Graphs). (2021). World Population Review.
[6] American Lung Association. (2020). Sacramento. State of the Air.
[7] UC Davis News and Media Relations. (2021, April 21). Decarbonizing California transportation by 2045. UC Davis.
[8] Solomon S. (2020, September 16). Leave the air alone: Leaf-blower ban in Davis passes unanimously. ABC 10.