Ground Level Ozone: Contributors to Western U.S. Ozone Pollution Include Fossil Fuel Combustion Pollutants (Mostly from Asia) But also International Shipping, Trees, Cooking Emissions, Volatile Chemical Products, Wildfires, and Fertilizers

        According to Wikipedia, average ground-level, or tropospheric ozone (O3) concentrations are about 20-30 parts per billion (ppb) and nearly 100 ppb in polluted areas. Stratospheric ozone is a wholly different issue since people are not exposed to it. About 90% of atmospheric ozone is in the stratosphere and about 10% is in the troposphere. Tropospheric ozone is at its highest levels in the summer months when there is more heat and more light.

     Ground-level ozone is often measured via remote sensing, including spectrometers aboard satellites and ground-based LIDAR, and with in-situ monitoring technology. It is one of the six primary criteria air pollutants. Measurements from ozone monitoring are derived from its UV-absorption properties. Precursors to ozone include NOx, carbon monoxide (CO), and VOCs. NOx emissions are the main culprit.

“Ground-level or tropospheric ozone is created by chemical reactions between NOx gases (oxides of nitrogen produced by combustion) and volatile organic compounds (VOCs). The combination of these chemicals in the presence of sunlight form ozone.”

“Ground-level ozone is created mostly by humans, formed by pollutants — such as nitrous oxide, methane, and volatile organic compounds — that are emitted by cars, trucks, refineries, power plants, and oil and gas development.”

An example is the chain reaction by which CO forms O3, which involves several steps. The net reaction is as follows: CO + 2O2 → CO2 + O3.

     The harmful effects of ozone are very well established and include respiratory irritation, coughing, reduced lung function, inflammation and lung damage, increased susceptibility to respiratory infections, and contributions to heart disease. Some people are exposed to ozone at higher levels or at higher frequencies and some are more vulnerable to it due to health predispositions. Below is a map showing the mostly urban areas where tropospheric ozone levels are the highest. Weather inversions in the Western U.S. can also keep ozone levels higher for longer. Thus, weather is also a big factor in exposure. Background concentrations are also naturally higher in the Western U.S. which can contribute significantly to high O3 levels there.

     According to a 2018 article in Scientific American:

“Ground-level ozone, or O3, is a hazard to human health and the environment, causing respiratory problems and exacerbating asthma while also harming some vegetation and wildlife. Created when by-products of combustion interact with sunlight, it may cause more than a million deaths each year around the world, along with tens of billions of dollars in crop losses.”

They also note that we have made progress in reducing ground-level ozone, citing a 31 percent decrease in the national average ozone concentration from 1980 to 2016. The following map shows that summer O3 concentrations are highest in the U.S. and Europe, and to a lesser extent China.

     Solutions to the ozone problem going forward include capturing and abating more combustion gases, switching to fuels that make less ozone precursors, and planting trees to help take up some of the urban O3. Levels around the world are regulated to stay below 50-70ppb. Ozone pollution can also travel through the atmosphere and drop to ground-level elsewhere as a study showed that ozone precursors from China was dropping to ground level in the Western U.S.

     While trees can abate ground-level ozone some can also create it. Trees release a VOC called isoprene which reacts to form formaldehyde, another smog precursor, when it is hot. Terpenes common to evergreen trees can also contribute to smog. This is the reason the Smoky Mountains are smoky. Ozone precursor VOC emissions are also known as reactive carbon emissions. Naturally occurring VOC emissions are known as biogenic emissions. Some trees often planted in urban environments like oaks and eucalyptus may contribute to the ozone problem by mixing with urban fossil fuel emissions to form ‘smog cocktails.’

     While tropospheric ozone is at its highest levels during the summer months it is increasing in the winter as well in some areas. A study from Hong Kong Polytechnic University found high ozone levels due to alkenes from Chinese petrochemical plants, especially in the early afternoon hours.

     A 2021 article by Yale Environment 360 notes that ozone levels at Sequoia National Park are often as high as they are in Los Angeles. This is due to the migration of the urban pollution as well as the agriculture and industry in the San Joaquin Valley.

     Smog has well-known negative effects on plants such as hampering photosynthesis. More detrimental effects on biodiversity are being discovered. According to the article:

“Ozone is the most damaging pollutant in the world,” said Evgenios Agathokleous, a professor of environmental resources at the Institute of Ecology at Nanjing University of Information Science & Technology in China and one of the top researchers in the field. “It induces the most widespread damage to plants, and it’s a very serious threat to biodiversity.” In some parts of Asia, he said, ozone levels are 10 times the critical thresholds.

     Ozone precursor emissions are rising due to combustion in China, India, and the rest of Southeast Asia. According to the USDA, ozone does more damage to plants than all the other pollutants combined and may even cause food scarcity in the future, according to some estimates. It damages plants in multiple ways. One is to make them more susceptible to diseases. It also disrupts their cycles and relationships to the environment. It shrinks leaf size. It can damage and stunt root growth as well. Some trees are more susceptible to ozone pollution. These include black cherry, white pine, and quaking aspen. Biodiversity is reduced. It can affect scent trails used by pollinators and other insects. The lungs of larger animals can be affected just as humans are.

     Agathokleous states that ground-level ozone is a hidden problem, but I would argue that is increasingly not the case. We can see smog as a haze for one. We can know where and when it is likely to occur, and we have air quality indices that we can pull up on our phones, often our weather app. Thus, measuring and monitoring devices can more or less inform us in real time of ozone dangers.  

     The Western U.S. urban areas are hotspots for ozone for several reasons including significant combustion sources, agriculture and industry, wildfires, and cooking emissions. Two sources, wildfires, and fertilizers, are hard to predict and abate. Their contributions are also difficult to quantify. Fertilizer emissions are NOx compounds emitted when nitrogen-based fertilizers form nitrous oxide compounds. Ian Faloona, in a January 2025 presentation to the American Meteorological Society wrote in his abstract:

“Despite significant progress over the past few decades, several areas of the US still exceed the current ozone National Ambient Air Quality Standard (NAAQS, a running 8-hour average of 70 ppb), including many parts of California, Texas, and the US Southwest. In fact, not only are O3 level improvements conspicuously absent over the last decade or so, in cities like Denver, Salt Lake City, and Phoenix, the trends appear to be increasing. We present here an argument using an observational-based analysis to quantitatively estimate the US anthropogenic, “background”, and wildfire contributions to the temporal and spatial distributions of maximum ozone concentrations throughout the southwestern US, including Texas & California. We find that the background ozone, which we further argue is becoming more and more impacted by soil and wildfire emissions of NOx (the most important ozone precursor), now dwarfs the traditional anthropogenic sources throughout the US Southwest, and precludes future attainment of the NAAQS. The upshot of this research is that the photochemical regime of our atmosphere is fundamentally shifting. Some of the consequences of such a shift include a call to reformulate our existing air pollution regulatory policies which might include an international effort to reduce background ozone concentrations, and/or the implementation of a standard based on the anthropogenic increment above the regionally varying US background. Finally, we point out that the predominant contribution of US background ozone across the southwestern US presents a profound challenge for current air quality modelling efforts because so many of the processes controlling it occur on small spatial scales, in statically stable environments, and evolve over hemisphere-wide distances. We therefore propose the development and use of a hierarchy of atmospheric chemistry models of varying complexity, including simplified conceptual models, to study these processes, similar to what is done in the field of geophysical fluid dynamics which includes ‘dynamical core’ and ‘aquaplanet’ models.

     According to Science News:

“Faloona developed a method to derive how much of the ozone came from various sources, and found a fundamental shift. A steady decrease over the past several decades has now stalled. The vast majority of ozone — 64 to 70 ppb — still wafts in from the Pacific Ocean from sources beyond U.S. borders, as it has since the 1990s. Meanwhile, now-regulated automobile and industrial sources, which once accounted for as much as 15 to 20 ppb in mid-sized cities, now contribute under 6 ppb in most urban areas (excluding megapolises like Los Angeles).”

“Wildfire and soil impacts boost ozone by another 1 to 7 ppb, he found, or up to 50 percent of the excess ozone. In a follow-up study focused on one air basin free of wildfire impacts, he found that some 2 ppb of NOx in the air came from agricultural fertilizers.”

I think one of the big takeaways, which I bolded out above, is the amount of these ozone precursors coming from other countries and “landing” on the West Coast. A 2017 article from NPR noted a paper in Atmospheric Chemistry and Physics that since 1990 NOx emissions from India and China have tripled. Scientists concluded that Asian air pollution contributed as much as 65 percent of an increase in Western ozone in recent years. This increase in Asian emissions served to offset emissions reduction gains in the U.S. The U.S. as a whole reduced Nox emissions by 50% in recent years. Now, it is time for China and India to further reduce their NOx emissions. As they do, less will come from there and more will come from tropical countries in Asia where the production of ozone is more efficient. Again, the takeaway is that most Western U.S. ozone emissions are coming from Asia. Most of that is from NOx precursors, with about 15% from methane as a precursor. Thus, it would be accurate to say that combustion pollution from India and China is harming people in the Western U.S.

     A 2020 paper in Atmospheric Chemistry and Physics modeled chemical transport to derive source-receptor relationships for ground-level ozone by separating sources into NOx and reactive carbon sources, including methane and other anthropogenic VOCs as well as biogenic VOCs. A section from the abstract also notes another important source of both NOx and VOCs – international shipping emissions. A few figures from the paper are given below that.

“Using our novel source attribution technique, we show that emissions of NOx (oxides of nitrogen) from international shipping over the high seas play a disproportionately strong role in our model system regarding the hemispheric-scale response of surface ozone to changes in methane, as well as to the springtime maximum in intercontinental transport of ozone and its precursors. We recommend a renewed focus on the improvement of the representation of the chemistry of ship NOx emissions in current-generation models. We demonstrate the utility of ozone source attribution as a powerful model diagnostic tool and recommend that similar source attribution techniques become a standard part of future model intercomparison studies.”

     While there is not much we can do about Asian pollution wafting in, the focus has shifted to minor local sources that add to the problem. Wildfires are always a concern and are not very predictable, unfortunately.

     A 2025 paper in Atmospheric Chemistry and Physics investigated urban ozone formation from volatile chemical products (VCPs) and cooking emissions, a subset of anthropogenic VOC emissions. The abstract noted significant contributions.

“VOC sensitivity analyses show that anthropogenic VOCs (AVOC) enhance the mean daily maximum 8 h average ozone in Pasadena by 13 ppb, whereas biogenic VOCs (BVOCs) contribute 9.4 ppb. Of the ozone influenced by AVOCs, VCPs represent the largest fraction at 45 %, while cooking and fossil fuel VOCs are comparable at 26 % and 29 %, respectively.”

Thus, they concluded that in the LA Basin about a quarter of anthropogenic VOCs come from cooking. About 45% came from volatile chemical products. VCPs include paints, adhesives, pesticides, and personal care products. Fossil fuel sources made up 29%. 

References:

 

Ozone Pollution: An Insidious and Growing Threat to Biodiversity. Jim Robbins. Yale Environment 360. October 7, 2021. Ozone Pollution: An Insidious and Growing Threat to Biodiversity - Yale e360

New Study Links Ozone Pollution to ‘Green’ Tech—And Its Making Conservation Efforts Worse. Yolisa Mjamba. Animal Planet HQ. March 31, 2025. New Study Links Ozone Pollution to ‘Green’ Tech—And Its Making Conservation Efforts Worse

Trees: Unlikely Culprits in Ozone Pollution. Stephanie Pappas. Live Science. July 23, 2014. Trees: Unlikely Culprits in Ozone Pollution | Live Science

Ground-level ozone. Wikipedia. Ground-level ozone - Wikipedia

Researchers make stunning discovery that shatters previous conceptions of ozone levels: 'Detrimental impacts on human health'. Beth Newhart. The Cool Down. February 16, 2025. Researchers make stunning discovery that shatters previous conceptions of ozone levels: 'Detrimental impacts on human health'

In LA, cooking emissions rival fossil fuels as ozone pollution source. Science X staff. Phys.org. March 13, 2025. In LA, cooking emissions rival fossil fuels as ozone pollution source

Urban ozone formation and sensitivities to volatile chemical products, cooking emissions, and NOx upwind of and within two Los Angeles Basin cities. Chelsea E. Stockwell, Matthew M. Coggon, Rebecca H. Schwantes, Colin Harkins, Bert Verreyken, Congmeng Lyu, Qindan Zhu, Lu Xu, Jessica B. Gilman, Aaron Lamplugh, Jeff Peischl, Michael A. Robinson, Patrick R. Veres, Meng Li, Andrew W. Rollins, Kristen Zuraski, Sunil Baidar, Shang Liu, Toshihiro Kuwayama, Steven S. Brown, Brian C. McDonald, and Carsten Warneke. Atmospheric Chemistry and Physics. Volume 25, issue 2. ACP, 25, 1121–1143, 2025. ACP - Urban ozone formation and sensitivities to volatile chemical products, cooking emissions, and NOx upwind of and within two Los Angeles Basin cities

Ozone Pollution Grows, but It Can Be Fixed. Dave Levitan & Ensia. Scientific American, February 22, 2018. Ozone Pollution Grows, but It Can Be Fixed | Scientific American

Wildfires and farm fertilizer use are fueling ozone pollution. Rachel Berkowitz. Science News. March 20, 2025. Wildfires and farm fertilizer use are fueling ozone pollution

Attribution of ground-level ozone to anthropogenic and natural sources of nitrogen oxides and reactive carbon in a global chemical transport model. Tim Butler, Aurelia Lupascu, and Aditya Nalam. Atmospheric Chemistry and Physics. Volume 20, issue 17. ACP, 20, 10707–10731, 2020. ACP - Attribution of ground-level ozone to anthropogenic and natural sources of nitrogen oxides and reactive carbon in a global chemical transport model

Implementation of the 2015 Primary Ozone NAAQS: Issues Associated with Background Ozone. White Paper for Discussion, December 30, 2015. U.S. EPA. whitepaper-bgo3-final.pdf

Soils, Wildfires, and Background Ozone: The Rise of a New Air Quality Photochemical Regime. Ian C. Faloona. American Meteorological Society. 105^th Annual Meeting. January 14, 2025. Soils, Wildfires, and Background Ozone: The Rise of a New Air Quality Photochemical Regime

Smog In Western U.S. Starts Out As Pollution In Asia, Researchers Say. Bill Chappell. NPR. March 3, 2017. Smog In Western U.S. Starts Out As Pollution In Asia, Study Says : The Two-Way : NPR

US surface ozone trends and extremes from 1980 to 2014: quantifying the roles of rising Asian emissions, domestic controls, wildfires, and climate. Meiyun Lin, Larry W. Horowitz, Richard Payton, Arlene M. Fiore, and Gail Tonnesen. European Geosciences Union. Atmospheric Physics and Chemistry. Volume 17, issue 4. ACP, 17, 2943–2970, 2017. ACP - US surface ozone trends and extremes from 1980 to 2014: quantifying the roles of rising Asian emissions, domestic controls, wildfires, and climate