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. 105th
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
