Western North America has spent decades reducing many of the emissions that help produce ground-level ozone. A new study suggests that those efforts have changed the atmosphere without making the region’s ozone background easier to control.
The research finds that relatively low ozone concentrations in the free troposphere above western North America have risen since the mid-1990s, even as local nitrogen-oxide emissions declined. The authors associate the change with a combination of emissions and transport involving Southeast Asia, the tropical Pacific, shipping and aircraft.
That conclusion needs to be stated carefully. The study does not show that local air-quality controls have failed, nor does it assign every ozone exceedance in the western United States to pollution arriving from overseas. Its importance is narrower and more consequential: the atmospheric baseline against which western air quality is measured is changing, and the remaining sources are increasingly difficult to assign to a single jurisdiction.
What the study measured
The paper, published in Atmospheric Chemistry and Physics, examined observations from 1994 through 2021 across western North America, roughly between 25 and 55 degrees north and 130 and 90 degrees west. Rather than concentrating only on average ozone levels or the most polluted episodes, the researchers examined observations below the 33rd percentile.
This lower tail is useful because it is less dominated by short-lived local pollution events. It offers a way to study what the researchers describe as the changing remote or background state of the atmosphere. The analysis combined aircraft, lidar and balloon-based observations with atmospheric-transport and chemical models. The published study is available from the journal Atmospheric Chemistry and Physics, with associated research records available through the NOAA Institutional Repository.
The researchers used FLEXPART-ERA5 source-receptor relationships to estimate where air reaching western North America had traveled. They also used chemical-model results to examine changes in ozone production in upwind regions. This is a serious evidentiary framework: the result does not rest on a single monitor, a short episode or a press-release interpretation.
What appears to be changing
The study found that low ozone over western North America was commonly associated with air originating over the tropical Pacific and Southeast Asia. The lower and middle troposphere did not show a clear, uniform trend in transport pathways across the entire period. But the upper-troposphere and lower-stratosphere analysis showed increasing source-receptor relationships, suggesting a stronger potential influence from high-altitude transport.
The authors report that chemical modeling found net ozone production over Southeast Asia increased by 157 percent in the lower troposphere and 7 percent in the free troposphere between 2007 and 2019. Those are model-derived changes in ozone production over a source region. They are not a direct measurement of how much ozone reached Los Angeles, Salt Lake City, Phoenix or any other particular city.
The study also identifies shipping and aviation as relevant contributors. Shipping emissions can affect ozone chemistry over the Pacific, while aircraft emissions influence the upper troposphere and lower stratosphere. The authors link rising aircraft emissions and changing trans-Pacific transport to the increase in low-ozone conditions observed aloft over western North America.
The most defensible summary is therefore that remote pollution appears to be contributing more to the western North American ozone background, with Southeast Asian emissions, shipping and aircraft among the likely contributors. The evidence does not establish a fixed percentage attributable to any one country, industry or transportation sector.
Why local progress still matters
It would be a mistake to read this study as evidence that domestic pollution controls no longer work. They do. Reducing nitrogen oxides and other ozone precursors changes local chemistry and reduces pollution that communities can control directly.
But local success can make the remaining background more visible. If locally produced ozone declines while imported or remotely generated ozone rises, the total concentration may fall more slowly than regulators or residents expect. A changing background can therefore make additional local reductions appear less effective even when those reductions are producing real atmospheric benefits.
This is a measurement problem as much as a regulatory one. A state may be responsible for meeting a health-based standard at a monitor while controlling only part of the emissions that influence that monitor. The public still experiences the ozone regardless of where its precursors originated.
Background ozone is an accounting category, not a separate chemical
The phrase “background ozone” can sound more precise than it is. It does not describe a separate kind of ozone molecule. Depending on the definition being used, background ozone can include natural ozone, stratospheric intrusions, wildfire-related pollution, pollution transported from outside North America and pollution formed in one region before moving to another.
That distinction matters because different sources create different regulatory questions. A stratospheric intrusion is not treated in the same way as routine emissions from an overseas industrial region. International transport may be relevant to a regulatory demonstration, but the scientific showing must be specific enough to address a particular location, time period and standard.
The Environmental Protection Agency’s explanation of international transport recognizes that ozone entering the United States from outside the country is one component of the broader ozone background. The agency’s framework does not turn a regional model into an automatic exemption from local controls. It requires evidence about what can and cannot reasonably be controlled.
The central uncertainty is attribution
The study’s broad observation is stronger than its most specific attribution. The researchers combine long-term observations, transport histories, emissions inventories and chemical modeling. That combination supports the conclusion that remote influences are becoming more important.
But models do not place a physical label on each ozone molecule. Their results depend on meteorological reanalysis, emissions inventories, chemical mechanisms and assumptions about how air moves through different atmospheric layers. Foreign emissions inventories may be less complete or precise than inventories for regulated domestic facilities. Shipping routes, aircraft activity, fuel composition and industrial emissions can all change over time.
There is also a vertical gap between the study and the public’s ordinary experience of air pollution. Much of the analysis concerns ozone several kilometers above the surface. Ozone aloft can contribute to surface concentrations, but that contribution depends on weather, mixing, terrain and the structure of the atmosphere. An increase in free-tropospheric ozone is not identical to a measured increase in human exposure at ground level.
For the same reason, the study does not establish that aircraft or ships are the dominant cause of surface ozone exceedances in any particular western city. Nor does it show that every high-ozone episode in California, Nevada, Utah, Arizona or the Pacific Northwest is imported.
What the study does—and does not—change
The research supports several conclusions. Western North American ozone is not determined solely by local emissions. Declining local nitrogen-oxide emissions do not guarantee that every component of the regional ozone background will decline. The free troposphere and upper atmosphere matter to western ozone trends, and trans-Pacific transport is part of the region’s changing atmospheric baseline.
It does not establish a new nationwide ozone crisis, prove that local controls are ineffective or determine whether any particular western area would meet federal standards if foreign emissions disappeared. The study period also ends in 2021, leaving later changes in aviation, shipping, wildfire activity and regional emissions outside the analysis.
Those limitations are not reasons to dismiss the finding. They define what must be measured next. Independent observations at high-elevation sites, urban monitors, aircraft platforms and balloon stations can help test whether the modeled changes aloft are appearing at the surface. Additional work can also test whether the source attribution holds across seasons, locations and weather patterns.
The policy problem is becoming harder, not simpler
Air-quality regulation is usually organized territorially: identify sources, regulate emissions and assess conditions within a jurisdiction. That structure is most effective when the principal sources are nearby and controllable. It becomes less straightforward when the remaining pollution baseline is shaped by emissions crossing an ocean, moving through the upper atmosphere or mixing with natural processes.
The answer is not to abandon local controls. Local emissions remain among the parts of the problem that regulators can directly influence. The answer is to judge local progress against a changing atmospheric baseline and to improve the evidence used to distinguish local, regional, international and natural contributions.
That distinction matters for public health as well as administrative responsibility. The EPA identifies ground-level ozone as harmful to health, particularly for people with asthma, children, older adults and people who work or exercise outdoors. The source of the ozone may determine who is responsible for reducing it, but it does not make exposure harmless.
The new study therefore points toward a more complicated conclusion than either “American air-quality rules no longer work” or “the pollution is foreign, so nothing can be done.” Local controls have changed the atmosphere. They have not stopped the atmosphere from changing elsewhere.
The next stage of western ozone policy will require both efforts: continued reductions in emissions that states and the federal government can control, and better international and atmospheric measurements of what arrives from beyond their jurisdiction. The scientific challenge is no longer merely detecting ozone. It is assigning responsibility without mistaking a regional model for a local verdict.
